The stainless steels owe their resistance to corrosion to the presence of chromium. Brearley discovered this fact more or less accidentally in 1913. Today, there is a range of steels from the plain chromium variety to those containing up to six alloying elements in addition to the usual impurities. A simple classification of the steels follow:

Hardenable alloys

1. 12-14 % chromium, iron and steels, whose mechanical properties are largely dependent on the carbon content. High strength is combined with considerable corrosion resistance:
a) Stainless iron,
b) Stainless steel, (mild, medium and hard)

2. Secondary hardening ,10-12% chromium, 0.12% C, with small additions Mo, V, Nb, Ni; a steel with ultimate stress of 927 MPa is used for gas turbine blades.

3. High chromium steel 17% Cr, 0.15% C, 2.5% Ni (431 S29). It has a higher resistance to corrosion than iron, due to higher chromium content. It is used for pump shafts, valves and fittings subjected to high temperature and high-pressure steam, but is unsuitable for acid conditions.

High carbon, 0.8 C, 16.5 Cr, 0.5 Mo steel (oil quenched at 1025°C and tempered at 100„aC to give hardness of 700 HB) is used for stainless ball bearings and instruments such as scalpels.

Ferritic iron
- 16/18% chromium rustless iron with low carbon content (430 S15). It has high resistance to corrosion but low impact and cannot be refined by heat-treatment alone. Prolonged service at 480°C can cause embrittlement. It is used for motor car trim.

- 25/30% chromium iron for furnace parts, resistant to sulphur compounds. Forms sigma phase additions of Nb and Mo prevent excessive grain growth.

Austenitic steels

1. Plain 18/8 Austenitic Steels,
2. Soft Austenitic,
3. Decay-proof Steel,
4. Special Purpose Austenitic Steels,
5. High Manganese Steel,
6. Heat-resisting Steels,
7. Precipitation-hardening high tensile steels.
(a) martensitic,
(b) Semi Austenitic
(c) Austenitic,

Heat-treatment

The hardening alloys possess critical ranges comparable with ordinary carbon steels, and can, therefore, be hardened, tempered and refined by heat-treatment which does not depend on recrystallisation after cold working.

The ferritic and normal austenitic steels, on the other hand, are not amenable to such treatment. Only cold work with subsequent heat-treatment involving recrystallisation can be employed to refine large grained material.

Effects of chromium and nickel

It will be readily appreciated that chromium is the chief alloying element in iron and steel for inhibiting corrosion. This resistance is not due to the inertness of the chromium, for it combines with oxygen with extreme rapidity, but the oxide so formed is very thin and stable, continuous and impervious to further attack.This property is, fortunately, conferred upon its solid solution in iron, becoming very marked as the amount exceeds 12 % in low carbon steels.

Thus, in oxidising environments, such as nitric acid, the high chromium steel is initially attacked at the same rate as ordinary plain steel, but it rapidly builds up an oxide film, known as a self-healing passive-film, which efficiently protects the underlying metal. This film has actually been isolated by U. R. Evans. The thickness of the film and its Cr2O3 content increases with the degree of polish.

In oxidising media any defect in the film which may arise through abrasion will be quickly repaired and such steel is quite satisfactory in the atmosphere, but the film does not offer sufficient permanent resistance to the less oxidising action of hydrochloric and sulphuric acids, except in very dilute solutions. Nickel has a low solubility in these acids and thus, with 8 to 10 % of nickel in addition to chromium, the steel is immune from attack by nitric acid and the resistance to the other acids is markedly increased. Hence it is very evident why the 18/8 steels have such extensive uses. Their resistance to particular acids have been further improved by additions of elements such as molybdenum and copper.

Steels are now used for a wide variety of conditions entailing heat and corrosion under both static and dynamic stresses, such as aero engine valves, furnace conveyers, retorts, oil cracking units and gas turbines. Three important properties are necessary in material used at elevated temperatures:

1. Resistance to oxidation and to scaling.
2. Retention of strength at the working temperature.
3. Structural stability as regards carbide precipitation, spheroidisation, sigma formation and temper embrittlement.

Other properties may also be important in particular applications, such as specific resistance and temperature coefficient for electrical purposes, coefficient of expansion for constructional units and resistance to penetration by products of combustion in many furnace applications. In the case of gas turbine steels additional characteristics have to be considered; internal damping capacity and fatigue strength, notch sensitivity and impact strength (hot and cold), machining and welding characteristics, especially as large rotors may have to be built of small sections welded together.

The scale which forms on iron is porous and loosely adherent, but it is rendered adherent and protective by the addition of certain elements to the steel. These are usually chromium, silicon and aluminium, and they are characterised by their great affinity for combining with oxygen, but the reaction is rapidly stifled by the formation of the inert oxide films.

The resistance of mild steel to oxidation is vastly improved by forming an aluminium-iron alloy on the surface. This is done by heating at 1000°C in contact with powdered aluminium (calorising) or by metal-Spraying the steel surface with aluminium, coating with bitumastic paint to prevent oxidation and heating to 780°C (aluminising).

Improved creep strength may be attained by

* raising the softening temperature by the solution of alloying elements;
* judicious use of precipitation hardening at the working temperature without readily over-ageing. Second phase hardening is critically dependent on the degree and uniformity of the dispersion achieved and creep rate is related to a critical range of particle spacing;
* controlled degree of work-hardening in the appropriate temperature ranges which often reduces the extent of the primary creep stage;
* variations in the process of manufacture; deoxidisers and particles in the crystal boundary can have a marked effect on creep properties;
* vacuum melting permits the use of advantageous compositions which cannot be melted by conventional methods. lt also improves ductility in the transverse direction.

Mechanical properties, are improved by the addition of various elements.
Cobalt, tungsten and molybdenum cause the steels to withstand the action of tempering. High alloy austenitic steels have no change points and therefore do not harden by air cooling, but their resistance to wear resistance is not great. A sufficiency of elements such as silicon and chromium raise the Ac, point to temperatures above those reached in service and prevent the steel from air hardening on cooling.

Steels with high nickel content should not be used at high temperatures in contact with gases containing sulphur dioxide or other sulphur compounds, since intercrystalline films of nickel sulphide are formed.

In high chromium steels the carbides coalesce into large particles which have less obstructive action on grain growth of the ferrite at temperatures above 700°C. The excessive grain growth reduces still further the toughness which these steels possess.

Grain growth also occurs in the austenitic steels above 1000°C, but no trouble arises since they remain tough and ductile even in the coarse grained condition. When heated in the range 500-900°C austenitic steels precipitate carbides along the boundaries of the austenite and as a result intercrystalline cracks are liable to develop if the steel is stressed continuously in tension in this temperature range. With certain compositions both ferritic and austenitic steels are embrittled by the formation of sigma phase.

Cold rolled steels provide excellent press formability, surface finish, and thickness and flatness tolerances. Steel companies manufacture three groups of low- or ultra-low-carbon grades to meet a variety of customer formability requirements: CS Type B, DS Type B, EDDS, and EDDS+. They also produce HSLA steels and structural steel grades for those applications that require specified strength levels.

Cold rolled steels can also be specified as dent resistant or bake hardenable for applications that require dent resistance after forming and painting. Each grade can be processed with several surface finishes depending on customer requirements. Lubricants can be applied to enhance formability and to avoid at-press lubrication.

Cold rolled steels have the following features:

* Excellent Surface Appearance. Cold Rolled Steels have manufacturing controls in place assuring consistent surface quality to satisfy customer requirements.
* Formability. Cold Rolled Steels can be used to produce parts containing simple bends to parts with extreme deep drawing requirements.
* Paintability. Due to stringent surface roughness controls, Cold Rolled Steels are readily paintable using essentially any paint system.
* Weldability. Cold Rolled Steels can be joined using virtually any accepted welding practice.

Standard grades for cold rolled steels are:

* Commercial Steel (CS Type B). May be moderately formed; a specimen cut in any direction can be bent flat on itself without cracking.
* Drawing Steel (DS Type B). DS Type B is made by adding aluminum to the mol steel and may be used in drawing applications.
* Extra Deep Drawing Steel (EDDS). Interstitial Free (I-F) steels are made Drawing Steel by adding titanium and/or niobium to the molten steel after vacuum degassing and offer excellent drawability.
* Extra Deep Drawing Steel Plus (EDDS+). Interstitial Free (I-F) steels are made by adding titanium and/or niobium to the molten steel after vacuum degassing and offer excellent drawability.

Surface Finish
Cold rolled steels are manufactured with a matte finish obtained by rolling with specially roughened rolls on the cold mill and the temper mill. This finish helps to maintain effective lubrication during metal forming and improves the appearance of painted surfaces. Non-standard matte finishes can be provided that optimize the opposing effects of surface roughness on painted part appearance and lubrication during press forming.

Surface Protection and Lubrication
To prevent rusting in transit and storage, cold rolled steels can be supplied with a rust protective oil film or press forming lubricants. A pre-applied press forming lubricant provides uniform lubrication and eliminates the housekeeping problems.

A dry film (acrylic/polymer) lubricant can also be supplied by further processing the cold rolled product through a coil coating facility. These specialty organic coatings are easily removed with a mild alkaline cleaner.

Formability and Mechanical Properties
The formability of all steel products is a result of the interaction of many variables, the main ones being the mechanical properties of the steel, the forming system (tooling) used to manufacture parts, and the lubrication used during forming.

Tight control over chemical composition, hot rolling parameters, amount of cold reduction, annealing time and temperature, and the amount of temper rolling allow the production of high-quality cold rolled steel products to meet customers requirements. Commercial Steel (CS Type B) should be used for moderate forming or bending applications. CS Type B products are produced from aluminum-killed continuously cast slabs and, unless otherwise specified, have a carbon content of less than 0.15%.

To prevent the occurrence of fluting or stretcher strains during forming, CS products are tempered as a normal step in the mill processing.

For more severe forming applications, Drawing Steel Type B (DS Type B) should be used. DS Type B has a controlled carbon content (<0.06%) and is produced in such a manner that parts formed from DS Type B Steel should not exhibit stretcher strain.

Extra Deep Drawing Steel (EDDS) or Extra Deep Drawing Steel plus (EDDS+) should be used for the most demanding forming applications. These steels (also known as Interstitial Free or I-F steels) are produced from a vacuum degassed, titanium stabilized grade. EDDS and EDDS+ have the lowest carbon content available (<0,010%) and have been specially formulated to be most ductile products.

For high strength or structural applications, cold rolled steels are also available in yield strengths up to 50 ksi.

Paintability
Cold rolled steels can be easily painted using a variety of paint systems provided proper care is taken in preparing the material. Prior to painting, the surface should be carefully cleaned with either a solvent or alkaline cleaner.

Cleaning should be followed by a pre-treatment prior to painting. Zinc or iron phosphates give good results on cold rolled steels. Mild abrasion prior to pre-treating may also be used to enhance mechanical bonding of the paint.

Cold rolled steels can be in general supplied as pre-painted or pre-primed.

Carbon steels contain only carbon as the principal alloying element. Other elements are present in small quantities, including those added for deoxidation. Silicon and manganese in cast carbon steels typically range from 0.25 to about 0.80% Si, and 0.50 to about 1.00% Mn.

Carbon steels can be classified according to their carbon content into three broad groups:

* Low-carbon steels: < 0.20% C
* Medium-carbon steels: 0.20 to 0.50% C
* High-carbon steels: > 0.50% C

Low-alloy steels contain alloying elements, in addition to carbon, up to a total alloy content of 8%. Cast steels containing more than the following amounts of a single alloying element are considered low-alloy cast steels:

Element Mn Si Ni Cu Cr Mo V W
Amount (%) 1.00 0.80 0.50 0.50 0.25 0.10 0.05 0.05

For deoxidation of carbon and low-alloy steels, aluminum, titanium, and zirconium are used. Aluminum is more frequently used because of its effectiveness and low cost. Unless otherwise specified, the normal sulphur limit for carbon and low-alloy steels is 0.06%, and the normal phosphorus limit is 0.05%.

Structure and Property Correlations
Carbon steel castings are produced to a great variety of properties because composition and heat treatment can be selected to achieve specific combinations of properties, including hardness, strength, ductility, fatigue resistance, and toughness. Although selections can be made from a wide range of properties, it is important to recognize the interrelationships among these properties.

For example, higher hardness, lower toughness, and lower ductility values are associated with higher strength values. The relationships among these properties and mechanical properties will be discussed in the following text. Property trends among carbon steels are illustrated as a function of the carbon content in Fig. 1.

Strength and Hardness. Depending on alloy choice and heat treatment, ultimate tensile strength levels from 414 to 1724 MPa can be achieved with cast carbon and low-alloy steels. For carbon steels, the hardness and strength values are largely determined by carbon content and the heat treatment.

Figure 1: Properties of cast carbon steels as a function of carbon content and heat treatment. (a) Tensile strength and reduction of area; (b) Yield strength and reduction of area; (c) Brinell hardness; (d) Charpy V-notch impact energy

Strength and Ductility. Ductility depends greatly on the strength, or hardness, of the cast steel (Fig. 2). Actual ductility requirements vary with the strength level and the specification to which steel is ordered. Quenched-and-tempered steels exhibit higher ductility values for a given yield strength level than normalized, normalized-and-tempered, or annealed steels.

Figure 2: Tensile properties of cast carbon steels as a function of Brinell hardness.

Strength and Toughness. Several test methods are available for evaluating the toughness of steels or the resistance to sudden or brittle fracture. These include the Charpy V-notch impact test, the drop-weight test, the dynamic tear test, and specialized procedures to determine plane-strain fracture toughness.

Charpy V-notch impact energy trends at room temperature reveal the distinct effect of strength and heat treatment on toughness. Higher toughness is obtained when steel is quenched and tempered, rather than normalized and tempered. Quenching, followed by tempering, produces superior toughness as indicated by the shift of the impact energy transition curve to lower temperatures.

Nil ductility transition temperatures (NDTT) ranging from 38°C to as low as -90°C have been recorded in tests on normalized-and-tempered cast carbon and low-alloy steels in the yield strength range of 207 to 655 MPa. When cast steels are quenched and tempered, the range of strength and of toughness is broadened. Depending on alloy selection, NDTT values of as high as 10°C to as low as -107°C can be obtained in the yield strength range of 345 to 1345 MPa.

An approximate relationship exists between the Charpy V-notch impact energy temperature behavior and the NDTT value. The NDTT value frequently coincides with the energy transition temperature determined in Charpy V-notch tests.

Plane-strain fracture toughness (KIc) data for a variety of steels reflect the important strength-toughness relationship. Fracture mechanics tests have the advantage over conventional toughness tests of being able to yield material property values that can be used in design equations.

Strength and Fatigue. The most basic method of presenting engineering fatigue data is by means of the S-N curve, which relates the dependence of the life of the fatigue specimen in terms of the number of cycles to failure N to the maximum applied stress. Other tests have been used, and the principal findings for constant amplitude tests and fatigue notch sensitivity for cast carbon steels are highlighted below.

The endurance ratio (endurance limit divided by the tensile strength) of cast carbon and low-alloy steels as determined by rotating-beam bending fatigue tests is generally taken to be approximately 0.40 to 0.50 for smooth bars. The results indicate that this endurance ratio is largely independent of strength, alloying additions, and heat treatment.

The fatigue notch sensitivity factor determined in rotating-beam bending fatigue tests is related to the microstructure of the steel (composition and heat treatment) and the strength. The quenched-and-tempered steels with a martensitic structure are less notch-sensitive than the normalized-and-tempered steels with a ferrite-pearlite microstructure.

Section Size and Mass Effects. Mass effects are common to steels, whether rolled, forged, or cast, because the cooling rate during heat treating varies with section size and because the microstructure constituents, grain size, and nonmetallic inclusions increase in size from surface to center. Mass effects are metallurgical in nature and are distinct from the effect of discontinuities, which are discussed in the following section in this article.

The section size or mass effect is of particular importance in steel castings because mechanical properties are typically assessed from test bars machined from standardized coupons having fixed dimensions and are cast separately from or attached to the castings. The removal of test bars from the casting is impractical because removal of material for testing would destroy the usefulness of the component.

Test specimens removed from a casting will not routinely exhibit the same properties as test specimens machined from the standard test coupon designs for which minimum properties are established in specifications. The mass effect discussed above, shows that the difference in cooling rate between the test coupons and the part being produced, is the fundamental reason for this situation. Several specifications such as ASTM E 208, A 356, and A 757 provide for the mass effect by permitting the testing of coupons that are larger than normal and that have cooling rates more representative of those experienced by the part being produced.

Carbon steels are produced in greater tonnage and have wider use than any other metal because of their versatility and low cost. There were several reasons why carbon steels proved satisfactory on reappraisal:

1. their hardenability, though less than that of alloy steels, was adequate for many parts, and for some parts shallower hardening was actually an advantage because of minimized quench cracking;
2. refinements in heat treating methods, such as induction hardening, flame hardening, and "shell quenching", made it possible to obtain higher properties from carbon steels than previously; and
3. new compositions were added to the carbon steel group, permitting more discriminating selection.

There are now almost 50 grades available in the nonresulfurized series 1000 carbon steels and nearly 30 grades in the resulfurized series 1100 and 1200. The versatility of the carbon steel group has also been extended by availability of the various grades with lead additions.

Carbon steels can be divided into three arbitrary classifications based on carbon content.

Steels with 0.10 to 0.25% C. Three principal types of heat treatment are used for this group of steels:

1. conditioning treatments, such as process annealing, that prepare the steel for certain fabricating operations,
2. case hardening treatments, and
3. quenching and tempering to improve mechanical properties.

The improvement in mechanical properties that can be gained by straight quenching and tempering of the low-carbon steels is usually not worth the cost.

An example of process annealing is in the treatment of low-carbon cold-headed bolts made from cold drawn wire. Sometimes the strains introduced by cold working weaken the heads so much that they break through the most severely worked portion under slight additional strain. Process annealing overcomes this condition. Since the temperatures used are close to the lower transformation temperature, this treatment results in considerable reduction of the normal mechanical properties of the shank of the cold headed bolt.

A more suitable treatment is stress relieving at about 1000oF (540oC). This treatment is used in order to retain much of the strength acquired in cold working and to provide ample toughness. A common practice is to combine a stress-relieving treatment with a quench from the upper transformation temperature, or slightly above, producing mechanical properties that approach those of cold drawn stock. A common quenching medium is a water solution of soluble oil, the use of which produces two desirable results:

1. the surface of the parts acquires a pleasing black color accepted as a commercial finish, and
2. the speed of the quench is slowed to the point where fully quenched hardness is not produced, so it is not necessary to temper the parts.

Heat treatments are frequently employed to improve machinability. The generally poor machinability of the low-carbon steels, except those containing sulfur or other special alloying elements, results principally from the fact that the proportion of free ferrite to carbide is high. This situation cannot be changed fundamentally, but the machinability can be improved by putting the carbide in its most voluminous form, pearlite, and dispersing this pearlite evenly throughout the ferrite mass. Normalizing is commonly used with success, but best results are obtained by quenching the steel in oil from 1500 to 1600oF (815-870oC). With the exception of steels 1024 and 1025, no martensite is formed, and the parts do not require tempering.

Steels with 0.25 to 0.55% C. Because of their higher carbon content, these steels are usually used in the hardened and tempered condition. By selection of quenching medium and tempering temperature a wide range of mechanical properties can be produced. They are the most versatile of the three groups of carbon steels and are most commonly used for crankshafts, couplings, tie rods and many other machinery parts where the required hardness values are within the range from 229 to 447 HB. This group of steels shows a continuous change from water-hardening to oil-hardening types. The hardenability is very sensitive to changes in chemical composition, particularly to the content of manganese, silicon and residual elements, and to grain size; the steels are sensitive to section changes.

The rate of heating parts for quenching has a marked effect on hardenability under certain conditions. If the structure is non-uniform, as a result of severe banding or lack of proper normalizing or annealing, extremely rapid heating such as may be obtained in liquid baths, will not allow sufficient time for diffusion of carbon and other elements in the austenite. As a result, non-uniform or low hardness will be produced unless the duration of heating is extended. In heating steels that contain free carbide (for example, spheroidized material), sufficient time must be allowed for the solution of the carbides; otherwise the austenite at the time of quenching will have a lower carbon content than is represented by the chemical composition of the steel, and disappointing results may be obtained.

These medium-carbon steels should usually be either normalized or annealed before hardening, in order to obtain the best mechanical properties after hardening and tempering. Parts made from bar stock are frequently given no treatment prior to hardening, but it is common practice to normalize or anneal forgings. Most of bar stocks, both, hot finished and cold finished, are machined as received, except the higher-carbon grades and small sizes, which require annealing to reduce the as-received hardness. Forgings are usually normalized, since this treatment avoids the extreme softening and consequent reduction of machinability that result from annealing.

In some instances a "cycle treatment" is used. In this practice the parts are heated as for normalizing, and are then cooled rapidly in the furnace to a temperature somewhat above the nose of the S-curve - that is, within the transformation range that produces pearlite. Then the parts are held at temperature or cooled slowly until the desired amount of transformation has taken place; thereafter they are cooled in any convenient manner. Specially arranged furnaces are usually required. Details of the treatments vary widely and are frequently determined by the furnace equipment available.

Cold headed products are commonly made from these steels, especially from the ones containing less than 0.40% C. Process treating before cold working is usually necessary because the higher carbon decreases the workability. For certain uses, these steels are normalized or annealed above the upper transformation temperature, but more frequently a spheroidizing treatment is used. The degree of spheroidization required depends on the application. After shaping operations are finished, the parts are heat treated by quenching and tempering.

These medium-carbon steels are widely used for machinery parts for moderate duty. When such parts are to be machined after heat treatment, the maximum hardness is usually held to 321HB, and is frequently much lower.

Salt solutions are often successfully used. Salt solutions are not dangerous to operators but their corrosive action on iron or steel parts of equipment is very serious.

When the section is light or the properties required after heat treatment are not high, oil quenching is often used. This nearly always eliminates the breakage problem and is very effective in reducing distortion.

A wide range in austenitizing temperatures is made necessary in order to meet required conditions. Lower temperatures should be used for the higher-manganese steels, light sections, coarse-grained material and water quenching; higher temperatures are required for lower manganese, heavy sections, fine grain and oil quenching.

From these steels are made many common hand tools, such as pliers, open-end wrenches, screwdrivers, and a few edged tools - for example, tin snips and brush knives. The cutting tools are necessarily quenched locally on the cutting edges, in water, brine or caustic, and are subsequently given suitable tempering treatments. In some instances the edge is time quenched; then the remainder of the tool is oil quenched for partial strengthening. When made of these grades of steel, pliers, wrenches and screwdrivers are usually quenched in water, either locally or completely, and are then suitably tempered.

Steels with 0.55 to 1.00% C. Carbon steels with these higher carbon contents are more restricted in application than the 0.25 to 0.55% C steels since they are more costly to fabricate, because of decreased machinability, poor formability and poor weldability. They are also more brittle in the heat treated condition.

Higher-carbon steels such as 1070 to 1095 are especially suitable for springs where resistance to fatigue and permanent set are required. They are also used in the nearly fully hardened condition (Rockwell C 55 and higher) for applications where abrasion resistance is the primary requirement, as for agricultural tillage tools such as plowshares, and knives for cutting hay or grain.

Forged parts should be annealed because refinement of the forging structure is important in producing a high-quality hardened product, and because the parts come from the hammer too hard for cold trimming of the flash or for economical machining. Ordinary annealing practice, followed by furnace cooling to 1100oF (590oC), is satisfactory for most parts.

Most of the parts made from steels in this group are hardened by conventional quenching. However, special technique is necessary sometimes. Both oil and water quenching are used - water, for heavy sections of the lower-carbon steels and for cutting edges and oil, for general use. Austempering and martempering are often successfully applied; the principal advantages from such treatments are considerably reduced distortion, elimination of breakage, in many instances, and greater toughness at high hardness.

For heavy machinery parts, such as shafts, collars and the like, steels 1055 and 1061 may be used, either normalized and tempered for low strength, or quenched and tempered for moderate strength. Other steels in the list may be used, but the combination of carbon and manganese in the two mentioned makes them particularly well adapted for such applications.

It must be remembered that even with all hardenability factors favorable, including the use of a drastic quench, these steels are essentially shallow hardening, as compared with alloy steels, because carbon alone, or in combination with manganese in the amounts involved here, does not promote deep hardening to any significant extent. Therefore, the sections for which such steels are suited will be definitely limited. In spite of this limitation the danger of breakage is real and must be carefully guarded against when such parts are being treated, especially whenever changes in section are involved.

Hand tools made from these steels include open-end wrenches, Stillson wrenches, hammers, mauls, pliers and screw drivers and cutting tools, such as hatchets, axes, mower knives and band knives. The combination of carbon and manganese in the steels used may vary widely for the same type of tool, depending partly on the equipment available for manufacture and partly on personal experience with, or preference for, certain combinations. A manganese content lower than standard will be used in some tools. This is justified when it makes a particular carbon range easier to handle, but it should be understood that for many applications, a combination of lower carbon and higher manganese would serve just as well.

Cast irons are alloys of iron, carbon, and silicon in which more carbon is present than can be retained in solid solution in austenite at the eutectic temperature. In gray cast iron, the carbon that exceeds the solubility in austenite precipitates as flake graphite.

Gray irons usually contain 2.5 to 4% C, 1 to 3% Si, and additions of manganese, depending on the desired microstructure (as low as 0.1% Mn in ferritic gray irons and as high as 1.2% in pearlitics). Sulphur and phosphorus are also present in small amounts as residual impurities.

The composition of gray iron must be selected in such a way to satisfy three basic structural requirements:

* The required graphite shape and distribution
* The carbide-free (chill-free) structure
* The required matrix

For common cast iron, the main elements of the chemical composition are carbon and silicon. High carbon content increases the amount of graphite or Fe3C. High carbon and silicon contents increase the graphitization potential of the iron as well as its castability.

The combined influence of carbon and silicon on the structure is usually taken into account by the carbon equivalent (CE):

CE = %C + 0.3x(%Si) + 0.33x(%P) - 0.027x(%Mn) + 0.4x(%S)

Although increasing the carbon and silicon contents improves the graphitization potential and therefore decreases the chilling tendency, the strength is adversely affected. This is due to ferrite promotion and the coarsening of pearlite.

The manganese content varies as a function of the desired matrix. Typically, it can be as low as 0.1% for ferritic irons and as high as 1.2% for pearlitic irons, because manganese is a strong pearlite promoter.

The effect of sulfur must be balanced by the effect of manganese. Without manganese in the iron, undesired iron sulfide (FeS) will form at grain boundaries. If the sulfur content is balanced by manganese, manganese sulfide (MnS) will form, which is harmless because it is distributed within the grains. The optimum ratio between manganese and sulfur for a FeS-free structure and maximum amount of ferrite is:

%Mn = 1.7x(%S) + 0.15

Other minor elements, such as aluminum, antimony, arsenic, bismuth, lead, magnesium, cerium, and calcium, can significantly alter both the graphite morphology and the microstructure of the matrix.

In general, alloying elements can be classified into three categories. Silicon and aluminum increase the graphitization potential for both the eutectic and eutectoid transformations and increase the number of graphite particles. They form colloid solutions in the matrix. Because they increase the ferrite/pearlite ratio, they lower strength and hardness.

Nickel, copper, and tin increase the graphitization potential during the eutectic transformation, but decrease it during the eutectoid transformation, thus raising the pearlite/ferrite ratio. This second effect is due to the retardation of carbon diffusion. These elements form solid solution in the matrix. Since they increase the amount of pearlite, they raise strength and hardness.

Chromium, molybdenum, tungsten, and vanadium decrease the graphitization potential at both stages. Thus, they increase the amount of carbides and pearlite. They concentrate in principal in the carbides, forming (FeX)nC-type carbides, but also alloy the aFe solid solution. As long as carbide formation does not occur, these elements increase strength and hardness. Above a certain level, any of these elements will determine the solidification of a structure with Fe3C (mottled structure), which will have lower strength but higher hardness.

Generally, it can be assumed that the following properties of gray cast irons increase with increasing tensile strength from class 20 to class 60:

* All strengths, including strength at elevated temperature
* Ability to be machined to a fine finish
* Modulus of elasticity
* Wear resistance.

On the other hand, the following properties decrease with increasing tensile strength, so that low-strength irons often perform better than high-strength irons when these properties are important:

* Machinability
* Resistance to thermal shock
* Damping capacity
* Ability to be cast in thin sections.

Successful production of a gray iron casting depends on the fluidity of the molten metal and on the cooling rate, which is influenced by the minimum section thickness and on section thickness variations.

Casting design is often described in terms of section sensitivity. This is an attempt to correlate properties in critical sections of the casting with the combined effects of composition and cooling rate. All these factors are interrelated and may be condensed into a single term, castability, which for gray iron may be defined as the minimum section thickness that can be produced in a mold, cavity with given volume/area ratio and mechanical properties consistent with the type of iron being poured.

Scrap losses resulting from missruns, cold shuts, and round corners are often attributed to the lack of fluidity of the metal being poured.

Mold conditions, pouring rate, and other process variables being equal, the fluidity of commercial gray irons depends primarily on the amount of superheat above the freezing temperature (liquidus). As the total carbon content decreases, the liquidus temperature increases, and the fluidity at a given pouring temperature therefore decreases. Fluidity is commonly measured as the length of flow into a spiral-type fluidity test mold.

The significance of the relationships between fluidity, carbon content, and pouring temperature becomes apparent when it is realized that the gradation in strength in the ASTM classification of gray iron is due in large part to differences in carbon content (~3.60 to 3.80% for class 20; ~2.70 to 2.95% for class 60). The fluidity of these irons thus resolves into a measure of the practical limits of maximum pouring temperature as opposed to the liquidus of the iron being poured.

The usual microstructure of gray iron is a matrix of pearlite with graphite flakes dispersed throughout. Foundry practice can be varied so that nucleation and growth of graphite flakes occur in a pattern that enhances the desired properties. The amount, size, and distribution of graphite are important. Cooling that is too rapid may produce so-called chilled iron, in which the excess carbon is found in the form of massive carbides. Cooling at intermediate rates can produce mottled iron, in which carbon is present in the form of both primary cementite (iron carbide) and graphite.

Flake graphite is one of seven types (shapes or forms) of graphite established in ASTM A 247. Flake graphite is subdivided into five types (patterns), which are designated by the letters A through E. Graphite size is established by comparison with an ASTM size chart, which shows the typical appearances of flakes of eight different sizes at l00x magnification.

Type A flake graphite (random orientation) is preferred for most applications. In the intermediate flake sizes, type A flake graphite is superior to other types in certain wear applications such as the cylinders of internal combustion engines.

Type B flake graphite (rosette pattern) is typical of fairly rapid cooling, such as is common with moderately thin sections (about 10 mm) and along the surfaces of thicker sections, and sometimes results from poor inoculation.

The large flakes of type C flake graphite are formed in hypereutectic irons. These large flakes enhance resistance to thermal shock by increasing thermal conductivity and decreasing elastic modulus. On the other hand, large flakes are not conducive to good surface finishes on machined parts or to high strength or good impact resistance.

The small, randomly oriented interdendritic flakes in type D flake graphite promote a fine machined finish by minimizing surface pitting, but it is difficult to obtain a pearlitic matrix with this type of graphite. Type D flake graphite may be formed near rapidly cooled surfaces or in thin sections. Frequently, such graphite is surrounded by a ferrite matrix, resulting m soft spots in the casting.

Type E flake graphite is an interdendritic form, which has a preferred rather than a random orientation. Unlike type D graphite, type E graphite can be associated with a pearlitic matrix and thus can produce a casting whose wear properties are as good as those of a casting containing only type A graphite in a pearluic matrix. There are, of course, many applications in which flake type has no significance as long as the mechanical property requirements are met.

The response of carbon steel to soil corrosion depends primarily on the nature of the soil and certain other environmental factors, such as the availability to moisture and oxygen. These factors can lead to extreme variations in the rate of the attack. For example, under the worst condition a buried vessel may perforate in less than one year, although archeological digs in arid desert regions have uncovered iron tools that are hundreds of years old.

Some general rules can be formulated. Soils with high moisture content, high electrical conductivity, high acidity, and high dissolved salts will be most corrosive. The effect of aeration on soils is somewhat different from the effect of aeration in water because poorly aerated conditions in water can lead to accelerated attack by sulfate-reducing anaerobic bacteria.

The effect of low levels of alloying additions on the soil corrosion of carbon steels is modest. Some data seems to show a small benefit of 1%Cu and 2.5% Ni on plain carbon steel.

The weight loss and maximum pit depth in soil corrosion can be represented by an equation of the form:

Z = a·tm

Where:
Z - either the weight of loss of maximum pit depth
T - time of exposure
a and m - constants that depend on the specific soil corrosion situation.

Carbon steel pipes and vessels are often required to transport water or are submerged in water to some extent during service. This exposure can be under conditions varying temperature, flow rate, pH, and other factors, all of which can alter the rate of corrosion. The relative acidity of the solution is probably the most important factor to be considered. At low pH the evolution of hydrogen tends to eliminate the possibility of protective film formation so that steel continues to corrode but in alkaline solutions, the formation of protective films greatly reduces the corrosion rate. The greater alkalinity, the slower the rate of attack becomes. In neutral solutions, other factors such as aeration, became determining so that generalization becomes more difficult.

The corrosion of steels in aerated seawater is about the same overall as in aerated freshwater, but this is somewhat misleading because the improved electrical conductivity of seawater can lead to increased pitting. The concentration cells can operate over long distance, and this leads to a more nonuniform attack than in fresh water. Alternate cycling through immersion and exposure to air produces more pitting attack than continuous immersion. The effect of various alloying addition and exposure conditions on the corrosion behavior is shown in Table 1.

Table 1. Comparison of results under different type of exposure

Effects of alloy selection, chemical composition and alloy additions	Sea air	Freshwater	Alternately wet with seawater or Spray and dry	Continuously wet with seawater
Ferrous alloys	Pockmarked	Vermiform on cleaned bars	Pitting, particularly on bars with scale	Pitting, particularly on bars with scale
Wrought iron versus carbon steel	Steel superior to wrought and ingot irons	Iron and steel equal in low-moor areas	Low-moor iron superior to carbon steel	Low-moor iron superior to carbon steel
Sulfur and phosphorus content	Best results when S and P are low	Best results when S and P are low	Best results when S and P are low	Apparently little influence
Addition of copper	Beneficial: Effect increasing with copper content	Beneficial: 0.635% Cu almost as good as 2.185% Cu	Beneficial: 0.635% and 2.185% Cu much the same	0.635% Cu slightly beneficial: 2.185% Cu somewhat less so
Addition of nickel	3.75% Ni superior even to 2% Cu; 36% Ni almost perfect after 15-year exposure	3.75%Ni superior even to 2%Cu; 36%Ni excellent resistance	3.75%Ni beneficial usually more so than Cu: 36%Ni the best metal in the set	3.75% Ni slightly beneficial and slightly superior to Cu: 36% Ni the best metal in the set
Addition of 13.5% Cr	Excellent resistance to corrosion: cold blast metal perfect after 15-year exposure: equal to 36% Ni steel	Excellent resistance to corrosion: equal to 36% Ni steel	Subject to severe localized corrosion that virtually destroys the metal	Subject to severe localized corrosion that virtually destroys the metal
Behavior of cast irons	Excellent resistance to corrosion: cold blast metal superior to hot: no graphitic corrosion	Undergoes graphitic corrosion	Undergoes graphitic corrosion	Undergoes graphitic corrosion

Interestingly, the corrosion rates of specimens completely immersed in seawater do not appear to depend on the geographical location of the test site; therefore, by inference, the mean temperature does not appear to play an important role.

This constancy of the corrosion rate in seawater has been attributed to the more rapid fouling of the exposed steel by marine organisms, such as barnacles and algae, in warmer seas. It is further speculated that this fouling offsets that increases expected from the temperature rise.

Atmospheres are often classified as being rural, industrial or marine in nature. Two decidedly rural environments can differ widely in average yearly temperature and rainfall patterns, mean temperature, and perhaps acid rain, can make extrapolations from past behavior less reliable.

The corrosion of carbon steel in the atmosphere and in many aqueous environments is best understood from a film formation and brake down standpoint. It is an inescapable fact that iron in the presence of oxygen and water is thermodynamically unstable with respect to its oxides. Because atmospheric corrosion is an electrolytic process, the presence of an electrolyte is required. This should not be taken to mean that the steel surface must be awash in water; a very thin adsorbed film of water is all that is required.

During the actual exposure, the metal spends some portion of the time awash with water because of rain or splashing and a portion of the time covered with a thin adsorbed water film. The portion of time spent covered with the thin water film depends quite strongly on relative humidity at the exposure site. This fact has led many corrosion scientists to investigate the influence of the time of wetness on the corrosion rate.

Rusting of iron depends on relative humidity and time of exposure in atmosphere containing 0.01% SO2. The increase in corrosion rate produced by the addition of SO2 is substantial. Oxides of nitrogen in the atmosphere would also exhibit an accelerating effect on the corrosion of steel. Indeed, any gaseous atmospheric constituent capable of strong electrolytic activity should be suspected as being capable of increasing the corrosion rate of steel.

Because carbon steels are not very highly alloyed, it is not surprising that most grades do not exhibit large differences in atmospheric-corrosion rate. Nevertheless, alloying can make changes in the atmospheric-corrosion rate of carbon steel. The elements generally found to be most beneficial in this regard are copper, nickel, silicon, chromium and phosphorus. Of these, the most striking example is that of copper, increases from 0.01-0.05%, decrease the corrosion rate by a factor of two to three. Additions of the above elements in combination are generally more effective than when added singly, although the effects are not additive.

Carbon steel, the most widely used engineering material, accounts for approximately 85%, of the annual steel production worldwide. Despite its relatively limited corrosion resistance, carbon steel is used in large tonnages in marine applications, nuclear power and fossil fuel power plants, transportation, chemical processing, petroleum production and refining, pipelines, mining, construction and metal-processing equipment.

The cost of metallic corrosion to the total economy must be measured in hundreds of millions of dollars (or euros) per year. Because carbon steels represent the largest single class of alloys in use, both in terms of tonnage and total cost, it is easy to understand that the corrosion of carbon steels is a problem of enormous practical importance. This is the reason for the existence of entire industries devoted to providing protective systems for irons and steel.

Carbon steels are by their nature of limited alloy content, usually less than 2% by weight for total of additions. Unfortunately, these levels of addition do not generally produce any remarkable changes in general corrosion behavior. One possible exception to this statement would be weathering steels, in small additions of copper, chromium, nickel and phosphorus produce significant reduction in corrosion rate in certain environments.

Because corrosion is such a multifaceted phenomenon, it is generally useful to attempt to categorize the various types. This is usually done on environmental basis. In this article, atmospheric corrosion, aqueous corrosion and some other corrosion types of interest, such as corrosion in soils, concrete and boilers and heating plants will be addressed.
Atmospheric corrosion
Atmospheres are often classified as being rural, industrial or marine in nature. Two decidedly rural environments can differ widely in average yearly temperature and rainfall patterns, mean temperature, and perhaps acid rain, can make extrapolations from past behavior less reliable.

The corrosion of carbon steel in the atmosphere and in many aqueous environments is best understood from a film formation and brake down standpoint. It is an inescapable fact that iron in the presence of oxygen and water is thermodynamically unstable with respect to its oxides. Because atmospheric corrosion is an electrolytic process, the presence of an electrolyte is required. This should not be taken to mean that the steel surface must be awash in water; a very thin adsorbed film of water is all that is required.

During the actual exposure, the metal spends some portion of the time awash with water because of rain or splashing and a portion of the time covered with a thin adsorbed water film. The portion of time spent covered with the thin water film depends quite strongly on relative humidity at the exposure site. This fact has led many corrosion scientists to investigate the influence of the time of wetness on the corrosion rate.

Rusting of iron depends on relative humidity and time of exposure in atmosphere containing 0.01% SO2. The increase in corrosion rate produced by the addition of SO2 is substantial. Oxides of nitrogen in the atmosphere would also exhibit an accelerating effect on the corrosion of steel. Indeed, any gaseous atmospheric constituent capable of strong electrolytic activity should be suspected as being capable of increasing the corrosion rate of steel.

Because carbon steels are not very highly alloyed, it is not surprising that most grades do not exhibit large differences in atmospheric-corrosion rate. Nevertheless, alloying can make changes in the atmospheric-corrosion rate of carbon steel. The elements generally found to be most beneficial in this regard are copper, nickel, silicon, chromium and phosphorus. Of these, the most striking example is that of copper, increases from 0.01-0.05%, decrease the corrosion rate by a factor of two to three. Additions of the above elements in combination are generally more effective than when added singly, although the effects are not additive.
Aqueous Corrosion
Carbon steel pipes and vessels are often required to transport water or are submerged in water to some extent during service. This exposure can be under conditions varying temperature, flow rate, pH, and other factors, all of which can alter the rate of corrosion. The relative acidity of the solution is probably the most important factor to be considered. At low pH the evolution of hydrogen tends to eliminate the possibility of protective film formation so that steel continues to corrode but in alkaline solutions, the formation of protective films greatly reduces the corrosion rate. The greater alkalinity, the slower the rate of attack becomes. In neutral solutions, other factors such as aeration, became determining so that generalization becomes more difficult.

The corrosion of steels in aerated seawater is about the same overall as in aerated freshwater, but this is somewhat misleading because the improved electrical conductivity of seawater can lead to increased pitting. The concentration cells can operate over long distance, and this leads to a more nonuniform attack than in fresh water. Alternate cycling through immersion and exposure to air produces more pitting attack than continuous immersion. The effect of various alloying addition and exposure conditions on the corrosion behavior is shown in Table 1.

Table 1. Comparison of results under different type of exposure

Effects of alloy selection, chemical composition and alloy additions	Sea air	Freshwater	Alternately wet with seawater or Spray and dry	Continuously wet with seawater
Ferrous alloys	Pockmarked	Vermiform on cleaned bars	Pitting, particularly on bars with scale	Pitting, particularly on bars with scale
Wrought iron versus carbon steel	Steel superior to wrought and ingot irons	Iron and steel equal in low-moor areas	Low-moor iron superior to carbon steel	Low-moor iron superior to carbon steel
Sulfur and phosphorus content	Best results when S and P are low	Best results when S and P are low	Best results when S and P are low	Apparently little influence
Addition of copper	Beneficial: Effect increasing with copper content	Beneficial: 0.635% Cu almost as good as 2.185% Cu	Beneficial: 0.635% and 2.185% Cu much the same	0.635% Cu slightly beneficial: 2.185% Cu somewhat less so
Addition of nickel	3.75% Ni superior even to 2% Cu; 36% Ni almost perfect after 15-year exposure	3.75%Ni superior even to 2%Cu; 36%Ni excellent resistance	3.75%Ni beneficial usually more so than Cu: 36%Ni the best metal in the set	3.75% Ni slightly beneficial and slightly superior to Cu: 36% Ni the best metal in the set
Addition of 13.5% Cr	Excellent resistance to corrosion: cold blast metal perfect after 15-year exposure: equal to 36% Ni steel	Excellent resistance to corrosion: equal to 36% Ni steel	Subject to severe localized corrosion that virtually destroys the metal	Subject to severe localized corrosion that virtually destroys the metal
Behavior of cast irons	Excellent resistance to corrosion: cold blast metal superior to hot: no graphitic corrosion	Undergoes graphitic corrosion	Undergoes graphitic corrosion	Undergoes graphitic corrosion

Interestingly, the corrosion rates of specimens completely immersed in seawater do not appear to depend on the geographical location of the test site; therefore, by inference, the mean temperature does not appear to play an important role.

This constancy of the corrosion rate in seawater has been attributed to the more rapid fouling of the exposed steel by marine organisms, such as barnacles and algae, in warmer seas. It is further speculated that this fouling offsets that increases expected from the temperature rise.

Soil Corrosion

The response of carbon steel to soil corrosion depends primarily on the nature of the soil and certain other environmental factors, such as the availability to moisture and oxygen. These factors can lead to extreme variations in the rate of the attack. For example, under the worst condition a buried vessel may perforate in less than one year, although archeological digs in arid desert regions have uncovered iron tools that are hundreds of years old.

Some general rules can be formulated. Soils with high moisture content, high electrical conductivity, high acidity, and high dissolved salts will be most corrosive. The effect of aeration on soils is somewhat different from the effect of aeration in water because poorly aerated conditions in water can lead to accelerated attack by sulfate-reducing anaerobic bacteria.

The effect of low levels of alloying additions on the soil corrosion of carbon steels is modest. Some data seems to show a small benefit of 1%Cu and 2.5% Ni on plain carbon steel.

The weight loss and maximum pit depth in soil corrosion can be represented by an equation of the form:

Z = a·t^m

Where:

Z - either the weight of loss of maximum pit depth

T - time of exposure

a and m - constants that depend on the specific soil corrosion situation.

The essential difference between ordinary steel and pure iron is the amount of carbon in the former, which reduces the ductility but increases the strength and the susceptibility to hardening when rapidly cooled from elevated temperatures. On account of the various micro-structures which may be obtained by different heat-treatments, it is necessary to emphasise the fact that the following structures are for "normal" steels, i.e. slowly cooled from 760-900°C depending on the carbon contents.

The appearance of pure iron is illustrated in Fig. 1. It is only pure in the sense that it contains no carbon, but contains very small quantities of impurities such as phosphorus, silicon, manganese, oxygen, nitrogen, dissolved in the solid metal. In other words, the structure is typical of pure metals and solid solutions in the annealed condition. It is built up of a number of crystals of the same composition, given the name ferrite in metallography (Brinell hardness 80).

The addition of carbon to the pure iron results in a considerable difference in the structure (Fig. 2), which now consists of two constituents, the white one being the ferrite, and the dark parts representing the constituent containing the carbon, the amount of which is therefore an index of the quantity of carbon in the steel. Carbon is present as a compound of iron and carbon (6-67 %) called cementite, having the chemical formula Fe3 C. This cementite is hard (Brinell hardness 600 +), brittle and brilliantly white.
On examination the dark parts will be seen to consist of two components occurring as wavy or parallel plates alternately dark and light. These two phases are ferrite and cementite which form a eutectic mixture, containing 0,87% carbon and known as pearlite. The appearance of this pearlite depends largely upon the objective employed in the examination and also on the rate of cooling from the elevated temperature.

Allotropy of iron

Certain substances can exist in two or more crystalline forms; for example charcoal, graphite and diamonds are allotropic modifications of carbon. Allotropy is characterized by a change in atomic structure which occurs at a definite transformation temperature.

Four changes occur in iron, which give rise to forms known as alpha, beta, gamma and delta. Of these, a, b and d forms have the same atomic structure (body centred cubic) while g -iron has a face centred cubic structure. Iron can, therefore, be considered to have two allotropic modifications.

The A2 change at 769°C, at which the a-iron loses its magnetism, can be ignored from a heat-treatment point of view. These changes in structure are accompanied by thermal changes, together with discontinuities in other physical properties such as electrical, thermo-electric potential, magnetic, expansion and tenacity. The A3 change from a b.c.c. to an f.c.c. atomic structure at 937°C is accompanied by a marked contraction while the reverse occurs at 1400°C. These changes in structure are accompanied by recrystallisation, followed by grain growth.

Critical points

The addition of carbon to iron, however, produces another change at 695°C, known as A1 and associated with the formation of pearlite. These structural changes, which occur during cooling, give rise to evolutions of heat, which cause arrests on a cooling curve. The temperatures of these arrests are known as critical points or "A" points. These arrests occur at slightly higher temperatures on heating, as compared with cooling, and this lag effect, increased by rapid cooling, is known as thermal hysteresis.

To differentiate between the arrests obtained during heating and cooling, the letters c and r respectively are added to the symbol A (from chauffage and refroidissement). In a steel containing about 0,8-0,9% carbon the evolution of the heat at Ar1 is sufficient to cause the material to become visibly hotter and the phenomenon is called "recalescence".

Iron-cementite equilibrium diagram

The addition of carbon to iron not only gives rise to the A1 point but also influences the critical points in pure iron. The A4 point is raised; and the A3 point lowered until it coincides with A1. The a, b and d modifications, which may be called ferrite, have only slight solubility for carbon, but up to 1,7% of carbon dissolves in y-iron to form a solid solution called Austenite. These effects are summarised in the iron-Fe3 C equilibrium diagram (Fig. 4), which is of much importance in the study of steels.

The iron-iron carbide system is not in true equilibrium, the stable system is iron-graphite, but special conditions are necessary to nucleate graphite. Will be seen that the complicated Fe-Fe3C diagram can be divided into several simple diagrams:

Peritectic transformation CDB - d-iron transforms to austenite. Eutectic at E - austenite and cementite. Solid solution D to F - primary dendrites of austenite form. Eutectic point at P - formation of pearlite.

The ferrite solubility line, A3P, denotes the commencement of precipitation of ferrite from austenite. The cementite solubility line, FP, indicates the primary deposition of cementite from austenite. The pearlite line, A1PG, indicates the formation of the eutectic at a constant temperature. Let us consider the freezing of alloys of various carbon contents.

0,3% carbon

Dendrites of d-iron form, the composition of which is represented eventually by C (0,07 %), and the liquid, enriched in carbon, by B. The solid crystals then react with the liquid to form austenite of composition D. Diffusion of carbon occurs as the solid alloy cools to line A3P. Here a-ferrite commences to be ejected from the austenite, consequently the remaining solid solution is enriched in carbon, until point P is reached at which cementite can be also precipitated.

The alternate formation of ferrite and cementite at 695°C gives rise to pearlite. The structure finally consists of masses of pearlite embedded in the ferrite.

0,6% carbon

When line BE is reached dendrites of austenite form, and finally the alloy completely freezes as a cored solid solution, which, on cooling through the critical range (750-695°C), decomposes into ferrite and pearlite.

1,4% carbon

Again, the alloy solidifies as a cored solid solution, but on reaching line FP, cementite starts to be ejected and the residual alloy becomes increasingly poorer in carbon until point P is reached, when both cementite and ferrite form in juxtaposition. The structure now consists of free cementite and pearlite.

Crucible and high-frequency methods

The Huntsman crucible process has been superseded by the high frequency induction furnace in which the heat is generated in the metal itself by eddy currents induced by a magnetic field set up by an alternating current, which passes round water-cooled coils surrounding the crucible. The eddy currents increase with the square of the frequency, and an input current which alternates from 500 to 2000 hertz is necessary. As the frequency increases, the eddy currents tend to travel nearer and nearer the surface of a charge (i.e. shallow penetration). The heat developed in the charge depends on the cross-sectional area which carries current, and large furnaces use frequencies low enough to get adequate current penetration.

Automatic circulation of the melt in a vertical direction, due to eddy currents, promotes uniformity of analysis. Contamination by furnace gases is obviated and charges from 1 to 5 tonnes can be melted with resultant economy. Consequently, these electric furnaces are being used to produce high quality steels, such as ball bearing, stainless, magnet, die and tool steels.
Acid and basic steels

The remaining methods for making steel do so by removing impurities from pig iron or a mixture of pig iron and steel scrap. The impurities removed, however, depend on whether an acid (siliceous) or basic (limey) slag is used. An acid slag necessitates the use of an acid furnace lining (silica); a basic slag, a basic lining of magnesite or dolomite, with line in the charge. With an acid slag silicon, manganese and carbon only are removed by oxidation, consequently the raw material must not contain phosphorus and sulphur in amounts exceeding those permissible in the finished steel.

In the basic processes, silicon, manganese, carbon, phosphorus and sulphur can be removed from the charge, but normally the raw material contains low silicon and high phosphorus contents. To remove the phosphorus the bath of metal must be oxidised to a greater extent than in the corresponding acid process, and the final quality of the steel depends very largely on the degree of this oxidation, before deoxidisers-ferro-manganese, ferro-silicon, aluminium-remove the soluble iron oxide and form other insoluble oxides, which produce non-metallic inclusions if they are not removed from the melt:

2Al + 3FeO (soluble) « 3Fe + Al2O3 (solid)

In the acid processes, deoxidation can take place in the furnaces, leaving a reasonable time for the inclusions to rise into the slag and so be removed before casting. Whereas in the basic furnaces, deoxidation is rarely carried out in the presence of the slag, otherwise phosphorus would return to the metal. Deoxidation of the metal frequently takes place in the ladle, leaving only a short time for the deoxidation products to be removed. For these reasons acid steel is considered better than basic for certain purposes, such as large forging ingots and ball bearing steel. The introduction of vacuum degassing hastened the decline of the acid processes.

Bessemer steel

In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity (Fig. 1). The oxidation of the impurities raises the charge to a suitable temperature; which is therefore dependent on the composition of the raw material for its heat: 2% silicon in the acid and 1,5-2% phosphorus in the basic process is normally necessary to supply the heat. The "blowing" of the charge, which causes an intense flame at the mouth of the converter, takes about 25 minutes and such a short interval makes exact control of the process a little difficult.

The Acid Bessemer suffered a decline in favour of the Acid Open Hearth steel process, mainly due to economic factors which in turn has been ousted by the basic electric arc furnace coupled with vacuum degassing.

The Basic Bessemer process is used a great deal on the Continent for making, from a very suitable pig iron, a cheap class of steel, e.g. ship plates, structural sections. For making steel castings a modification known as a Tropenas converter is used, in which the air impinges on the surface of the metal from side tuyeres instead of from the bottom. The raw material is usually melted in a cupola and weighed amounts charged into the converter.

Open-hearth processes

In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas. But the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating.

The furnaces have a saucer-like hearth, with a capacity which varies from 600 tonnes for fixed, to 200 tonnes for tilting furnaces (Fig. 1). The raw materials consist essentially of pig iron (cold or molten) and scrap, together with lime in the basic process. To promote the oxidation of the impurities iron ore is charged into the melt although increasing use is being made of oxygen lancing. The time for working a charge varies from about 6 to 14 hours, and control is therefore much easier than in the case of the Bessemer process.

The Basic Open Hearth process was used for the bulk of the cheaper grades of steel, but there is a growing tendency to replace the OH furnace by large arc furnaces using a single slag process especially for melting scrap and coupled with vacuum degassing in some cases.

Electric arc process

The heat required in this process is generated by electric arcs struck between carbon electrodes and the metal bath (Fig. 1). Usually, a charge of graded steel scrap is melted under an oxidising basic slag to remove the phosphorus. The impure slag is removed by tilting the furnace. A second limey slag is used to remove sulphur and to deoxidise the metal in the furnace. This results in a high degree of purification and high quality steel can be made, so long as gas absorption due to excessively high temperatures is avoided. This process is used extensively for making highly alloyed steel such as stainless, heat-resisting and high-speed steels.

Oxygen lancing is often used for removing carbon in the presence of chromium and enables scrap stainless steel to be used. The nitrogen content of steels made by the Bessemer and electric arc processes is about 0,01-0,25% compared with about 0,002-0,008% in open hearth steels.

Oxygen processes

The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air. In Austria the LID process (Linz-Donawitz) converts low phosphorus pig iron into steel by top blowing with an oxygen lance using a basic lined vessel (Fig. 2b). To avoid excessive heat scrap or ore is added. High quality steel is produced with low hydrogen and nitrogen (0,002%). A further modification of the process is to add lime powder to the oxygen jet (OLP process) when higher phosphorus pig is used.

The Kaldo (Swedish) process uses top blowing with oxygen together with a basic lined rotating (30 rev/min) furnace to get efficient mixing (Fig. 2a). The use of oxygen allows the simultaneous removal of carbon and phosphorus from the (P, 1,85%) pig iron. Lime and ore are added. The German Rotor process uses a rotary furnace with two oxygen nozzles, one in the metal and one above it (Fig. 2c). The use of oxygen with steam (to reduce the temperature) in the traditional basic Bessemer process is also now widely used to produce low nitrogen steel. These new techniques produce steel with low percentages of N, S, P, which are quite competitive with open hearth quality.

Other processes which are developing are the Fuel-oxygen-scrap, FOS process, and spray steelmaking which consists in pouring iron through a ring, the periphery of which is provided with jets through which oxygen and fluxes are blown in such a way as to "atomise" the iron, the large surface to mass ratio provided in this way giving extremely rapid chemical refining and conversion to steel.

Vacuum degassing is also gaining ground for special alloys. Some 14 processes can be grouped as stream, ladle, mould and circulation (e.g. DH and RH) degassing methods, Fig. 3. The vacuum largely removes hydrogen, atmospheric and volatile impurities (Sn, Cu, Pb, Sb), reduces metal oxides by the C – O reaction and eliminates the oxides from normal deoxidisers and allows control of alloy composition to close limits. The clean metal produced is of a consistent high quality, with good properties in the transverse direction of rolled products. Bearing steels have greatly improved fatigue life and stainless steels can be made to lower carbon contents.

Vacuum melting and ESR. The aircraft designer has continually called for new alloy steels of greater uniformity and reproducibility of properties with lower oxygen and sulphur contents. Complex alloy steels have a greater tendency to macro-segregation, and considerable difficulty exists in minimising the non-metallic inclusions and in accurately controlling the analysis of reactive elements such as Ti, Al, B. This problem led to the use of three processes of melting.

(a) Vacuum induction melting within a tank for producing super alloys (Ni and Co base), in some cases for further remelting for investment casting. Pure materials are used and volatile tramp elements can be removed.
(b) Consumable electrode vacuum arc re-melting process (Fig. 4) originally used for titanium, was found to eliminate hydrogen, the A and V segregates and also the large silicate inclusions. This is due to the mode of solidification. The moving parts in aircraft engines are made by this process, due to the need for high strength cleanness, uniformity of properties, toughness and freedom from hydrogen and tramp elements.
(c) Electroslag refining (ESR) This process, which is a larger form of the original welding process, re-melts a preformed electrode of alloy into a water-cooled crucible, utilising the electrical resistance heating in a molten slag pool for the heat source (Fig. 5). The layer of slag around the ingot maintains vertical unidirectional freezing from the base. Tramp elements are not removed and lead may be picked up from the slag.

The equilibrium diagram does not tell us what form is taken by the ferrite or cementite ejected from the austenite on cooling. Without going too deeply into the matter, it may be considered that the ferrite has a choice of three different positions, which, in order of degree of supercooling or ease of forming nuclei, are:

(1) boundaries of the austenite crystals
(2) certain crystal planes (octahedral)
(3) about inclusions.

Thus, ferrite starts to form at the grain boundaries, and if sufficient time is allowed for the diffusion phenomena a ferrite network structure is formed, while the pearlite occupies the centre, as in Fig. 1. The size of the austenite grains existing above A3 is thereby betrayed.

If the rate of cooling is faster, the complete separation of the ferrite at the boundaries of large austenite grains is not possible, and ejection takes place within the crystal along certain planes, forming a mesh-like arrangement known as a Widmanstätten structure, shown in Fig. 2. In steels containing more than 0,9% carbon, cementite can separate in a similar way and Widmanstätten structures are also found in other alloy systems.

Steel with Widmanstätten structures are characterised by (1) low impact value, (2) low percentage elongation since the strong pearlite is isolated in ineffective patches by either weak ferrite or brittle cementite, along which cracks can be readily propagated. This structure is found in overheated steels and cast steel, but the high silicon used in steel castings modifies.

It is highly desirable that Widmanstätten and coarse network structures generally be avoided, and as these partly depend upon the size of the original austenite grain, the methods of securing small grains are of importance. Large austenite grains may be refined by (a) hot working, (b) normalising.

Such refined austenite grains are liable to coarsen when the steel is heated well above the Ac3 temperature, in such operations as welding, forging and carburising unless the grain growth is restrained. This restraint can be brought about by a suitable mode of manufacture of the steel.

Controlled grain size

It is now possible to produce two steels of practically identical analysis with inherently different grain growth characteristics so that at a given temperature each steel has an "inherent austenite grain size", one being fine relative to the other. The so-called "fine-graine" steel increases its size on heating above Ac3 but the temperature at which the grain size becomes relatively coarse is definitely higher than that at which a "coarse-grained" steel develops a similar size.

The fine-grained steels are "killed" with silicon together with a slight excess of aluminium which forms aluminium nitride as submicroscopic particles that obstruct austenite grain growth and is an example of a general phenomenon.

At the coarsening temperature the AIN goes into solution rapidly above 1200°C and virtually completely at 1350°C. The austenite grain size is frequently estimated by the following tests:

(1) McQuaid-Ehn Test. Micro-sections of structural steels carburised for not less than 8 hours at 925°C and slowly cooled to show cementite networks are photographed at a magnification of 100. Comparison is made with a grain-size chart issued by the American Society for Testing Materials. This test is also valuable in detecting "abnormality" of pearlite.

(2) The Quench and Fracture test consists in heating normalised sections of the steel, above Ac3 quenching them at intervals of 30°C. An examination of the fractured surface enables the depth of hardness and grain size to be estimated by comparison with standard frac tures.

Below some basics of European and Japanese designation systems are explained. Please refer to articles about corresponding national and international standards for more details.

DIN standards are developed by Deutsches Institut fur Normung in the Federal Republic of Germany. All West German steel specifications are preceded by the uppercase letters DIN followed an alphanumeric or numeric code. The latter method, known as the Werkstoff number, uses numbers only with a decimal point after the first digit.

JIS standards are developed by the Japanese Industrial Standards Committee, which is part of the Ministry of International Trade and Industry in Tokyo. The JIS steel specifications begin with the uppercase letters JIS and are followed by an uppercase letter (G in the case of carbon and low-alloy steels) designating the division (product form) of the standard. This letter is followed by a series of numbers and letters that indicate the specific steel.

British standards (BS) are developed by the British Standards Institute in London, England. Similar to the JIS standards, each British designation includes a product form and an alloy code.

AFNOR standards are developed by the Association Francaise de Normalisation in Paris, France. The correct format for reporting AFNOR standards is as follows. An uppercase NF is placed to the left of the alphanumeric code. This code consists of an uppercase letter followed by a series of digits, which are subsequently followed by an alphanumeric sequence.

UNI standards are developed by the Ente Nazionale Italiano di Unificazione in Milan, Italy. Italian standards are preceded by the uppercase letter UNI followed by a four-digit product form code subsequently followed by an alphanumeric alloy identification.

Swedish standards (SS) are prepared by the Swedish Standards Institution in Stockholm. Designations begin with the letters SS followed by the number 14 (all Swedish carbon and low-alloy steels are covered by SS14). What subsequently follows is a four digit numerical sequence similar to the German Werkstoff number.

A designation is the specific identification of each grade, type, or class of steel by a number, letter, symbol, name, or suitable combination. Unique to a particular steel grade, type and class are terms used to classify steel products. Within the steel industry, they have very specific uses: grade is used to denote chemical composition; type is used to indicate deoxidation practice; and class is used to describe some other attribute, such as strength level or surface smoothness.

In ASTM specifications, however, these terms are used somewhat interchangeably. In ASTM A 533, for example, type denotes chemical composition, while class indicates strength level. In ASTM A 515, grade identifies strength level; the maximum carbon content permitted by this specification depends on both plate thickness and strength level. In ASTM A 302 grade denotes requirements for both chemical composition and mechanical properties. ASTM A 514 and A 5117 are specifications for high-strength quenched and tempered plate for structural and pressure vessel applications, respectively, each contains several compositions that can provide the required mechanical properties. However, A 514 type A has the identical composition limits as A 517 grade.

Chemical composition is by far the most widely used basis for classification and/or designation of steels. The most commonly used system of designation in the United States is that of the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI). The Unified Numbering System (UNS) is also being used with increasing frequency.

SAE-AISI Designations
As stated above, the most widely used system for designating carbon and alloy steels is the SAE-AISI system. As a point of technicality, there are two separate systems, but they are nearly identical and have been carefully coordinated by the two groups. It should be noted, however, that AISI has discontinued the practice of designating steels.

The SAE-AISI system is applied to semi-finished forgings, hot-rolled and cold-finished bars, wire rod and seamless tubular goods, structural shapes, plates, sheet, strip, and welded tubing.

Carbon steels contain less than 1.65% Mn, 0.60% Si, and 0.60% Cu; they comprise the lxxx groups in the SAE-AISI system and are subdivided into four distinct series as a result of the difference in certain fundamental properties among them.

Designations for merchant quality steels include the prefix M. A carbon steel designation with the letter B inserted between the second and third digits indicates the steel contains 0.0005 to 0.003% B. Likewise, the letter L inserted between the second and third digits indicates that the steel contains 0.15 to 0.35% Pb for enhanced machinability. Resulfurized carbon steels of the 11xx group and resulfurized and rephosphorized carbon steels of the 12xx group are produced for applications requiring good machinability. Steels that having nominal manganese contents of between 0.9 and 1.5% but no other alloying additions now have 15xx designations in place of the 10xx designations formerly used.

Alloy steels contain manganese, silicon, or copper in quantities greater than those listed for the carbon steels, or they have specified ranges or minimums for one or more of the other alloying elements. In the AISI-SAE system of designations, the major alloying elements are indicated by the first two digits of the designation. The amount of carbon, in hundredths of a percent, is indicated by the last two (or three) digits.

For alloy steels that have specific hardenability requirements, the suffix H is used to distinguish these steels from corresponding grades that have no hardenability requirement. As with carbon steels, the letter B inserted between the second and third digits indicates that the steel contains boron. The prefix E signifies that the steel was produced by the electric furnace process.

HSLA Steels. Several grades of HSLA steel are described in SAE Recommended Practice J410. These steels have been developed as a compromise between the convenient fabrication characteristics and low cost of plain carbon steels and the high strength of heat-treated alloy steels. These steels have excellent strength and ductility as-rolled.

UNS Designations The Unified Numbering System (UNS) has been developed by ASTM and SAE and several other technical societies, trade associations, and United States government agencies.

A UNS number, which is a designation of chemical composition and not a specification, is assigned to each chemical composition of a metallic alloy. The UNS designation of an alloy consists of a letter and five numerals. The letters indicate the broad class of alloys; the numerals define specific alloys within that class. Existing designation system, such as the AISI-SAE system for steels, have been incorporated into UNS designations. UNS is described in greater detail in SAE J1086 and ASTM E 527.
AMS Designation
Aerospace Materials Specifications (AMS), published by SAE, are complete specifications that are generally adequate for procurement purposes. Most of the AMS designations pertain to materials intended for aerospace applications; the specifications may include mechanical property requirements significantly more severe than those for grades of steel having similar compositions but intended for other applications. Processing requirements, such as for consumable electrode remelting, are common in AMS steels.

ASTM (ASME) Specifications The most widely used standard specifications for steel products in the United States are those published by ASTM. These are complete specifications, generally adequate for procurement purposes. Many ASTM specifications apply to specific products, such as A 574 for alloy steel socket head cap screws. These specifications are generally oriented toward performance of the fabricated end product, with considerable latitude in chemical composition of the steel used to make the end product.

ASTM specifications represent a consensus among producers, specifiers, fabricators, and users of steel mill products. In many cases, the dimensions, tolerances, limits, and restrictions in the ASTM specifications are similar to or the same as the corresponding items of the standard practices in the AISI Steel Products Manuals.

Many of the ASTM specifications have been adopted by the American Society of Mechanical Engineers (ASME) with little or no modification; ASME uses the prefix S and the ASTM designation for these specifications. For example, ASME-SA213 and ASTM A 213 are identical.

Steel products can be identified by the number of the ASTM specification to which they are made. The number consists of the letter A (for ferrous materials) and an arbitrary, serially assigned number. Citing the specification number, however, is not always adequate to completely describe a steel product. For example, A 434 is the specification for heat-treated (hardened and tempered) alloy steel bars. To completely describe steel bars indicated by this specification, the grade (SAE-AISI designation in this case) and class (required strength level) must also be indicated. The ASTM specification A 434 also incorporates, by reference, two standards for test methods (A 370 for mechanical testing and E 112 for grain size determination) and A 29, which specifies the general requirements for bar products.

SAE-AISI designations for the compositions of carbon and alloy steels are sometimes incorporated into the ASTM specifications for bars, wires, and billets for forging. Some ASTM specifications for sheet products include SAE-AISI designations for composition. The ASTM specifications for plates and structural shapes generally specify the limits and ranges of chemical composition directly, without the SAE.AISI designations.

General Specifications. Several ASTM specifications, such as A 20 covering steel plate used for pressure vessels, contain the general requirements common to each member of a broad family of steel products. These general specifications are often supplemented by additional specifications describing a different mill form or intermediate fabricated product.

The Annual Book of ASTM Standards for Steel consists of 8 volumes. It contains formally approved ASTM standard classifications, guides, practices, specifications, test methods and terminology and related material such as proposals. These terms are defined as follows in the Regulations Governing ASTM Technical Committees.

Covers:

* Steel Pipes, Tubes and Fittings
* Steel Plates for General Structure
* Steel Plates for Boiler and Pressure Vessels
* Steels for Machine Structural Use
* Steels for Special Purposes.

The following data is given for each standard:

* Standard number and year
* Grade
* Chemical composition
* Mechanical properties (yield point, tensile strength, notch toughness).

When deemed useful, steel type, manufacturing method, thickness of plate, heat treatment, and other data are described.

The study of steels is important because steels represent by far the most widely used metallic materials, primarily due to the fact that they can be manufactured relatively cheaply in large quantities to very precise specifications. They also provide an extensive range of mechanical properties from moderate strength levels (200-300MPa) with excellent ductility and toughness, to very high strengths (2000 MPa) with adequate ductility. It is, therefore, not surprising that irons and steels comprise well over 80% by weight of the alloys in general industrial use.

Steels form perhaps the most complex group of alloys in common use. Therefore, in studying them it is useful to consider the behavior of pure iron first, then iron-carbon alloys, and finally examine the many complexities which arise when further alloying additions are made.

Pure iron is not an easy material to produce. However, it has recently been made with a total impurity content not exceeding 60 ppm (parts per million), of which 10 ppm is accounted for by non-metallic impurities such as carbon, oxygen, sulphur, phosphorus, while 50 ppm represents the metallic impurities. Iron of this purity is extremely weak: the resolved shear stress of a single crystal at room temperature can be as low as 10 MPa, while the yield stress of a polycrystalline sample at the same temperature can be well below 150 MPa.

The phase transformation: α- and γ- iron
Pure iron exists in two crystal forms, one body-centred cubic (bcc) (α-iron, ferrite) which remains stable from low temperatures up to 910°C (the A3 point), when it transforms to a face-centred cubic (fcc) form (γ-iron, austenite). The γ-iron on remains stable until 1390°C, the A4 point, when it reverts to bcc form, (now δ-iron) which remains stable up to the melting point of 1536°C.

The detailed geometry of unit cells of α- and γ-iron crystals is particularly relevant to, for example, the solubility in the two phases of non-metallic elements such as carbon and nitrogen, the diffusivity of alloying elements at elevated temperatures, and the general behavior on plastic deformation.

The bcc structure of α-iron is more loosely packed than that of fcc γ-iron. The largest cavities in the bcc structure are the tetrahedral holes existing between two edge and two central atoms in the structure, which together form a tetrahedron.

It is interesting that the fcc structure, although more closely-packed, has larger holes than the bcc-structure. These holes are at the centers of the cube edges, and are surrounded by six atoms in the form of an octagon, so they are referred to as octahedral holes.

The α↔γ transformation in pure iron occurs very rapidly, so it is impossible to retain the high-temperature fcc form at room temperature. Rapid quenching can substantially alter the morphology of the resulting α-iron, but it still retains its bcc structure.

Carbon and nitrogen in solution in α- and γ- iron
The addition of carbon to iron is sufficient to form a steel. However, steel is a generic term which covers a very large range of complex compositions. The presence of even a small concentration of carbon, e.g. 0.1-0.2 weight per cent (wt%); approximately 0.5-1.0 atomic per cent, has a great strengthening effect on iron, a fact known to smiths over 2500 years ago since iron heated in a charcoal fire can readily absorb carbon by solid state diffusion. However, the detailed processes by which the absorption of carbon into iron converts a relatively soft metal into a very strong and often tough alloy have only recently been fully explored.

The atomic sizes of carbon and nitrogen are sufficiently small relative to that of iron to allow these elements to enter the α- iron and &gamma- iron lattices as interstitial solute atoms. In contrast, the metallic alloying elements such as manganese, nickel and chromium have much larger atoms, i.e. nearer in size to those of iron, and consequently they enter into substitutional solid solution.

However, comparison of the atomic sizes of C and N with the sizes of the available interstices makes it clear that some lattice distortion must take place when these atoms enter the iron lattice. Indeed, it is found that C and N in α-iron occupy not the larger tetrahedral holes, but the octahedral interstices which are more favorably placed for the relief of strain, which occurs by movement of two nearest neighbor iron atoms. In the case of tetrahedral interstices, four iron atoms are of nearest-neighbor status and the displacement of these would require more strain energy. Consequently these interstices are not preferred sites for carbon and nitrogen atoms.

The solubility of both C and N in austenite should be greater than in ferrite, because of the larger interstices available. It is, therefore, reasonable to expect that during simple heat treatments, excess carbon and nitrogen will be precipitated. This could happen in heat treatments involving quenching from the γ state, or even after treatments entirely within the α field, where the solubility of C varies by nearly three orders of magnitude between 720°C and 20°C.

Precipitation of carbon and nitrogen from α-iron. α-iron containing about 0.02 wt % C is substantially supersaturated with carbon if, after being held at 700°C, it is quenched to room temperature. This supersaturated solid solution is not stable, even at room temperature, because of the ease with which carbon can diffuse in α-iron. Consequently, in the range 20-300°C, carbon is precipitated as iron carbide. This process has been followed by measurement of changes in physical properties such as electrical resistivity, internal friction, and by direct observation or the structural changes in the electron microscope.

The process of ageing is a two-stage one. The first stage takes place at temperatures up to 200°C and involves the formation or a transitional iron carbide phase (ε) with a close-packed hexagonal structure which is often difficult to identify, although its morphology and crystallography have been established. It forms as platelets on {100}α planes, apparently homogenously in the α-iron matrix, but at higher ageing temperatures (150-200°C) nucleation occurs preferentially on dislocations. The composition is between Fe2.4C and Fe3C.

Ageing at 200°C and above leads to the second stage of ageing in which orthorhombic cementite Fe3C is formed as platelets on {110}α. Often the platelets grow on several {110} planes from a common centre giving rise to structures which appear dendritic in character. The transition from ε-iron carbide to cementite is difficult to study, but it appears to occur by nucleation of cementite at the ε-carbide/α interlaces, followed by re-solution of the metastable ε-carbide precipitate.

The maximum solubility of nitrogen in ferrite is 0.10 wt %, so a greater volume fraction of nitride precipitate can be obtained. The process is again two-stage with a be tetragonal α" phase, Fe16N2, as the intermediate precipitate, forming as discs on {100}α, matrix planes both homogeneously and on dislocations. Above about 200°C, this transitional nitride is replaced by the ordered fcc γ’, Fe4N.

The ageing of α-iron quenched from a high temperature in the α-range is usually referred to as quench ageing, and there is substantial evidence to show that the process can cause considerable strengthening, even in relatively pure iron. In commercial low carbon steels, nitrogen is usually combined with aluminium, or present in too low concentration to make a substantial contribution to quench ageing, with the result that the major effect is due to carbon. This behavior should be compared with that of strain ageing.

Some practical aspects. The very rapid diffusivity of carbon and nitrogen in iron compared with that of the metallic alloying elements is exploited in the processes of carburizing and nitriding.

Carburizing can be carried out by heating a low carbon steel in contact with carbon to the austenitic range, e.g. 1000°C, where the carbon solubility, c1, is substantial. The result is a carbon gradient in the steel, from c1 at the surface in contact with the carbon, to c at a depth.

The diffusion coefficient D of carbon in iron actually varies with carbon content, so the above relationship is not rigorously obeyed. Carburizing, whether carried out using carbon, or more efficiently using a carburizing gas (gas carburizing), provides a high carbon surface on a steel, which, after appropriate heat treatment, is strong and wear resistant.

Nitriding is normally carried out in an atmosphere of ammonia, but at a lower temperature (500-550°C) than carburizing, consequently the reaction occurs in the ferrite phase, in which nitrogen has a substantially higher solubility than carbon.

Nitriding steels usually contain chromium (≈1%), aluminum (≈1%), vanadium or molybdenum (≈0.2%), which are nitride-forming elements, and which contribute to the very great hardness of the surface layer produced.