Austenitizing Ductile Cast Iron

The usual objective of austenitizing is to produce an austenitic matrix with as uniform carbon content as possible prior to thermal processing. For a typical hypereutectic ductile cast iron, an upper critical temperature must be exceeded so that the austenitizing temperature is in two-phase (austenite and graphite) field. This temperature varies with alloy content.

The "equilibrium" austenite carbon content in equilibrium with graphite increases with an increase in austenitizing temperature. This ability to select (within limits) the matrix austenite carbon content makes austenitizing temperature control important in processes that depend on carbon in the matrix to drive a reaction. This is particularly true in structures to be austempered, in which the hardenability (or austemperability) depends to a significant degree on matrix carbon content. In general, alloy content, the original microstructure, and the section size determine the time required for austenitizing. The sections to follow on annealing, normalizing, quenching and tempering, and austempering discuss austenitizing when it is of concern.

Annealing Ductile Cast Iron

When maximum ductility and good machinability are desired and high strength is not required, ductile iron castings are generally given a full ferritizing anneal. The microstructure is thus converted to ferrite, and the excess carbon is deposited on the existing nodules. This treatment produces ASTM grade 60-40-18. Amounts of manganese, phosphorus, and alloying elements such as chromium and molybdenum should be as low as possible if superior machinability is desired because these elements retard the annealing process.

Recommended practice for annealing ductile iron castings is given below for different alloy contents and for castings with and without eutectic carbides:

• Full anneal for unalloyed 2 to 3% Si iron with no eutectic carbide: Heat and hold at 870 to 900°C (1600 to 1650°F) for 1 h per inch of section. Furnace cool at 55°C/h (100°F/h) to 345°C (650°F). Air cool.
• Full anneal with carbides present: Heat and hold at 900 to 925°C (1650 to 1700°F) for 2 h minimum, longer for heavier sections. Furnace cool at 110°C/h (200°F/h) to 700°C (1300°F). Hold 2 h at 700°C (1300°F). Furnace cool at 55°C/h (100°F/h) to 345°C (650°F). Air cool.
• Subcritical anneal to convert pearlite to ferrite: Heat and hold at 705 to 720°C (1300 to 1330°F), 1 h per inch of section. Furnace cool at 55°C/h (100°F/h) to 345°C (650°F). Air cool. When alloys are present, controlled cooling times through the critical temperature range down to 400°C (750°F) must be reduced to below 55°C/h (100°F/h).

However, certain carbide-forming elements, mainly chromium, form primary carbides that are very difficult, if not impossible, to decompose. For example, the presence of 0.25% Cr results in primary intercellular carbides that cannot be broken down in 2 to 20 h heat treatments at 925°C (1700°F). The resulting matrix after pearlite breakdown is carbides in ferrite with only 5% elongation. Other examples of carbide stabilizers are molybdenum contents greater than 0.3%, and vanadium and tungsten contents exceeding 0.05%.

Hardenability of Ductile Cast Iron

The hardenability of ductile cast iron is an important parameter for determining the response of a specific iron to normalizing, quenching and tempering, or austempering.

Hardenability is normally measured by the Jominy test, in which a standard-sized bar (1 inch diameter by 4 inch in length) is austenitized and water quenched from one end. The variation in cooling rate results in micro-structural variations, giving hardness changes that are measured and recorded.

The higher matrix carbon content resulting from the higher austenitizing temperature results in an increased hardenability (the Jominy curve is shifted to larger distances from the quenched end) and a greater maximum hardness.

The purpose of adding alloy elements to ductile cast irons is to increase hardenability. Manganese and molybdenum are much more effective in increasing hardenablitty, per weight percent added, than nickel or copper. However, as is the case with steel, combinations of nickel and molybdenum, or copper and molybdenum, or copper, nickel, and manganese are more effective than the separate elements. Thus heavy-section castings that require through hardening or austempering usually contain combinations of these elements. Silicon, apart from its effect on matrix carbon content, does not have a large effect on hardenability.

Normalizing Ductile Cast Iron

Normalizing (air cooling following austenitizing) can result in a considerable improvement in tensile strength and may be used in the production of ductile iron of ASTM type 100-70-03.

The microstructure obtained by normalizing depends on the composition of the castings and the cooling rate. The composition of the casting dictates its hardenability that is, the relative position of the fields in the time-temperature CCT diagram. The cooling rate depends on the mass of the casting, but it also may be influenced by the temperature and movement of the surrounding air, during cooling.

Normalizing generally produces a homogeneous structure of fine pearlite, if the iron is not too high in silicon content and has at least a moderate manganese content (0.3 to 0.5% or higher). Heavier castings that require normalizing usually contain alloying elements such as nickel, molybdenum, and additional manganese, for higher hardenability to ensure the development of a fully pearlitic structure after normalizing. Lighter castings made of alloyed iron may be martensitic or may contain an acicular structure after normalizing.

The normalizing temperature is usually between 870 and 940°C (1600 and 1725°F). The standard time at temperature of 1 h per inch of section thickness or 1 h minimum is usually satisfactory. Longer times may be required for alloys containing elements that retard carbon diffusion in the austenite. For example, tin and antimony segregate to the nodules, effectively preventing the solution of carbon from the nodule sites.

Normalizing is sometimes followed by tempering to attain the desired hardness and relieve residual stresses that develop upon air cooling when various parts of a casting, with different section sizes, cool at different rates. Tempering after normalizing is also used to obtain high toughness and impact resistance. The effect of tempering on hardness and tensile properties depends on the composition of the iron and the hardness level obtained in normalizing. Tempering usually consists of reheating to temperatures of 425 to 650°C (800 to 1200°F) and holding at the desired temperature for 1 h per inch of cross section. These temperatures are varied within the above range to meet specification limits.

Quenching and Tempering Ductile Cast Iron

An austenitizing temperature of 845 to 925°C (1550 to 1700°F) is normally used for austenitizing commercial castings prior to quenching and tempering. Oil is preferred as a quenching medium to minimize stresses and quench cracking, but water or brine may be used for simple shapes. Complicated castings may have to be oil quenched at 80 to 100°C (180 to 210°F) to avoid cracks.

The influence of the austenitizing temperature on the hardness of water-quenched cubes of ductile iron shows that the highest range of hardness (55 to 57 HRC) was obtained with austenitizing temperatures between 845 and 870°C (1550 and 1600°F). At temperatures above 870°C, the higher matrix carbon content resulted in a greater percentage of retained austenite and therefore a lower hardness.

Castings should be tempered immediately after quenching to relieve quenching stresses. Tempered hardness depends on as-quenched hardness level, alloy content, and tempering temperature, as well as time. Tempering in the range from 425 to 600°C (800 to 1100°F) results in a decrease in hardness, the magnitude of which depends upon alloy content, initial hardness, and time. Vickers hardness of quenched ductile iron alloys change with tempering temperature and time.

Tempering ductile iron is a two-stage process. The first involves the precipitation of carbides similar to the process in steels. The second stage (usually shown by the drop in hardness at longer times) involves nucleation and the growth of small, secondary graphite nodules at the expense of the carbides. The drop in hardness accompanying secondary graphitization produces a corresponding reduction in tensile and fatigue strength as well. Because alloy content affects the rate of secondary graphitization, each alloy will have a unique range of useful tempering temperatures.

Austempering Ductile Cast Iron

When optimum strength and ductility are required, the heat treater has the opportunity to produce an austempered structure of austenite and ferrite. The austempered matrix is responsible for a significantly better tensile strength-to-ductility ratio than is possible with any other grade of ductile cast iron. The production of these desirable properties requires careful attention to section size and the time-temperature exposure during austenitizing and austempering.

Section Size and Alloying. As section size increases, the rate of temperature change between the austenitizing temperature and austempering temperature decreases. Quenching and austempering techniques include the hot-oil quench (up to 240°C, or 460°F, only), nitrate/nitrite sail quenches, fluidized-bed method (for thin, small parts only), and, in tool-type applications, lead baths.

In order to avoid high-temperature reaction products (such as pearlite in larger section sizes), salt bath quench severities can be increased with water additions or with alloying elements (such as copper, nickel, manganese, or molybdenum) that enhance pearlite hardenability. It is important to understand that these alloying elements tend to segregate during solidification so that a nonuniform distribution exists throughout the matrix. This has a potentially detrimental effect on the austempering reaction and therefore on mechanical properties. Ductility and impact toughness are the most severely affected.

Manganese and molybdenum have the most powerful effect upon pearlite hardenability but will also segregate and freeze into intercellular regions of the casting to promote iron or alloy carbides. While nickel and copper do not affect hardenability nearly as much, they segregate to graphite nodule sites and do not form detrimental carbides. Combinations of these elements, which segregate in opposite fashions, are selected for their synergistic effect on hardenability.

Austenitizing Temperature and Time. Usual schematic phase diagram shows that as austenitizing temperature increases, so does the matrix carbon content; the actual matrix carbon content depends in a complex way on the alloy elements present, their amount, and their location (segregation) within the matrix.

The most important determinant of matrix carbon content in ductile irons is the silicon content; as silicon content increases for a given austenitizing temperature, the carbon content in the matrix decreases. Austenitizing temperatures between 845 and 925°C (1550 and 1700°F) are normal, and austenitizing times of approximately 2 h have been shown to be sufficient to recarburize the matrix fully. Austenitizing temperature, through its effect upon matrix carbon, has a significant effect on hardenability. The higher austenitizing temperature with its higher carbon content promotes increased hardenability, which causes a slower rate of isothermal austenite transformation.

Austempering Temperature and Time. The austempering temperature is the primary determinant of the final microstructure and therefore the hardness and strength of the austempered product. As the austempering temperature increases, the strength and impact toughness vary.

The attainment of maximum ductility at any given austempering temperature is a sensitive function of time. The initial increase in elongation occurs as stage I and elongation progresses to completion, at which point the fraction of austenite is a maximum. Further austempering merely serves to reduce ductility as the stage II reaction causes decomposition to the equilibrium bainite product. Typical austempering times vary from 1 to 4 h.

Nodular cast irons (or ductile, or spheroidal graphite iron) are primarily heat treated to create matrix microstructures and associated mechanical properties not readily obtained in the as-cast condition. As-cast matrix microstructures usually consist of ferrite or pearlite or combinations of both, depending on cast section size and/or alloy composition.

The most important heat treatments and their purposes are:

• Stress relieving, a low-temperature treatment, to reduce or relieve internal stresses remaining after casting
• Annealing, to improve ductility and toughness, to reduce hardness, and to remove carbides
• Normalizing, to improve strength with some ductility
• Hardening and tempering, to increase hardness or to improve strength and raise proof stress ratio
• Austempering, to yield a microstructure of high strength, with some ductility and good wear resistance
• Surface hardening, by induction, flame, or laser, to produce a locally selected wear-resistant hard surface

The normalizing, hardening, and austempering heat treatment, which involve austenitization, followed by controlled cooling or isothermal reaction, or a combination of the two, can produce a variety of microstructures and greatly extend the limits on the mechanical properties of ductile cast iron.

These microstructures can be separated into two broad classes:

• Those in which the major iron-bearing matrix phase is the thermodynamically stable body-centered cubic (ferrite) structure
• Those with a matrix phase that is a meta-stable face-centered cubic (austenite) structure.
The former are usually generated by the annealing, normalizing, normalizing and tempering, or quenching and tempering processes. The latter are generated by austempering, an isothermal reaction process resulting in a product called austempered ductile iron (ADI).

Other heat treatments in common industrial use include stress-relief annealing and selective surface heat treatment. Stress-relief annealing does not involve major micro-structural transformations, whereas selective surface treatment (such as flame and induction surface hardening) does involve microstructural transformations, but only in selectively controlled parts of the casting.

The basic structural differences between the ferritic and austenitic classes are explained in the Fig 1 and 2. Figure 1 shows a continuous cooling transformation (CCT) diagram and cooling curves for furnace cooling, air-cooling, and quenching.

It can be seen from Fig 1 that slow furnace cooling results in a ferritic matrix (the desired product of annealing), whereas the cooling curve for air cooling, or normalizing, results in a pearlitic matrix, and quenching produces a matrix microstructure consisting mostly of martensite with some retained austenite. Tempering softens the normalized and quenched conditions, resulting in microstructures consisting of the matrix ferrite with small panicles of iron carbide (or secondary graphite).

Fig.1: CCT diagram showing annealing, normalizing and quenching;
Ms stand martensite start, Mf for martensite finish.

Figure 2 shows an isothermal transformation (IT) diagram for a ductile cast iron, together with a processing sequence depicting the production of ADI. In this process, austenitizing is followed by rapid quenching (usually in molten salt) to an intermediate temperature range for a time that allows the unique metastable carbon-rich (≈2% C) austenitic matrix (γH) to evolve simultaneously with nucleation and growth of a plate-like ferrite (α) or of ferrite plus carbide, depending on the austempering temperature and time at temperature.

This austempering reaction progresses to a point at which the entire matrix has been transformed to the metastable product (stage I in Fig 2), and then that product is "frozen in" by cooling to room temperature before the true bainitic ferrite plus carbide phases can appear (stage II in Fig 2).

In ductile cast irons the presence of 2 to 3 wt% Si prevents the rapid formation of iron carbide (Fe₃C). Hence the carbon rejected during ferrite formation in the first stage of the reaction (stage I in Fig 2) enters the matrix austenite, enriching it and stabilizing it thermally to prevent martensite formation upon subsequent cooling. Thus the processing sequence in Fig 2 shows that the austempering reaction is terminated before stage II begins and illustrates the decrease in the martensite start (Ms) and martensite finish (Mf) temperatures as γH forms in stage I. Typical austempering times range from 1 to 4 h depending on alloy content and section size. If the part is austempered too long, undesirable bainite will form. Unlike steel, bainite in cast iron microstructures exhibits lower toughness and ductility.

Fig.2: IT diagram of a processing sequence for austempering.

Ferritic and pearlitic malleable irons are both produced by annealing white iron of controlled composition. Malleable irons have largely been replaced by ductile iron in many applications. This is due in part to the necessity of lengthy heat treatments for malleable iron and the difficulty in cooling thick sections rapidly enough to produce white iron. Malleable iron is still often preferred for thin section castings and parts that require maximum machinability and wear resistance.

The annealing of malleable iron should be done in a furnace with a controlled atmosphere of dry nitrogen, hydrogen (1.5%), and carbon monoxide (1.5%). The dew point of this mixture should be between -40 and -70°C (-40 and -20°F). These conditions eliminate the possibility of decarburization and loss of learner carbon nodules below the casting surface.

The annealing treatment involves three important steps:

* The first causes nucleation of temper carbon. It is initiated during heating to a high holding temperature and occurs very early during the holding period.
* The second step consists of holding at 900 to 970°C (1650 to 1780°F); this step is called first-stage graphitization (FSG). During FSG, massive carbides are eliminated from the iron structure. Long holding periods at 955°C (1750°F) will reduce the solubility of nitrogen in iron (which should be kept at 80 to 120 ppm), thereby reducing the mechanical properties of the iron. This occurrence should be kept in mind for long, or "weekend" holding periods. When the carbides are eliminated, the iron is rapidly cooled to 740°C.
* The third step in the annealing treatment consists of slow cooling through the allotropic transformation range of the iron; this step is called second-stage graphitization (SSG). During SSG a completely ferrule matrix free of pearlite and carbides is obtained when the cooling rate is 2 to 28°C/h (3 to 50°F/h). This cooling rate, which depends on the silicon content of the iron and the temper carbon nodule count, may be increased to 85°C/min (150°F/min) by air quenching from 900°C (1650°F) to form a pearlitic matrix. Oil quenching from 900°C (1650°F) will produce a martensitic matrix. However, unless the temperature in the furnace is lowered to 845°C (1550°F) for at least 4 h (plus 1 h for each 25 mm or inch of section casting thickness), prior to uniform quenching in oil, the matrix microstructure will not be uniform in combined carbon. This nonuniformity reduces machinability. If the hardness is reduced by extended tempering, the resulting structure may not have a good response to selective hardening.

Hardening and Tempering of Malleable Iron
A typical procedure for producing a hardened pearlitic malleable iron consists of:

* first air quenching castings after first-stage annealing, which results in retention of about 0.75% combined carbon in the matrix;
* second, reheating and holding for 1 hour at 885°C (l625°F) to reaustenitize the matrix and homogenize the combined carbon; and then
* quenching in heated and agitated oil, thereby developing a matrix consisting of martensite without bainite and having a hardness of 555 to 627 HB.

The appropriate austenitizing temperature for pearlitic malleable iron is 885°C (1625°F) and for ferritic malleable iron it is 900°C (1650°F). If direct oil quenching is used, caution must be exercised to prevent cracking due to high combined carbon.

Air-quenched and pearlitic malleable iron has a matrix consisting of a ferrite ring around the temper carbon (which produces a lower yield strength) and partially broken lamely pearlite. The remaining lamellar pearlite reduces machinability to a limit of 240 HB.

Increasing the austenitizing time and temperature increases the amount dissolved carbon, which is measured as combined carbon in the matrix after quenched to room temperature. Austenitizing temperatures in the range of 900 to 930°C (1650 to 1700°F) result in a more homogeneous austenite, which is desirable for more uniform martensite. Higher temperatures can result in a greater tendency toward distortion or cracking. Tempering of pearlite is time and temperature dependent. Tempering of martensite is primarily temperature dependent, while time being secondary.

Hardened and tempered pearlitic malleable iron can also be produced from fully annealed ferritic malleable iron, the matrix of which is essentially carbon-free: graphite can be dissolved in austenite by holding at 900 to 930°C (1650 to 1700°F) for a time sufficiently long for the production of an austenite matrix of uniform carbon content. In general, the combined carbon content of the matrix produced by this procedure is slightly lower than they of a pearlitic malleable iron made by air quenching directly from 900°C (1650°F).

Tempering treatments consist of cycles of no less than 2 h at temperature to ensure uniformity of product. Tempering times must also be adjusted for section thickness and quenched microstructures. Fine pearlite and bainite require longer tempering times than that for martensite. In general, final hardness is controlled with process controls approximately the same as those encountered in the heat treatment of medium-carbon and higher-carbon steels. This is particularly true when the specification requires final hardnesses in the range from 241 to 321 HB.

The effects of tempering on the hardness of alloyed and unalloyed malleable irons illustrate the beneficial effects of alloying on as-quenched hardness and stability at elevated temperatures. During all tempering treatments, carbide has a tendency to decompose, with resulting deposition of graphite on existing temper carbon nodules. This tendency is least at the lower tempering temperatures or in suitably alloyed pearlitic malleable irons.

Martempering and tempering develops mechanical properties similar to those resulting from conventional oil quenching and tempering: typical tensile strength 860 MPa (125 ksi), yield strength 760 MPa (110 ksi), and hardness 300 HB.

Pearlitic malleable iron castings that are susceptible to cracking when quenched in warm oil (40 to 95°C, or 100 to 200°F) from the austenitizing temperature may be safely quenched in salt or oil at about 200°C (400°F). Elevator camshafts varying in length from 0.3 to 0.45 m (12 to 18 in.) and various sizes of wear-chain components are examples of martempered pearlitic malleable iron.

Bainitic Heat Treatment of Pearlitic Malleable Iron
Both upper and lower bainite can be formed in pearlitic malleable iron with a marked increase in tensile strength and hardness but with a decrease in ductility.

A pearlitic malleable iron (2.6C-1.4Si-0.5Mn-0.1S), annealed at 930°C (1700°F) for 16 h, air quenched and tempered at 680°C (1250°F) for 4 h, developed an ultimate tensile strength of 650 MPa (94.2 ksi), a yield strength of 460 MPa (66.5 ksi), and a 3.4% elongation at 217 HB.

This same iron austenitized at 900°C (1650°F) in molten salt for 1 h, quenched in molten salt at 295°C (560°F) for 3 h, and air cooled gave an ultimate strength of 995 MPa (144.2 ksi), a yield strength of 920 MPa (133.4 ksi), and 388 HB.

Surface Hardening of Pearlitic Malleable Iron
Fully pearlitic malleable iron may be surface hardened by either induction heating and quenching or flame heating and quenching. Laser and electron beam techniques also have been used for hardening selected areas on the surface of pearlitic and ferritic malleable iron castings that are free from decarburization.

Generally, hardness in the range from 55 to 60 HRC is attainable, with the depth of penetration being controlled by the rate of heating and by the temperature developed at the surface of the part being hardened. In induction hardening, this is accomplished by the close regulation of power output, operating frequency, heating time, and alloy content of the iron.

The maximum hardness obtainable in the matrix of a properly hardened part is 67 HRc; however, conventional hardness measurements show less than the true matrix hardness because of the temper carbon nodules that are averaged into the hardness. Generally, a casting with a matrix microhardness of 67 HRc will have average hardness of about 62 HRc, as measured with the standard Rockwell tester.

Rocker arms and clutch hubs are examples of automotive production parts that are surface hardened by induction. Flame hardening requires close control for these applications in order to avoid distortion that would interfere with their operation. The two examples that follow describe the successful application of induction and flame hardening to other production parts.

High-alloy cast irons are an important group of materials whose production should be considered separately from that of the ordinary types of cast irons. The producing foundries usually have the equipment needed to handle the heat treatment and other thermal processing unique to the production of these alloys.

The high-alloy white irons are primarily used for abrasion-resistant applications and are readily cast in the shapes needed in machinery used for crushing, grinding, and general handling of abrasive materials. The large volume of eutectic carbides in their microstructures provides the high hardness needed for crushing and grinding other materials. The metallic matrix supporting the carbide phase in these irons can be adjusted by alloy content and heat treatment to develop the proper balance between resistance to abrasion and the toughness needed to withstand repeated impact.

All high-alloy white irons contain chromium to prevent formation of graphite on solidification and to ensure the stability of the carbide phase. Most also contain nickel, molybdenum, copper, or combinations of these alloying elements to prevent the formation of pearlite in the microstructure. While low-alloyed pearlitic white iron castings develop hardness in the range 350 to 550 HB, the high-alloyed white irons range from 450 to 800 HB.

ASTM Specification A 532 covers the composition and hardness of white iron grades used for abrasion-resistant applications. Many castings are ordered according to these specifications: however, a large number of castings are produced with modifications to composition for specific applications. It is most desirable that the designer, metallurgist, and foundry-man work together to specify the composition, heat treatment, and foundry practice to develop the most suitable alloy and casting design for a specific application.

The high-alloy white cast irons fall into three major groups:

* The Ni-Cr white irons, which are low-chromium alloys containing 3 to 5% Ni and 1 to 4% Cr with one alloy modification which contains 7 to 11% Cr.
* The chromium-molybdenum irons containing 11 to 23% Cr, up to 3% Mo, and often additionally alloyed with nickel or copper.
* The 25% Cr or 28% Cr white irons, which may contain other alloying additions of molybdenum and/or nickel up to 1.5%

Nickel-Chromium White Irons
One of the oldest groups of high-alloy irons of industrial importance, the Ni-Cr white irons, or Ni-Hard irons, have been produced for more than 50 years and are very cost-effective materials for crushing and grinding.

In these martensitic white irons, nickel is the primary alloying element because at levels of 3 to 5% it is effective in suppressing the transformation of the austenite matrix to pearlite, and thus ensuring that a hard, martensitic structure will develop on cooling in the mold. Chromium is included in these alloys, at levels from 1.4 to 4% to ensure that the irons will solidify with carbides to counteract the graphitizing effect of nickel. The optimum composition of the Ni-Cr white iron alloy depends on the properties required for the service conditions and the dimensions and weight of the casting. Abrasion resistance is generally a function of the bulk hardness and the volume of carbide in Cr-Mo iron.

Carbon is varied according to properties needed for the intended service. Carbon contents in the range of 3.2 to 3.6% are prescribed when maximum abrasion resistance is desired. Where impact loading is present, carbon content should be held in the range of 2.7 to 3.2%.

Silicon is needed for two reasons. A minimum amount of silicon is necessary to improve fluidity and produce a fluid slag. But of equal importance is its effect on as-cast hardness. Increased levels of silicon, in the range of 1 to 1.5%, have been found to increase the amount of martensite and the resulting hardness. Late additions of ferrosilicon have been reported to increase toughness. Note that higher silicon contents can promote pearlite and may increase the nickel requirement.

Manganese is usually held to 0.8% max. While it provides increased hardenability to avoid pearlite formation, it is also a potent austenite stabilizer, more so than nickel, and will promote increased amounts of retained austenite and lower as-cast hardness. For this reason higher manganese levels are undesirable. In considering the nickel content required to avoid pearlite in a given casting, the level of manganese present should be a factor.

Copper increases hardenability and the retention of austenite and, therefore, must be controlled for the same reason manganese is limited. Copper should be treated as a nickel substitute and, when properly included in the calculation of the amount of nickel required to inhibit pearlite in a given casting, it reduces the nickel requirement. Molybdenum is a potent hardenability agent in these alloys and is used in heavy section castings to augment hardenability and inhibit pearlite.

Heat Treatment or Nickel-Chromium White Irons. Nickel-chromium white iron castings are given a stress-relief heat treatment because, properly made, they have a martensitic matrix structure, as-cast. Tempering is performed between 205 to 260°C (400 to 450°F) for at least 4 h. This tempers the martensite, relieves some of the transformation stresses, and increases the strength and impact toughness by 50 to 80%. Some additional martensite may form on cooling from the tempering temperature. This heat treatment does not reduce hardness or abrasion resistance.

In the heat treatment of any white cast iron, care must be taken to avoid cracking by thermal shock; never place the castings in a hot furnace or otherwise subject them to rapid heating or cooling. The risk of cracking increases with the complexity of the casting shape and section thickness.

An austenitizing heat treatment usually comprised heating at temperatures between 750 and 790°C (1380 and 1450°F) with a soak time of 8 h. Air or furnace cooling, not over 30°C/h, was conducted followed by a tempering/stress-relief heat treatment. Refrigeration heat treatment is the more commonly practiced remedy for low hardness today.

High-Chromium White Irons
The oldest high-alloy white irons produced commercially were the high-chromium (28% Cr) white irons. The high-chromium white irons have excellent abrasion resistance and are used effectively in slurry pumps, brick molds, coal-grinding mills, rolling mill rolls, shot blasting equipment, and components for quarrying, hard-rock mining and milling. In some applications they must also be able to withstand heavy impact loading.

These alloyed white irons are recognized as providing the best combination of toughness and abrasion resistance attainable among the white cast irons. Through variations in composition and heat treatment these properties can be adjusted to meet the needs of most abrasive applications.

Special High-Chromium Iron Alloys for Corrosion Resistance. Alloys with improved resistance to corrosion, for applications such as pumps handling, are produced with high chromium contents (26 to 28% Cr) and low carbon contents (1.6 to 2.0% C). These high-chromium, low-carbon irons will provide the maximum chromium content in the matrix. Addition of 2% Mo is recommended for improving resistance to chloride-containing environments. Chromium causes the formation of an adherent, complex, chromium-rich oxide film providing resistance to scaling at temperatures up to 1040°C (1900°F).

The high-chromium irons designated for use at elevated temperatures fall into one of three categories, depending upon the matrix structure:

* The martensitic irons alloyed with 12 to 28% Cr
* The ferritic irons alloyed with 30 to 34% Cr
* The austenitic irons which in addition to containing 15 to 30% Cr also contain 10 to 15% Ni to stabilize the austenite phase

Carbon contents of these alloys range from 1 to 2%.

Optimum performance is usually achieved with heat treated martensitic structures. As described in the previous section, alloying must be sufficient to ensure that a pearlite-free microstructure is obtained in heat treatment. Of necessity, the heat treatment requires an air quench from the austenitizing temperature. Faster cooling rates should not be used, because the casting can develop cracks due to high thermal and/or transformation stresses. Thus the alloy must have sufficient hardenability to allow air hardening. Over-alloying with manganese, nickel, and copper will promote retained austenite, which detracts from resistance to abrasion and spalling.

Austenitization. There is an optimum austenitizing temperature to achieve maximum hardness, which varies for each composition. The austenitizing temperature determines the amount of carbon that remains in solution in the austenite matrix. Too high a temperature increases the stability of the austenite, and the higher retained austenite content reduces hardness. Low temperatures result in low-carbon martensite reducing both hardness and abrasion resistance. Class II irons containing 12 to 20% Cr are austenitized in the temperature range 950 to 1010°C (1750 to 1850°F). Class III irons containing 23 to 28% Cr are austenitized in the temperature range 1010 to 1090°C (1850 to 2000°F).

Quenching. Air quenching (vigorous fan cooling) the castings from the austenitizing temperature to below the pearlite temperature range (that is, between 550 and 600°C, or 1020 and 1110°F) is highly recommended. The subsequent cooling rate should be substantially reduced to minimize stresses; still-air or even furnace cooling to ambient is common. Complex and heavy section castings are often placed back into the furnace, which is at 550 to 600°C, and allowed sufficient time to reach uniform temperature within the casting. After temperature is equalized, the castings are either furnace or still-air cooled to ambient temperature.

Tempering. Castings can be put into service in the hardened (as cooled) condition without further tempering or subcritical heat treatments; however, tempering in the range of 200 to 230°C (400 to 450°F) for 2 to 4 h is recommended to restore some toughness in the martensitic matrix and to further relieve residual stresses.

Sub critical Heat Treatment. Sub critical heat treatment (tempering) is sometimes performed, particularly in large heat-treated martensitic castings, to reduce retained austenite contents and increase resistance to spalling. The tempering parameters necessary to eliminate retained austenite are very sensitive to time and temperature and vary depending on the castings composition and prior thermal history. Typical tempering temperatures range from 480 to 540°C (900 to 1000°F) and times range from 8 to 12 h. Excess time or temperature results in softening and a drastic reduction in abrasion resistance.

Annealing. Castings can be annealed to make them more machinable, either by sub-critical annealing or a full anneal. Subcritical annealing is accomplished by pearlitizing, via soaking in the narrow range between 690 and 705°C for from 4 to 12 h, which will produce hardness in the range 400 to 450 HB. Lower hardness can often be achieved with full annealing, whereby castings are heated in the range 955 to 1010°C followed by slow cooling to 760°C and holding at this temperature for 10 to 50 h depending on composition.

Stress-Relieving. Very little information is available on the amount of stress relief that occurs with tempering. The predominant stresses present in heat-treated castings develop as a result of the volume change accompanying austenite to martensite transformation. Low-temperature tempering, in the range of 200 to 230°C, is particularly desirable because a substantial improvement (20%) in fracture toughness occurs when tempering the martens lie phase. Tempering at temperatures sufficient to significantly relieve stresses, that is, above 540°C, will substantially reduce abrasion resistance.

High-alloy cast irons are an important group of materials whose production should be considered separately from that of the ordinary types of cast irons. In these cast iron alloys, alloy content is well above 4% and, consequently, they cannot be produced by ladle additions to irons of otherwise standard compositions. The producing foundries usually have the equipment needed to handle the heat treatment and other thermal processing unique to the production of these alloys.

The cast iron alloys discussed in this article are alloyed for increased abrasion resistance, for strength and oxidation resistance at elevated temperatures, and for improved corrosion resistance. They include the high-alloy graphitic irons and the high-alloy white irons.

The heat treatment practices for the following high-alloy graphitic irons are described:

• Austenitic gray and ductile irons
• High-silicon irons for heat resisting applications
• High-silicon irons for corrosion resisting applications

The high-alloy graphitic cast irons have found special use primarily in applications requiring (1) corrosion resistance or (2) strength and oxidation resistance in high-temperature service. Those alloys used in applications requiring corrosion resistance comprise the nickel-alloyed (13 to 36% Ni) gray and ductile irons, and the high-silicon (14.5% Si) gray irons.

The alloyed irons produced for high-temperature service comprise the austenitic, nickel-alloyed gray and nodular irons, the high-silicon (4 to 6% Si) gray and nodular irons and the aluminum-alloyed gray and nodular irons. Two groups of aluminum-alloyed irons are recognized: the 1 to 7% Al irons and the 18 to 25% Al irons.

Austenitic Nickel-Alloyed Graphitic Irons

These nickel-alloyed austenitic irons have found usefulness in applications requiring corrosion resistance, wear resistance, and high-temperature stability and strength. Additional properties of benefit are low thermal expansion coefficients, nonmagnetic properties, and cast iron materials having good toughness at low temperatures. The procedures and temperatures of the heat treatments for these ductile irons with nodular graphite are similar to those for gray (flake-graphite), corrosion-resistant austenitic cast irons.

ASTM Specification A 436 defines eight grades of austenitic gray iron alloys, four of which are designed to be used in elevated-temperature applications and four types are used in applications requiring corrosion resistance.

The ASTM Specification A 439 defines the group of austenitic ductile irons. There are nine alloys listed in the specification. The austenitic ductile iron alloys have similar compositions to the austenitic gray iron alloys but have been treated with magnesium to produce nodular graphite. The ductile iron alloys have high strength and ductility combined with the same desirable properties of the gray iron alloys. They provide resistance to frictional wear, corrosion resistance, strength and oxidation resistance at high temperatures, nonmagnetic characteristics and, in some alloys, low thermal expansivity at ambient temperatures.

Heat Treatment of Austenitic Ductile Irons. Heat treatment of the nickel-alloyed austenitic irons serves to reduce residual stresses and to stabilize the microstructure for increased durability. Heat treatments are similar with the graphite in nodular form (ductile iron) or flake form (gray iron).

Stress Relieving. For most applications, it is recommended that austenitic cast irons be stress relieved at 620 to 675°C (1150 to 1250°F), for 1 h per 25 mm (1 in.) of section, to remove residual stresses resulting from casting or machining, or both. Stress relieving should follow rough machining, particularly for castings that must conform to close dimensional tolerances, that have been extensively welded, or that are to be exposed to high stresses in service. Stress relieving does not affect tensile strength, hardness, or ductility. For large, relatively thin-section castings, mold-cooling to below 315°C (600°F) is recommended rather than stress relief heat treatment.

Spheroidize Annealing. Castings with hardness above 190 HB may be softened by heating to 980 to 1040°C (1800 to 1900°F) for 1/2 to 5 h except those alloys containing 4% or more chromium. Excessive carbides cause this high hardness and may occur in rapidly cooled castings and thin sections. Annealing dissolves or spheroidizes carbides. Although it lowers hardness, spheroidize annealing does not adversely affect strength.

High-Temperature Stabilization. This treatment consists of holding at 760°C (1400°F) for 4 h minimum or at 870°C (1600°F) for 2 h minimum, furnace cooling to 540°C (1000°F), and then cooling in air. This treatment stabilizes the microstructure and minimizes growth and warpage in service. The treatment is designed to reduce carbon levels in the matrix and some growth and distortion often accompanies heat treatment. Thus, it is usually advisable to stabilize castings prior to final machining.

Dimensional Stabilization. This treatment normally is limited to castings that require true dimensional stability, such as those used in precision machinery or scientific instruments. The treatment is not applicable to castings of type I alloys. Other alloys may be dimensionally stabilized by the following treatment:

• Heat to 870°C (1600°F), and hold for 2 h minimum plus 1 h per 25 mm (1 in.) of section
• Furnace cool, at a maximum rate of 50°C/h (100°F/h), to 540°C (1000°F)
• Hold at 540°C (1000°F) for 1 h per 25 mm (1 in.) of section, and then cool uniformly in air
• After rough machining, reheat to 455 to 480°C (850 to 900°F) and hold for 1 h per 25 mm (1 in.) of section, and cool uniformly in air
• Finish machine and reheat to 260 to 315°C (500 to 600°F), and cool uniformly in air.

Solution Treating. Although this treatment is seldom used, quenching from high temperatures is capable of producing higher-than-normal strength levels and slightly higher hardnesses by dissolving some carbon in austenite at elevated temperatures and by preventing precipitation of the carbon by rapid cooling. This treatment consists of heating to 925 to 1010°C (1700 to 1850°F) and quenching in oil or water. Because no metallurgical phase change occurs, the possibility of cracking is lessened.

High-Silicon Irons for High-Temperature Service

Graphitic irons alloyed with from 4 to 6% Si have provided good service, and low cost, in many elevated-temperature applications. These irons, whether gray or nodular, provide good oxidation resistance and stable ferritic matrix structures that will not go through a phase change at temperatures up to 815°C.

The elevated silicon content of these otherwise normal cast iron alloys reduces the rate of oxidation at elevated temperatures, because it promotes the formation of a dense, adherent film at the surface, which consists of iron silicate rather than iron oxide. This layer is much more resistant to oxygen penetration and its effectiveness improves with increasing silicon content.

High-Silicon Nodular Irons. The advent of ductile iron led to the development of high-silicon nodular irons, which currently represent the greatest tonnage of these types of irons being produced. Converting the eutectic flake graphite network to isolated graphite nodules further improved resistance to oxidation and growth. The higher strength and ductility of the nodular iron versions of these alloys qualifies them for more rigorous service.

The high-silicon nodular iron alloys are designed to extend the upper end of the range of service temperatures viable for ferritic nodular irons. These irons are used to temperatures of 900°C. At 5 to 6% Si, oxidation resistance is improved and critical temperature is increased, but the iron can be very brittle at room temperature. For most applications, alloying with 0 to 1% Mo provides adequate strength at elevated temperatures and creep resistance.

The high-silicon gray and nodular irons are predominantly, ferritic as-cast, but the presence of carbide stabilizing elements will result in a certain amount of pearlite and often intercellular carbides. These alloys are inherently more brittle than standard grades of iron and usually have higher levels of internal stress due to lower thermal conductivity and higher elevated-temperature strength. These factors should be taken into account where deciding on heat treatment requirements.

For the high-silicon nodular irons, high-temperature heat treatment is advised in all cases to anneal any pearlite and stabilize the casting against growth in service. A normal graphitizing (full) anneal in the austenitic temperature range is recommended where undesirable amounts of carbide are present.

For the 4 to 5% Si irons this will require heating to at least 900°C (1650°F) for several hours, followed by slow cooling to below 700°C (1300°F). At higher silicon contents (>5%), in which carbides readily break down, and in castings relatively carbide-free, subcritical annealing in the temperature range 720 to 790°C (1325 to 1450°F) for 4 h is effective in ferritizing the matrix. Compared to full annealing, the subcritically annealed material will have somewhat higher strength, but ductility and toughness will be reduced.

High-Silicon Irons for Corrosion Resistance. Irons with high silicon content (14.5% Si) comprise a unique corrosion-resistant ferritic cast iron group. These alloys are widely used in the chemical industry for processing and for transporting highly corrosive liquids. The most common of the high-silicon iron alloys are covered in ASTM Specification A 518M.

Because of the very brittle nature of high-silicon cast iron, castings are usually shaken out only after mold cooling to ambient temperature. However, some casting geometries demand hot shakeout so that the castings can be immediately stress-relieved and furnace cooled to prevent cracking.

Castings are stress relieved by heating in the range of 870 to 900°C (1600 to 1650°F) followed by slow cooling to ambient temperatures to minimize the likelihood of cracking. Heat treatments have no significant effect on corrosion resistance.

Gray irons are a group of cast irons that form flake graphite during solidification, in contrast to the spheroidal graphite morphology of ductile irons. The flake graphite in gray irons is dispersed in a matrix with a microstructure that is determined by composition and heat treatment.

The heat treatment of gray irons can considerably alter the matrix microstructure with little or no effect on the size and shape of the graphite achieved during casting. The matrix microstructures resulting from heat treatment can vary from ferrite-pearlite to tempered martensite. However, even though gray iron can be hardened by quenching from elevated temperatures, heat treatment is not ordinarily used commercially to increase the overall strength of gray iron castings because the strength of the as-cast metal can be increased at less cost by reducing the silicon and total carbon contents or by adding alloying elements.

Hardening and Tempering

Gray irons are hardened and tempered to improve their mechanical properties, particularly strength and wear resistance. After being hardened and tempered, these irons usually exhibit wear resistance approximately five times greater than that of pearlitic gray irons.

Furnace or salt bath hardening can be applied to a wider variety of gray irons than can either flame or induction hardening. In flame and induction hardening, a relatively large content of combined carbon is required because of the extremely short period available for the solution of carbon in austenite. In furnace or salt bath hardening, however, a casting can be held at a temperature above the transformation range for as long as is necessary; even an iron initially containing no combined carbon can be hardened.

Unalloyed gray iron of low combined carbon content must be austenitized for a longer time to saturate austenite with carbon. With increased time, more carbon is dissolved in austenite and hardness after quenching is increased.

Because of its higher silicon content, an unalloyed gray iron with a combined carbon content of 0.60% exhibits higher hardenability than a carbon steel with the same carbon content. However, because of the effect of silicon in reducing the solubility of carbon in austenite, unalloyed irons with higher silicon contents necessarily require higher austenitizing temperatures to attain the same hardness.

Manganese increases hardenability; approximately 1.50% Mn was found to be sufficient for through hardening a 38 mm section in oil or for through hardening a 64 mm section in water.

Manganese, nickel, copper, and molybdenum are the recognized elements for increasing the hardenability of gray iron. Although chromium, by itself, does not influence the hardenability of gray iron, its contribution to carbide stabilization is important, particularly in flame hardening.

Austenitizing. In hardening gray iron, the casting is heated to a temperature high enough to promote the formation of austenite, held at that temperature until the desired amount of carbon has been dissolved, and then quenched at a suitable rate.

The temperature to which a casting must be heated is determined by the transformation range of the particular gray iron of which it is made. The transformation range can extend more than 55°C above the At (transformation-start) temperature. A formula for determining the approximate A, transformation temperature of unalloyed gray iron is:

A (°C) = 730 + 28.0 (% Si) - 25.0 (% Mn) Chromium raises the transformation range of gray iron. In high-nickel, high-silicon irons, for example, each percent of chromium raises the transformation range by about 10 to 15°C. Nickel, on the other hand, lowers the critical range. In a gray iron containing from 4 to 5% Ni, the upper limit of the transformation range is about 710°C.

Castings should be treated through the lower temperature range slowly, in order to avoid cracking. Above a range of 595 to 650°C, which is above the stress-relieving range, heating may be as rapid as desired. In fact, time may be saved by heating the casting slowly to about 650°C in one furnace and then transferring it to a second furnace and bringing it rapidly up to the austenitizing temperature.

Quenching. Molten salt and oil are the quenching media used most frequently for gray iron. Water is not generally a satisfactory quenching medium for furnace-heated gray iron; it extracts heat so rapidly that distortion and cracking are likely in all parts except small ones of simple design. Recently developed water-soluble polymer quenches can provide the convenience of water quenching, along with lower cooling rates, which can minimize thermal shock.

The least severe quenching medium is air. Unalloyed or low-alloy gray iron castings usually cannot be air quenched because the cooling rate is not sufficiently high to form martensite. However, for irons of high alloy content, forced-air quenching is frequently the most desirable cooling method.

Tempering. After quenching, castings are usually tempered at temperatures well below the transformation range for about 1h per inch of thickest section. As the quenched iron is tempered, its hardness decreases, whereas it usually gains in strength and toughness.

Austempering

In austempering, the microstructural end product of the gray iron matrix formed below the pearlite range but above the martensite range is an acicular or bainitic fer-rite, plus varying amounts of austenite depending on the transformation temperature. The iron is quenched from a temperature above the transformation range in a hot quenching bath and is maintained in the bath at constant temperature until the austempering transformation is complete.

In all hot quenching processes, the temperatures to which castings must be heated for austenitizing and the required holding times at temperature prior to quenching in the hot bath correspond to the temperatures and times used in conventional hardening, that is, temperatures between 840 and 900°C (1550 and 1650°F). The holding time depends on the size and chemical composition of the casting.

Gray iron is usually quenched in salt, oil, or lead baths at 230 to 425°C for austempering. When high hardness and wear resistance are the ultimate aim of this treatment, the temperature of the quench bath is usually held between 230 and 290°C. The effect of iron composition on the holding time may be considerable. Alloy additions, such as nickel, chromium, and molybdenum, increase the time required for transformation.

Martempering

Martempering is used to produce martensite without developing the high stresses that usually accompany its formation. It is similar to conventional hardening except that distortion is minimized. Nevertheless, the characteristic brittleness of the martensite remains in a gray iron casting after martempering, and martempered castings are almost always tempered. The casting is quenched from above the transformation range in a salt, oil, or lead bath: held in the bath at a temperature slightly above the range at which martensite forms (200 to 260°C or 400 to 500°F. for unalloyed irons) only until the casting has reached the bath temperature; and then cooled to room temperature.

If a wholly martensitic structure is desired, the casting must be held in the hot quench bath only long enough to permit ii to reach the temperature of the bath. Thus, the size and shape of the casting dictate the duration of martempering.

Flame Hardening

Flame hardening is the method of surface hardening most commonly to gray iron. After flame hardening, a gray iron casting consists of a hard, wear-resistant outer layer of martensite and a core of softer gray iron, which during treatment does not reach the At transformation temperature.

Both unalloyed and alloyed gray irons can be successfully flame hardened. However, some compositions yield much better results than do others. One of the most important aspects of composition is the combined carbon content, which should be in the range of 0.50 to 0.70%, although irons with as little as 0.40% combined carbon can be flame hardened. In general, flame hardening is not recommended for irons that contain more than 0.80% combined carbon because such irons (mottled or white irons) may crack in surface hardening.

Effects of Alloying Elements. In general, alloyed gray irons can be flame hardened with greater ease than can unalloyed irons, partly because alloyed gray irons have increased hardenability. Final hardness also may be increased by alloying additions. The maximum hardness obtainable by flame hardening an unalloyed gray iron containing approximately 3% total carbon, 1.7% Si, and 0.60 to 0.80% Mn ranges from 400 to 500 HB. This is because the Brinell hardness value for gray iron is an average of the hardness of the matrix and that of the relatively soft graphite flakes. Actually, the matrix hardness on which wear resistance depends approximates 600 HB. With the addition of 2.5% Ni and 0.5% Cr, an average surface hardness of 550 HB can be obtained. The same result has been achieved using 1.0 to 1.5% Ni and 0.25% Mo.

Stress Relieving. Whenever practicable or economically feasible, flame-hardened castings should be stress relieved at 150 to 200°C.

Induction Hardening

Gray iron castings can be surface hardened by the induction method when the number of castings to be processed is large enough to warrant the relatively high equipment cost and the need for special induction coils.

Considerable variation in the hardness of the cast irons may be expected because of a variation in the combined carbon content. A minimum combined carbon content of 0.40 to 0.50% C is recommended for cast iron to be hardened by induction, with the short heating cycles that are characteristic of this process. Heating castings with lower combined carbon content to high hardening temperatures for relatively long periods of time may dissolve some free graphite, but such a procedure is likely to coarsen the grain.

Gray Irons are a group of cast irons that form flake graphite during solidification, in contrast to the spheroidal graphite morphology of ductile irons. The flake graphite in gray irons is dispersed in a matrix with a microstructure that is determined by composition and heat treatment. The usual microstructure of gray iron is a matrix of pearlite with the graphite flakes dispersed throughout. In terms of composition, 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 pearlitic). Other alloying elements include nickel, copper, molybdenum, and chromium.

The heat treatment of gray irons can considerably alter the matrix microstructure with little or no effect on the size and shape of the graphite achieved during casting. The matrix microstructures resulting from heat treatment can vary from ferrite-pearlite to tempered martensite. However, even though gray iron can be hardened by quenching from elevated temperatures, heat treatment is not ordinarily used commercially to increase the overall strength of gray iron castings because the strength of the as-cast metal can be increased at less cost by reducing the silicon and total carbon contents or by adding alloying elements. The most common heat treatments of gray iron are annealing and stress relieving.

Chemical composition is another important parameter influencing the heat treatment of gray cast irons. Silicon, for example, decreases carbon solubility, increases the diffusion rate of carbon in austenite, and usually accelerates the various reactions during heat treating. Silicon also raises the austenitizing temperature significantly and reduces the combined carbon content (cementite volume). Manganese, in contrast, lowers the austenitizing temperature and increases hardenability. It also increases carbon solubility, slows carbon diffusion in austenite, and increases the combined carbon content. In addition, manganese alloys and stabilizes pearlitic carbide and thus increases the pearlite content.

Annealing

The heat treatment most frequently applied to gray iron, with the possible exception of stress relieving, is annealing. The annealing of gray iron consists of heating the iron to a temperature high enough to soften it and/or to minimize or eliminate massive eutectic carbides, thereby improving its machinability. This heat treatment reduces mechanical properties substantially. It reduces the grade level approximately to the next lower grade: for example, the properties of a class 40 gray iron will be diminished to those of a class 30 gray iron. The degree of reduction of properties depends on the annealing temperature, the time at temperature, and the alloy composition of the iron.

Gray iron is commonly subjected to one of three annealing treatments, each of which involves heating to a different temperature range. These treatments are ferritizing annealing, medium (or full) annealing, and graphitizing annealing.

Ferritizing Annealing. For an unalloyed or low-alloy cast iron of normal composition, when the only result desired is the conversion of pearlitic carbide to ferrite and graphite for improved machinability, it is generally unnecessary to heat the casting to a temperature above the transformation range. Up to approximately 595°C (1100°F), the effect of short times at temperature on the structure of gray iron is insignificant. For most gray irons, a ferritizing annealing temperature between 700 and 760°C (1300 and 1400°F) is recommended.

Medium (full) annealing. It is usually performed at temperatures between 790 and 900°C (1450 and 1650°F). This treatment is used when a ferritizing anneal would be ineffective because of the high alloy content of a particular iron. It is recommended, however, to test the efficacy of temperatures below 760°C (1400°F) before a higher annealing temperature is adopted as part of a standard procedure.

Holding times comparable to those used in ferritizing annealing are usually employed. When the high temperatures of medium annealing are used, however, the casting must be cooled slowly through the transformation range, from about 790 to 675°C (1450 to 1250°F).

Graphitizing Annealing. If the microstructure of gray iron contains massive carbide particles, higher annealing temperatures are necessary. Graphitizing annealing may simply serve to convert massive carbide to pearlite and graphite, although in some applications it may be desired to carry out a ferritizing annealing treatment to provide maximum machinability.

The production of free carbide that must later be removed by annealing is, except with pipe and permanent mold castings, almost always an accident resulting from inadequate inoculation or the presence of excess carbide formers, which inhibit normal graphitization; thus, the annealing process is not considered part of the normal production cycle.

To break down massive carbide with reasonable speed, temperatures of at least 870°C (1600°F) are required. With each additional 55°C (100°F) increment in holding temperature, the rate of carbide decomposition doubles. Consequently, it is general practice to employ holding temperatures of 900 to 955°C (1650 to 1750°F).

Normalizing

Gray iron is normalized by being heated to a temperature above the transformation range, held at this temperature for a period of about 1 hour per inch of maximum section thickness, and cooled in still air to room temperature. Normalizing may be used to enhance mechanical properties, such as hardness and tensile strength, or to restore as-cast properties that have been modified by another heating process, such as graphitizing or the preheating and postheating associated with repair welding.

The temperature range for normalizing gray iron is approximately 885 to 925°C (1625 to 1700°F). Austenitizing temperature has a marked effect on microstructure and on mechanical properties such as hardness and tensile strength.

The tensile strength and hardness of a normalized gray iron casting depend on the following parameters:

• Combined carbon content
• Pearlite spacing (distance between cementite plates)
• Graphite morphology.

The graphite morphology does not change to any significant extent during normalization, and its effect on hardness and tensile strength is omitted in this discussion on normalizing.

Combined carbon content is determined by the normalizing (austenitizing) temperature and the chemical composition of the casting. Higher normalizing temperatures increase the carbon solubility in austenite (that is, the cementite volume in the resultant pearlite). A higher cementite volume, in turn, increases both the hardness and the tensile strength. The alloy composition of a gray iron casting also influences carbon solubility in austenite. Some elements increase carbon solubility, some decrease it, and others have no effect on it. The carbon content of the matrix is determined by the combined effects of the alloying elements.

The other parameter affecting hardness and tensile strength in a normalized gray iron casting is the pearlite spacing. Pearlite spacing is determined by the cooling rate of the casting after austenitization and the alloy composition. Fast cooling results in small pearlite spacing, higher hardness, and higher tensile strength. Too high a cooling rate may cause partial or full martensitic transformation. The addition of alloying elements may change hardness and tensile strength significantly.

Malleable cast iron is a heat-treated iron-carbon alloy, which solidifies in the as-cast condition with a graphite-free structure, i.e. the total carbon content is present in the cementite form (Fe3C).

Two groups of malleable cast iron are specified, differentiated by chemical composition, temperature and time cycles of the annealing process, the annealing atmosphere and the properties and microstructure resulting therefrom.

Whiteheart malleable cast iron

The microstructure of whiteheart malleable cast iron depends on section size. Small sections contain pearlite and temper carbon in ferritic substrate. In the large sections exists three different zones:

• surface zone which contains pure ferrite,
• intermediate zone which has pearlite, ferrite and temper carbon,
• core zone containing pearlite, temper carbon and ferritic inclusions.

The microstructure shall not contain flake graphite.

Blackheart and pearlitic malleable cast iron

The microstructure of blackheart malleable cast iron has a matrix essentially of ferrite. The microstructure of pearlitic malleable cast iron has a matrix, according to the grade specified, of pearlite or other transformation products of austenite.

Graphite is present in the form of temper carbon nodules. The microstructure shall not contain flake graphite.

Malleable cast iron designation system

The designation according to ISO 5922 (1981) of malleable cast iron consists of one letter designating the type of iron, two figures designating the tensile strength and two figures designating the minimum elongation.

1. Letters designating the type of malleable cast iron can be:
• W for whiteheart malleable cast iron,
• B for blackheart malleable cast iron,
• P for peariitic malleable cast iron.
This letter is followed by a space.
2. The first two figures designating the minimum tensile strength, in Newtons per square millimetre, of a 12 mm diameter test piece, divided by ten. For example if the minimum tensile strength were 350 N/mm², the designation would be 35.
3. The next two figures designating the minimum elongation (L₀ = 3d) as a percentage of a 12 mm diameter test piece. A nought (0) shall be the first figure when the value is less than 10%, for example if the minimum elongation is 4%, the designation is 04, and if the minimum elongation is 12%, the designation is 12.

For example: The designation of a whiteheart malleable cast iron having a minimum tensile strength of 400 N/mm² and minimum elongation of 5% when measured on a 12 mm diameter test piece, would be W 40-05.

Chemical composition of malleable iron

The chemical composition of malleable iron generally conforms to the ranges given in the Table 1. Small amounts of chromium (0.01 to 0.03%), boron (0.0020%), copper (≤ 1.0%), nickel (0.5 to 0.8%), and molybdenum (0.35 to 0.5%) are also sometimes present.

Table 1. Chemical composition of malleable iron

Element	Composition %
Carbon	2.16-2.90
Silicon	0.90-1.90
Manganese	0.15-1.25
Sulfur	0.02-0.20
Phosphorus	0.02-0.15

Mechanical properties of malleable iron

Malleable iron, like ductile iron, possesses considerable ductility and toughness because of its combination of nodular graphite and low-carbon metallic matrix. Because of the way in which graphite is formed in malleable iron, however, the nodules are not truly spherical as they are in ductile iron but are irregularly shaped aggregates.

Malleable iron and ductile iron are used for some of the applications in which ductility and toughness are important. In many cases, the choice between malleable and ductile iron is based on economy or availability rather than on properties. In certain applications, however, malleable iron has a distinct advantage. It is preferred for thin-section castings:

• for parts that are to be pierced, coined, or cold formed,
• for parts requiring maximum machinability,
• for parts that must retain good impact resistance at low temperatures, and
• for parts requiring wear resistance (martensitic malleable iron only).

Ductile iron has a clear advantage where low solidification shrinkage is needed to avoid hot tears or where the section is too thick to permit solidification as white iron (Solidification as white iron throughout a section is essential to the production of malleable iron). Malleable iron castings are produced in section thicknesses ranging from about 1.5 to 100 mm and in weights from less than 0.03 to 180 kg or more.

The mechanical properties of test pieces of malleable cast iron shall be in accordance with the values listed below:

Table 2. Mechanical properties of whiteheart malleable cast iron

Designation	Diameter of test piece mm	Tensile strength N/mm²	0,2% proof stress N/mm²	Elongation (L0 = 3d) % min	Hardness HB
W 35-04	9 - 15	340 - 360	-	5 - 3	230
W 38-12	9 - 15	320 - 380	170 - 210	15 - 8	200
W 40-05	9 - 15	360 - 420	200 - 230	8 - 4	220
W 45-07	9 - 15	400 - 480	230 - 280	10 - 4	220

Table 3. Mechanical properties of blackheart and pearlitic malleable cast iron

Designation	Diameter of test piece mm	Tensile strength N/mm²	0,2% proof stress N/mm²	Elongation (L0 = 3d) % min	Hardness HB
B 30-06	12 - 15	300	-	6	150 max
B 32-12	12 - 15	320	190	12	150 max
B 35-10	12 - 15	350	200	10	150 max
P 45-06	12 - 15	450	270	6	150-200
P 50-05	12 - 15	500	300	5	160-220
P 55-04	12 - 15	550	340	4	180-230
P 60-03	12 - 15	600	390	3	200-250
P 65-02	12 - 15	650	430	2	210-260
P 70-02	12 - 15	700	530	2	240-290
P 80-01	12 - 15	800	600	1	270-310

Melting Practices

Melting can be accomplished by batch cold melting or by duplexing. Cold melting is done in coreless or channel-type induction furnaces, electric arc furnaces, or cupola furnaces. In duplexing, the iron is melted in a cupola or electric arc furnace, and the molten metal is transferred to a coreless or channel-type induction furnace for holding and pouring.

Charge materials (foundry returns, steel scrap, ferroalloys, and, except in cupola melting, carbon) are carefully selected, and the melting operation is well controlled to produce metal having the desired composition and properties. Minor corrections in composition and pouring temperature are made in the second stage of duplex melting, but most of the process control is done in the primary melting furnace.

Molds are produced in green sand, silicate CO₂ bonded sand, or resin bonded sand (shell molds). Equipment ranges from highly mechanized or automated machines to that required for floor or hand molding methods, depending on the size and number of castings to be produced. In general, the technology of molding and pouring malleable iron is similar to that used to produce gray iron. Heat treating is done in high-production controlled-atmosphere continuous furnaces or batch-type furnaces, again depending on production requirements.

After solidification and cooling, the metal is in a white iron state, and gates, sprues, and feeders can be easily removed from the castings by impact. This operation, called spruing, is generally performed manually with a hammer because the diversity of castings produced in the foundry makes the mechanization or automation of spruing very difficult. After spruing, the castings proceed to heat treatment, while gates and risers are returned to the melting department for reprocessing.

Standard specifications for engineering grades of ductile iron castings classify the grades according to the tensile strength of a test bar cut from a prescribed test casting. The International Standards Organization (ISO) specification ISO 1083:1976 and most national specifications also specify the ductility in terms of percentage of elongation and the 0.2% proof strength or offset yield strength. The impact values of those grades with the highest ductility are frequently specified in the ISO, UK, and German specifications, and a guide to microstructure is included in most specifications. Hardness is usually specified, but is only mandatory in SAE J434C.

The actual values of properties to be expected from good-quality ductile irons produced to meet any given specified grade will normally cover a range that more than satisfies the requirements of the specification.

Specifications for the highest-strength grades usually mention the possibility of hardened-and-tempered structures, but for the most recently reported austempered ductile irons, which have the highest combinations of tensile strength and ductility, there are as yet only tentative unofficial specifications.

Factors That Affect Properties
Graphite Structures. The amount and form of the graphite in ductile iron are determined during solidification and cannot be altered by subsequent heat treatment. All of the mechanical and physical properties of this class of materials are a result of the graphite being substantially or wholly in the spheroidal nodular shape, and any departure from this shape in a proportion of the graphite will cause some deviation from these properties. It is common to attempt to produce greater than 90% of the graphite in this form (>90% nodularity), although structures between 80 and 100% nodularity are sometimes acceptable.

All properties relating to strength and ductility decrease as the proportion of non-nodular graphite increases, and those relating to failure, such as tensile strength and fatigue strength, are more affected by small amounts of such graphite than properties not involving failure, such as proof strength.

The form of non-nodular graphite is important because thin flakes of graphite with sharp edges have a more adverse effect on strength properties than compacted forms of graphite with rounded ends. For this reason, visual estimates of percentage of nodularity are only a rough guide to properties. Graphite form also affects modulus of elasticity, which can be measured by resonant frequency and ultrasonic velocity measurements, and such measurements are therefore often a better guide to nodularity and its effects on other properties. A low percentage of nodularity also lowers impact energy in the ductile condition, reduces fatigue strength, increases damping capacity, increases thermal conductivity, and reduces electrical resistivity.

Graphite Amount. As the amount of graphite increases, there is a relatively small decrease in strength and elongation, in modulus of elasticity, and in density. In general, these effects are small compared with the effects of other variables because the carbon equivalent content of spheroidal graphite iron is not a major variable and is generally maintained close to the eutectic value.

Matrix Structure. The principal factor in determining the different grades of ductile iron in the specifications is the matrix structure. In the as-cast condition, the matrix will consist of varying proportions of pearlite and ferrite, and as the amount of pearlite increases, the strength and hardness of the iron also increase. Ductility and impact properties are principally determined by the proportions of ferrite and pearlite in the matrix.

The matrix structure can be changed by heat treatments, and those most often carried out are annealing to produce a fully ferritic matrix and normalizing to produce a substantially pearlitic matrix. In general, annealing produces a more ductile matrix with a lower impact transition temperature than is obtained in as-cast ferritic irons. Normalizing produces a higher tensile strength with a higher amount of elongation than is obtained in fully pearlitic as-cast irons.

Section Size. As section size decreases, the solidification and cooling rates in the mold increase. This results in a fine-grain structure that can be annealed more rapidly. In thinner sections, however, carbides may be present, which will increase hardness, decrease machinability, and lead to brittleness. To achieve soft ductile structures in thin sections, heavy inoculation, probably at a late stage, is desirable to promote graphite formation through a high nodule number.

As the section size increases, the nodule number decreases, and micro segregation becomes more pronounced. This results in a large nodule size, a reduction in the proportion of as-cast ferrite, and increasing resistance to the formation of a fully ferritic structure upon annealing. In heavier sections, minor elements, especially carbide formers such as chromium, titanium, and vanadium, segregate to produce a segregation pattern that reduces ductility, toughness, and strength. The effect on proof strength is much less pronounced. It is important for heavy sections to be well inoculated and to be made from a composition low in trace elements.

Composition. In addition to the effects of elements in stabilizing pearlite or retarding transformation (which facilitates heat treatment to change matrix structure and properties), certain aspects of composition have an important influence on some properties. Silicon hardens and strengthens ferrite and raises its impact transition temperature; therefore, silicon content should be kept as low as practical, even below 2%, to achieve maximum ductility and toughness.

Nickel also strengthens ferrite, but has much less effect than silicon in reducing ductility. When producing as-cast grades of iron requiring fairly high ductility and strength such as ISO Grade 500-7, it is necessary to keep silicon low to obtain high ductility, but it may also be necessary to add some nickel to strengthen the iron sufficiently to obtain the required tensile strength.

Almost all elements present in trace amounts combine to reduce ferrite formation, and high-purity charges must be used for irons to be produced in the ferritic as-cast condition. Similarly, all carbide-forming elements and manganese must be kept low to achieve maximum ductility and low hardness. Silicon is added to avoid carbides and to promote ferrite as-cast in thin sections.

The electrical, magnetic, and thermal properties of ductile irons are influenced by the composition of the matrix. In general, as the amount of alloying elements increases, resistivity and the magnetic hardness of the material increase and thermal conductivity decreases.

Heat Treatment of Ductile Iron
The first stage of most heat treatments designed to change the structure and properties of ductile iron consists of heating to, and holding at, a temperature between 850 and 950oC for about 1h plus 1h for each 25 mm of section thickness to homogenize the iron. When carbides are present in the structure, the temperature should be approximately 900 to 950oC, which decomposes the carbides prior to subsequent stages of heat treatment. The time may have to be extended to 6 or 8h if carbide-stabilizing elements are present. In castings of complex shape in which stresses could be produced by nonuniform heating, the initial heating to 600oC should be slow, preferably 50 to 100oC per hour.

To prevent scaling and surface decarburization during this stage of treatment, it is recommended that a nonoxidizing furnace temperature be maintained using a sealed furnace; a controlled atmosphere may be necessary. Care must also be taken to support castings susceptible to distortion and to avoid packing so that castings are not distorted by the weight of other castings placed above them.

The most important heat treatments and their purposes are:

* Stress relieving - a low-temperature treatment, to reduce or relieve internal stresses remaining after casting
* Annealing - to improve ductility and toughness, to reduce hardness and to remove carbides
* Normalizing - to improve strength with some ductility
* Hardening and tempering - to increase hardness or to give improved strength and higher proof stress ratio
* Austempering - to yield bainitic structures of high strength, with some ductility and good wear resistance
* Surface hardening - by induction, flame, or laser to produce a local wear-resistant hard surface

High-alloy white cast irons are an important group of materials whose production must be considered separately from that of ordinary types of cast irons. In these cast iron alloys, the alloy content is well above 4%, and consequently they cannot be produced by ladle additions to irons of otherwise standard compositions. They are usually produced in foundries specially equipped to produce highly alloyed irons.

The high-alloy white irons are primarily used for abrasion-resistant applications and are readily cast into the parts needed in machinery for crushing, grinding, and handling of abrasive materials. The chromium content of high-alloy white irons also enhances their corrosion-resistant properties. The large volume fraction of primary and/or eutectic carbides in their microstructures provides the high hardness needed for crushing and grinding other materials. The metallic matrix supporting the carbide phase in these irons can be adjusted by alloy content and heat treatment to develop the proper balance between the resistance to abrasion and the toughness needed to withstand repeated impact.

While low-alloy white iron castings, which have alloy content below 4%, develop hardnesses in the range of 350 to 550 HB, the high-alloy irons range in hardness is from 450 to 800 HB.

Specification ASTM A 532 covers the composition and hardness of the abrasion-resistant white iron grades. Many castings are ordered according to these specifications. However, a large number of castings are produced with composition modifications for specific applications. It is most desirable that the designer, metallurgist, and foundry man work together to specify the composition, heat treatment, and foundry practice to develop the most suitable alloy and casting design for a specific application.

The high-alloy white cast irons fall into two major groups:

* Nickel-chromium white irons, which are low-chromium alloys containing 3 to 5% Ni and 1 to 4% Cr, with one alloy modification that contains 7 to 11% Cr,

* Chromium-molybdenum irons containing 11 to 23% Cr, up to 3% Mo and often additionally alloyed with nickel or copper.

A third group comprises the 25% or 28% Cr white irons, which may contain other alloying additions of molybdenum and/or nickel up to 1.5%. The nickel-chromium irons are also commonly identified as Ni-Hard types 1 to 4.

Nickel-Chromium White Irons
The oldest group of high-alloy irons of industrial importance, the nickel-chromium white irons, or Ni-Hard irons, have been produced for more than 50 years and are very cost-effective materials for crushing and grinding.

In these martensitic white irons, nickel is the primary alloying element because at levels of 3 to 5% it is effective in suppressing the transformation of the austenite matrix to pearlite, thus ensuring that a hard martensitic structure (usually containing significant amounts of retained austenite) will develop upon cooling in the mold. Chromium is included in these alloys, at levels from 1.4 to 4%, to ensure that the irons solidify carbidic, that is, to counteract the graphitizing effect of nickel.

The optimum composition of a nickel-chromium white iron alloy depends on the properties required for the service conditions and the dimensions and weight of the casting. Abrasion resistance is generally function of the bulk hardness and the volume of carbide in the microstructure. When abrasion resistance is the principal requirement and resistance to impact loading is secondary, alloys having high carbon contents, ASTM A 532 class I type A (Ni-Hard 1), are recommended. When conditions of repeated impact are anticipated, the lower carbon alloys, class I type B ( Ni-Hard 2) are recommended because they have less carbide and, therefore, greater toughness. A special grade, class J type C, has been developed for producing grinding balls and slugs. Here, the nickel-chromium alloy composition has been adapted for chill casting and specialized sand casting processes.

The Class I type D (Ni-Hard 4) alloy is a modified nickel-chromium iron that contains higher levels of chromium, ranging from 7 to 11%, and increased levels of nickel, ranging from 5 to 7%. Carbon is varied according to the properties needed for the intended service. Carbon contents in the range of 3.2 to 3.6% are prescribed when maximum abrasion resistance is desired. When impact loading is expected, carbon content should be held in the range of 2.7 to 3.2%.

Nickel content increases with section size or cooling time of the casting to inhibit pearlitic transformation. For castings of 38 to 50 mm thick, 3.4 to 4.2% Ni is sufficient to suppress pearlite formation upon mold cooling. Heavier sections may require nickel levels up to 5.5% to avoid the formation of pearlite. It is important to limit nickel content to the level needed for control of pearlite; excess nickel increases the amount of retained austenite and lowers hardness.

Silicon is needed for two reasons. A minimum amount of silicon is necessary to improve fluidity of the melt and to produce a fluid slag, but of equal importance is its effect on as-cast hardness. Increased levels of silicon, in the range of 1 to 1.5%, have been found to increase the amount of martensite and the resulting hardness. Late additions of ferrosilicon (0.2% as 75% Si grade ferrosilicon) have been reported to increase toughness. It is important to note that higher silicon contents can promote pearlite and may increase the nickel requirement.

Chromium is primarily added to offset the graphitizing effects of nickel and silicon in types A, B, and C alloys, ranges from 1.4 to 3.5%. Chromium content must be increased with increasing section size. In type D alloy, chromium levels range from 7 to 11% (typically 9%) for the purpose of producing eutectic carbides of the M7C3 chromium carbide type, which are harder and less deleterious to toughness.

Manganese is typically held to a maximum of 0.8% even though 1.3% maximum is allowed according to ASTM A 532 specification. While it provides increased harden-ability to avoid pearlite formation, it is a more potent austenite stabilizer than nickel, and promotes increased amounts of retained austenite and lower as-cast hardness. For this reason, higher manganese levels are undesirable. When considering the nickel content required to avoid pearlite in a given casting, the level of manganese present should be a factor.

Copper increases both hardenability and the retention of austenite and therefore must be controlled for the same reason that manganese must be limited. Copper should be treated as a nickel substitute and, when properly included in the calculation of the amount of nickel required to inhibit pearlite, it reduces the nickel requirement.

Molybdenum is a potent hardenability agent in these alloys and is used in heavy-section castings to augment hardenability and inhibit pearlite.

High-Chromium White Irons
The high-chromium white irons have excellent abrasion resistance and are used effectively in slurry pumps, brick molds, coal-grinding mills, shot-blasting equipment, and components for quarrying, hard-rock mining, and milling. In some applications they must also be able to withstand heavy impact loading. These alloyed white irons are recognized as providing the best combination of toughness and abrasion resistance attainable among the white cast irons.

In the high-chromium irons, as with most abrasion-resistant materials, there is a trade-off between wear resistance and toughness. By varying composition and heat treatment, these properties can be adjusted to meet the needs of most abrasive applications. Specification ASTM A 532 covers the compositions and hardnesses of two general classes of the high-chromium irons. The chromium-molybdenum irons (Class II of ASTM A532) contain 11 to 23% Cr and up to 3.5% Mo and can be supplied either as-cast with an austenitic or austenitic-martensitic matrix, or heat-treated with a martensitic matrix microstructure for maximum abrasion resistance and toughness. They are usually considered the hardest of all grades of white cast irons. Compared to the lower-alloy nickel-chromium white irons, the eutectic carbides are harder and can be heat-treated to achieve castings of higher hardness. Molybdenum, as well as nickel and copper when needed, is added to prevent pearlite and to ensure maximum hardness.

The high-chromium irons (class III of ASTM A 532) represent the oldest grade of high-chromium irons, with the earliest patents dating back to 1917. These general-purpose irons, also called 25% Cr and 28% Cr irons, contain 23 to 28% Cr with up to 1.5% Mo. To prevent pearlite and attain maximum hardness, molybdenum is added in all but the lightest-cast sections. Alloying with nickel and copper up to 1% is also practiced. Although the maximum attainable hardness is not as high as in the class II chromium-molybdenum white irons, these alloys are selected when resistance to corrosion is also desired.

Nickel markets should hold up fairly well over the next 12 months thanks to a careful balance between supply and demand. Stainless and nickel's steadily improving performance should continue over the next year, especially if many manufacturing segments begin to improve. However, continuing problems in the aerospace industry concern many vendors.

GROUNDED

During the Institute of Scrap Recycling Industries Inc.'s (ISRI) Nickel/ Stainless Roundtable, held this fall in Pittsburgh, several speakers noted continuing problems with the aerospace industry are putting downward pressure on markets. Ed Newman, vice president of Keywell LLC, Pittsburgh, rattled off a list of stories that highlight the difficult situation.

"When we look at today's business picture we can somewhat understand how hard it can be to be an optimist in a pessimistic time," said Newman. "Honestly, it was hard to find a bright spot," he noted in regard to the headlines. "Boeing delivered 527 airplanes in 2001. This year they expect to deliver 380, a drop of almost 30 percent. Next year they project to deliver between 275 and 300 airplanes--another 25 percent drop. They have let nearly 30,000 workers go to date.

"American Airlines announced in August that it is retiring another 74 airplanes and is deferring delivery on 35 new planes that they had planned to take delivery of this year. They also plan another 7,000 layoffs. US Air is in Chapter 11, and there are reports that United might not be far behind," he added.

Newman continued by noting that General Electric, the world's largest maker of jet engines, expects engine deliveries to fall 15 percent to 20 percent this year and a similar amount next year.

STAYING STEADY

Even with such a bleak forecast for one of the main end markets for the specialty grades of nickel and stainless steel, many handlers of the metal have seen steadily improving prices. The main reason for the improved pricing is nickel's balance between supply and demand.

During the ISRI meeting, Yuri Sobolev, senior vice president of Norimet-Norilsk Nickel, the large Russian mining concern, noted that his company has taken a significant amount of production off line. This move reverses an earlier trend when Russian metals producers were accused of dumping their commodities on the world market, resulting in much lower prices.

Several scrap nickel handlers note that Norilsk has a large block of nickel warehoused at the port of Rotterdam, Netherlands, as collateral for loans. This move has tightened supply and demand for the material.

It is not likely that this nickel will be placed on the open market until prices climb much higher, according to Patricia Mohr with Scotia Bank, an investment company that tracks commodity markets.

The lack of available scrap nickel is not limited to the U.S. Reports indicate that scrap nickel supplies are limited in Europe and Asia as well. The result has improved the overall pricing environment for the metal.

"Western World stainless steel production will rise by almost 5 percent in 2002, after last year's inventory correction," says Mohr. "Though global stainless steel output may falter temporarily in the fourth quarter, the impact on primary nickel demand will be limited by tight scrap supplies."

In its October meeting the International Nickel Study Group (INSG) reported that nickel production and consumption should end up sharply higher this year. For 2002 the group forecasts that world primary nickel production should increase by around 3.6 percent, with production in the West increasing by 4.6 percent. The INSG is an autonomous, intergovernmental organization whose members are comprised of nickel producing, consuming and trading countries.

The INSG also expects nickel production to grow by 3.5 percent next year, with production in the West climbing by 4.7 percent.

While production should show some promise, nickel consumption should increase by 5.4 percent in 2002 and by 6.6 percent next year.

With the conflicting information for the nickel market, it is no surprise that the overall trend is pointing upward. Despite the recession in the manufacturing segment over the past two years, the strength in consumer purchases has helped prop up prices for nickel over the past several quarters. With the expectation that the manufacturing segment will start to turnaround, the feeling is that prices, after a temporary lull leading into early 2003, should begin improving.

While long term the fundamentals look good for stainless, short term the market for scrap nickel will fluctuate significantly.

Al Goodman, a trader with ELG Metals, Chicago, says that nickel is a thinly traded commodity, making it more volatile.

While volatility makes it difficult to predict market conditions for nickel and stainless in the short term, the two metals show a definite growth pattern. Goodman notes that China is a strong end market. As China seeks to modernize, growth in demand for nickel, stainless steel and other metals will continue to outpace demand in the U.S.