For the successful hard surfacing or overlaying operation a welding procedure should be established. The procedure should be related to the particular part being surfaced and the composition or analysis of the part. It should specify the welding process to be used, the method of application, the prewelding operations such as cleaning, undercutting, etc.

The welding procedure should also give the preheat and interpass temperature and any special techniques that should be employed, such as the pattern of hardsurfacing, the method of welding whether beading or weaving, the interface between adjacent beads, and finally, any postwelding operations such as peening and the method of cooling. When a properly developed procedure is followed the service life of the job will be predictable.

The selection of the surfacing alloy was discussed. In many cases two separate materials may be required: the buildup alloy, which is used when the part is to be reclaimed or is excessively worn, and the hardfacing alloy. In general, over three layers of hardfacing alloys are not deposited. The hardfacing alloys are considerably more expensive than buildup alloys. The hard-surfacing should be replaced when the hardfacing alloy is worn away.

When deposit exceeds three layers other problems may be encountered such as cracking, etc., which will influence the service life of the deposit. The other factor to be considered is dilution. This is the diluting of the hardfacing alloy with base metal. Excessive dilution will reduce the effectiveness of the hard-facing material. Excessive penetration and poor tie-in of adjacent beads should be avoided.

In common with all welding operations, fabrication, repair, or surfacing, the base metal to be welded must be clean. Shot blasting, grinding, machining, and brushing are all methods for cleaning the base metal prior to welding. A major consideration is the location of finished surface with respect to the worn surface. In many cases, the first layer of surfacing may have sufficient dilution of base metal so that it is unsuitable for the desired service.

In this case, the worn surface should be further removed so that there is sufficient room for two layers of surfacing metal. This will provide a better service life. There are other situations in which the part is to be re-machined after surfacing and it is not recommended that the machining surface be at the interface between weld surfacing metal and the base metal. Here again, pre-machining may be required. This is particularly important when the base metal is of hardenable material.

Preheating, interpass temperature, and cooling of the part being surfaced are as important in surfacing as they are in repair welding or in original fabrication. The factors that apply to the welding of the base metal in normal fabrication should be followed when overlaying with surfacing weld metal. Preheating is used to minimize distortion, to avoid thermal shock, and to prevent surfacing cracking. The temperature of preheat depends on at least two factors, the carbon and the alloy content of the base metal and the mass of the part being surfaced.

A soak type preheat should be used and sufficient time must be allowed for the preheat temperature to stabilize throughout the part. If it is extremely complex in shape, preheat should be increased. If the ambient temperature is low, preheat should also be increased. Any part that is preheated should be maintained at that temperature throughout the entire welding operation and should then be allowed to slow cool.

The base metal composition must be known in order to provide proper preheat temperatures. Certain materials such as austenitic manganese steel should be treated in accordance with the requirements of the steel. In this case preheat should not exceed 260°C (500 °F). Cast iron should be given sufficient preheat. Cast iron is crack sensitive and normally it is not hardsurfaced because it is relatively inexpensive compared to the cost of surfacing metal and it may be more economical to replace the part than to surface it.

The thickness of the surfacing deposit is extremely important. If the deposit is too heavy, problems can be encountered. Hardfacing alloys should be restricted to two layers. The first will include dilution from the base metal, but the second layer should provide the properties expected. Some types of alloys can be used in three layers.

Surfacing for corrosion resistance and others are recommended for use of one layer only. When edges are being built up with surfacing material make sure that sufficient material is removed so that the edge has at least two layers of surfacing metal prior to re-machining or grinding. Consult the manufacturer’s data for the particular product involved.

A weaving technique is recommended instead of stringer bead welding. In addition, the pass thickness or layer thickness should not exceed 5 mm. The adjacent beads must fair into the previous bead to provide as smooth a surface as possible.

There is considerable controversy concerning the exact pattern of welds that should be made when applying the surfacing deposits. In general, the direction of welding should not be transverse to the load on the part. This can create stress concentrations and may affect service life of the part. Diagonally-shaped welds have an advantage in this regard. In certain types of metal, peening is recommended but this is based on the metal. The manufacturer’s instructions should be followed.

Hardfacing by welding is an excellent method of reclaiming parts and will save considerable time and money. It will often reduce downtime of equipment and may keep equipment going without as much downtime. It is considerably cheaper than replacing original parts and should be used whenever possible. It is now becoming popular for original equipment manufacturers to actually hardface wear parts on new equipment to provide better service life of the equipment.

The corrosion of metals is one of the less known but more expensive of the factors that cause premature failures of many things. These range from automobile bodies to chemical plant equipment to ship hulls. The cost of corrosion is difficult to measure but should include the loss of efficiency of operating equipment such as pumps, mixers, valves, etc., as well as total failures. Fortunately, corrosion can be prevented or at least substantially reduced so that metal parts will have a longer life cycle.

One of the best ways to reduce corrosion is to protect the metal with an overlay or surface of a material less susceptible to corrosion in a specific environment. Coated metals such as galvanized steel and clad metals with nonferrous facings have long been used to reduce the effect of corrosion. In more and more applications, surfaces that are sprayed or welded are contributing to longer service life of parts exposed to corrosive atmospheres.

It was mentioned previously that the deterioration of metal surfaces is caused by the combination of factors, such as corrosion and oxidation, corrosion and erosion, or cavitation.

In repairing corroded or deteriorated surfaces it is necessary to analyze the reason for the deterioration. These factors should be considered in designing new surfaces for specific types of service. That is, consideration should be made in selecting a material for overlays to prevent corrosion.

There is considerable confusion in this field concerning the proper terms to use for the weld surface that is applied. The general term surfacing is sometimes used but more often the term cladding is used. The terms weld buildup and buttering have no official status; however, the term corrosion-resistant weld-overlay cladding does provide an understanding of what is being covered in this section.

Cladding of this type is applied for many reasons:

1. to produce a corrosion-resistant surface as on the inside of a nuclear pressurized vessel,
2. to produce a corrosion-resistant material to replace a higher-priced or unavailable high-alloy material,
3. to produce a metallurgical structural composition that is more weldable,
4. to deposit weld metal which would later be used as a filler metal such as in a tube-to-tube sheet weld, or
5. to produce a wear- or erosion-resistant surface.

Various methods or techniques can be used to provide these surfaces, such as explosive clad metal, roll bond clad, and loose cladding liners, plug and seam welded to the inside of a vessel or tank. Many of the welding processes can be used for applying liners. When attaching liner plates or sheets to carbon mild steel the problems of welding dissimilar metals must be considered. This involves the metallurgical requirements of the clad material and the compatibility of the two materials.

When the solid solubility, that is, ability of one element to be dissolved in another, is exceeded, cracking may occur. In addition, the effects of elements such as sulfur and phosphorous from dilution can be a source of trouble. The welding technique and procedure involving the selection of filler metals, coatings, fluxes, etc., must be considered as with any dissimilar welding operations. These same factors apply whether the material is being applied as a weld surfacing or as separate sheets or plates welded to the carbon steel structure.

There are a number of alloys that are used for overlays or clads for corrosion and oxidation resistance. These are usually standardized compositions commonly used by themselves for the same requirements. These are summarized as follows.

The copper-based alloys are used for certain corrosion requirements. The copper silicon alloys and the copper tin alloys are used for certain corrosion-resistance requirements.

The austenitic stainless steels, which include the standard alloy types 308, 309, 310, 316, and 347, are all used for corrosion-resistant surfaces. These alloys exhibit moderate resistance to high-stress abrasion and have excellent oxidation-resistance and impact properties.

The nickel-base alloys are also used for this purpose. This includes 100% nickel, the Monel (67 Ni-30 Cu) and Inconel (72 Ni-7 Fe-16 Cr). These alloys are frequently used as overlays on carbon and low-alloy steels for cladding of tanks and vessels.

The high-cobalt chromium alloys are used for specific overlays when corrosion is a major problem. These are used quite often in refineries where high pressures, high temperatures, and corrosive materials are pumped and stored. These alloys can be applied in several ways; as a powder applied by the plasma process, as a cold wire, or by covered electrodes with the shielded metal arc welding process. The selection of the overlay is based entirely on the requirements of the materials to which the product is exposed. The selection must be based on normal metallurgical factors.

A unique but rather typical application of weld overlay is used in the repairing of digesters used in pulp and paper mills. Digesters are tanks or pressure vessels ranging in height from 25-50 ft and in diameter from 8-12 ft. They are used for the first chemical processing step of converting wood chips into pulp for paper manufacturing, primarily in the sulphate or kraft paper process.

The wood chips are placed in the digester and are cooked in a highly corrosive alkaline solution. The mixture of wood chips and alkaline liquor is under pressure and operates at a relatively high temperature. The digester is made of carbon steel of from 1 to 2 inches thick. The internal surfaces of digesters corrode at a high rate at the surface of the liquor due to the corrosive action of the alkaline solution. The steel walls of the vessel will gradually deteriorate until they become so thin that pressures and temperatures must be reduced for safety. Unless the metal is replaced by welding the digester will eventually become unsafe and will have to be abandoned.

Welding has been employed to repair the pitted or corroded areas and to rebuild wall thickness to original dimension. Originally, carbon steel weld metal was used. It was found, however, that stainless steel electrodes provide a surface that is less subject to the corrosive action. Tests revealed that stainless overlay outlasts the original carbon steel many times.

Recently the gas metal arc welding (GMAW) process has been used for this overlaying operation. Automatic methods have been used to make the overlay welds more rapidly than with manual application. The automatic application will deposit weld metal in horizontal beads on the vertical inside circumference of the tank. The automatic welding heads are mounted on a boom that rotates about the centerline of the tank and deposits metal as it revolves inside the tank. It is possible to utilize two or even three automatic heads that automatically travel around the inside circumference of the tank. This work is done starting at the lower portion to be welded and moves upwards as it revolves. The most popular procedure uses either 316 or 310 stainless alloy in the 0.035-in. diameter electrode wire with argon for shielding.

In normal applications the inside diameter of the digester is prepared for welding by grit blasting the entire surface to be welded. This may be followed by an acid wash and water rinse. The welding operation, once it is begun, is usually continuous to eliminate any voids in the surface. Each pass must fair smoothly into the previous one and the depth of the surface should be from 1/8 to 3/16 of an inch thick.

This technique is used occasionally for new digesters to reduce the rate of corrosion and the length of time between maintenance repair work. Penetration must be closely controlled so that dilution will not appreciably lower the alloy content of the deposit.

Other procedures for accomplishing an overlay on the inside diameter of smaller tanks are done by rotating the tank and doing the welding in the flat position. In this case, the welding is done by the submerged arc process using one or more electrode wires. There are some situations in which the strip overlay method is used. The submerged arc welding process increases the speed of making the overlay. In some cases the gas tungsten arc welding (GTAW) or plasma hot wire process is used. The process should be selected which is most appropriate for the position and the job to be done.

Normally single layers are used; however, for certain applications such as pump linings and wear areas a second layer of surfacing is applied. The second layer can be made with an electrode of lower alloy content since the dilution factor is drastically reduced.

Wear

The deterioration of surfaces is a very real problem in many industries. Wear is the result of impact, erosion, metal-to-metal contact, abrasion, oxidation, and corrosion, or a combination of these. The effects of wear, which are extremely expensive, can be repaired by means of welding. Surfacing with specialized welding filler metals using the normal welding processes is used to replace worn metal with metal that can provide more satisfactory wear than the original. Hardfacing applies a coating for the purpose of reducing wear or loss of material by abrasion, impact, erosion, oxidation, cavitations, etc.

In order to properly select a hard facing alloy for a specific requirement it is necessary to understand the wear that has occurred and what caused the metal deterioration. The various types of wear can be categorized and defined as follows:

Impact wear is the striking of one object against another. It is a battering, pounding type of wear that breaks, splits, or deforms metal surfaces. It is a slamming contact of metal surfaces with other hard surfaces or objects. A good example is the impact encountered by a shovel dipper lip or tamper.

Abrasion is the wearing away of surfaces by rubbing, grinding, or other types of friction. It usually occurs when a hard material is used on a softer material. It is a scraping or grinding wear that rubs away metal surfaces. It is usually caused by the scouring action of sand, gravel, slag, earth, and other gritty material.

Erosion is the wearing away or destruction of metals and other materials by the abrasive action of water, steam or slurries that carry abrasive materials. Pump parts are subject to this type of wear.

Compression is a deformation type of wear caused by heavy static loads or by slowly increasing pressure on metal surfaces. Compression wear causes metal to move and lose its dimensional accuracy. This can be damaging when parts must maintain close dimensional tolerances.

Cavitation wear results from turbulent flow of liquids, which may carry small suspended abrasive particles.

Metal-to-metal wear is a seizing and galling type of wear that rips and tears out portions of metal surfaces. It is often caused by metal parts seizing together because of lack of lubrication. It usually occurs when the metals moving together are of the same hardness. Frictional heat helps create this type of wear.

Corrosion wear is the gradual eating away or deterioration of unprotected metal surfaces by the effects of the atmosphere, acids, gases, alkalies, etc. This type of wear creates pits and perforations and may eventually dissolve metal parts.

Oxidation is a special type of wear indicated by the flaking off or crumbling of metal surfaces, which takes place when unprotected metal is exposed to a combination of heat, air, moisture. Rust is an example of oxidation.

Corrosion (erosion) wear takes place at the same time. This can happen when corrosive liquids flow over unprotected surfaces.

Thermal shock is a problem indicated by cracking or splintering, which is caused by rapid heating and cooling cycles. While not exactly a wear problem it is a deterioration problem and is thus considered here.

Many of the above types of wear occur in combination with one another. It is wise to consider not only one factor, but to look for a combination of factors that create the wear problem in order to best determine the type of hard facing material to apply. This is done by studying the worn part, the job it does, how it works with other parts of the equipment and the environment in which it works. With these factors in mind it is then possible to make a hardfacing alloy selection.

Hardfacing Alloy Selection
Unfortunately, there is no standardized method of classifying and specifying the different surfacing weld rods and electrodes. Many of the hard facing electrodes commercially available are not covered by any of most used specifications. Various filler metal suppliers provide data setting forth classes of service and have categorized their own products within these classes. Many suppliers also provide complete information for using their specific products for various applications and for different industries such as quarrying, steel mills, foundries, etc.

The best system of classification has been established by the American Society for Metals Committee on Hardfacing. In this system, there are five major groups classed according to total alloy content other than iron, with subdivisions based on the major alloying elements. Most of these alloys are available as solid bare filler rod in straightened lengths or in coils or covered electrodes. Some of the materials are available as powder for special applications.

The following is a brief description of the five major groups, what they contain as alloys, and where they are recommended.

Group 1 is the low-alloy steels that, with few exceptions, contain chromium as the principal alloying element. The subgroup 1A has alloy content 2-6% including carbon. These alloys are often used as buildup materials under higher-alloy hard facing materials. The Group 1B is similar except that they have a higher alloy content, ranging from 6-12%. Several alloys in the group have higher carbon content exceeding 2%, and include several alloy cast irons.

The alloys of Group 1 have the greatest impact resistance of all hardfacing alloys except the austenitic manganese steels (Group 2D) and have better wear resistance than low or medium carbon steels. They are the least expensive of the alloy surfacing materials and are extremely popular. They are machinable and have a moderate improvement over the wear properties of the base metal to which they are welded. They have a high compressive strength and fair resistance to erosion and scratch abrasion.

Group 2 contains higher alloyed steels. Group 2A has chromium (Cr) as the chief alloying element with total alloy content of 12-25%. Many of these alloys also contain molybdenum. Those with over 1.75% carbon are medium-alloy cast irons. Group 2B has molybdenum (Mo) as the principal alloying element but many of these also contain appreciable amounts of chromium. The hardfacing alloys of Groups 2A and 2B are more wear resistant, less shock resistant, and more expensive than those in Group 1.

Groups 2A and 2B are quite strong and have relatively high compressive strengths. They are effective for rebuilding severely worn parts and are used for buildup prior to using higher alloy facing materials. They provide high impact resistance and good abrasion resistance at normal temperatures.

Group 2C contains tungsten and modified high-speed tool steels. They are excellent choices at service temperatures up to 590°C (1100°F) and when good resistance coupled with toughness is required. They are not considered as good high abrasion-resistant types but are resistant to hot abrasion up to 590°C (1100°F) and exhibit good metal-to-metal wear at elevated temperatures.

Group 2D are the austenitic manganese steels, which contain either nickel or molybdenum as stabilizers. The alloys in Group 2D are highly shock resistant but have limited wear resistance unless subjected to work hardening. The total alloy content ranges from 12-25%. This group is excellent for metal-to-metal wear and impact when the deposit is work hardened in use. The as-welded deposit hardness is low, from 70 to 230 BHN, but will work harden to 450-550 BHN. The deposit may deform under battering but it will not crack. The deposit should not be heated to above 260°C (500°F), which would cause embrittlement.

Group 3 contains higher-alloyed compositions ranging from 25-50% total alloy. They are all high-chromium alloys and some contain nickel, molybdenum, or both. The carbon can range from slightly under 2% to over 4%. The alloys in this group exhibit better impact, erosion resistance, metal-to-metal wear, and shock resistance than the previous groups. The 3B grouping will withstand elevated temperatures of up to 540°C (1000°F). The 3C group is high in cobalt which improves high-temperature properties. The Group 3 alloys are more expensive than Groups 1 and 2.

The compositions within Group 4 are nonferrous alloys either cobalt base or nickel base with total content of nonferrous metals from 50 to 99%.

The Group 4A alloys are the high-cobalt-based alloys with high percentage of chromium. These alloys are used exclusively for applications subjected to a combination of heat, corrosion, erosion, and oxidation. They are considered the most versatile of the hard facing materials. The alloys with higher carbon are used for applications requiring high hardness and abrasion resistance but when impact is not as important. These alloys are excellent when service temperatures are above 650°C (1200°F). They resist oxidation temperatures of up to 980°C (1800°F).

The Group 4B alloys are the nickel-based alloys which contain relatively high percentages of chromium. This group of alloys is excellent for metal-to-metal resistance, exhibits good scratch abrasion resistance, and corrosion resistance. They will retain hardness to 540°C (1000°F). The alloys with higher carbon content provide higher hardnesses but are more difficult to machine and provide for less toughness. These alloys show good oxidation resistance up to 950°C (1750°F).

The Group 4C alloys are the chrome-nickel cobalt alloys and all are recommended for elevated temperatures. The high-nickel alloy has excellent resistance to hot impact, abrasion, and corrosion and moderate resistance to wear and deformation at elevated temperatures. The medium-nickel alloy has high-temperature wear resistance and impact resistance. It also provides resistance to erosion, corrosion, and oxidation. The low-nickel alloy is used for moderate high temperatures and provides good edge strength, corrosion resistance, and moderate strength.

The Group 5 alloys provide a tungsten carbide weld deposit. This deposit consists of tungsten carbide particles distributed in a metal matrix. The matrix metals include iron, carbon steel, nickel-based alloys, cobalt-based alloys, and copper-based alloys. The tungsten carbide particles are crushed to mesh sizes varying from 8 to 10 down to 100 and have excellent resistance to abrasion and corrosion, and moderate resistance to impact. The matrix material determines the resistance to corrosion and high-temperature resistance. The finish of the deposit depends on the tungsten carbide particle size: the finer the particles the smoother the finish. The deposits are not machinable and are very difficult to grind.

There are three most important set of operations for repair welding:

1. Preparation for welding
2. Repair welding
3. Postweld operation

This article introduces actions and procedures for repair welding and surfacing.
Preparation for Welding
A large number of factors should be considered and decisions made before starting to weld.

Safety. The repair welding location or area must be surveyed and all safety considerations satisfied. This can include the posting of the area required by certain regulations, removal of all combustible materials from the area, the draining of fuel tanks of construction equipment, aircraft, boats, trucks, etc. Other precautions include the elimination of toxic materials such as thick coats of lead paint, plastic coverings of metals, etc.

Cleaning. The immediate work area must be clean from all contaminants and this includes removal of dirt, grease, oil, rust, paint, plastic coverings, etc., from the surface of the parts being welded. The method of cleaning depends on the material to be removed and the location of the work piece. For most construction and production equipment, steam cleaning is recommended. When this is not possible solvent cleaning can be used. Blast cleaning with abrasives is also used. For small parts pickling or solvent dip cleaning can be used and, finally, power tool cleaning with brushes, grinding wheels, disc grinding, etc., can be employed. The time spent cleaning a weld repair area will pay off in the long run.

Disassembly. Except for the simplest repair jobs disassembly may be required. This can be related to items mentioned above but also applies to lubrication lines, instrument tubing, wiring, etc. Sometimes it is necessary to disassemble major components such as machinery from machinery frames, etc.

Protection of adjacent machinery and machined surfaces. When repair welding is done on machinery many parts that are not removed should be protected from weld spatter, flame cutting sparks, and other foreign material generated by the repair process. Sheet metal guards or baffles are used to protect adjacent machinery. For machined surfaces, asbestos cloth can be employed. It is wise to secure protective material with wire, clamps, or other temporary bracing. Machined surfaces within five feet of the welding operation should be protected.

Bracing and clamping. On complex repair jobs bracing or clamping may be required. This is because of the heavy weight of parts or the fact that loads may be exerted on the part being weld repaired. If main structural members are to be cut the load must be carried by temporary braces. The braces can be temporarily welded to the structure being repaired.

Lay out repair work. In most repair jobs it is necessary to remove metal so that a full-penetration weld can be made. A layout should be made to show the metal that is to be removed by cutting or gouging to prepare the part for welding. The minimum amount of metal should be removed to obtain a full-penetration weld. The layout should be selected so that welding can be balanced, if possible, and that the bulk of the welding can be made from the more comfortable welding position.

Preheating. The preheating and flame cutting or gouging are parts of the preparation for welding but can be considered part of the welding operation. When flame cutting or gouging is required, preheating should be the same as when welding. It might not be quite as important since stresses are much smaller; however, the thermal shock on the metal can occur in gouging as well as in welding.

Cutting and gouging. The oxygen fuel gas-cutting torch is most often used for this application. Special gouging tips are available and they should be selected based on the particular geometry of the joint preparation. It is possible, by closely watching the cut surface, to find and follow cracks during the flame gouging operation. The edges of the cracks will show since they become slightly hotter. The air carbon arc cutting and gouging process is also widely used for weld repair preparation. Proper power sources and carbons should be selected for the volume of metal to be removed. The technique should be selected to avoid carbon deposit on the prepared metal surface. For some metals the torch or carbon arc might not be appropriate and in these cases mechanical chipping and grinding may be employed.

Grinding and cleaning. The resulting surfaces may not be as smooth as desired and may include burned areas, oxide, etc. Grind the surfaces to clean bright metal prior to starting to weld. For critical work or where there is a suspicion of additional cracks it is wise to check the surface by magnetic particle inspection to make sure that all cracks and defects have been removed.

The above nine steps constitute a listing of steps for weld preparation. Some of these may be eliminated but they should all be considered to properly prepare the joint for welding.

Repair Welding
Successful repair welding also involves following a logical sequence to make sure that all factors are considered and adequately provided.

Welding procedure. The welding procedure must be available for the use of the welders. It must include the process to be used, the specific filler metals, the preheat required, and any other specific information concerning the welding joint technique.

Welding equipment. Sufficient welding equipment should be available so that there will be no delays. Standby equipment might also be required. This not only includes welding equipment but includes sufficient electrode holders, grinders, wire feeders if required, cables, etc.

Materials. Sufficient materials must also be available for the entire job. This includes the filler metals stored properly for use on the repair. It also includes materials such as insert pieces, reinforcing pieces, etc. Materials also include fuel for maintaining preheat and interpass temperature, shielding gases if used, and fuel for engine powered welding machines.

Alignment markers. Prior to making the weld alignment markers are sometimes used. These can be nothing more than center punch marks made across the joint in various locations.

Welding sequences. The welding sequence should be well described in the welding procedure and can include block welding, back-step sequence welding, wandering sequence welding, and peeling.

Finally, there should be a sufficient number of welders assigned to the job so that the job can be completed quickly.

Safety. Finally, safety cannot be overlooked throughout the welding operation. For example, ventilation must be provided when fuel gases are used for preheating, etc.

Weld Quality. The quality of the weld should be continually checked. The final weld should be smooth, there should be no notches, and reinforcing, if used, should fair smoothly into the existing structure. If necessary, grinding should be done to maintain smooth flowing contours.

Postweld Operation
After the weld has been completed, it should be allowed to slow cool. It should not be exposed to winds or drafts, nor should the machinery loads be placed on the repaired part until the temperature has returned to the normal ambient temperature.

Inspection. The finished weld should be inspected for smoothness and quality. This can include nondestructive testing such as magnetic particle, ultrasonic, or X-ray. The repair weld should be of high quality since it is replacing original metal of high quality.

Clean up operation. This includes the removal of strong backs and the smooth grinding of the points where they were attached. It also involves the removal of other bracing and protective covers, etc. In addition, all weld stubs, weld spatter, weld slag, and other residue should be removed from the repair area to make it cleaner than it was originally. Grinding dust is particularly troublesome and every effort should be made to remove it entirely since it is abrasive and can get into working joints, bearings, etc., and create future problems.

Repainting. After the weld and adjacent repair area has been cleaned it should be repainted and other areas should be re-greased in preparation for the re-operation of the machinery.

Reassembly. After cleaning and painting, etc., the pieces of machinery that were taken away are returned. This involves the reassembly of machinery.
Rebuilding and overlaying
Rebuilding and overlaying with weld metal or spray metal are both considered surfacing operations. Surfacing is the deposition of filler metal on a base metal to obtain desired dimensions or properties. Overlay is considered to be a weld or spray metal deposit that has specific properties sometimes unlike the original surface properties.

Rebuilding is used to bring parts back to their original dimensions and properties, such as the rebuilding of worn shafts, repair of parts that were machined undersize, etc. Overlay surfacing is used to return the part to original dimensions but with weld metal having particular properties to reduce wear, erosion, corrosion, etc.

Rebuilding and overlay, or the all-embracing term, surfacing, can be done by many of the welding processes and by the thermal spraying processes. The selection of the process is based on the same factors that are used to select a welding process for fabricating or repairing.

There are some situations in which the thermal spray processes should be selected. The thermal spray processes do not introduce as much heat into the work as the welding processes. Where this is an important requirement, the thermal spray method should be used. It is possible to thermal spray certain materials that cannot be deposited with the welding processes. This applies particularly to the ceramic sprayed coatings or other nonmetallic materials.

Selection of the welding process
The selection of the welding process and the welding procedure and technique is as important as the selection of the deposit alloy. Almost all the arc welding processes and several others can be used for the application of hard facing weld metal.

The shielded metal arc welding process is probably the most commonly used of any of the welding processes for hard facing. It can be used in the field and in the shop and can be applied to small and large parts in any position.

Submerged arc welding is also used for many applications but it is restricted to welding in the flat position. Most often it is used for plant operations and not used in the field. It is often used for repeating applications when the same part is surfaced on a routine basis.

Flux-cored arc welding with and without shielding gas is a popular semiautomatic welding process. It can be used in the field or in the shop and is not restricted to the flat position.

The gas tungsten arc welding process is used for many smaller applications, usually for shop work in which the part can be brought to the shop and manipulated and moved for ease of welding. Gas tungsten arc can be used manually or in an automatic mode with automatic wire feeders, oscillators, etc. It is more expensive than the other processes and for this reason is restricted to the more technical type jobs.

Plasma arc welding is also used much in the same manner as gas tungsten arc. It does have a higher temperature and for this reason can be used in certain cases where gas tungsten arc welding is not applicable. It is again restricted to the smaller types of jobs.

The electroslag welding process is also used for certain special applications. It has been widely used for rebuilding crusher hammers. These can be rebuilt with special fixturing and done quite rapidly with the electroslag process.

Oxyacetylene welding is also used for certain applications. It is widely used for application of specialized cobalt alloys on relatively thin edges.

In general, the process is selected based on normal process selection factors and modified by some of the above comments. Once the process is selected, the next requirement is the selection of the deposited metal to provide the necessary properties.

The need for weld repair and surfacing
There are probably more welders employed doing maintenance and repair welding than there are in any other industry grouping. The work done in the primary metal industry is primarily maintenance and repair. This is true also of the utility services category and by combining these with repair services you find that approximately 18% of the welders are engaged in this type of work.

In addition, it has prime importance to welding since the earliest use of welding was for repair work. The most famous incident happened at the outbreak of World War I when German ships were interned in New York harbor. Their crews, hoping to make the ships inoperable, sabotaged the engines and machinery. However, by means of welding, repairs were quickly made and the ships were placed in transatlantic service to deliver material from the U.S. to Europe.

Repair welding and surfacing are both considered in the field of maintenance welding and are covered together since they are both done by the same welders. Often it is extremely difficult to separate what is considered repair welding from maintenance welding, and surfacing can be included in both situations. The same basic factors apply to both weld repair and surfacing.

Parts break and wear out continually. It may be impossible to obtain another part exactly like the one that broke or wore out. This is particularly true of older industrial machinery, construction machinery, agricultural machinery, machine tool parts, and even automobiles. Repaired parts may be more serviceable than the original part, since they can be reinforced and the weaknesses of the original part corrected. It is often more economical to weld repair since the delay in obtaining the replacement part could be excessive and the cost of the new part would normally exceed the cost of repairing the damaged part.

Weld repair is commonly used to improve, update, and rework parts so that they equal or exceed the usefulness of the original part. This is normally attained, with the possible exception of weld-repaired cast iron parts that are subjected to heating and cooling. Weld repairs on cast iron parts subjected to repetitive heating and cooling may or may not provide adequate service life. The problem is that cast iron parts subjected to high-temperature heating and cooling, such as machinery brakes, furnace sections, etc., fail originally from this type of service and due to metallurgical changes the weld may fail again without providing adequate service life. Except for emergency situations, it is not wise to repair cast iron parts of this type.

The metal that the part to be repaired is made of has a great influence on the service life of the repaired parts. Parts made of low-carbon and low-alloy steels can be repaired without adversely affecting the service life of the part. On the other hand, high-carbon steels may be weld repaired but must be properly heat treated if they are to provide adequate service life.

It is absolutely essential that we know the type, specification, or composition of the metal that we are planning to weld. As mentioned above, it may be unwise to weld repair certain metals. But we should not weld on any metal unless we know its composition.

The economics of weld repairing are usually very favorable and this applies to the smallest or the largest weld repair job. Some weld repair jobs may take only a few minutes and others may require weeks for proper preparation and welding. Even so, the money involved in a repair job may be less than the cost of a new part.

A part made of any metal that can be welded can be repair welded or surfaced. In fact, some of the metals that are not normally welded can be given special surfacing coatings by one process or another. All the arc welding processes are used for repair and maintenance work. In addition the brazing processes, the oxy-fuel gas welding processes, soldering, thermit welding, electro slag welding, electron beam welding, and laser beam welding are also used. The thermal spraying processes are all widely used for surfacing applications. In addition, the various thermal cutting processes are used for preparing parts for repair welding.

The selection of the appropriate preparation process and welding process depends on the same factors that are considered in selecting a welding or cutting process for the original manufacturing operation.

In the case of repair welding, there are usually limitations, such as the availability of equipment for a one-time job and the necessity of obtaining equipment quickly for emergency repair work. This limits the selection and it is for this reason that the shielded metal arc welding process, the gas metal arc welding process, the gas tungsten arc welding process, and oxyacetylene welding and torch brazing are most commonly used.

However, for many routine and continuous types of repair work some of the other welding processes may be the most economical. For example, submerged arc welding is widely used for building up the surface of worn parts. The electro slag process has been used to repair and resurface parts for hammer mills, for construction equipment, and for rebuilding rolls for steel mills. Thus there is a difference in the selection of the welding process for the routine, continuing types of repair and surfacing work versus the one-of-a-type or breakdown emergency repair job.

Analyze and develop rework procedure
The success of a repair or surfacing job depends on the thought and preparation prior to doing any actual work on the project. Many factors must be considered in making a thorough analysis. A thorough analysis as outlined may not be required in many situations. This is due to experience gained by welders and others in analyzing jobs, making repairs, and then checking on the service life of the repaired part. As experience is gained many short cuts can be taken, but it is the intent to provide a detailed method of analyzing jobs so that the repair will be as successful as possible.

One of the reasons for such an investigation is to establish the cause of the failure in the case of a broken part or the cause of wear or erosion in the case of a part to be surfaced. The four points outlined are:

* Make a detailed study of the actual parts that failed.
* Learn the background information concerning the specifications and design.
* Make an investigation of the materials used.
* Make a listing of all of the facts so that at the conclusion the reason of failure will be as accurate as possible.

There are certain situations and certain types of equipment for which repair welding may not be done or may be done only with prior approvals.

Certain types of containers and transportation equipment must not be weld repaired or may be welded only with special permission and approval. These include railroad locomotive and car wheels, high-alloy high-strength truck frames, and compressed gas cylinders. Most pieces of power-generating machinery, including turbines, generators, and large engines, are covered by casualty insurance. Weld repair on such machinery can be done only with the prior approval of the welding procedure by the insurance underwriters. In some cases, approval may not be granted. An example of this can be cast iron crankshafts in large stationary diesel engines. Certain weld repairs may be made but it is necessary to develop a written procedure which must be approved in writing by the underwriting company’s representative.

Repairs by welding to boilers and pressure vessels require special attention. Pressure vessels that carry an ASME stamp or are under the jurisdiction of any state or province or government agency must be repaired in accordance with the National regulations issued by responsible authorities.

Repairs by welding are limited to steels having known weldable quality. It provides a maximum carbon content of 0.35% for carbon steels and a carbon content of 0.25% for low-alloy steels.

For welding high-alloy materials and nonferrous materials the work must be done in accordance with the ASME code. Welders making such repairs must be qualified based on the thickness of the material and the type of material being welded. Full-penetration welds are required with welding recommended from both sides. Permissible welded repairs are defined as cracks, corroded surfaces, and seal welding, patches, and the replacement of stays.

A repair is the work necessary to return a boiler or pressure vessel to a safe and satisfactory operating condition. Alterations are also permitted and this is a change in a boiler or pressure vessel that substantially alters the original design and in this case work can be done only by a manufacturer possessing a valid certificate authorization from ASME. All alterations must comply with the section of code to which the original boiler or pressure vessel was constructed.

A written repair procedure is required for doing either repair work or alterations. In the case of an alteration a record must be made and all alteration work must be approved. These records must be filed with the inspection agency or the jurisdictional agency, the National Board of Boiler and Pressure Vessel Inspectors, and all work must be inspected.

Alterations on bridges, large steel frame buildings, and ships may be done only with special authorization. The alteration work must be designed and approved. The welders must be qualified according to the code used and the work must be inspected. Written welding procedures are required.

Once the decision has been made to make a weld repair it is then necessary to establish why the part failed or wore out. This relates to the type of repair job since it also determines whether reinforcing may be required. Reasons for the part to fail or wear out can be among the following:

* Accident
* Misapplication
* Abuse
* Overload.

If the part failed because of an accident or an overload, it may be returned to service with the weld repair made to bring it back to its original strength. The same consideration applies if the part has been abused or misapplied. It may be necessary to reinforce the part so that it will stand temporary overloads, misapplication, or abuse. This decision should be made prior to the weld repair.

In the case of poor workmanship, poor design, or incorrect material the weld repair should eliminate the poor workmanship that was responsible for the failure. In this case, the part would be returned to its original design. If failure is due to poor design, design changes may be required and reinforcement may be added. In a case of wrong material it will be assumed that the material was of a lower strength level which contributed to the failure. In this case reinforcing would be required. If the repair or alteration job is to modify the part, it is necessary that the modification be designed by competent designers who have the knowledge of the design conditions of the original part. This may require reinforcing to make sure the modification or alteration is satisfactory.

Another important factor that must be considered is what results are expected of the repaired or reworked part. Should it be reinforced or should it be redesigned and altered to provide necessary service life? Finally, in the case of surfacing, what better surface could be provided to withstand the service that caused the premature wear or failure?

Rework Procedure
A written repair procedure is required for all but the most simple jobs. It is absolutely necessary that the type of material being welded is known. This can be found in several ways. If possible, refer to the drawing of the part and the specifications that are shown for the part or parts to be welded.

If this is not possible, particularly in the field or at the maintenance shop, look for clues as to the type of metal involved. Analyze the application of the metal, for clues. For example, an automobile engine block is normally cast iron except for some which might be cast aluminum. Aluminum and iron are easily distinguishable. The spring of an automobile or truck would normally be high-carbon steel. The body structure of a car or truck would be mild steel. The appearance often helps provide clues.

As a final resort it may be necessary to obtain a laboratory analysis of the metal. Filings or a piece of the metal must be sent to a laboratory capable of making such determinations.

The normal method of selecting the welding process will be followed once the material to be welded has been identified. This involves the type of metal, the thickness of the metal, the position of welding, etc. This also leads into the question of filler metal to be used. After this, the normal method of filler metal selection is followed. This involves matching base metal composition, matching the base metal properties, particularly strength, and providing weld metal that will withstand the service involved.

In surfacing, the surface characteristics desired for the finished job depend entirely on the service to which the surface will be exposed. This is based on knowledge and experience and on the fact that the surface has deteriorated to the point that it needs to be reworked or resurfaced. When wear is involved, surfaces can be rebuilt many times without reducing the strength of the part and the service life will be greatly extended.

The term high-strength steel is often applied to all steels other than mild low-carbon steels. The steels which have yield strength over 560 MPa are sometimes called the ultra-high-strength steels or super alloys.

The groups of steels that fall into this category are:

* Medium-carbon low-alloy hardenable steels
* Medium-alloy hardenable or tool and die steels
* High-alloy hardenable steels
* High-nickel maraging steels
* Martensitic stainless steels
* Semi austenitic precipitation-hardenable stainless steels

Medium-Carbon Low-Alloy Hardenable Steels
The best-known steels in this class are AISI 4130 and AISI 4140 steels. Also in this class are the higher-strength AISI 4340 steel and the AMS 6434 steel. These steels obtain their high strength by heat treatment to a full martensitic microstructure, which is tempered to improve ductility and toughness.

Tempering temperatures greatly affect the strength levels of these steels. The carbon is in the medium range and as low as possible but sufficient to give the required strength. Impurities are kept to an absolute minimum because of high-quality melting and refining methods.

These steels are available as sheets, bars, tubing, and light plate. The steels in this group can be mechanically cut or flame cut. However, when they are flame cut they must be preheated to 316°C. Flame-cut parts should be annealed before additional operations in order to reduce the hardness of the flame-cut edges.

These steels are suitable for welding only when they are in the annealed or normalized condition. After welding, they have to be heat treated to obtain the desired strength. The gas tungsten arc, the gas metal arc, the shielded metal arc, and the gas welding process are all used for welding these steels. The composition of the filler metal is designed to produce a weld deposit that responds to a heat treatment in approximately the same manner as the base metal.

In order to avoid brittleness and the possibility of cracks during welding, relatively high preheat and interpass temperatures are used. Preheating is in the order of 316°C. Complex weldments are heat treated immediately after welding.

Aircraft engine parts, aircraft tubular frames, and racing car frames are made from AISI 4130 tubular sections. These types of structures are normally not heat treated after welding.

Medium-Alloy Hardenable Steels
These steels are used largely in the aircraft industry for ultra-high-strength structural applications. They have carbon in the low to medium range and possess good fracture toughness at high-strength levels. In addition, they are air hardened, which reduces the distortion that is encountered with more drastic quenching methods. Some of the steels in this group are known as hot work die steels and another grade has become known as 5Cr-Mo-V aircraft quality steel. These steels are available as forging billets, bars, sheet, strip, and plate.

There is another type of steel in this general class which is a medium-alloy quenched and tempered steel known as high-yield or HY 130/150. This type of steel is used for submarines, aerospace applications, and pressure vessels, and is normally available as plate. This steel has good notch toughness properties at 0°C and below. These types of steels have much lower carbon than the grades mentioned previously.

When flame cutting or welding the aircraft quality steels, preheating is absolutely necessary since the steels are air hardening. A preheating on 316°C is used before flame cutting and then annealed immediately after the flame-cutting operation. This will avoid a brittle layer at the flame-cut edge, which is susceptible to cracking.

These types of steel should only be welded in the annealed condition. The steel should be preheated to 316°C and this temperature must be maintained throughout the welding operation. After welding, the work must be cooled slowly. This can be done by post heating, or by furnace cooling. The weldment is then stress relieved at 704°C and air cooled to obtain a fully tempered microstructure suitable for additional operations. It is usually annealed, after all welding is done, prior to final heat treatment. The filler metal should be of the same com-position as the base metal. The gas tungsten arc and gas metal arc processes are most widely used. However, shielded metal arc welding, plasma arc, and electron beam welding processes can be used.

The medium-alloy quenched and tempered high-yield strength steels are usually welded with the shielded metal arc, gas metal arc, or the submerged arc welding process. The filler metal must provide deposited metal of a strength level equal to the base material. In all cases, a low-hydrogen or no-hydrogen process is required.

For shielded metal arc welding the low-hydrogen electrodes of the E-13018 type are recommended. Electrodes must be properly stored. In the case of the other processes, precautions should be taken to make sure that the gas is dry and that the submerged arc flux is dry. By employing the proper heat input-heat output procedure yield strength and toughness are maintained. Preheating should be at least at 38°C for thinner materials. For heavier materials preheating temperature has to be higher.

The heat input should be such that the adjacent base metal does not become overheated while the heat output is sufficient to maintain the proper microstructure in the heat-affected zone. There may be some softening in the intermixing zone. The properties of welded joints that are properly made will be in the same order as the base metal. Subsequent heat-treating is usually not required or desired.

High-Alloy Hardenable Steels
The steels in this group develop high strength by standard hardening and tempering heat treatments. The steels possess extremely high strength in the range of 1240 MPa yield and have a high degree of toughness. This is obtained with a minimum carbon content usually in the range of 0.20%; however, these steels contain relatively high amounts of nickel and cobalt, and they are sometimes called the 9 Ni-4 Co steels. These steels also contain small amounts of other alloying elements.

They are normally welded in the quenched and tempered condition by the gas tungsten arc welding process. No post-heat treatment is required. The filler metal must match the analysis of the base metal.

High-Nickel Maraging Steels
This type of steel has relatively high nickel, and low carbon content. It obtains its high strength from a special heat treatment called maraging. These steels possess an extraordinary combination of ultra-high-strength and fracture toughness and at the same time are formable, weldable, and easy to heat treat. There are three basic types: the steels with 18% nickel, 20% nickel, and 25% nickel. These steels are available in sheet, forging billets, bars, strip, and plate. Some are available as tubing.

The extra special properties of these steels are obtained by heating the steel to 482°C and allowing it to cool to room temperature. During this heat treatment all of the austenite transforms to martensite. The heating time at the 482°C temperature is extremely important and usually is in the range of three hours. The steels derive their strength while aging at this temperature in the martensitic condition and for this reason are known as maraging steels.

These steels are supplied in the soft or annealed condition. They can be cold worked in this condition and can be flame cut or plasma arc cut. Plasma arc cutting is preferred.

These steels are usually welded by the gas tungsten arc or the gas metal arc welding process. The shielded metal arc and submerged arc process can also be used with special electrode-flux combinations. The filler metal should have the same composition as the base metal. In addition, the filler metal must be of high purity with low carbon. Preheat or postheat is not required; however, the welding must be followed by the maraging heat treatment which produces weld joints of an extremely high strength.

Martensitic Stainless Steels
These steels are of the straight chromium type, such as AISI 420. They contain 12-14% chromium and up to 0.35% carbon. This composition combines stainlessness with high strength. Numerous variations of this basic composition are available, all of which are in the martensitic classification.

This type of steel has been used for compressor and turbine blades of jet engines and for other applications in which moderate corrosion resistance and high strength are required. The strength level of these steels is obtained by a quenching and tempering heat treatment. They can be obtained as sheet, strip, tubing, and plate. The compositions are also used for castings. These steels can be heat treated to strengths as high as 1750 MPa yield strength.

These stainless steels can be flame cut by the powder cutting system normally used for flame cutting stainless steels. They can also be cut with the oxy-arc process. Flame cutting should be done with the steel in the annealed condition. Most grades should be preheated to 316°C because they are air hardenable. They should be annealed after cutting to restore softness and ductility. These materials can also be cold worked in the annealed condition.

The martensitic stainless steels can be welded in the annealed or fully hardened condition, usually without preheat or postheat. The gas tungsten arc welding process is normally used. The filler metal must be of the same analysis as the base metal. Following welding the weldment should be annealed and then heat treated to the desired strength level.

Semiaustenitic Precipitation-Hardenable Stainless Steels
The steels in this group are chrome-nickel steels that are ductile in the annealed condition but can be hardened to high strength by proper heat treatment. In the annealed condition the steels are austenitic and can be readily cold worked. By special heat treatment the austenite is transformed to martensite and later a precipitant is formed in the martensite. The outstanding extra high strength is obtained by a combination of these two hardening processes.

The term semi austenitic type was given these steels to distinguish them from normal stainless steels. They are also called precipitation hardening steels or PH steels. The heat treatment for these steels is based on heating the annealed material to a temperature between 927°C and 954°C, followed by a tempering or aging treatment in the range of 454-593°C. These steels are available as billets, sheets, tubing, and plates.

These steels are normally not flame cut. Welding is performed using the gas tungsten arc or the gas metal arc welding process. The shielded metal arc welding process is rarely used. The filler metal should have the same composition as the base metal. No preheat or postheat is required if the parts are welded in the annealed condition. After welding, the steel has to be heat-treated to develop optimum strength levels.

However, there is a loss of joint strength due to heating of the heat-affected zone above the aging temperature. In view of this, it is not possible to produce a 100% efficient joint. Extra reinforcing must be utilized to develop full-strength joints. These steels are also brazed using nickel alloy filler metal.

When welding on any of these high-strength steels, weld quality must be of the highest degree. Root fusion must be complete, and there should be no undercut or any type of stress risers. The weld metal should be free of porosity and any weld cracking is absolutely unacceptable. All precautions must be taken in order to produce the highest weld quality.

Stainless steels or, more precisely, corrosion-resisting steels are a family of iron-base alloys having excellent resistance to corrosion. These steels do not rust and strongly resist attack by a great many liquids, gases, and chemicals. Many of the stainless steels have good low-temperature toughness and ductility. Most of them exhibit good strength properties and resistance to scaling at high temperatures. All stainless steels contain iron as the main element and chromium in amounts ranging from about 11% to 30%. Chromium provides the basic corrosion resistance to stainless steels. There are about 15 types of straight chromium stainless steels.

Nickel is added to certain of the stainless steels, which are known as chromium-nickel stainless steel. The addition of nickel reduces the thermal conductivity and decreases the electrical conductivity. The chromium-nickel steels belong to AISI/SAE 300 series of stainless steels. They are nonmagnetic and have austenitic microstructure. These stainless steels contain small amounts of carbon because this element has tendency to make chromium carbides, which are not corrosion resistant. Carbon is undesirable particularly in the 18% chromium, 8% nickel group.

Manganese is added to some of the chromium-nickel alloys. Usually these steels contain slightly less nickel since the chromium-nickel-manganese alloys were developed originally to conserve nickel. In these alloys, a small portion of nickel is replaced by manganese, generally in a two-to-one relationship. The AISI/SAE 200 series of stainless steels are the chromium-nickel-manganese series. These steels have an austenitic microstructure and they are nonmagnetic.

Molybdenum is also included in some stainless steel alloys. Molybdenum is added to improve the creep resistance of the steel at elevated temperatures. It will also increase resistance to pitting and corrosion in many applications.

Stainless steels can be welded using several different procedures such as shielded metal arc welding, gas tungsten arc welding, and gas metal arc welding.

These steels are slightly more difficult to weld than mild carbon steels. The physical properties of stainless steel are different from mild steel and this makes it weld differently. These differences are:

* Lower melting temperature,
* Lower coefficient of thermal conductivity,
* Higher coefficient of thermal expansion,
* Higher electrical resistance.

The properties are not the same for all stainless steels, but they are the same for those having the same microstructure. Regarding this, stainless steels from the same metallurgical class have the similar welding characteristics and are grouped according to the metallurgical structure with respect to welding.

Austenitic Type. Manganese steels are not hardenable by heat treatment and are nonmagnetic in the annealed condition. They may become slightly magnetic when cold worked or welded. This helps to identify this class of stainless steels. All of the austenitic stainless steels are weldable with most of the welding processes, with the exception of Type 303, which contains high sulphur and Type 303Se, which contains selenium to improve machinability.

The austenitic stainless steels have about 45% higher thermal coefficient of expansion, higher electrical resistance, and lower thermal conductivity than mild-carbon steels. High travel speed welding is recommended, which will reduce heat input and carbide precipitation, and minimize distortion.

The melting point of austenitic stainless steels is slightly lower than melting point of mild-carbon steel. Because of lower melting temperature and lower thermal conductivity, welding current is usually lower. The higher thermal expansion dictates that special precautions should be taken with regard to warping and distortion. Tack welds should be twice as often as normal. Any of the distortion reducing techniques such as back-step welding, skip welding, and wandering sequence should be used. On thin materials it is very difficult to completely avoid buckling and distortion.

Ferritic Stainless Steels. The ferritic stainless steels are not hardenable by heat treatment and are magnetic. All of the ferritic types are considered weldable with the majority of the welding processes except for the free machining grade 430F, which contains high sulphur content. The coefficient of thermal expansion is lower than the austenitic types and is about the same as mild steel. Welding processes that tend to increase carbon pickup are not recommended. This would include the oxy-fuel gas process, carbon arc process, and gas metal arc welding with CO2 shielding gas.

The lower chromium types show tendencies toward hardening with a resulting martensitic type structure at grain boundaries of the weld area. This lowers the ductility, toughness, and corrosion resistance at the weld. For heavier sections preheat of 200°C is beneficial. To restore full corrosion resistance and improve ductility after welding, annealing at 760-820°C, followed by a water or air quench, is recommended. Large grain size will still prevail, however, and toughness may be impaired. Toughness can be improved only by cold working the weld.

If heat treating after welding is not possible and service demands impact resistance, an austenitic stainless steel filler metal should be used. Otherwise, the filler metal is selected to match the base metal.

Martensitic Stainless Steels. The martensitic stainless steels are hardenable by heat treatment and are magnetic. The low-carbon type can be welded without special precautions. The types with over 0.15% carbon tend to be air hardenable and, therefore, preheat and postheat of weldments are required. A preheat temperature range of 230-290°C is recommended. Postheating should immediately follow welding and be in the range of 650-760°C, followed by slow cooling.

If preheat and postheat are not possible, an austenitic stainless steel filler metal should be used. Type 416Se is the free-machining composition and should not be welded. Welding processes that tend to increase carbon pickup are not recommended. Increased carbon content increases crack sensitivity in the weld area.

Welding filler metals
The selection of the filler metal alloy for welding the stainless steels is based on the composition of the stainless steel. The various stainless steel filler metal alloys are normally available as covered electrodes and as bare solid wires. Recently flux-cored electrode wires have been developed for welding stainless steels.

Filler metal alloy for welding the various stainless steel base metals are: Cr-Ni-Mn (AISI No. 308); Cr-Ni-Austenitic (AISI No. 309, 310, 316, 317, 347); Cr-Martensitic (AISI No. 410, 430); Cr-Ferritic (AISI No. 410, 430, 309, 502). It is possible to weld several different stainless base metals with the same filler metal alloy.

Welding procedures
For shielded metal arc welding, there are two basic types of electrode coatings. These are the lime type indicated by the suffix 15 and the titanium type designated by the suffix 16. The lime type electrodes are used only with direct current electrode positive (reverse polarity). The titanium-coated electrode with the suffix 16 can be used with alternating current and with direct current electrode positive. Both coatings are of the low-hydrogen type and both are used in all positions. However, the type 16 is smoother, has more welder appeal, and operates better in the flat position. The lime type electrodes are more crack resistant and are slightly better for out-of-position welding. The width of weaving should be limited to two-and-one-half (2,5) times the diameter of the electrode core wire.

Covered electrodes for shielded metal arc welding must be stored at normal room temperatures in dry area. These electrode coatings, of low hydrogen type, are susceptible to moisture pickup. Once the electrode box has been opened, the electrodes should be kept in a dry box until used.

The gas tungsten arc welding process is widely used for thinner sections of stainless steel. The 2% tungsten is recommended and the electrode should be ground to a taper. Argon is normally used for gas shielding; however, argon-helium mixtures are sometimes used for automatic applications.

The gas metal arc welding process is widely used for thicker materials since it is a faster welding process. The spray transfer mode is used for flat position welding and this requires the use of argon for shielding with 2% or 5% oxygen or special mixtures. The oxygen helps producing better wetting action on the edges of the weld. The short-circuiting transfer can also be used on thinner materials. In this case, CO2 shielding or the 25% CO2 plus 75% argon mixture is used. The argon-oxygen mixture can also be used with small-diameter electrode wires. With extra low-carbon electrode wires and CO2 shielding the amount of carbon pickup will increase slightly. This should be related to the service life of the weldment. If corrosion resistance is a major factor, the CO2 gas or the CO2-argon mixture should not be used.

For all welding operations, the weld area should be cleaned and free from all foreign material, oil, paint, dirt, etc. The welding arc should be as short as possible when using any of the arc processes.

Standard EN 12072
The standard EN12072 specifies requirements for classification of wire electrodes, wires and rods for gas shielded metal arc welding, gas tungsten arc welding, plasma arc welding and submerged arc welding of stainless and heat resisting steels. The classification of the wire electrodes, wires and rods is based on their chemical composition.

For stainless steel welding consumables there is no unique relationship between the product form (wire electrode, wire or rod) and the welding process used (gas shielded metal arc welding, gas tungsten arc welding, plasma arc welding or submerged arc welding). For this reason the wire electrodes, wires or rods can be classified on the basis of any of the above product forms and can be used as appropriate, for more than on of the above processes.

A wire electrode, wire or rod is classified in accordance with its chemical composition. The classification is divided into two parts, as follows:

1. The first part gives a symbol indicating the product/process to be identified, as follows:
G = Gas shielded metal arc welding.
W = Gas tungsten arc welding.
P = Plasma arc welding.
S = Submerged arc welding.
2. The second part gives a symbol indicating the chemical composition of the wire electrode, wire or rod. This grade has the symbol 25 20 Mn and is a heat resisting type.

The influence of the shielding gas or flux on the chemical composition of the all-weld metal is considered. Differences between the chemical composition of the all-weld metal and the wire electrode, wire or rod can occur.

Proof and tensile strength of the weld metal made by this grade is expected to conform with the minimum requirements contained in the mechanical properties table. Elongation and impact properties of the weld metal can deviate from the minimum values specified for the corresponding parent metal as a result of variations in the microstructure.

Standard EN 758
This specification specifies the requirements for classification of tubular cored electrodes in the as-welded condition for metal arc welding, with or without a gas shield, of non alloy and fine grain steels with a minimum yield strength of up to 500 N/mm. One tubular cored electrode can be tested and classified with different gases.

The designation contains 6 compulsory and 2 optional parts.

Compulsory Section:

The first part, T, is a symbol denoting that it is a tubular cored electrode used in the metal arc welding process.

The second part, 50, is a symbol denoting the yield strength, tensile strength and elongation of the all-weld metal in the as-welded condition.

The third part is a symbol denoting the temperature at which minimum average impact energy of 47 J of all-weld metal can be achieved, as follows:
Z = No requirement
A = +20°C
0 = 0°C
2 = -20°C
3 = -30°C
4 = -40°C
5 = -50°C
6 = -60°C

The fourth part, Z, is a symbol indicating the chemical composition of all-weld metal. The symbol Z denotes any agreed composition other than those grades already contained in the specification.

The fifth part is a symbol indicating the type of tubular cored electrode relative to its core composition and slag characteristics, as follows:
R = Rutile, slow freezing slag, single and multiple pass types of weld, requiring a shielding gas.
P = Rutile, fast freezing slag, single and multiple pass types of weld, requiring a shielding gas.
B = Basic, single and multiple pass types of weld, requiring a shielding gas.
M = Metal powder, single and multiple pass types of weld, requiring a shielding gas.
V = Rutile or basic/fluoride, single pass type of weld, not requiring a shielding gas.
W = Basic/fluoride, slow freezing slag, single and multiple pass types of weld, not requiring a shielding gas.
Y = Basic/fluoride, fast freezing slag, single and multiple pass types of weld, not requiring a shielding gas.
Z = Other types.

The sixth part is a symbol indicating the type of shielding gas as follows:
M = mixed gases: EN 439 - M2 but without helium.
C = EN 439 - C1, carbon dioxide
N = This symbol shall be used for tubular cored electrodes without a gas shield.

Optional Section (the next two parts have not been included in the designation for this grade):

The seventh part gives a symbol for the welding position as follows:
1 = all positions;
2 = all positions, except vertical down;
3 = flat butt weld, flat fillet weld, horizontal-vertical fillet weld;
4 = flat butt weld, flat fillet weld;
5 = vertical down and positions according to symbol 3.

The eighth part gives a symbol indicating the hydrogen content of deposited metal as follows:
Symbol Hydrogen content ml/100 g deposited metal
H5 5 maximum
H10 10 maximum
H15 15 maximum

Standard EN 499
This specification specifies the requirements for classification of covered electrodes and deposited metal in the as-welded condition for manual metal arc welding of non alloy and fine grain steels with a minimum yield strength of up to 500 N/mm in the welded condition.

The designation contains 5 compulsory and 3 optional parts.

Compulsory Section: The first part, E, is a symbol denoting that it is a covered electrode used in the manual metal arc welding process.

The second part, 38, is a symbol denoting the yield strength, tensile strength and elongation of the all-weld metal in the as-welded condition.

The third part is a symbol denoting the temperature at which minimum average impact energy of 47 J of all-weld metal can be achieved, as follows:
Z = No requirement
A = +20°C
0 = 0°C
2 = -20°C
3 = -30°C
4 = -40°C
5 = -50°C
6 = -60°C

The fourth part, 1Ni, is a symbol indicating the chemical composition of all-weld metal.

The fifth part is a symbol indicating the type of electrode covering as follows:
A = acid covering
C = cellulosic covering
R = rutile covering
RR = rutile thick covering
RC = rutile-cellulosic covering
RA = rutile-acid covering
RB = rutile-basic covering
B = basic covering

Optional Section (the next three parts have not been included in the designation for this grade):

The sixth part gives a symbol for the weld metal recovery and type of current as follows:
Symbol % weld metal recovery Type of current
1 less than or equal to 105 a.c. + d.c.
2 less than or equal to 105 d.c.
3 over 105 up to and inc.125 a.c. + d.c.
4 over 105 up to and inc.125 d.c.
5 over 125 up to and inc.160 a.c. + d.c.
6 over 125 up to and inc.160 d.c.
7 over 160 a.c. + d.c.
8 over 160 d.c.

The seventh part gives a symbol for welding position as follows:
1 = all positions
2 = all positions, except vertical down
3 = flat butt weld, flat fillet weld, horizontal vertical fillet weld
4 = flat butt weld, flat fillet weld
5 = vertical down and positions according to symbol 3

The eighth part gives a symbol indicating the hydrogen content of all-weld metal as follows:
Symbol Hydrogen content ml/100 g all-weld metal
H5 5 maximum
H10 10 maximum
H15 15 maximum

These process groups are shown by the Table, along with each process name and letter designations. Each process group will be briefly described.

Group	Allied Process	Letter Designation
Adhesive bonding	Dextrin cements	AB-D
	Solvent or rubber cements	AB-RC
	Synthetic resins	AB-SR
	Expoxys	AB-E
Arc cutting (thermal)	Air carbon arc cutting	AAC
	Carbon arc cutting	CAC
	Gas tungsten arc cutting	GTAC
	Metal arc cutting	MAC
	Plasma arc cutting	PAC
Oxygen cutting (thermal)	Chemical flux cutting	FOC
	Metal powder cutting	POC
	Oxygen arc cutting	AOC
	Oxy Fuel gas cutting	OFC
	Oxygen lance cutting	LOG
Other thermal cutting processes	Electron beam cutting	EEC
	Laser beam cutting	LBC
Thermal spraying	Electric arc spraying	EASP
	Flame spraying	FLSP
	Plasma spraying	PSP

Adhesive bonding

Adhesive bonding (AB) is a joining process in which an adhesive is placed between the faying surfaces which solidifies to produce an adhesive bond. The adhesive bond is the attractive force, generally physical in character, between an adhesive and the base materials.

The two principle interactions that contribute to the adhesion are the van der Waals bond and the diepole bond. The van der Waals bond is defined as a secondary bond arising from the fluctuating-diepole nature of an atom with all occupied electron shells filled. The diepole bond is a pair of equal and opposite forces that hold two atoms together and results from a decrease in energy as two atoms are brought closer to one another.

Adhesive bonding of metal-to-metal applications accounts for less than 2% of the total metal joining requirements. The bonding of metals to nonmetals, especially plastics, is very important and is the major use of adhesive bonding.

Dextrins belong to the family of starch-derived adhesives ranging in color from white to dark brown and are normally fluid filmy materials. These are glues and pastes used to bond porous materials. They are spread in a thin film.

Rubber cements or solvent cements are adhesives that contain organic solvents rather than water. They are based on nitro cellulose or polyvinyl acetate, normally elastomeric products, dispersed in solvent. They are free flowing, thin set materials that dry to hard tack free films. They are used in pressure-sensitive labeling operations and in contact bonding for the woodworking industry.

Synthetic resins are composed of synthetic organic materials and are relatively expensive. They are used when a high-quality bond is required and they are relatively heat and moisture resistant. They can be applied by automatic or semiautomatic equipment, are used for sealing cartons and for wood, and for vinyl film laminations. One of the major groups is the hot melts which are combinations of waxes and resins that form a bond by applying heat and then cooling.

Epoxy Adhesives are the newest of the adhesives and can be used to bond metal-to-metal, metal to plastics, and plastics to plastics. They are a family of materials characterized by reactive epoxy chemical groups on the ends of resin molecules. They consist of two components, a liquid resin and the hardener to convert the liquid resins to solid. They may contain other modifiers to produce specific properties for special applications. Some epoxies will bond to concrete. One of the newer advances is the oily metal epoxy that bonds directly to oily metals "as received" with normal protective films on them. The oily coating need not be removed. They achieve intimate molecular contact with the surface to be bonded and will achieve high adhesion on almost any surface. Epoxies are the most expensive of the adhesives; however, they offer more advantages.

Arc cutting

These processes utilize heat and thus differ from mechanical cutting processes such as sawing, shearing, blanking, etc.

The arc cutting processes are a group of thermal cutting processes which melt the metals to be cut with the heat of an arc between an electrode and the base metal. Within this group is air carbon arc cutting; carbon arc cutting; gas tungsten arc cutting, shielded metal arc, gas metal arc, and plasma arc cutting. Each will be briefly described.

The thermal cutting processes can be applied by means of manual, semiautomatic, machine, or automatic methods in the same manner as the arc welding processes. Air Carbon Arc Cutting(AAC) is "an arc cutting process in which metals to be cut are melted by the heat of a carbon arc and the molten metal is removed by a blast of air."

Principle of operation is the following: a high velocity air jet traveling parallel to the carbon electrode strikes the molten metal puddle just behind the arc and blows the molten metal out of the immediate area. It shows the arc between the carbon electrode and the work and the air stream parallel to the electrode coming from the special electrode holder.

The process is not recommended for weld preparation for stainless steel, titanium, zirconium, and other similar metals without subsequent cleaning.

Carbon Arc Cutting (CAC) is "an arc cutting process in which metals are severed by melting them with the heat of an arc between a carbon electrode and the base metal."

The process is identical to air carbon arc cutting except that the air blast is not employed. The process depends strictly upon the heat input of the carbon arc to cause the metal to melt. The molten metal falls away by gravity to produce the cut. The process is relatively slow, a very ragged cut results and it is used only when other cutting equipment is not available. It has little industrial significance.

Metal Arc Cutting (MAC) is "an arc cutting process which severs metals by melting them with the heat of an arc between a metal electrode and the base metal." When covered electrodes are used it is known as shielded metal arc cutting (SMAC).

The equipment required is identical to that required for shielded metal arc welding. When the heat input into the base metal exceeds the heat losses the molten metal pool becomes large and unmanageable. If the base metal is not too thick, the molten metal will fall away and create a hole or cut. The cut produced by the shielded metal arc cutting process is rough and is not normally used for preparing parts for welding. The metal arc cutting process using covered electrodes is used only where a small cutting job is required and other means are not available for the purpose.

Gas Tungsten Arc Cutting (GTAC) is "an arc cutting process in which metals are severed by melting them with an arc between a single tungsten (nonconsumable) electrode and the work. Shielding is obtained from a gas or gas mixture."

This process has largely been supplanted by plasma arc cutting and is of little industrial significance except for the small jobs when other equipment is not available.

Plasma Arc Cutting (PAC) is an arc cutting process which severs metal by melting a localized area with a constricted arc and removing the molten material with a high-velocity jet of hot ionized gas.

There are three major variations: (1) low-current plasma cutting which is a rather recent development, (2) the original relatively high current plasma cutting, and (3) plasma cutting with water added. The low-current plasma variation is gaining in popularity because it can be manually applied.

The principle operation of plasma cutting is almost identical with the keyhole mode of plasma welding. The difference is that the cut is maintained and the keyhole is not allowed to close as in the case of welding. Heat input at the plasma arc is so high and the heat losses cannot carry the heat away quickly enough so that the metal is melted and a hole is formed. The plasma gas at a high velocity helps cut through the metal.

The secondary gas can also assist the jet in removing molten metal and limits the formation of drops at the cutting edge. Plasma cutting is ideal for gouging and for piercing. For some operations air is used as the plasma gas. A higher arc voltage is normally used for cutting than for welding.

The plasma arc cutting process can be used to cut metals underwater.

Oxygen cutting

Oxygen Cutting (OC) is a group of thermal cutting processes used to sever or remove metals by means of the chemical reaction of oxygen with the base metal at elevated temperatures. In the case of oxidation-resistant metals the reaction is facilitated by the use of a chemical flux or metal powder. Five basic processes are involved: (1) oxy fuel gas cutting, (2) metal powder cutting, (3) chemical flux cutting, (4) oxygen lance cutting, and (5) oxygen arc cutting. Each of these processes is different and will be described.

Oxy Fuel Gas Cutting (OFC) is used to sever metals by means of the chemical reaction of oxygen with the base metal at elevated temperatures. The necessary temperature is maintained by means of gas flames obtained from the combustion of a fuel gas and oxygen.

Metal Powder Cutting (POO) is an oxygen-cutting process which severs metals through the use of powder, such as iron, to facilitate cutting. This process is used for cutting cast iron, chrome nickel stainless steels, and some high-alloy steels.

Chemical Flux Cutting (FOC) is an oxygen-cutting process in which metals are severed using a chemical flux to facilitate cutting and powdered chemicals are utilized in the same way as iron powder is used in the metal powder cutting process. This process is sometimes called flux injection cutting.

Oxygen Lance Cutting (LOC) is an oxygen-cutting process used to sever metals with oxygen supplied through a consumable tube. The preheat is obtained by other means. This is sometimes called oxygen lancing. The oxygen lance is a length of pipe or tubing used to carry oxygen to the point of cutting.

Oxygen Arc Cutting (AOC) is an oxygen-cutting process used to sever metals by means of the chemical reaction of oxygen with the base metal at elevated temperatures. The necessary temperature is maintained by means of an arc between a consumable tubular electrode and the base metal.

Other thermal cutting processes

Electron Beam Cutting (EBC) is a thermal cutting process which uses the heat obtained from a concentrated beam composed of high-velocity electrons which impinge upon the work piece to be cut. The difference between electron beam welding and cutting is the heat input-to-heat output relationship.

The electron beam generates heat in the base metal, which vaporizes the metal and allows it to penetrate deeper until the depth of the penetration, based on the power input, is achieved. In welding the electron beam actually produces a hole, known as a keyhole. The metal flows around the keyhole and fills in behind. In the case of cutting the heat input is increased so that the keyhole is not closed.

Laser Beam Cutting (LBC) is a thermal cutting process which severs materials with the heat obtained in the application of a concentrated coherent light beam impinging on the workpiece to be cut. The process can be used without an externally supplied gas.

Thermal spraying

Thermal spraying (THSP) is a group of allied processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semi-molten condition to form a coating. The coating material may be in the form of powder, ceramic rod, or wire.

There are three separate processes within this group: electric arc spraying, flame spraying, and plasma spraying. These three processes differ considerably, since each uses a different source of heat. The apparatus is different and their capabilities are different.

The selection of the spraying process depends on the properties desired of the coating. Thermal spraying is utilized to provide surface coatings of different characteristics, such as coatings to reduce abrasive wear, cavitation, or erosion. The coating may be either hard or soft. It may be used to provide high temperature protection. Thermal sprayed coatings improve atmosphere and water corrosion resistance. One of the major uses is to provide coatings resistance to salt water atmospheres. Another use is to restore dimensions to worn parts.

Thermit welding (TW) is a welding process which produces coalescence of metals by heating them with superheated liquid metal from a chemical reaction between a metal oxide and aluminum with or without the application of pressure.

Filler metal is obtained from an exothermic reaction between iron oxide and aluminum. The temperature resulting from this reaction is approximately 2500°C. The superheated steel is contained in a crucible located immediately above the weld joint. The superheated steel runs into a mold which is built around the parts to be welded. Since it is almost twice as hot as the melting temperature of the base metal melting occurs at the edges of the joint and alloys with the molten steel from the crucible. Normal heat losses cause the mass of molten metal to solidify, coalescence occurs, and the weld is completed.

The thermit welding process is apply only in the automatic mode. Once the reaction is started it goes to completion.

Laser beam welding (LBW) is a welding process which produces coalescence of materials with the heat obtained from the application of a concentrated coherent light beam impinging upon the surfaces to be joined.

The focused laser beam has the highest energy concentration of any known source of energy. The laser beam is a source of electromagnetic energy or light that can be projected without diverging and can be concentrated to a precise spot. The beam is coherent and of a single frequency.

Producing a laser beam is extremely complex. The early laser utilized a solid-state transparent single crystal of ruby made into a rod approximately an inch in diameter and several inches long. The end surfaces of the ruby rod were ground flat and parallel and were polished to extreme smoothness.

The laser can be compared to solar light beam for welding. The laser can be used in air. The laser beam can be focused and directed by special optical lenses and mirrors. The laser can operate at considerable distance from the work piece.

When using the laser beam for welding the electromagnetic radiation impinges on the surface of the base metal with such a concentration of energy that the temperature of the surface is melted and volatilized. The beam penetrates through the metal vapor and melts the metal below. One of the original questions concerning the use of the laser was the possibility of reflectivity of the metal so that the beam would be reflected rather than heat the base metal. It was found, however, that once the metal is raised to its melting temperature the surface conditions have little or no effect.

The welding characteristics of the laser and of the electron beam are similar. The concentration of energy by both beams is similar, with the laser having a power density in the order of 106 watts per square centimeter. The power density of the electron beam is only slightly greater. This is compared to a current density of only 104 watts per square centimeter for arc welding.

Laser beam welding has a tremendous temperature differential between the molten metal and the base metal immediately adjacent to the weld. Heating and cooling rates are much higher in laser beam welding than in arc welding, and the heat-affected zones are much smaller. Rapid cooling rates can create problems such as cracking in high carbon steels.

The laser beam has been used to weld carbon steels, high strength low alloy steels, aluminum, stainless steel and titanium. Laser welds made in these materials are similar in quality to welds made in the same materials by electron beam process.

Electron beam welding (EBW) is a welding process which produces coalescence of metals with the heat obtained from a concentrated beam composed primarily of high-velocity electrons impinging upon the surfaces to be joined. Heat is generated in the workpiece as it is bombarded by a dense stream of high-velocity electrons. Virtually all of the kinetic energy-the energy of motion-of the electrons is transformed into heat upon impact.

The electron beam welding process had its inception in the 1950s in the nuclear field. There were many requirements to weld refractory and reactive metals. These metals, because of their affinity for oxygen and nitrogen of the air, are very difficult to weld.

The original work was done in a high vacuum. The process utilized an electron gun similar to that used in an X-ray tube. In an X-ray tube the beam of electrons is focused on a target of either tungsten or molybdenum which gives off X-rays. The target becomes extremely hot and must be water-cooled. In welding, the target is the base metal which absorbs the heat to bring it to the molten stage. In electron beam welding, X-rays may be produced if the electrical potential is sufficiently high.

As developments continued, two basic designs evolved: (1) the low-voltage electron beam system, which uses accelerating voltages in 30,000 volts or (30 kV) to 60,000-volt (60 kV) range and (2) the high-voltage system with accelerating voltages in the 100,000- volt (100 kV) range. The higher voltage system emits more X-rays than the lower voltage system.

In both systems, the electron gun and the work piece are housed in a vacuum chamber. There are three basic components in an electron beam-welding machine. These are (1) the electron beam gun, (2) the power supply with controls, and (3) a vacuum work chamber with work-handling equipment. The electron beam gun emits electrons, accelerates the beam of electrons, and focuses it on the work piece.

Recent advances in equipment allow the work chamber to operate at a medium vacuum or pressure. In this system, the vacuum in the work chamber is not as high. It is sometimes called a "soft" vacuum. This vacuum range allowed the same contamination that would be obtained in atmosphere of 99.995% argon. Mechanical pumps can produce vacuums to the medium pressure level.

One of the major advantages of electron beam welding is its tremendous penetration. This occurs when the highly accelerated electron hits the base metal. It will penetrate slightly below the surface and at that point release the bulk of its kinetic energy which turns to heat energy. The addition of the heat brings about a substantial temperature increase at the point of impact. The succession of electrons striking the same place causes melting and then evaporation of the base metal. This creates metal vapors but the electron beam travels through the vapor much easier than solid metal. This causes the beam to penetrate deeper into the base metal. The width of the penetration pattern is extremely narrow. The depth-to-width can exceed a ratio of 20 to 1. As the power density is increased penetration is increased.

The heat input of electron beam welding is controlled by four variables: (1) the number of electrons per second hitting the work piece or beam current, (2) the electron speed at the moment of impact, the accelerating potential, (3) the diameter of the beam at or within the work-piece, the beam spot size, and (4) the speed of travel or the welding speed. The first two variables, beam current and accelerating potential, are used in establishing welding parameters. The third factor, the beam spot size, is related to the focus of the beam, and the fourth factor is also part of the procedure.

Since the electron beam has tremendous penetrating characteristics, with the lower heat input, the heat-affected zone is much smaller than that of any arc welding process. In addition, because of the almost parallel sides of the weld nugget, distortion is greatly minimized. The cooling rate is much higher and for many metals this is advantageous; however, for high-carbon steel this is a disadvantage and cracking may occur.

The weld joint details for electron beam welding must be selected with care. In high vacuum chamber welding special techniques must be used to properly align the electron beam with the joint. Welds are extremely narrow and therefore preparation for welding must be extremely accurate.

Filler metal is not used in electron beam welding; however, when welding mild steel highly deoxidized filler metal is sometimes used. This helps deoxidize the molten metal and produce dense welds.

Almost all metals can be welded with the electron beam welding process. The metals that are most often welded are the super alloys, the refractory metals, the reactive metals, and the stainless steels. Many combinations of dissimilar metals can also be welded.

One of the disadvantages of the electron beam process is its high capital cost. The price of the equipment is very high and it is expensive to operate due to the need for vacuum pumps. In addition, fit up must be precise and locating the parts with respect to the beam must be perfect.