Welding Procedures

As welding becomes a modern engineering technology it requires that the various elements involved be identified in a standardized way. This is accomplished by writing a procedure which is simply a "manner of doing" or "the detailed elements (with prescribed values or range of values) of a process or method used to produce a specific result." The AWS definition for a welding procedure is "the detailed methods and practices including all joint welding procedures involved in the production of a weldment." The joint welding procedure mentioned includes "the materials, detailed methods and practices employed in the welding of a particular joint."

A welding procedure is used to make a record of all of the different elements, variables, and factors that are involved in producing a specific weld or weldment. Welding procedures should be written whenever it is necessary to:

* Maintain dimensions by controlling distortion
* Reduce residual or locked up stresses
* Minimize detrimental metallurgical changes
* Consistently build a weldment the same way
* Comply with certain specifications and codes.

Welding procedures must be tested or qualified and they must be communicated to those who need to know. This includes the designer, the welding inspector, the welding supervisor, and last but not least, the welder.

When welding codes or high-quality work is involved this can become a welding procedure specification, which lists in detail the various factors or variables involved. Different codes and specifications have somewhat different requirements for a welding procedure, but in general a welding procedure consists of three parts as follows:

* A detailed written explanation of how the weld is to be made
* A drawing or sketch showing the weld joint design and the conditions for making each pass or bead
* A record of the test results of the resulting weld.

If the weld meets the requirements of the code or specification and if the written procedure is properly executed and signed it becomes a qualified welding procedure.

The variables involved in most specifications are considered to be essential variables. In some codes the term nonessential variables may also be used. Essential variables are those factors which must be recorded and if they are changed in any way, the procedure must be retested and requalified. Nonessential variables are usually of less importance and may be changed within prescribed limits and the procedure need not be requalified.

Essential variables involved in the procedure usually include the following:

* The welding process and its variation
* The method of applying the process
* The base metal type, specification, or composition
* The base metal geometry, normally thickness
* The base metal need for preheat or postheat
* The welding position
* The filler metal and other materials consumed in making the weld
* The weld joint, that is, the joint type and the weld
* Electrical or operational parameters involved
* Welding technique.

Some specifications also include nonessential variables and these are usually the following:

* The travel progression (uphill or downhill)
* The size of the electrode or filler wire
* Certain details of the weld joint design
* The use and type of weld backing
* The polarity of the welding current.

The procedure write-up must include each of the listed variables and describe in detail how it is to be done. The second portion of the welding procedure is the joint detail sketch and table or schedule of welding conditions.

Tests are performed to determine if the weld made to the procedure specification meets certain standards as established by the code or specification. If the destructive tests meet the minimum requirements the procedure then becomes a qualified procedure specification. The writing, testing, and qualifying procedures become quite involved and are different for different specifications and will be covered in detail in a later chapter.

In certain codes, welding procedures are prequalified. By using data provided in the code individual qualified procedure specifications are not required, for the standard joints on common base materials using the shielded metal arc welding process.

The factors included in a procedure should be considered in approaching any new welding job. By means of knowledge and experience establish the optimum factors or variables in order to make the best and most economical weld on the material to be welded and in the position that must be welded.

Welding procedures take on added significance based on the quality requirements that can be involved. When exact reproducibility and perfect quality are required, the procedures will become much more technical with added requirements, particularly in testing. Tests will become more complex to determine that the weld joint has the necessary properties to withstand the service for which the weld is designed.

Procedures are written to produce the highest-quality weld required for the service involved, but at the least possible cost and to provide weld consistency. It may be necessary to try different processes, different joint details, and so on, to arrive at the lowest-cost weld which will satisfy the service requirements of the weldment.
The Physics and Chemistry of Welding

Welding follows all of the physical laws of nature and a good understanding of physics and chemistry will help you better understand how welds are made.

The science of sound is important to welding since one welding process and one weld nondestructive examination technique is based on the use of sound. Sound is transmitted through most materials: metals, gases, liquids, etc., but it will not pass through a vacuum. Sound is an alternating type of energy based on vibrations, which are regions of compaction and rarification.

The science of light also involves welding. The laser beam welding process utilizes light energy at very high concentrations to create heat sufficient to cause melting, which can be used for welding or cutting. Light is a by-product of the arc welding processes. Light is given off by the arc and by heated electrodes and base metals.

The science of friction also involves welding. Here we are interested in dynamic friction, better known as sliding friction. This is the force between two moving bodies and if sufficient force is available heat will be generated. This is the basis for the friction-welding process.

Several chemical definitions relate to welding. One is known as burning or oxidation. This takes place when any substance combines with oxygen usually at high temperatures. An example of this is the combining of acetylene with oxygen. This produces carbon dioxide plus water plus a large amount of heat. We use the heat produced by the burning of acetylene in the flame of the oxyacetylene torch to make welds. In all oxidation reactions heat is given off. Oxidation can occur very slowly as in the case of rusting. If iron is exposed to oxygen at high temperature rapid oxidation or burning will occur with the liberation of more heat. Rapid oxidation or burning does not occur until the kindling temperature of the material is reached. In the case of a liquid this term is called the flash point. Oxidation is very important in welding operations since oxygen of the air is usually present as well as heat.

Resistance welding is a group of welding processes in which coalescence is produced by the heat obtained from resistance of the work piece to electric current in a circuit of which the work piece is a part and by the application of pressure. There are at least seven important resistance-welding processes. These are flash welding, high-frequency resistance welding, percussion welding, projection welding, resistance seam welding, resistance spot welding, and upset welding. They are alike in many respects but are sufficiently different.

Resistance spot welding (RSW) is a resistance welding process which produces coalescence at the faying surfaces in one spot by the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes.

The size and shape of the individually formed welds are limited primarily by the size and contour of the electrodes. The equipment for resistance spot welding can be relatively simple and inexpensive up through extremely large multiple spot welding machines. The stationary single spot welding machines are of two general types: the horn or rocker arm type and the press type.

The horn type machines have a pivoted or rocking upper electrode arm, which is actuated by pneumatic power or by the operator`s physical power. They can be used for a wide range of work but are restricted to 50 kVA and are used for thinner gauges. For larger machines normally over 50 kVA, the press type machine is used. In these machines, the upper electrode moves in a slide. The pressure and motion are provided on the upper electrode by hydraulic or pneumatic pressure, or are motor operated.

For high-volume production work, such as in the automotive industry, multiple spot welding machines are used. These are in the form of a press on which individual guns carrying electrode tips are mounted. Welds are made in a sequential order so that all electrodes are not carrying current at the same time.

Projection welding (RPW) is a resistance welding process which produces coalescence of metals with the heat obtained from resistance to electrical current through the work parts held together under pressure by electrodes.

The resulting welds are localized at predetermined points by projections, embossments, or intersections. Localization of heating is obtained by a projection or embossment on one or both of the parts being welded. There are several types of projections: (1) the button or dome type, usually round, (2) elongated projections, (3) ring projections, (4) shoulder projections, (5) cross wire welding, and (6) radius projection.

The major advantage of projection welding is that electrode life is increased because larger contact surfaces are used. A very common use of projection welding is the use of special nuts that have projections on the portion of the part to be welded to the assembly.

Resistance seam welding (RSEW) is a resistance welding process which produces coalescence at the faying surfaces the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes.

The resulting weld is a series of overlapping resistance spot welds made progressively along a joint rotating the electrodes. When the spots are not overlapped enough to produce gaslight welds it is a variation known as roll resistance spot welding. This process differs from spot welding since the electrodes are wheels. Both the upper and lower electrode wheels are powered. Pressure is applied in the same manner as a press type welder. The wheels can be either in line with the throat of the machine or transverse. If they are in line it is normally called a longitudinal seam welding machine. Welding current is transferred through the bearing of the roller electrode wheels. Water cooling is not provided internally and therefore the weld area is flooded with cooling water to keep the electrode wheels cool.

In seam welding a rather complex control system is required. This involves the travel speed as well as the sequence of current flow to provide for overlapping welds. The welding speed, the spots per inch, and the timing schedule are dependent on each other. Welding schedules provide the pressure, the current, the speed, and the size of the electrode wheels.

This process is quite common for making flange welds, for making watertight joints for tanks, etc. Another variation is the so-called mash seam welding where the lap is fairly narrow and the electrode wheel is at least twice as wide as used for standard seam welding. The pressure is increased to approximately 300 times normal pressure. The final weld mash seam thickness is only 25% greater than the original single sheet.

Flash Welding (FW) is a resistance welding process which produces coalescence simultaneously over the entire area of abutting surfaces, by the heat obtained from resistance to electric current between the two surfaces, and by the application of pressure after heating is substantially completed.

Flashing and upsetting are accompanied by expulsion of metal from the joint. During the welding operation there is an intense flashing arc and heating of the metal on the surface abutting each other. After a predetermined time the two pieces are forced together and coalescence occurs at the interface, current flow is possible because of the light contact between the two parts being flash welded.

The heat is generated by the flashing and is localized in the area between the two parts. The surfaces are brought to the melting point and expelled through the abutting area. As soon as this material is flashed away another small arc is formed which continues until the entire abutting surfaces are at the melting temperature. Pressure is then applied and the arcs are extinguished and upsetting occurs.

Upset welding (UW) is a resistance welding process which produces coalescence simultaneously over the entire area of abutting surfaces or progressively along a joint, by the heat obtained from resistance to electric current through the area where those surfaces are in contact.

Pressure is applied before heating is started and is maintained throughout the heating period. The equipment used for upset welding is very similar to that used for flash welding. It can be used only if the parts to be welded are equal in cross-sectional area. The abutting surfaces must be very carefully prepared to provide for proper heating.

The difference from flash welding is that the parts are clamped in the welding machine and force is applied bringing them tightly together. High-amperage current is then passed through the joint, which heats the abutting surfaces. When they have been heated to a suitable forging temperature an upsetting force is applied and the current is stopped. The high temperature of the work at the abutting surfaces plus the high pressure causes coalescence to take place. After cooling, the force is released and the weld is completed.

Percussion welding (PEW) is a resistance welding process which produces coalescence of the abutting members using heat from an arc produced by a rapid discharge of electrical energy.

Pressure is applied progressively during or immediately following the electrical discharge. This process is quite similar to flash welding and upset welding, but is limited to parts of the same geometry and cross section. It is more complex than the other two processes in that heat is obtained from an arc produced at the abutting surfaces by the very rapid discharge of stored electrical energy across a rapidly decreasing air gap. This is immediately followed by application of pressure to provide an impact bringing the two parts together in a progressive percussive manner. The advantage of the process is that there is an extremely shallow depth of heating and time cycle is very short. It is used only for parts with fairly small cross-sectional areas.

High frequency resistance welding (HFRW) is a resistance welding process which produces coalescence of metals with the heat generated from the resistance of the work pieces to a high-frequency alternating current in the 10,000 to 500,000 hertz range and the rapid application of an upsetting force after heating is substantially completed. The path of the current in the work piece is controlled by the proximity effect.

This process is ideally suited for making pipe, tubing, and structural shapes. It is used for other manufactured items made from continuous strips of material. In this process the high frequency welding current is introduced into the metal at the surfaces to be welded but prior to their contact with each other.

Current is introduced by means of sliding contacts at the edge of the joint. The high-frequency welding current flows along one edge of the seam to the welding point between the pressure rolls and back along the opposite edge to the other sliding contact.

The current is of such high frequency that it flows along the metal surface to a depth of several thousandths of an inch. Each edge of the joint is the conductor of the current and the heating is concentrated on the surface of these edges. At the area between the closing rolls the material is at the plastic temperature, and with the pressure applied, coalescence occurs.

The American Welding Society has made each welding process definition as complete as possible so that it will suffice without reference to another definition. They define a process as "a distinctive progressive action or series of actions involved in the course of producing a basic type of result".

The official listing of processes and their grouping is shown by Figure 1., the AWS Master Chart of Welding and Allied Processes. The welding society formulated process definitions from the operational instead of the metallurgical point of view. Thus the definitions prescribe the significant elements of operation instead of the significant metallurgical characteristics.

Figure 1. AWS master chart of welding and allied processes.

The AWS definition for a welding process is "a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material".

AWS has grouped the processes together according to the "mode of energy transfer" as the primary consideration. A secondary factor is the "influence of capillary attraction in effecting distribution of filler metal" in the joint. Capillary attraction distinguishes the welding processes grouped under "Brazing" and "Soldering" from "Arc Welding", "Gas Welding", "Resistance Welding", "Solid State Welding", and "Other Processes."

The welding processes, in their official groupings, are shown by Table 1. This table also shows the letter designation for each process. The letter designation assigned to the process can be used for identification on drawings, tables, etc. Allied and related processes include adhesive bonding, thermal spraying, and thermal cutting.

Arc Welding
The arc welding group includes eight specific processes, each separate and different from the others but in many respects similar.

The carbon arc welding (CAW) process is the oldest of all the arc welding processes and is considered to be the beginning of arc welding. The Welding Society defines carbon arc welding as "an arc welding process which produces coalescence of metals by heating them with an arc between a carbon electrode and the work-piece. No shielding is used. Pressure and filler metal may or may not be used." It has limited applications today, but a variation or twin carbon arc welding is more popular. Another variation uses compressed air for cutting.

The development of the metal arc welding process soon followed the carbon arc. This developed into the currently popular shielded metal arc welding (SMAW) process defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a covered metal electrode and the work-piece. Shielding is obtained from decomposition of the electrode covering. Pressure is not used and filler metal is obtained from the electrode."

Automatic welding utilizing bare electrode wires was used in the 1920s, but it was the submerged arc welding (SAW) process that made automatic welding popular. Submerged arc welding is defined as "an arc welding process which produces coalescence of metals by heating them with an arc or arcs between a bare metal electrode or electrodes and the work piece. Pressure is not used and filler metal is obtained from the electrode and sometimes from a supplementary welding rod." It is normally limited to the flat or horizontal position.

The need to weld nonferrous metals, particularly magnesium and aluminum, challenged the industry. A solution was found called gas tungsten arc welding (GTAW) and was defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a tungsten (non-consumable) electrode and the work piece. Shielding is obtained from a gas or gas mixture."

Plasma arc welding (PAW) is defined as "an arc welding process which produces a coalescence of metals by heating them with a constricted arc between an electrode and the work piece (transferred arc) or the electrode and the constricting nozzle (non-transferred arc). Shielding is obtained from the hot ionized gas issuing from the orifice which may be supplemented by an auxiliary source of shielding gas." Shielding gas may be an inert gas or a mixture of gases. Plasma welding has been used for joining some of the thinner materials.

Another welding process also related to gas tungsten arc welding is known as gas metal arc welding (GMAW). It was developed in the late 1940s for welding aluminum and has become extremely popular. It is defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a continuous filler metal (consumable) electrode and the work piece. Shielding is obtained entirely from an externally supplied gas or gas mixture." The electrode wire for GMAW is continuously fed into the arc and deposited as weld metal. This process has many variations depending on the type of shielding gas, the type of metal transfer, and the type of metal welded.

A variation of gas metal arc welding has become a distinct welding process and is known as flux-cored arc welding (FCAW). It is defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a continuous filler metal (consumable) electrode and the work piece. Shielding is provided by a flux contained within the tubular electrode." Additional shielding may or may not be obtained from an externally supplied gas or gas mixture.

The final process within the arc welding group of processes is known as stud arc welding (SW). This process is defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a metal stud or similar part and the work piece". When the surfaces to be joined are properly heated they are brought together under pressure. Partial shielding may be obtained by the use of ceramic ferrule surrounding the stud.
Brazing (B)
Brazing is "a group of welding processes which produces coalescence of materials by heating them to a suitable temperature and by using a filler metal, having a liquidus above 450oC and below the solidus of the base materials. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction."

A braze is a very special form of weld, the base metal is theoretically not melted. There are seven popular different processes within the brazing group. The source of heat differs among the processes. Braze welding relates to welding processes using brass or bronze filler metal, where the filler metal is not distributed by capillary action.

Oxy Fuel Gas Welding (OFW)

Oxy fuel gas welding is "a group of welding processes which produces coalescence by heating materials with an oxy fuel gas flame or flames with or without the application of pressure and with or without the use of filler metal."

There are four distinct processes within this group and in the case of two of them, oxyacetylene welding and oxyhydrogen welding, the classification is based on the fuel gas used. The heat of the flame is created by the chemical reaction or the burning of the gases. In the third process, air acetylene welding, air is used instead of oxygen, and in the fourth category, pressure gas welding, pressure is applied in addition to the heat from the burning of the gases. This welding process normally utilizes acetylene as the fuel gas. The oxygen thermal cutting processes have much in common with this welding processes.

Resistance Welding (RW)

Resistance welding is "a group of welding processes which produces coalescence of metals with the heat obtained from resistance of the work to electric current in a circuit of which the work is a part, and by the application of pressure". In general, the difference among the resistance welding processes has to do with the design of the weld and the type of machine necessary to produce the weld. In almost all cases the processes are applied automatically since the welding machines incorporate both electrical and mechanical functions.

Other Welding Processes

This group of processes includes those, which are not best defined under the other groupings. It consists of the following processes: electron beam welding, laser beam welding, thermit welding, and other miscellaneous welding processes in addition to electroslag welding which was mentioned previously.
Soldering (S)
Soldering is "a group of joining processes which produces coalescence of materials by heating them to a suitable temperature and by using a filler metal having a liquidus not exceeding 450 oC (840 oF) and below the solidus of the base materials. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction." There are a number of different soldering processes and methods.

Solid State Welding (SSW)

Solid state welding is "a group of welding processes which produces coalescence at temperatures essentially below the melting point of the base materials being joined without the addition of a brazing filler metal. Pressure may or may not be used."

The oldest of all welding processes forge welding belongs to this group. Others include cold welding, diffusion welding, explosion welding, friction welding, hot pressure welding, and ultrasonic welding. These processes are all different and utilize different forms of energy for making welds.

Almost 85% of the metal produced and used is steel. The term steel encompasses many types of metals made principally of iron. Steel is an alloy of iron and carbon, but steels most often contain other metals such as manganese, chromium, nickel, etc., and nonmetals such as carbon, silicon, phosphorus, sulfur, and others.

There are so many different types and kinds of steels that it is sometimes confusing just to be able to identify the steel that is being used. For example, there are structural steels, cast steels, stainless steels, tool steels, hot rolled steel, reinforcing, steel, low alloy high strength steel, etc. Steels are sometimes given names based on their principal alloy such as carbon steel, chrome-manganese steel, chrome-molybdenum steel, etc.

Low-Carbon Steels and Low-alloy Steels
Low-carbon steels include those in the AISI series C-1008 to C-1025. Carbon ranges from 0.10 to 0.25%, manganese ranges from 0.25 to 1.5%, phosphorous is 0.4% maximum, and sulfur is 0.5% maximum. Steels in this range are most widely used for industrial fabrication and construction. These steels can be easily welded with any of the arc, gas, and resistance welding processes.

The low-alloy high-strength steels represent the bulk of the remaining steels in the AISI designation system. These steels are welded with E-80XX, E-90XX, and E-100XX class of covered welding electrodes. It is also for these types of steels that the suffix to the electrode classification number is used. These steels include the low-manganese steels, the low-to-medium nickel steels, the low nickel-chromium steels, the molybdenum steels, the chromium-molybdenum steels, and the nickel-chromium-molybdenum steels.

These alloys are included in AISI series 2315, 2515, and 2517. Carbon ranges from 0.12-0.30%, manganese from 0.40-0.60%, silicon from 0.20-0.45% and nickel from 3.25-5.25%. If the carbon does not exceed 0.15% preheat is not necessary, except for extremely heavy sections. If the carbon exceeds 0.15% preheat of up to 260oC, depending on thickness is required.

For the shielded metal arc welding process, attention was directed toward the selection of the class of covered electrodes based on their usability factors. All the electrodes described in AWS specification A5.1 are applicable to the mild and low-alloy steels. The E-60XX and E-70XX classes of electrodes provide sufficient strength to produce 100% weld joints in the steels. The yield strength of electrodes, in these classes, will overmatch the yield strength of the mild and low alloy steels. The E-60XX class should be used for steels having yield strength below 350 MPa and the E-70XX class should be used for welding steels having yield strength below 420 MPa. Low-hydrogen electrodes should be used and preheat is suggested when welding heavier materials, or restrained joints. The electrode that provides the desired operational features should be selected.

When welding the low-alloy high-strength steels, the operating characteristics of the electrode are not considered since the E-80XX and higher-strength electrodes are all of the low-hydrogen type. There is one exception, which is the E-XX10 class. These are shown in the AWS specification for low-alloy steel-covered arc welding electrodes, AWS 5.5. This specification is more complex than the one for mild steel electrodes, even though there are only two basic classes in each strength level. The lower strength level includes the E-8010, E-XX15, E-XX16, and the more popular E-XX18 classes.

This new information now allows the selection of the covered electrode to match not only the mechanical properties of the base metal, but also to approximately match the composition of the base metal. From this reason, the base metal composition and the mechanical properties must be know in order to select the correct covered electrode to be used. The only E-80XX or higher-strength electrodes that do not have low-hydrogen coverings are the E-8010 type electrodes which are designed specifically for welding pipes.

These high strength, cellulose-covered, electrodes are matched to specific alloy of the steel pipes. The deep penetrating characteristics of the cellulose-covered electrodes make them suitable for cross-country pipe welding. The theory and practice is that alloy steel pipe is relatively thin and it is welded with cellulose-covered electrodes at relatively high currents. In addition, each welding pass is very thin and the weld metal is aged for a considerable length of time prior to putting the pipeline into service. This allows for hydrogen, which might be absorbed, to escape from the metal and not adversely affect the service life of the pipeline.

Medium-Carbon Steels
The medium-carbon steels include those in the AISI series C-1020 to C-1050. The composition is similar to low-carbon steels, except that the carbon ranges from 0.25 to 0.50% and manganese from 0.60 to 1.65%.

With higher carbon and manganese the low-hydrogen type electrodes are recommended, particularly in thicker sections. Preheating may be required and should range from 150-260oC. Postheating is often specified to relieve stress and help stress and help reduce hardness that may have been caused by rapid cooling. Medium-carbon steels are readily weldable provided the above precautions are observed.

These steels can be welded with all of the processes mentioned above.

High-Carbon Steels
High-carbon steels include those in the AISI series from C-1050 to C-1095. The composition is similar to medium-carbon steels, except that carbon ranges from 0.30 to 1.00%.

Special precautions must be taken when welding steels in these classes. The low-hydrogen electrodes must be employed and preheating of from 300-320oC is necessary, especially when heavier sections are welded. A postheat treatment, either stress relieving or annealing, is usually specified.

High-carbon steels can be welded with the same processes mentioned previously.

Low-Nickel Chrome Steels
Steels in this group include the AISI 3120, 3135, 3140, 3310, and 3316. In these steels, carbon ranges from 0.14-0.34%, manganese from 0.40-0.90%, silicon from 0.20-0.35%, nickel from 1.10-3.75% and chromium from 0.55-0.75%.

Thin sections of these steels in the lower carbon ranges can be welded without preheat. A preheat of 100-150oC is necessary for carbon in the 0.20% range, and for higher carbon content a preheat of up 320oC should be used. The weldment must be stress relived or annealed after welding.

Low-Manganese Steels
Included in this group are the AISI type 1320, 1330, 1335, 1340, and 1345 designations. In these steels, the carbon ranges from 0.18-0.48%, manganese from 1.60-1.90%, and silicon from 0.20-0.35%.

Preheat is not required at the low range of carbon and manganese. Preheat of 120-150oC is desirable as the carbon approaches 0.25%, and mandatory at the higher range of manganese. Thicker sections should be preheated to double the above figure. A stress relief postheat treatment is recommended.

Low-Alloy Chromium Steels
Included in this group are the AISI type 5015 to 5160 and the electric furnace steels 50100, 51100, and 52100. In these steels carbon ranges from 0.12-1.10%, manganese from 0.30-1.00%, chromium from 0.20-1.60%, and silicon from 0.20-0.30%. When carbon is at low end of the range, these steels can be welded without special precautions. As the carbon increases and as the chromium increases, high hardenability results and a preheat of as high 400oC will be required, particularly for heavy sections.

When using the submerged arc welding process, it is also necessary to match the composition of the electrode with the composition of the base metal. A flux that neither detracts nor adds elements to the weld metal should be used. In general, preheat can be reduced for submerged arc welding because of the higher heat input and slower cooling rates involved. To make sure that the submerged arc deposit is low hydrogen, the flux must be dry and the electrode and base metal must be clean.

When using the gas metal arc welding process, the electrode should be selected to match the base metal and the shielding gas should be selected to avoid excessive oxidation of the weld metal. Preheating with the gas metal arc welding (GMAW) process should be in the same order as with shielded metal arc welding (SMAW) since the heat input is similar.

When using the flux-cored arc welding process, the deposited weld metal produced by the flux-cored electrode should match the base metal being welded. Preheat requirements would be similar to gas metal arc welding.

When low-alloy high-strength steels are welded to lower-strength grades the electrode should be selected to match the strength of the lower-strength steel. The welding procedure, that is, preheat input, etc., should be suitable for the higher-strength steel.

* Galvanic corrosion occurs between weld metal and base metal, between different areas of the same metal and between different metals in water.
* The intensity of galvanic corrosion is determined by the conductivity, oxygen content and the effective anode-to-cathode area ratio.
* Galvanic effects are spread over a large area in brackish and seawater; are confined to the immediate area of the junction in fresh water; and are often negligible in deaerated brines.
* Steel, Ni-Resist, zinc and aluminum are very effective in suppressing crevice corrosion on stainless steels except types 303 and 303Se.
* Carbon, graphite-lubricated gaskets, packing, greases etc. are very effective in initiating severe corrosion of stainless steels.
* Galvanic effects can be significantly reduced by removing coatings from the anode and by coating the more noble (cathodic) material.
* Stainless steel (or titanium) tubing increases copper alloy tube sheet attack to the point where impressed current cathodic protection is normally required to control tube sheet corrosion.
* Galvanic corrosion between different grades of nickel stainless steels mechanically joined is rare, but can be severe when welded. Caution and exposure tests are suggested.
* Avoid use of types 303 and 303Se.

Galvanic processes occur between different metals and between different areas of the same metal in the water environment. Water is an electrolyte, a poorly conductive one at the low dissolved solids content of fresh waters, and a highly conductive one at the high dissolved solids content of sea water.

When two different metals are immersed in an electrolyte and connected through a metallic path, current will flow. Oxidation occurs at the anode and reduction (normally oxygen reduction) occurs at the cathode. These reactions and the hydrogen reduction reaction that occurs in deaerated waters are represented in the usual form below.

Oxidation (corrosion) M → M+ + 2e
Reduction (deaerated waters) O2 (dissolved) + 2H2O + 4e → 4(OH-)
Reduction (deaerated waters) 2H+ + 2e → 2e

The electrons flow through the metal path from the anode to the cathode. The circuit is completed by transport (migration) of the ionic species (OH) from the vicinity of the cathode to the vicinity of the anode. In the absence of other species, the rate at which these reactions occur, and consequently the rate at which the anode corrodes, is controlled by the rate at which oxygen can be reduced at the cathode.

The rate of reduction of oxygen at the cathode in turn is determined primarily by the resistance to electron flow in the circuit, the cathodic surface area available for oxygen reduction and the amount of oxygen available at the cathodic area. The galvanic current (corrosion) is directly proportional to the cathodic area when the cell is under cathodic control as it normally is in water.

Conductivity plays a major role by limiting galvanic corrosion to the immediate area of contact in low conductivity fresh water and by spreading the galvanic effect over rather large areas in highly conductive waters such as seawater.

Painting the anode requires all of the anodic corrosion to occur in the very small areas where coating breakdown at scratches, welds, etc. occurs and exposes the steel. Painting the cathode reduces the area available for the rate-controlling oxygen reduction reaction and the amount of oxidation (corrosion) that can occur at the anode. Stainless steels are positioned towards the cathodic end of the galvanic series in seawater.

The potential range shown for each alloy should be interpreted as the range within which the metal to sea water potential is likely to vary for each alloy, not as an indication that alloys close to each other in the series are likely to change position. This rarely happens. The second more anodic potential band for stainless steels should be interpreted as the potential that develops in a shielded area where crevice corrosion has initiated.

Lee and Tuthill have developed quantitative guidelines for the amount of carbon steel or Ni-Resist required suppressing crevice corrosion of types 304 and 316 stainless steels in seawater.

These data indicate that carbon steel is very effective in suppressing crevice corrosion of 304 and 316 stainless steel in up to 100:1 SS to CS area ratios at 14°C (57°F) in seawater. At 28°C there is complete protection at 10:1 and a tenfold reduction in the percentage of sites where initiation occurs at 50:1 SS:CS area ratios. Ni-Resist (NR) is found to provide full protection for 316 and substantial protection for 304 at 50:1 SS: NR area ratios at temperatures up to 28°C.

The position of the copper alloys in the galvanic series suggests that copper alloys will not suppress crevice corrosion in stainless steels and, in fact, may accelerate crevice attack once it has started. Experience indicates copper alloys provide no useful galvanic protection for stainless steels.

Type 304 is the least noble of the nickel stainless steels and alloy 825 the most noble, being separated by about 0.05 volt. The various nickel stainless steels are generally coupled mechanically to each other and to nickel-base alloys without serious galvanic effects. There are two major qualifications:

* Should type 316L inadvertently be welded with type 308L filler metal instead of 316L, the weld metal will suffer severe localized corrosion. Hard facing overlays for rotating seal faces and weld overlay of tail shafts are other applications where close attention must be given to the position of individual alloys with respect to each other in the galvanic series in order to avoid costly failures.
* Both type 303 and 303Se suffer extraordinarily severe corrosion in seawater. The high density of manganese sulfide or selenide inclusion in these free-machining alloys create a surface with numerous built in austenite-to-inclusion galvanic cells.

Carbon is 0.2-0.3 volts or more positive than the nickel stainless steels. Carbon in the form of graphite; containing gaskets, packing and lubricants has been responsible for serious galvanic corrosion of stainless steels in seawater. Graphite in any form should never be used in contact with stainless steels in brackish or seawater.

Carbon filled rubber O-rings and gaskets are widely used in contact with stainless steels in seawater. The corrosion that occurs under O-rings and black rubber gaskets is normally crevice corrosion. However, in some instances acids used for chemical cleaning have softened these carbon-filled rubbers sufficiently to release carbon and set up adverse galvanic cell action, greatly accelerating the crevice attack that occurs in these rubber-to-stainless crevices.

Galvanic corrosion is a two way street and the effect on the other material coupled to stainless steels must always be considered. Investigating why copper alloy tube sheets were being so severely corroded when copper alloy condensers were retubed with stainless steel and titanium tubes, Gehring, Kuester and Maurer found that the whole inside surface area of the tube of the more noble alloys became effective as a cathode in copper alloy to stainless steel or titanium couples.

The more noble materials are so easily polarized that the cathodic area available for the reduction reaction (the rate controlling process) is multiplied far beyond the old two or four diameters rule of thumb, which was based on copper alloy behavior. Later work by Gehring and Kyle indicated that the intensity of the galvanic effect decreased with salinity. The increasing resistance of the lower salinity waters limits the effective cathodic area.

Galvanic corrosion can occur between different constituents of the same metal as well as between different metals. Iron embedded in the surface of stainless steel, manganese sulfide stringers and less highly alloyed weld metal are common examples.

Some elements extend the γ-loop in the iron-carbon equilibrium diagram, e.g. nickel and manganese. When sufficient alloying element is added, it is possible to preserve the face-centered cubic austenite at room temperature, either in a stable or metastable condition.

Chromium added alone to plain carbon steel tends to close the γ-loop and favor the formation of ferrite. However, when chromium is added to a steel containing nickel it retards the kinetics of the γ → α transformation, thus making it easier to retain austenite at room temperature.

The presence of chromium greatly improves the corrosion resistance of the steel by forming a very thin stable oxide film on the surface, so that chromium-nickel stainless steels are now the most widely used materials in a wide range of corrosive environments both at room and elevated temperatures.

Added to this, austenitic steels are readily fabricated and do not undergo a ductile/brittle transition which causes so many problems in ferritic steels. This has ensured that they have become a most important group of construction steels, often in very demanding environments.

Simple austenitic steels usually contain between 18 and 30% Cr, 8 to 20% Ni, and between 0.03 and 0.1% carbon. The binary iron-chromium equilibrium diagram (Fig. 1) shows that chromium restricts the occurrence of the γ-loop to the extent that above 13% Cr the binary alloys are ferritic over the whole temperature range, while there is a narrow (α + γ) range between 12% and 13% Cr. The ferrite is normally referred to as delta ferrite, because in these steels the phase can have a continuous existence from the melting point to room temperature.

Fig.1: The Fe-Cr equilibrium diagram

The addition of carbon to the binary alloy extends the γ-loop to higher chromium contents (Fig. 2), and also widens the (α + γ) phase field up to 0.3% C. When carbon is progressively added to an 18% Cr steel, in the range up to about 0.04% C, the steel is fully ferritic (Fig. 2) and cannot be transformed. Between 0.08 and 0.22% C, partial transformation is possible leading to (α + γ) structures, while above 0.40% C the steel can be made fully austenitic if cooled rapidly from the γ - loop region.

Fig.2: Effect of carbon on the Fe-Cr diagram

In austenitic steels, M23C6 is the most significant carbide formed and it can have a substantial influence on corrosion resistance.

If nickel is added to a low carbon iron-18% Cr alloy, the γ -phase field is expanded until at about 8% Ni the γ-phase persists to room temperature leading to the familiar group of austenitic steels based on 18% Cr 8% Ni. This particular composition arises because a minimum nickel content is required to retain γ at room temperature. With both lower and higher Cr contents more nickel is needed. For example, with more corrosion resistant, higher Cr steels, e.g. 25% Cr, about 15% Ni is needed to retain the austenite at room temperature.

Lack of complete retention is indicated by the formation of martensite. A stable austenite can be defined as one in which the Ms is lower than room temperature. The 18Cr8Ni steel, in fact, has an Ms just below room temperature and, on cooling, e.g. in liquid air, it will transform very substantially to martensite.

Manganese expands the γ -loop and can, therefore, be used instead of nickel. However, it is not as strong a γ -former but about half as effective, so higher concentrations are required. In the absence of chromium, around 12% Mn is required to stabilize even higher carbon (1-1.2%) austenite, achieved in Hadfields steel which approximates to this composition. Typically Cr-Mn steels require 12-15% Cr and 12-15% Mn to remain austenitic at room temperature if the carbon content is low.

Like carbon, nitrogen is a very strong austenite former. Both elements, being interstitial solutes in austenite, are the most effective solid solution strengtheners available. Nitrogen is more useful in this respect as it has less tendency to cause intergranular corrosion. Concentrations of nitrogen up to 0.25 %are used, which can nearly double the proof stress of Cr-Ni austenitic steel.

One of the most convenient ways of representing the effect of various elements on the basic structure of chromium-nickel stainless steels is the Schaeffler diagram, often used in welding. It plots the compositional limits at room temperature of austenite, ferrite and martensite, in terms of nickel and chromium equivalents (Fig. 3).

At its simplest level, the diagram shows the regions of existence of the three phases for iron-chromium-nickel alloys. However, the diagram becomes of much wider application when the equivalents of chromium and of nickel are used for the other alloying elements. The chromium equivalent has been empirically determined using the most common ferrite-forming elements:

Cr equivalent = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) + 1.75(Nb) + 1.5(Ti) + 0.75(W)

while the nickel equivalent has likewise been determined with the familiar austenite-forming elements:

Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)

All concentrations being expressed in weight percentages. Fig.3: Schaeffler diagram. Effect of alloying elements on the basic structure of Cr- Ni stainless steels. The large influence of C and N relative to that of the metallic elements should be particularly noted. The diagram is very useful in determining whether particular steel is likely to be fully austenitic at room temperature. This is relevant to bulk steels, particularly to weld metal where it is frequently important to predict the structure in order to avoid weld defects and excessive localized corrosive attack.

Cast high alloy steels are widely used for their corrosion resistance in aqueous media at or near room temperature and for service in hot gases and liquids at elevated and high temperatures (> 650°C). High-alloy cast steels are most often specified on the basis of composition using the designation system, which has been replaced by the Alloy Casting Institute (ACI), which formerly administered these designations.

Mechanical properties of these grades (for example, hardness and tensile strength) can be altered by suitable heat treatment. The cast high-alloy grades that contain more than 20 to 30% Cr+Ni, however, do not show the phase changes observed in plain carbon and low-alloy steels during heating or cooling between room temperature and the melting point. These materials are therefore non hardenable, and their properties depend on composition rather than heat treatment. Therefore, special consideration must be given to each grade of high-alloy cast steel with regard to casting design, foundry practice, and subsequent thermal processing.

Corrosion-resistant high-alloy cast steels, more commonly referred to as cast stainless steels, have grown steadily in technological and commercial importance during the past 40 years. The principal applications for these steels are for chemical-processing and power-generating equipment involving corrosion service in aqueous or liquid-vapor environments at temperatures normally below 315°C. These alloys are also used for special services at temperatures up to 650°C.

Cast stainless steels are defined as ferrous alloys that contain a minimum of 17% Cr for corrosion resistance. Most cast stainless steels are of course considerably more complex compositionally than this simple definition implies. Stainless steels typically contain one or more alloying elements in addition to chromium (for example, nickel, molybdenum, copper, niobium, and nitrogen) to produce a specific microstructure, corrosion resistance, or mechanical properties for particular service requirements.

Corrosion-resistant high-alloy cast steels are usually classified on the basis of composition or microstructure. It should be recognized that these bases for classification are not completely independent in most cases; that is, classification by composition also often involves microstructural distinctions.

Alloys are grouped as chromium steels, chromium-nickel steels in which chromium is the predominant alloying element, and nickel-chromium steels in which nickel is the predominant alloying element. The serviceability of cast corrosion-resistant steels depends greatly on the absence of carbon, and especially precipitated carbides, in the alloy microstructure.

The high-alloy cast steels can also be classified on the basis of microstructure. Structures may be austenitic, ferritic, martensitic, or duplex; the structure of a particular grade is primarily determined by composition. Chromium, nickel, and carbon contents are particularly important in this regard. In general, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic.

Martensitic grades include alloys CA-15, CA-40, CA-I5M, and CA-6NM. The CA-15 alloy contains the minimum amount of chromium necessary to make it essentially rustproof. It has good resistance to atmospheric corrosion as well as to many organic media in relatively mild service. A higher-carbon modification of CA-15, CA-40 can be heat treated to higher strength and hardness levels. Alloy CA-15M is a molybdenum-containing modification of CA-15 that provides improved elevated-temperature strength. Alloy CA-6NM is an iron-chromium-nickel-molybdenum alloy of low carbon content.

Austenitic grades include CH-20, CK-20, and CN-7M. The CH-20 and CK-20 alloys are high-chromium, high-carbon, wholly austenitic compositions in which the chromium exceeds the nickel content. The more highly alloyed CN-7M has excellent corrosion resistance in many environments and is often used in sulfuric acid service.

Ferritic grades are designated CB-30 and CC-50. Alloy CB-30 is practically nonhardenable by heat treatment. As this alloy is normally made, the balance among the elements in the composition results in a wholly ferritic structure similar to wrought AISI type 442 stainless steel. Alloy CC-50 has substantially more chromium than CB-30 and has relatively high resistance to localized corrosion in many environments.

Austenitic-ferritic alloys include CE-30, CF-3, CF-3A, CF-8, CF-SA, CF-20, CF-3M, CF-3MA, CF-8M, CF-8C, CF-16F, and CG-8M. The microstructures of these alloys usually contain 5 to 40% ferrite, depending on the particular grade and the balance among the ferrite-promoting and austenite-promoting elements in the chemical composition.

Duplex Alloys. Two duplex alloys CD-4MCu and Ferralium are currently of interest. Alloy CD-4MCu is the most highly alloyed duplex alloy. Ferralium was developed by Langley Alloys and is essentially CD-4MCu with about 0.15% N added. With high levels of ferrite (about 40 to 50%) and low nickel, the duplex alloys have better resistance to stress-corrosion cracking (SCC) than CF-3M. Alloy CD-4MCu, which contains no nitrogen and has relatively low molybdenum content, has only slightly better resistance to localized corrosion than CF-3M. Ferralium, which has nitrogen and slightly higher molybdenum than CD-4MCu, exhibits better-localized corrosion resistance than either CF-3M or CD-4MCu.

Improvements in stainless steel production practices (for example, electron beam refining, vacuum and argon-oxygen decarburization, and vacuum induction melting) have created a second generation of duplex stainless steels. These steels offer excellent resistance to pitting and crevice corrosion, significantly better resistance to chloride SCC than the austenitic stainless steels, good toughness, and yield strengths two to three times higher than those of type 304 or 316 stainless steels.

First generation duplex stainless steels, for example, AISI type 329 and CD-4MCu, have been in use for many years. The need for improvement in the weldability and corrosion resistance of these alloys resulted in the second-generation alloys, which are characterized by the addition of nitrogen as an alloying element.

Second generation duplex stainless steels are usually about a 50-50 blend of ferrite and austenite. The new duplex alloys combine the near immunity to chloride SCC of the ferritic grades with the toughness and ease of fabrication of the austenitics. Among the second-generation duplexes, Alloy 2205 seems to have become the general-purpose stainless.

Precipitation-Hardening Grades. The alloys in this group are CB-7Cu and CD-4MCu. Alloy CB-7Cu is a low-carbon martensitic alloy that may contain minor amounts of retained austenite or ferrite. The copper precipitates in the martensite when the alloy is heat treated to the hardened (aged) condition.

Heat-resistant high-alloy steel castings are extensively used for applications involving service temperatures in excess of 650°C. Strength at these elevated temperatures is only one of the criteria by which these materials are selected, because applications often involve aggressive environments to which the steel must be resistant. The atmospheres most commonly encountered are air, flue gases, or process gases; such atmospheres may be either oxidizing or reducing and may be sulfidizing or carburizing if sulfur or carbon are present.

Carbon and low-alloy steels seldom have adequate strength and corrosion resistance at elevated temperatures in the environments for which heat-resistant cast steels are normally selected. Only heat-resistant steels exhibit the required mechanical properties and corrosion resistance over long periods of time without excessive or unpredictable degradation. In addition to long-term strength and corrosion resistance, some cast heat-resistant steels exhibit special resistance to the effects of cyclic temperatures and changes in the nature of the operating environment.

These alloy types resemble high-alloy corrosion-resistant steels except for their higher carbon contents, which impart greater strength at elevated temperature. The higher carbon content and, to a lesser extent, alloy composition ranges distinguish cast heat-resistant steel grades from their wrought counterparts.

Iron-chromium alloys contain 8 to 30% Cr and little or no nickel. They are ferritic in structure and exhibit low ductility at ambient temperatures. Iron-chromium alloys are primarily used where resistance to gaseous corrosion is the predominant consideration because they possess relatively low strength at elevated temperatures.

Iron-chromium-nickel alloys contain more than 18% Cr and more than 8% Ni, with the chromium content always exceeding that of nickel. They exhibit an austenitic matrix, although several grades also contain some ferrite. These alloys exhibit greater strength and ductility at elevated temperatures than those in the iron-chromium group and withstand moderate thermal cycling. Examples of these alloys are the HE, HF, HH, HI, HK, and HL grades.

Iron-nickel-chromium alloys contain more than 10% Cr and more than 23% Ni, with the nickel content always exceeding that of chromium. These alloys are wholly austenitic and exhibit high strength at elevated temperatures. They can withstand considerable temperature cycling and severe thermal gradients and are well suited to many reducing, as well as oxidizing, environments. Examples of iron-nickel-chromium alloys are the HN, HP, HT, HU, HW, and HX grades. Even though nickel is the major element in the HW and HX grades, these grades are ordinarily referred to as high-alloy steels rather than nickel-base alloys.

Heat-resisting alloys useful at temperatures above 1200oF are based on iron, on nickel and on cobalt and contain elements that form precipitates that harden the matrix after solution treating and aging. Structural stability and resistance to oxidation and corrosion at elevated temperatures are required of these alloys.

Iron-base (actually, iron-chromium-nickel-base) alloys are the least costly and are applied in the lower temperature range, 1200 to 1500oF. Nickel-base and cobalt-base alloys are both applicable within the range of 1500 to 2000oF, and at temperatures below 1500oF as well. The hardening phase in nickel-base alloys is a nickel-aluminum-titanium phase called gamma prime. The hardening phase in cobalt-base alloys is complex carbide.

Vacuum melting permits accurate adjustment of composition and deoxidation with carbon, thus permitting oxygen removal in gaseous combination with carbon and inhibiting the formation of solid oxides in the bath. Under vacuum, gaseous hydrogen and nitrogen are removed to trace residuals. Vacuum melting also removes volatile metals, such as lead and zinc. Final additions of reactive metals are facilitated by the absence of any reaction of the bath with either air or slag.

For the most complex alloy systems, powder metallurgy is employed to prevent gross segregation. The alloy is melted in a conventional way and atomized while still in the liquid state, to form spheres, which are ground to fine powders of homogeneous chemical composition. The powders are compacted into preforms, sintered and then forged in the conventional way to produce segregation-free forgings.

A great many cast and wrought heat-resisting alloys are available. Iron-base heat-resisting alloys are only slightly more alloyed than stainless steels. They maintain useful strength within the lower range of temperatures, up to 1200oF; some are used at up to 1500oF. Figures 1 show rupture strengths for about 40 different compositions as a function of temperature. Ascoloy, with the curve shown at the extreme left in Fig. 1 over a temperature range of 900 to 1200oF, is a martensitic chromium stainless steel.

The curves shown on diagrams are typical and reflect neither statistical distribution nor specified minimums. Variations in composition, melting, forging and heat treatment are not reflected by these smoothed, typical curves. The curves therefore provide only a first approximation for material selection. Creep characteristics, microstructural stability, and resistance to corrosion by sulfur-containing gas at high temperature must also be taken into consideration.

Although developed originally for use at high temperature, some heat-resisting alloys have also been used at cryogenic temperatures, as forged components for handling liquid oxygen and liquid hydrogen.

Mechanical-test results for Inconel 718 at room and cryogenic temperatures are shown in Fig. 2 for specimens cut from forged components of over-all dimensions 4x9x15 in. The forgings were produced from 6-in.-diameter billets broken down from an 18-in.-diameter ingot.

Test results shown in Fig. 2 include tensile, notch-tensile and Charpy impact values. Each plotted point is an average of four tests. Testing was at room temperature, at -110oF in gaseous nitrogen, at -320oF in liquid nitrogen, and at -423oF in liquid hydrogen. The test values that concern ductility (elongation, notch-tensile / smooth-tensile ratio, and impact toughness) are shown for both longitudinal and transverse directions. Longitudinal bars were machined parallel to the 15-in. dimension of the forging; long-transverse direction bars were machined parallel to the 9-in. dimension; and the short-transverse direction bars, parallel to the 4-in. dimension.

Stainless steels are a class of chromium-containing steels widely used for their corrosion resistance in aqueous environments and for service at elevated temperatures. Stainless steels are distinguished from other steels by the enhanced corrosion and oxidation resistance created by chromium additions. Chromium imparts passivity of ferrous alloys when present in amounts of more than about 11% particularly if conditions are strongly oxidizing. Consequently, steels with more than 10 or 12% Cr are sometimes defined as stainless steels.

Stainless steel castings are usually classified as either corrosion-resistant castings or heat-resistant. However this line of demarcation in terms of application is not always distinct, particularly for steel castings used in the range from 450 to 650oC. The usual distinction between heat-resistant and corrosion-resistant cast steels is based on carbon content.

In general, the cast and wrought stainless steels possess equivalent resistance to corrosive media and they are frequently used in conjunction with each other. One significant difference between the cast and wrought stainless steels is in the microstructure of cast austenitic stainless steels. There is usually small amount of ferrite present in austenitic stainless steel castings, in contrast to the single-phase austenitic structure of the wrought alloys.

The presence of ferrite in the castings is desirable for facilitating weld repair, but ferrite also increases resistance to stress-corrosion cracking. The principal reasons for this resistance are apparently:

* Silicon added for fluidity gives added benefit from the standpoint of stress-corrosion cracking.
* Sand castings are usually tumbled or sandblasted to remove molding sand and scale, this probably tends to put the surface in compression

Wrought and cast stainless steels may also differ in mechanical properties, magnetic properties, and chemical content. Because of the possible existence of large dendritic grains, intergranular phases, and alloy segregation, typical mechanical properties of cast stainless steels may vary more and generally are inferior to those of any wrought structure.

Cast stainless steels are most of ten specified on the basis of composition using the designation system of the High Alloy Product Group of the Steel Founders Society of America (the Alloy Casting Institute). The first letter of the designation indicates whether the alloy is intended primarily for liquid corrosion service (C) or high temperature service (H). The second letter denotes the nominal chromium-nickel type of the alloy. As nickel content increases, the second letter of the designation is changed from A to Z. The numeral or numerals following the first two letters indicate maximum carbon content (percentage x 100) of the alloy. Finally, if further alloying elements are present, these are indicated by the addition of one or more letters as a suffix.

Corrosion-Resistant Steel Castings. These steel castings for liquid corrosion service are often classified on the basis of composition, although it should be recognized that classification by composition often involves microstructural distinction.

Alloys are grouped as:

* Chromium steels
* Chromium-nickel steels, in which chromium is the predominant alloying element
* Nickel-chromium steels, in which nickel is the predominant alloying element.

The service ability of cast corrosion-resistant steels depends greatly on the absence of carbon, and especially precipitated carbides, in the alloy microstructure. Therefore, cast corrosion resistant alloys are generally low in carbon (usually lower than 0,20% and sometimes lower than 0,03%).

All cast corrosion-resistant steels contain more than 11% chromium, and most contain from 1 to 30% nickel (a few have less than 1% Ni).

In general, the addition of nickel to iron-chromium alloys improves ductility and imparts strength. An increase in nickel content increases resistance to corrosion by neutral chloride solutions and weakly oxidizing acids.

The addition of molybdenum increases resistance to pitting attack by chloride solutions. It also extends the range of passivity in solutions of low oxidizing characteristics.

The addition of copper to duplex (ferrite in austenite) nickel-chromium alloys produces alloys that can be precipitation hardened to higher strength and hardness. The addition of copper to single-phase austenitic alloys greatly improves their resistance to corrosion by sulfuric acid. In all iron-chromium-nickel stainless alloys, resistance to corrosion by environments that cause intergranular attack can be improved by lowering the carbon content.

Compositions of Heat-Resistant Steel Castings. Castings are classified as heat resistant if they are capable of sustained operation while exposed, either continuously or intermittently, to operating temperatures that result in metal temperatures in excess of 650oC. Heat-resistant steel castings resemble high-alloy corrosion-resistant steels except for their higher carbon content, which imparts greater strength at elevated temperature.

The three principal categories of this type cast steels, based on composition are:

* Iron-chromium alloys
* Iron-chromium-nickel alloys
* Iron-nickel-chromium alloys

In the cast stainless steels structures may be austenitic, ferritic, martensitic, or ferric-austenitic (duplex). The structure of a particular grade is primarily determined by composition. Chromium, molybdenum, and silicon promote the formation of ferrite (magnetic), while carbon, nickel, nitrogen, and manganese favor the formation of austenite (non-magnetic).

Chromium (a ferrite and martensite promoter), nickel, and carbon (austenite promoters) are particularly important in determining microstructure. In general, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic.

Cast austenitic alloys usually have from 5 to 20% ferrite distributed in discontinuous pools throughout the matrix, the percent of ferrite depending on the nickel, chromium, and carbon contents. The presence of ferrite in austenite may be beneficial or detrimental, depending on the application.

Ferrite can be beneficial in terms of weldability because fully austenitic stainless steels are susceptible to a weldability problem known as hot cracking, or microfissuring. The intergranular cracking occurs in the weld deposit and/or in the weld heat-affected zone and can be avoided if the composition of the filler metal is controlled to produce about 4% ferrite in the austenitic weld deposit. Duplex CF grade alloy castings are immune to this problem.

The presence of ferrite in duplex CF alloys improves the resistance to stress-corrosion cracking (SCC) and generally to intergranular attack. In the case of SCC, the presence of ferrite pools in the austenite matrix is thought to block or make more difficult the propagation of cracks. In the case of intergranular corrosion, ferrite is helpful in sensitized castings because it promotes the preferential precipitation of carbides in the ferrite phase rather than at the austenite grain boundaries, where they would increase susceptibility to intergranular attack.

The presence of ferrite also places additional grain boundaries in the austenite matrix, and there is evidence that intergranular attack is arrested at austenite-ferrite boundaries. It is important to note, however, that not all studies have shown ferrite to be unconditionally beneficial to the general corrosion resistance of cast stainless steels. Some solutions attack the austenite phase in heat-treated alloys, whereas others attack the ferrite.

Ferrite can be detrimental in some applications. One concern may be the reduced toughness from ferrite, although this is not a major concern, given the extremely high toughness of the austenite matrix. A much greater concern is for applications that require exposure to elevated temperatures, usually 315oC and higher, where the metallurgical changes associated with the ferrite can be severe and detrimental. In application requiring that these steels be heated in the range from 425 to 650oC, carbide precipitation occurs at the edges of the ferrite pools in preference to the austenite grain boundaries.

Carbon and alloy grades for low-temperature service are required to provide the high strength, ductility, and toughness in vehicles, vessels, and structures that must serve at -45°C and lower. Because a number of steels are engineered specifically for service at low temperature (about -100°C), selecting the optimum material calls for thorough understanding of the application and knowledge of the mechanical properties that each grade provides.

At temperature below ambient, a metals behavior is characterized somewhat by crystalline structure. The yield and tensile strengths of metals that crystallize in the body-centered cubic from iron, molybdenum, vanadium and chromium depend greatly on temperature. These metals display a loss of ductility in a narrow temperature region below room temperature.
The tensile strength of metals with face-centered cubic structures - aluminum, copper, nickel and austenitic stainless steel - is more temperature dependent than their yield strength, and the metals often increase in ductility as temperature decrease.

Transformation occurring in compositions that are normally stable at room temperature, but metastable at cryogenic temperatures, can greatly alter their behavior. For example, the combination of gross plastic deformation and cryogenic temperatures can cause a normally ductile and tough stainless steel, such as 301, 302, 304, 321, to partially transform to bcc structure, resulting in an impairment of ductility and toughness. A fully stable stainless steel 310 cannot be transformed at cryogenic temperatures.

The 300 series steels offer a fine combination of toughness and weldability for service to the lowest temperatures. In the annealed condition, their strength properties are adequate for ground-based equipment but inadequate for lightweight structures. For aerospace applications, fabricators can take advantage of the alloys strain-hardening characteristics and use them in highly cold-worked condition. The principal shortcomings of cold-worked materials are: low weld-joint efficiencies caused by annealing during welding and the transformation to martensite that occurs during cryogenic exposure. Selection of fully stable grade type 310, overcomes the transformation problem. Precipitation-hardening A286 stainless has even higher strength when cold worked before aging.

The only alloy steel recommended for cryogenic service is 9% nickel steel. It is satisfactory for service down to -195°C and is used for transport and storage of cryogenics because of its low cost and ease of fabrication. Other alloy steels are suitable for service in the low-temperature range. The steels A201 and T-1 can suffice to -45°C, nickel steels with 2.25% Ni can suffice to -59°C, and nickel steels with 3.5% Ni to -101°C.

Steel for Cryogenic Service: An Example

Designers of cryogenic assemblies base their stress calculations on the room-temperature properties of the material. The reason is that it is the highest temperature the material will encounter. And it stands that if a higher-strength material that stands up to super cold conditions were available, designers might specify it.

At 26°C austenitic stainless steel has tensile and yield strengths that are 172 MPa greater than the corresponding strengths for type 304 stainless. At -100°C its tensile and yield strength exceed those of type 304 by 550 MPa and 276 MPa respectively.

A grade with following chemical composition shows good mechanical properties at cryogenic temperatures:

C - 0.072%
Mn - 16%
P - 0.02%
S - 0.008%
Si - 0.41%
Ni - 5.85%
Cr - 17.8%
N - 0.36%
Fe - Remainder
(The composition is given for plates with 12.7mm thickness)

The material combination of high strength, good toughness, and weldability should prompt designers to specify it for welded pressure vessels for the storage of cryogens.

Steels for Low-Temperature

When designing low-temperature systems or equipment, the engineer finds that notch toughness ranks high in importance, because a part or structure will generally fail due to a notch or other stress concentration. Test results measure the steels capacity to absorb energy, and thus signify its ability to resist failure at points of local stress concentration.

Fatigue limit of steel also must be considered. At low temperatures, systems are usually subjected to dynamic loads, and structural members to cycle stresses. Examples include vessels that frequently undergo pressure changes and large structures and mobile equipment that experience extreme stress imposed by packed snow or high winds. Other considerations include heat conductivity and thermal expansion.

Carbon steels have a better weldability, greater toughness, and higher strenght with low coefficients of termal conductivity than alloy steels. The A 516, one of the most frequently used group of carbon steels, have tensile strengths ranging from 379 MPa to 586 MPa minimum. The big advantage of A 516 steels is their low initial cost.

Compared with A 516, A 442 class have higher carbon and manganese in plates less than 25.4 mm thickness, and lower manganese beyond 25.4 mm. However, applications for A 516 Grades 55 and 60 duplicate those of A 442. They are easier to fabricate than A 442 grades because carbon content is lower.

Higher strength with good notch toughness is available in carbon steels A 537 Grade A and A 537 grade B. Their can be earlier normalized or quenched and tempered to raise yield and tensile strength and impact toughness beyond those of the A 516`s. Table 1 shows mechanical properties at low temperatures for some typical ASTM carbon steels.

Table 1. Specifications for Low-temperature Steels

Designation	Lowest usual service temperature, (°C)	Min Yield Strength (MPa)	Tensile Strength (MPa)	Min Elongation, L0= 50 mm (%)	Uses
A442 Gr. 55	-45	221	379 - 448	26	Welded pressure vessels and storage tanks; refrigeration; transport equipment
A442 Gr. 60	-45	221	414 - 496	23
A516 Gr. 55	-45	207	379 - 448	27
A516 Gr. 60	-45	221	414 - 496	25
A516 Gr. 65	-45	241	448 - 531	23
A516 Gr. 70	-45	262	483 - 586	21
A517 Gr. F	-45	690	792 - 931	16	Highly stressed vessels
A537 Gr. A	-60	345	483 - 620	22	Offshore drilling platforms, storage tanks, earthmoving equipment
A537 Gr. B	-60	414	551 - 690	22
A203 Gr. A	-60	255	448 - 531	23	Piping for liquid propane, vessels, tanks
A203 Gr. B	-60	276	482 - 586	21
A203 Gr. D	-101	255	448 - 531	23	Land-based storage for liquid propane, carbon dioxide, acetylene, ethane and ethylene
A203 Gr. E	-101	276	482 - 586	21
A533 Gr. 1	-73	345	552 - 690	18	Nuclear reactor vessels where low ambient toughness required for hydrostatic testing; some chemical and petroleum equipment
A533 Gr. 2	-73	482	620 - 793	16
A533 Gr. 3	-73	569	690 - 862	16
A543 Gr. 1	-107	586	724 - 862	14	Candidate material with high notch toughness for heavy-wall pressure vessels
A543 Gr. 2	-107	690	793 - 931	14

Since a variety of low-temperature steels are available, the engineer must consider the advantages each has to offer according to the application. The cost-strength ratio is but one factor; others, such as welding and fabrication costs, have equal or greater bearing on final costs. However, heat-treated carbon grades are often used for low-temperature services. Besides offering excellent low-temperature toughness plus fabricability, these grades are lower in initial cost.

Pipeline Steels for Low Temperature Uses

Steels for natural gas pipelines must meet more demanding requirements than that used for oil. For example they carry compressed gas at -25°C to -4°C, making crack growth and brittleness a problem in the severe artic environment. Achieving low-temperature notch toughness, grain size control, and low sulfur content were among major problems in developing the steel, particularly since economic feasibility had to be considered.

Hot-rolled steels present a good opportunity to cut both cost and weight if the cost per unit strength could be reduced. As strength of high-strength, low-alloy steels rise, toughness usually drops.

In steel alloyed with molybdenum, manganese and columbium, which is use for these pipe-lines, molybdenum raises both strength and toughness. Carbon is reduced to make columbium more soluble, and to improve weldability and impact strength. Steels with small and large amount of columbium have similar precipitation kinetics; higher strengths are produced by larger quantities of columbium. Columbium also promotes hardenability, which is needed to develop an acicular-ferrite microstructure. Manganese, along with molybdenum, helps to inhibit transformation to polygonal ferrite on the steel.

Where sulphur cannot be kept low, however, rare earth additions will control the shape of the sulfide inclusions. During hot working, grain refinement is enhanced because columbium has a grain-boundary pinning effect. This effect makes it possible to produce a highly substructured austenite prior transformation, which helps in assuring transformation to fine grained acicular ferrite.

Contributing to high strength and good impact resistance is the transformation mechanism - austenite changes to fine-grained acicular ferrite, which is further strengthened by the precipitation of columbium carbonitride. Other advantages include good formability and most important, excellent weldability.

Aside from pipeline, this steel can be used in the automotive, railroad, heavy equipment, construction and shipbuilding industries, application areas which the keynote is low cost per unit strength. Because of their inherently good strength-toughness relationship, the manganese-molybdenum-columbium steels may well satisfy this requirement.

Ferritic stainless steels have certain useful corrosion properties, such as resistance to chloride stress-corrosion cracking, corrosion in oxidizing aqueous media, oxidation at high temperatures and pitting and crevice corrosion in chloride media.

These steels contain above approximately 13% Cr and precipitate a prime phase in 350oC to 540oC range, and the maximum effect is at about 470oC. Because precipitation hardening lowers temperature ductility, it must be taken into account in both processing and usage of ferritic stainless steels, especially those with higher chromium content.

Structures of these steels are kept completely ferritic at room and high temperature by adding titanium or columbium, or by melting to very low levels of carbon and nitrogen, or both. Such microstructures provide ductility and corrosion resistance in weldments. Molybdenum improves pitting corrosion resistance, while silicon and aluminum increase resistance to high temperature oxidation.

The newer ferritic steels with high content of chromium have become possible through vacuum and argon-oxygen decarburization, electron-beam melting, and large-volume vacuum induction melting. The representatives of this group include ASTM designations 409 and 439.

Type 409 with 12% Cr is relatively low-cost and has good formability and weldability. Recommended thickness is limited to approximately 3,8 mm maximum if ductile-to-brittle transition temperature (DBTT) at room temperature or lower is needed (Figure 1). Its atmospheric corrosion resistance is adequate for functional uses, so applications of this type of steel include automobile exhaust equipment, radiator tanks, catalytic reactors, containerization and dry fertilizer trunks.

Type 439 with 18-20% Cr resists chloride stress-corrosion cracking. Resistance to general and pitting corrosion is approximately equivalent to that of austenitic types 304 and 316. This grade is suitable for equipment exposed to the aqueous chloride environments, heat transfer applications, condenser tubing for fresh water power plants, food-handling uses and water tubing for domestic and industrial buildings. Sheet thickness cannot exceed approximately 3,2 mm if DBTT (Figure 1) at room temperature or lower is needed.

Figure 1. Ductile-to-brittle transition temperatures (DBTT) for ferritic stainless steel rise with section thickness. Bands for 409 and 439 indicate data scatter

Resistance to stress-corrosion cracking is the most obvious advantage of the ferritic stainless steels. Ferritic steels resist chloride and caustic stress corrosion cracking very well. Nickel and copper residuals lower resistance of these steels to stress corosion.

Susceptibility of the ferritic steels to intergranular corrosion is due to chromium depletion, caused by precipitation of chromium carbides and nitrides at grain boundaries. Because of the lower solubility for carbon and nitrogen and higher diffusion rates in ferrite, the synthesized zones of welds in ferritic steels are in the weld and adjacent to the weld. To eliminate the intergranular corrosion, it is necessary either to reduce carbon to very low levels, or to add titanium and columbium to tie up the carbon and nitrogen.

Pitting, an insidious localized type of corrosion occurring in halide media, can put complete installations out of operation in relatively short time. Resistance to this type of corrosion depends on chloride concentration, exposure time, temperature and oxygen content. In general, resistance to pitting increases with chromium content. Molybdenum also plays an important role and it is equivalent to several percentages of chromium.

General corrosion resistance: The atmospheric corrosion resistance of the ferritic steels is excellent. These steels have good corrosion resistance in strongly oxidizing environments, such as nitric acid. In organic acids, all ferritic steels are superior to austenitic, but in reducing media general corrosion resistance of ferritic steels is worst than austenitic.

High-chromium ferritic stainless steels
High-chromium ferritic stainless steels - such as types 442 and 446 - have excellent resistance to corrosion and to oxidation in many industrial environments. These alloys are included in ASTM specifications A176-74 (Chromium stainless flat products), A 511 (Seamless stainless steel mechanical tubing), A268-74 (Ferritic stainless steel tubing for general service) and also in ASME code and AISI and SAE specifications.

High-chromium ferritic steels have 18-30% Cr and low content of carbon and nitrogen. Titanium in these alloys prevents intergranular chromium-carbide and nitride precipitation during welding or processing. Because of the ferritic structure and controlled composition, the alloys exhibit good resistance to general, intergranular and pitting corrosion, and stress corrosion cracking. Similar to other high chromium stainless steels, types 442 and 446 have excellent oxidation resistance at elevated temperatures. They also have high thermal conductivity, higher yield strength than austenitic stainless steels, and lower tensile ductility.

The excellent resistance to chlorides, organic acids and chloride stress-corrosion indicates that these alloys should be suitable for a wide range of applications in which conventional stainless steels or other materials are either inadequate or uneconomical. High-chromium ferritic stainless steels are useful in heat exchanger tubing, feed-water tubing and in equipment that operate with chloride-bearing or brackish cooling waters.

Available in sheet, strip, tubing and welding wire, alloys are finding substantial application in replacing brass and cupronickel, corrosion-resistant high-nickel alloys, and other materials in the food processing, power, chemical, petrochemical, marine and pulp and paper industries.