Alloy Cast Irons
Alloy Cast Irons
Alloy cast irons are the casting alloys which are based on the iron (Fe) – carbon (C) – silicon (Si) system. They contain one or more alloying elements intentionally added to improve one or more properties. The addition to the ladle of small amounts of substances such as ferrosilicon (Fe-Si), cerium (Ce), or magnesium (Mg)) that are used to control the size, shape, and/or distribution of graphite particles is termed as inoculation. The quantities of material used for inoculation neither change the basic composition of the solidified cast iron nor alter the properties of individual constituents. Alloying elements, including Si when it exceeds about 3 %, are usually added to increase the strength, hardness, hardenability, or corrosion resistance of the basic iron and are often added in quantities sufficient to affect the occurrence, properties, or distribution of constituents in the microstructure.
In gray and ductile cast irons, small amounts of alloying elements such as chromium (Cr), molybdenum (Mo), or nickel (Ni) are added primarily to achieve high strength or to ensure the attainment of a specified minimum strength in heavy sections. Otherwise, alloying elements are used almost exclusively to enhance resistance to abrasive wear or chemical corrosion or to extend service life at elevated temperatures.
Classification of alloy cast irons
Alloy cast irons can be classified as (i) white cast irons, (ii) corrosion resistant cast irons, and (iii) heat resistant cast irons (Fig 1).
Fig 1 Classification of alloy cast irons
White cast irons
White cast irons are so named because of their characteristically white fracture surfaces. They do not have any graphite in their microstructures. Instead, the C is present in the form of carbides, mainly of the types Fe3C and Cr7C3. Frequently, complex carbides such as (Fe,Cr)3C and (Cr,Fe)7C3, or those containing other carbide forming elements, are also present.
White cast irons are usually very hard, which is the single property maximum responsible for their outstanding resistance to abrasive wear. White cast iron can be produced either throughout the section (mainly by adjusting the composition) or only partly inward from the surface (mainly by casting against a chill). The iron cast with a chill is sometimes called as chilled cast iron to distinguish it from the cast iron which is white throughout. Chilled cast iron castings are produced by casting the liquid metal against a metal or graphite chill, resulting in a surface virtually free from graphitic carbon. While producing chilled cast iron, the composition is so selected so that only the surfaces cast against the chill are free from graphitic C. The more slowly cooled portions of the casting are gray or mottled iron. The depth and hardness of the chilled portion is usually controlled by adjusting the composition of the iron, the extent of inoculation, and the pouring temperature.
White cast iron is a cast iron virtually free from graphitic C because of selected chemical composition. The composition is chosen so that, for the desired section size, graphite does not form as the casting solidifies. The hardness of white cast iron castings is controlled by further adjustment of composition.
The main difference in microstructure between chilled cast iron and white cast iron is that chilled cast iron is fine grained and shows directionality perpendicular to the chilled face, while white cast iron is usually coarse grained, randomly oriented, and white throughout, even in relatively heavy sections. Fine grain white cast iron is normally produced by casting a white iron composition against a chill.
The difference reflects the effect of composition difference between the two types of abrasion resistant cast irons. Chilled cast iron is directional only since the casting, made of a composition that is ordinarily gray, has been cooled through the eutectic temperature so rapidly at one or more faces that the iron solidified white, growing inward from the chilled face. White cast iron, on the other hand, has a composition so low in carbon equivalent (CE) or so rich in alloy content that gray cast iron is not produced even at the relatively low rates of cooling that exist in the centre of the heaviest section of the casting.
Corrosion resistant cast irons
Corrosion resistant cast irons derive their resistance to chemical attack mainly from their high alloy content. Depending on which of three alloying elements (Si, Cr, and Ni) dominates the composition, a corrosion resistant cast iron can be ferritic, pearlitic, martensitic, or austenitic in its microstructure. Depending on composition, cooling rate, and inoculation practice, a corrosion resistant cast iron can be white, gray, or nodular in both form and distribution of carbon.
Heat resistant cast irons
Heat resistant cast irons combine resistance to high temperature oxidation and scaling with resistance to softening or microstructural degradation. Resistance to scaling depends primarily on high alloy content, and resistance to softening depends on the initial microstructure along with the stability of the C containing phase. Heat resistant cast irons are generally ferritic or austenitic as cast. C exists predominantly as graphite, either in flake or nodular form, which subdivides heat resistant cast irons into either gray or ductile cast irons. There are also ferritic and austenitic white cast iron grades, although they are less frequently used.
Effects of alloying elements
In majority of the cast irons, it is the interaction among alloying elements (including C and Si) which has the greatest effect on properties. This influence is exerted mostly by effects on the amount and shape of graphitic C present in the casting. As an example, in low alloy cast irons, depth of chill or the tendency of the iron to be white as-cast depends greatly on the CE, the Si in the composition, and the state of inoculation. The addition of other elements can only modify the basic tendency established by the C-Si relationship.
On the other hand, abrasion resistant white cast irons are specifically alloyed with Cr to produce fully carbidic cast irons. One of the benefits of Cr is that it causes carbide, rather than graphite, to be the stable C rich eutectic phase upon solidification. At higher Cr contents (greater or equal to 10 %) M7C3 becomes the stable C rich phase of the eutectic reaction. In general, only small amounts of alloying elements are needed to improve depth of chill, hardness, and strength. High alloy contents are needed for the most significant improvements in abrasion resistance, corrosion resistance, or elevated temperature properties.
Alloying elements such as Ni, Cr, and Mo are used, singly or in combination, to provide specific improvements in properties compared to unalloyed cast irons. Since the use of such elements means higher cost, the improvement in service performance is to be adequate to justify the increased cost.
Effects of carbon
In chilled irons, the depth of chill decreases, and the hardness of the chilled zone increases, with increasing C content. C also increases the hardness of white cast irons. Low C white cast irons (2.50 % C) have a hardness of about 375 HB (Brinnel hardness), while white cast irons with fairly high total C (more than 3.50 %) have a hardness as high as 600 HB. In unalloyed white cast irons, high total C is essential for high hardness and maximum wear resistance. Carbon decreases the transverse breaking strength and increases the brittleness.
Carbon also increases the tendency for graphite to form during solidification, especially when the Si content is also high. As a result, it is very important to keep the Si content low in high C white cast irons. The usual range of C content for unalloyed or low alloy white cast irons is around 2.2 % to 3.6 %. For high Cr white cast irons, the normal range is from about 2.2 % to the C content of the eutectic composition, which is around 3.5 % for a 15 % Cr cast iron and around 2.7 % for a 27 % Cr cast iron.
The C content of gray and ductile alloy cast irons is generally somewhat higher than that of a white cast iron of similar alloy content. In addition, the Si content is usually higher, so that graphite is formed during solidification.
Effects of silicon
Silicon is present in all cast irons. In alloy cast irons, Si is the main reason that determines the C content of the eutectic. Increasing the Si content lowers the C content of the eutectic and promotes the formation of graphite upon solidification. Hence, the Si content is the prime reason for controlling the depth of chill in unalloyed or low Cr chilled and white cast irons.
In high alloy white cast irons, Si has a negative effect on hardenability. It tends to promote pearlite formation in martensitic irons. However, when sufficient amounts of pearlitic suppressing elements such as Mo, Ni, Cr, and manganese (Mn) are present, increasing the Si contest raises the Ms temperature of the alloy, thus tending to increase both the amount of martensite and the final hardness.
The Si silicon content of chilled and white cast irons is usually between 0.3 % and 2.2 %. In martensitic Ni-Cr white cast irons, the desired Si content is generally 0.4 % to 0.9 %.
Si additions of 3.5 % to 7 % improve high temperature properties by raising the eutectoid transformation temperature. Elevated levels of Si also reduce the rates of scaling and growth by forming a tight, adhering oxide scale. This occurs at Si contents above 3.5 % in ferritic cast irons and above 5 % in 36 % Ni austenitic cast irons. Additions of 14 % to 17 % (frequently accompanied by additions of about 5 % Cr and 1 % Mo) yield cast iron that is very resistant to corrosive acids, although resistance varies somewhat with acid concentration.
High Si cast irons (14 % to 17 %) are difficult to cast and are virtually not machinable. High Si cast irons have particularly low resistance to mechanical and thermal shock at room temperature or moderately elevated temperature. However, above around 260 deg C, the shock resistance exceeds that of ordinary gray cast iron.
Effects of manganese and sulphur
Manganese and sulphur (S) are normally considered together in their effects on gray or white cast iron. Alone, either Mn or S increases the depth of chill, but when one is present, addition of the other decreases the depth of chill until the residual concentration has been neutralized by the formation of manganese sulphide (MnS).
Normally, S is the residual element, and excess Mn can be used to increase chill depth and hardness. Also, since S promotes the formation of finer and harder pearlite, Mn is often preferred for decreasing or preventing mottling in heavy section castings. Mn, in excess of the amount needed to scavenge S, mildly suppresses pearlite formation. It is also a relatively strong austenite stabilizer and is normally kept below about 0.7 % in martensitic white cast irons. In some pearlitic or ferritic alloy cast irons, up to about 1.5 % Mn can be used to help ensure that specified strength levels are obtained. When Mn content exceeds about 1.5 %, the strength and toughness of martensitic cast irons begin to drop. Abrasion resistance also drops, mainly because of austenite retention. Liquid iron with a high Mn content tends to attack furnace and ladle refractories. Therefore, the use of Mn is limited in cast irons, even though it is one of the least expensive alloying elements.
The normal S contents of alloy cast irons are neutralized by Mn, but the S content is kept low in most of the alloy cast irons. In abrasion resistant cast irons, the S content is to be as low as is commercially feasible, since the sulphides in the microstructure degrade abrasion resistance. A sulphur content of 0.03 % is usually the maximum which can be tolerated when optimum abrasion resistance is needed.
Effects of phosphorus
Phosphorus (P) is a mild graphitizer in unalloyed cast irons. It mildly reduces chill depth in chilled cast irons. In alloyed cast irons, the effects of P are somewhat obscure. There is some evidence that it reduces the toughness of martensitic white cast irons. The effect, if any, on abrasion resistance has not been conclusively proved. In heavy section castings made from Mo containing cast irons, high P content is considered detrimental since it neutralizes part of the deep hardening effect of the Mo. It is considered desirable to keep the P content of alloy cast irons below around 0.3 %, and some specifications call for less than 0.1 %. In cast irons for high temperature or chemical service, it is normal to keep the P content below 0.15 %.
Effects of chromium
Chromium has three major uses in cast irons namely (i) to form carbides, (ii) to impact corrosion resistance, and (iii) to stabilize the structure for high temperature applications. Small amounts of Cr are routinely added to stabilize pearlite in gray cast iron, to control chill depth in chilled cast iron, or to ensure a graphite free structure in white cast iron containing less than 1 % Si. At such low percentages, generally not greater than 2 % to 3 %, Cr has little or no effect on hardenability, primarily because most of the Cr is tied up in carbides. However, Cr does influence the fineness and hardness of pearlite and tends to increase the amount and hardness of the eutectic carbides. Therefore, Cr is frequently added to gray cast iron to ensure that strength requirements can be met, particularly in heavy sections. Sometimes, it is added to ductile cast iron for the same purpose. Also, relatively low percentages of Cr are used to improve the hardness and abrasion resistance of pearlitic white cast irons.
When the Cr content of cast iron is greater than at around 10 %, eutectic carbides of the M7C3 are formed, rather than the M3C type that predominates at lower Cr contents. More significantly, however, the higher Cr content causes a change in solidification pattern to a structure in which the M7C3 carbides are surrounded by a matrix of austenite or its transformation products. At lower Cr contents, the M3C carbide forms the matrix. Because of the solidification characteristics, hypoeutectic irons containing M7C3 carbides are normally stronger and tougher than irons containing M3C carbides.
The relatively good abrasion resistance, toughness, and corrosion resistance found in high Cr white cast irons have led to the development of a series of commercial martensitic or austenitic white cast irons containing 12 % to 28 % Cr. Because much of the Cr in these cast irons is present in combined form as carbides, Cr is much less effective than Mo, Ni, Mn, or copper (Cu) in suppressing the eutectoid transformation to pearlite and therefore has a lesser effect on hardenability than it has in steels.
Martensitic white cast irons generally contain one or more of the elements Mo, Ni, Mn, and Cu to give the required hardenability. These elements ensure that martensite is formed during cooling from above the upper transformation temperature either while the casting is cooling in the mould or during subsequent heat treatment.
It is difficult to maintain low Si content in high Cr cast irons because of the Si introduced by high C ferro-chrome and other sources. Low Si content is advantageous in that it provides for ready response to annealing and yields high hardness when the alloy is air quenched from high temperatures. High Si content lessens response to this type of heat treatment. Although high Cr white cast irons are sometimes used as-cast, their optimum properties are obtained in the heat treated condition.
For developing resistance to the softening effect of heat and for protection against oxidation, Cr is the most effective element. It stabilizes iron carbide and therefore prevents the breakdown of carbide at high temperatures. 1 % Cr gives adequate protection against oxidation up to around 760 deg C in many applications. For temperatures of 760 deg C and above, Cr contents up to 5.5 % are found in austenitic ductile cast irons for added oxidation resistance. For long term oxidation resistance at elevated temperatures, white cast irons having Cr contents ranging from 15 % to 35 % are used. This percentage of Cr suppresses the formation of graphite and makes the alloy solidify as white cast iron.
High levels of Cr stabilize the ferrite phase up to the melting point. Typical high Cr ferritic irons contain 30 % to 35 % Cr. Austenitic grades of high Cr irons, which have significantly higher strength at high temperatures, contain 10 % to 15% Ni, along with 15 % to 30 % Cr.
Effects of nickel
Nickel is almost entirely distributed in the austenitic phase or its transformation products. Like Si, Ni promotes graphite formation, and in white and chilled cast irons, this effect is normally balanced by the addition of about one part Cr for every three parts Ni in the composition. If fully white castings are desired, the amount of Cr is increased. Some low- and medium-alloy cast irons have a ratio as low as one part Cr to 1.3 parts Ni. In high Cr cast irons, the Ni content may be as high as 15 % to stabilize the austenite phase.
When added to low Cr white cast iron in amounts up to about 2.5 %, Ni produces a harder and finer pearlite in the structure, which improves its abrasion resistance. Ni in somewhat larger amounts (up to around 4.5 %) is required to completely suppress pearlite formation, thus ensuring that a martensitic iron results when the castings cool in the moulds. This latter practice forms the basis for production of the Ni-Hard cast irons (which are usually identified in standard specification as Ni- Cr martensitic irons).
With small castings (e.g. grinding balls) which can be shaken out of the moulds while still hot, air cooling from the shakeout temperature produces the desired martensitic structure even when the Ni content is as low as 2.7 %. On the other hand, an excessively high Ni content (greater than around 6.5 %) will so stabilize the austenite that little martensite, if any, can be formed in castings of any size. Appreciable amounts of retained austenite in Ni-Hard cast irons can be transformed to martensite by refrigerating the castings at -55 deg C to -75 deg C or by using special tempering treatments. One of the Ni-Hard family of commercial alloy white cast irons contains 1.0 % to 2.2 % Si, 5 % to 7 % Ni, and 7 % to 11 % Cr. In the as-cast condition, it has a structure of M7C3 eutectic carbides in a martensitic matrix. If retained austenite is present, the martensite content and hardness of the alloy can be increased by refrigeration treatment or by re-austenitizing and air cooling. This Ni-Hard cast iron is often specified for pumps and other equipment used for handling abrasive slurries because of its combination of relatively good strength, toughness, and abrasion resistance.
Ni is used to suppress pearlite formation in large castings of high Cr white cast iron (12 % to 28 % Cr). The typical amount of Ni is about 0.2 % to 1.5 %, and it is usually added in conjunction with Mo. Ni contents higher than this range tend to excessively stabilize the austenite, leading to austenite retention. Control of composition is especially important for large castings that are intended to be martensitic, because their size dictates that they cool slowly regardless of whether they are to be used as-cast or after heat treatment.
Ni additions of more than 12 % are required for optimum resistance to corrosion or heat. High Ni gray or ductile cast irons usually contain 1 % to 6 % Cr and may contain as much as 10 % Cu. These elements act in conjunction with the Ni to promote resistance to corrosion and scaling, especially at elevated temperatures. All types of cast iron with Ni contents above 18 % are fully austenitic.
Effects of copper
Copper in moderate amounts can be used to suppress pearlite formation in both low and high Cr martensitic white cast irons. The effect of Cu is relatively mild compared to that of Ni. Because of the limited solubility of Cu in austenite, Cu additions probably are usually limited to around 2.5 % or less. This limitation means that Cu cannot completely replace Ni in Ni-Hard irons. When added to chilled cast iron without Cr, Cu contracts the zone of transition from white to gray iron, thus reducing the ratio of the mottled portion to the clear chilled portion.
Copper is most effective in suppressing pearlite when it is used in conjunction with around 0.5 % to 2.0 % Mo. The hardenability of this combination is generally good, which indicates that there is a synergistic effect when Cu and Mo are added together to cast iron. Combined additions are particularly effective in the martensitic high Cr cast irons. Here, Cu content is usually held to 1.2 % or less with larger amount tends to induce austenite retention.
Copper is used in amounts of around 3 % to 10 % in some high Ni gray and ductile cast irons which are normally specified for corrosion or high temperature service. Here, Cu enhances corrosion resistance, particularly resistance to oxidation or scaling.
Effects of molybdenum
Molybdenum in chilled and white cast irons is distributed between the eutectic carbides and the matrix. In graphitic irons, its main functions are to promote deep hardening and to improve high temperature strength and corrosion resistance. In chilled cast irons, Mo additions mildly increase depth of chill. It is, however, around one-third as effective as Cr. The primary purpose of small additions (0.25 % to 0.75 %) of Mo to chilled cast iron is to improve the resistance of the chilled face to spalling, pitting, chipping, and heat checking.
Molybdenum hardens and toughens the pearlitic matrix. Where a martensitic white cast iron is preferred for superior abrasion resistance, additions of 0.5 % to 3.0 % Mo effectively suppress pearlite and other high temperature transformation products. Mo is even more effective when used in combination with Cu, Cr, Ni, or both Cr and Ni. Mo has an advantage over Ni, Cu, and Mn in that it increases depth of hardening without appreciably over-stabilizing austenite, thus preventing the retention of undesirably large amounts of austenite in the final structure. When studying the influence of different amounts of Mo on the hardenability of high Cr white cast irons, It can be seen that the hardenability (measured as the critical diameter for air hardening) increases as the ratio of Cr to C increases.
The pearlite suppressing properties of Mo have been used to advantage in cast irons of high Cr content. White Cast irons with 12 % to 18 % Cr are used for abrasion resistant castings. The addition of 1 % to 4 % Mo is effective in suppressing pearlite formation, even when the castings are slowly cooled in heavy sections. Mo can replace some of the Ni in the Ni-Cr type of martensitic white cast irons. In heavy section castings in which 4.5 % Ni is generally used, the addition of 1 % Mo permits a reduction of Ni content to around 3 %. In light section castings of this type, where 3 % Ni is normally used, the addition of 1 % Mo permits a reduction of Ni to 1.5%.
Molybdenum, in quantities of around 1 % to 4 %, is effective in enhancing corrosion resistance, especially in the presence of chlorides. In quantities of 0.5 % to 2 %, Mo improves high temperature strength and creep resistance in gray and ductile cast irons with ferritic or austenitic matrices.
Effects of vanadium
Vanadium (V) is a strong carbide stabilizer and increases the depth of the chill. The magnitude of the increase of depth of chill depends on the amount of V, the composition of the cast iron, section size, and the conditions of casting. The powerful chilling effect of V in thin sections can be balanced by additions of Ni or Cu, by a large increase in C or Si, or both. In addition to its carbide stabilizing influence, V in amounts of 0.10 % to 0.50 % refines the structure of the chill and minimizes coarse columnar grain structure. Because of its strong carbide forming tendency, V is rarely used in gray or ductile cast irons for corrosion or high temperature service.
Effects of Inoculants
Certain elements, when added in very small amounts in the pouring ladle, have relatively strong effects on the size, shape, and distribution of graphite in graphitic cast irons. Other elements are equally powerful in stabilizing carbides. These elements, called inoculants, seem to act more as catalysts than as participants in the reactions. The main graphitizing inoculant is ferrosilicon, which is often added in detectable amounts (several kilograms per ton) as a final adjustment of CE in gray or ductile cast irons. In ductile cast irons, it is essential that the graphite be present in the final structure in nodular form rather than in the form of flakes. Mg, Ce, rare earth elements, and certain proprietary substances are added to the liquid iron just before pouring to induce the graphite to form in nodular shape of the desired size and distribution.
In white cast irons, tellurium (Te), bismuth (Bi), and sometimes V are the principal carbide inducing inoculants. Te is extremely strong. An addition of only about 5 gram/ton (5 ppm) is often sufficient. Te has one major drawback. It has been found to cause tellurium halitosis in the workers exposed to even minute traces of its fumes. Hence, its use as an inoculant has been discouraged and sometimes prohibited.
Bismuth, in amounts of 50 gram/ton to 100 gram/ton (50 ppm to 100 ppm), effectively suppresses graphite formation in unalloyed or low alloy white cast irons. Actually, Bi is used in the low C compositions intended for malleabilizing heat treatment. It has been reported that Bi produces a fine grain microstructure free from spiking, a condition that is sometimes favoured in abrasion resistant white cast irons.
Vanadium, in amounts up to 0.5 %, is sometimes considered useful as a carbide stabilizer and grain refiner in white or chilled cast irons. Nitrogen (N2) and boron (B) containing ferroalloys have also been used as inoculants with reported beneficial effects. In general, however, the economic usefulness of inoculants in abrasion resistant white cast irons has been inconsistent and remains not proven. Inoculants other than appropriate graphitizing or nodularizing agents are used rarely, if ever, in high alloy corrosion resistant or heat resistant cast irons.
Abrasion resistant cast Irons
It is normally presumed that parts subjected to abrasion wear out and hence need to be replaced from time to time. Also, for many applications, there are one or more types of relatively low cost materials which have adequate wear resistance and one or more types of higher cost materials which have measurably higher wear resistance. For both the situations, the ratio of wear rate to replacement cost need to be evaluated. This ratio can be a very effective means of evaluating the most economical use of materials. It is often more economical to use a less wear resistant material and replace it more often. However, in some cases, such as when frequent occurrences of downtime cannot be tolerated, material economy is less important than service life. Total cost effectiveness must take into account the actual cost of materials, heat treatment, time for removal of worn parts and insertion of new parts, and other production time lost.
In general, chilled cast iron and unalloyed white cast iron are less expensive than alloy cast irons. They are also less wear resistant. However, the abrasion resistance of chilled or unalloyed white cast iron is entirely adequate for many applications. Costly alloy cast irons ate used only when a clear performance advantage can be proved that alloy cast irons show an economic advantage over unalloyed cast irons. As an example, in a one year test in a mill for grinding cement clinker, grinding balls made of martensitic Ni-Cr white cast iron had to be replaced only about one fifth as often as forged and hardened alloy steel balls. In another test of various parts in a brick making plant, martensitic N- Cr white cast iron was found to last three to four times as long as unalloyed white cast iron, in terms of both tonnage handled and lifetime in days. In both cases, martensitic Ni-Cr white cast iron has shown a clear economic advantage as well as a clear performance advantage over the alternative materials.
Typical compositions ranges for the typical commercial unalloyed and low alloy grades of white and chilled cast irons used for abrasion resistant castings are nominally classed as pearlitic white cast irons. Historically, most of the early white cast iron castings produced for abrasion resistance was cast from low C, 1.0 % to 1.6 % Si unalloyed compositions, which were also used for malleable iron castings. As changes have occurred in demand and specific uses, the trend has been to produce a more abrasion resistant 2.8 % to 3.6 % C, low Si grade, which is usually alloyed with Cr to suppress graphite and to increase the fineness and hardness of the pearlite. Other alloying elements such as Ni, Mo, Cu, and Mn are used mainly to increase hardenability in order to obtain austenitic or martensitic structures.
Martensitic white cast irons have largely replaced pearlitic white cast irons for making many types of abrasion resistant castings, with the possible exception of chilled cast iron rolls and grinding balls. Although martensitic white cast irons cost more than pearlitic cast irons, their much superior abrasion resistance, combined with the increasing costs of all castings, makes martensitic alloy white cast irons economically attractive. The better strength and toughness of martensitic cast irons favour their use.
The iron alloys of class I are designed to be largely martensitic as-cast; the only heat treatment commonly applied is tempering. The iron alloys of classes II and III are either pearlitic or austenitic as-cast, except in slow-cooling heavy sections, which may be partially martensitic. The iron alloys of classes II and III are usually heat treated. There are several situations in which the abrasion resistance of the as-cast austenitic casting is very good and in such cases no heat treatment is applied.
Various high and low temperature heat treatments are being used to improve the properties of white and chilled cast iron castings. For the unalloyed or low Cr pearlite white cast irons, heat treatment is done primarily to relieve the internal stresses that develop in the castings as they cool in their moulds.
Generally, such heat treatments are used only on large castings such as mill rolls and chilled cast iron automobile wheels. Temperatures up to around 710 deg C are used without severely reducing abrasion resistance. In some cases, the castings can be removed from their moulds above the pearlitic formation temperature and then can be isothermally transformed to pearlite (or to ferrite and carbide) in an annealing furnace. As the tempering or annealing temperature is increased, the time at those temperatures are to be reduced to prevent graphitization.
Residual stresses in large castings result from volume changes during the transformation of austenite and during subsequent cooling of the casting to room temperature. Since these volume changes do not occur simultaneously in each part of the casting, they tend to set up residual stresses, which may be very high and may therefore cause the casting to crack during production in the foundry or in the service.
The Ni- Cr martensitic white cast irons, containing up to about 7 % Ni and 11 % Cr, are usually put into service after only a low temperature heat treatment at 230 deg C to 290 deg C to temper the martensite and to increase toughness. If retained austenite is present and the cast iron therefore has less than optimum hardness, a sub-zero treatment down to liquid N2 temperature can be employed to transform much of the retained austenite to martensite. Sub-zero treatment substantially raises the hardness, often as much as 100 BH points.
Following sub-zero treatment, the castings are almost always tempered at 230 deg C to 260 deg C. The austenite – martensite microstructures produced in Ni alloyed cast irons are frequently desirable for their intrinsic toughness. It is possible to transform additional retained austenite by heat treating Ni – Cr white cast irons at around 730 deg C. Such a treatment decreases matrix C and therefore raises the Ms temperature. However, high temperature treatments are usually less desirable than sub-zero treatments because the former are more costly and more likely to induce cracking due to transformation stresses.
The high Cr martensitic white cast irons (Cr more than 12 %) are subjected to a high temperature heat treatment to develop full hardness. They can be annealed to soften them for machining, and then hardened to develop the required abrasion resistance. Because of their high Cr content, there is no likelihood of graphitization while the castings are held at the re-austenitizing temperature. The normal re-austenitizing temperature for high chromium irons ranges from about 955 deg C for a 15 Cr-Mo iron to about 1065 deg C for a 27 % Cr iron. An appreciable holding time (3 to 4 hours minimum) at temperature is usually mandatory to permit precipitation of dispersed secondary carbide particles in the austenite. This lowers the amount of C dissolved in the austenite to a level that permits transformation to martensite during cooling to room temperature. Air quenching is usually used, although small, simply shaped castings can be quenched in oil or molten salt without producing quench cracks. Following quenching, it is advisable to stress relieve (temper) the castings at about 210 deg C to 260 deg C.
With rapid solidification, such as that which occurs in thin wall iron castings or when the liquid iron solidifies against a chill, the austenite dendrites and eutectic carbides are fine grained, which tends to increase fracture toughness. In low Cr white cast irons, rapid solidification also reduces any tendency towards formation of graphite. The presence of graphite severely degrades abrasion resistance. Chills in the mould are used to promote directional solidification and therefore reduce shrinkage cavities in the iron castings. Certain inoculants, notably Bi, may beneficially alter the solidification pattern by reducing spiking or by producing a finer as-cast grain size.
Immediately after solidification, the microstructure of unalloyed or low Cr white cast irons consists of austenite dendrites, containing up to around 2 % C, surrounded by M3C carbides. When the Cr content of the iron exceeds around 7 %, the structure contains M7C3 eutectic carbides surrounded by austenite. This reversal of the continuous phase in the structure tends to increase the fracture toughness of white cast irons, but only for those cast irons that have a hypoeutectic or eutectic CE. All hypereutectic white cast irons are relatively brittle and are seldom used.
After a white cast iron casting has solidified and begins to cool to room temperature, the carbide phase may decompose into graphite plus ferrite or austenite. This tendency to form graphite can be suppressed by rapid cooling or by the addition of carbide stabilizing alloying elements, usually Cr, although inoculating with Te or Bi is also very effective. Austenite in the solidified white cast iron structure normally undergoes several changes as it cools to ambient temperature. If it is cooled slowly enough, it tends to reject hypereutectoid C, either on existing eutectic carbide particles or as particles, platelets, or spines within the austenite grains. This precipitation occurs principally between around 1040 deg C and 760 deg C. The rate of precipitation depends on both time and temperature.
As the austenite cools further, through the range of 710 deg C to 540 deg C, it tends to transform to pearlite. This transformation, however, can be suppressed by rapid cooling and/or by the use of pearlite suppressing elements in the cast iron. Ni, Mn, and Cu are the principal pearlite suppressing elements. Cr does not contribute significantly to pearlitic suppression (hardenability) in many white cast irons, since most of the Cr is tied up in carbides. Mo, a strong carbide former, is also tied up in carbides. However, in high Cr irons, there is enough Cr and Mo remaining in the matrix to contribute significantly to hardenability.
Upon cooling below about 540 deg C, the austenite transforms to bainite or martensite, thus producing martensitic white cast iron, which is currently the most widely used type of abrasion resistant white cast iron. Martensitic white cast irons usually contain some retained austenite, which is not considered objectionable unless it exceeds around 15 %. Retained austenite is metastable and may transform to martensite when plastically deformed at the wearing surface of the casting.
Silicon has a substantial influence on the microstructure of any grade of white cast iron. Normally, Si content exceeds 0.3 %, and it may range as high as 2.2 % in some of the high Cr grades. During the solidification of unalloyed or low alloy irons, Si tends to promote the formation of graphite, an effect that can be suppressed by rapid solidification or by the addition of carbide stabilizing elements. After solidification, either while the casting is cooling to ambient temperature or during subsequent heat treatment, Si tends to promote the formation of pearlite in the structure if it is the only alloy present. However, in the presence of Cr and Mo, both of which suppress ferrite, Si has a minimal effect on ferrite and substantially suppresses bainite. In certain alloy white cast irons with high retained austenite contents, increasing the Si content raises the Ms temperature of the austenite, which in turn promotes the transformation of austenite to martensite. Si is also used to enhance the hardening response when the castings are cooled below ambient temperature.
Hardness is the principal mechanical property of white cast iron which is regularly determined and reported. Other (nonstandard) tests to determine strength, impact resistance, and fracture toughness are sometimes employed. Because of the difficulty of preparing test specimens, especially from heavy-section castings, these nonstandard tests are seldom used for regular quality control. Two exceptions are the tumbling breakage test and the repeated drop test, which have been normally used by certain manufacturers for testing grinding balls.
Minimum hardness values for pearlitic white cast irons are 321 HB for the low C grade and 400 HB for the high C grade. A chill cast high C 2 % Cr white cast iron may reach a hardness of around 550 HB. A typical hardness range for a sand cast high C grade is around 430 HB to 500 HB.
The minimum hardness specified for the hardened (heat treated) class II castings is well below the average expected hardness. These cast irons, when fully hardened so that they are free from high temperature products of austenite transformation, have hardness values ranging from around 800 HV to 950 HV (Vickers hardness) depending on retained austenite content. The 800 HV to 950 HV range is equivalent to 700 HB to 790 HB or 62.5 HRC to 67.5 HRC. For optimum abrasion resistance of the class I cast irons, the minimum Brinell hardness, as measured with a tungsten carbide ball or converted from HV or HRC values, is to be 700 HB.
Hardness conversions for white cast irons are somewhat different from the published data for steel. Because of inherent variations in structure for many cast irons, hardness conversion is to be made with caution. For example, Brinell hardness tests are more consistent and reliable for coarse structures such as those typical of heavy sections.
The tensile strength (TS, in reality, the fracture strength) of pearlitic white cast irons normally ranges from about 210 N/sq mm for high C grades to about 415 N/sq mm for low C grades. The TS of martensitic irons with M3C carbides ranges from around 345 N/sq mm to 415 N/sq mm, while high Cr irons, with their M7C3 type carbides, usually have TS values of 415 to 550 N/sq mm. Limited data indicate that the yield strength (YS) values of white cast irons are about 90 % of their TS values. These data are extremely sensitive to variations in specimen alignment during testing. Because of the near zero ductility of white cast irons, the usefulness of tensile test data for design or quality assurance is very limited.
Transverse strength, which is an indirect measurement of TS and tensile ductility, is generally determined with a moderate degree of accuracy on un-machined cast test bars. The product of transverse strength and deflection provides one measure of toughness. The values are normally considered very general.
The elastic modulus of a white cast iron is considerably influenced by its carbide structure. A cast iron with M3C eutectic carbides has a tensile modulus of 165 to 195 GPa, irrespective of whether it is pearlitic or martensitic, while a cast iron with M7C3 eutectic carbides has a modulus of 205 to 220 GPa.
The density of white cast irons ranges from 7.50 gram/cu cm to 7.75 gram/cu cm. Increase of the C content tends to decrease density while increasing the amount of retained austenite in the structure tends to increase density.
The relative abrasion resistance of various types of white cast iron has been widely studied. In general, martensitic white cast irons have substantially better abrasion resistance than pearlitic or austenitic white cast irons. There can be substantial differences in abrasion resistance among the various martensitic cast irons. The degree of superiority of one type over another can also vary considerably, depending on the application and also on whether abrasive wear is due to gouging, high stress (grinding) abrasion, or low stress scratching or erosion. In addition, performance in a dry environment is quite different from that in a wet environment.
For Ni-Cr martensitic white cast irons, there are conflicting data as to the relative serviceability of sand cast and chill cast parts subjected to abrasive wear. This is not particularly surprising, because many of the data are obtained in test using abrasive ores where the nature of the gangue was incompletely defined or largely ignored. The hardness of the abrasive material has a marked influence on relative abrasion rates. For example, when the abrasive is silicon carbide, which is hard enough to scratch M3C and M7C3 carbides as well as martensite and pearlite, there is little difference in relative wear rates among any of the white cast irons. However, with silica (the abrasive most commonly encountered in service), which is not hard enough to scratch M7C3 carbides but may scratch M3C carbides and definitely will scratch martensite and pearlite, high Cr white cast irons, with their M7C3 carbides, tend to provide superior performance. If the abrasive mineral is a silicate of intermediate hardness such as feldspar, which (theoretically) does not scratch fully hard martensite but scratches pearlite, any of the martensitic white cast irons can perform much better than any of the pearlitic white cast irons.
The relatively low hardness of the retained austenite in high Cr cast irons warrants special consideration. Because this austenite tends to work harden rapidly and may also transform to martensite, it is quite abrasion resistant when severely loaded. However, majority of abrasion tests and field experience indicate that cast irons containing considerable retained austenite are not as abrasion resistant as those put into service with fully martensitic microstructures.
Corrosion resistant cast irons
The corrosion resistance of gray cast iron is enhanced by the addition of appreciable amounts of Ni, Cr, and Cu, singly or in combination, or Si in excess of around 3 %.
Up to 3 % Si is normally present in all cast irons. In larger percentages, Si is considered an alloying element. It promotes the formation of a strongly protective surface film under oxidizing conditions such as exposure to oxidizing acids. Relatively small amounts of Mo and/or Cr can be added in combination with high Si. The addition of Ni to gray cast iron improves resistance to reducing acids and provides high resistance to caustic alkalis. Cr assists in forming a protective oxide that resists oxidizing acids, although it is of little benefit under reducing conditions. Cu has a smaller beneficial effect on resistance to sulphuric acid.
High Si cast irons are the most universally corrosion resistant alloys available at moderate cost. They are widely used for handling the corrosive media common in chemical plants, even when abrasive conditions are also encountered. When the Si content is 14.2 % or higher, these cast irons show a very high resistance to boiling sulphuric acid. They are especially useful when the concentration of sulphuric acid is above 50 %, at which point they are virtually immune to attack. The high Si cast irons are also very resistant to nitric acid. Increasing the Si content to 16.5 % makes the alloy quite resistant to corrosion in boiling nitric and sulphuric acids at nearly all concentrations, but this is accompanied by a reduction of mechanical strength.
The 14.5 % Si cast iron is less resistant to the corrosive action of hydrochloric acid, but this resistance is improved by additions of Cr and Mo and can be further enhanced by increasing the Si content to 17 %. The Cr bearing Si cast irons are very useful in contact with solutions containing Cu salts, free wet chlorine, or other strongly oxidizing impurities.
The high Si cast irons are very resistant to organic acid solutions at any concentration or temperature. However, their resistance to strong hot caustics is not satisfactory for many purposes. They are resistant to caustic solutions at lower temperatures and concentrations, and (although they are no better than unalloyed gray cast iron in this regard) they can be used where caustics and other corrosives are mixed or alternately handled. They do not have useful resistance to hydrofluoric or sulphurous acids.
High Si cast irons have poor mechanical properties and particularly low thermal and mechanical shock resistance. These alloys are typically very hard and brittle, with a TS of around 110 N/sq mm and a hardness of 480 to 520 HB. They are difficult to cast and are virtually not machinable. Their considerable use stems from their outstanding resistance to acids. They are widely used for drain pipe. High Si cast iron towers, tubes, and fittings are standard equipment for concentrating sulphuric and nitric acids in the explosives and fertilizer industries. High Si cast iron pumps, valves, mixing nozzles, tank outlets, and steam jets are widely used for handling severe corrosive materials such as chromic acid, sulphuric acid slurries, bleach solutions, and acid chloride slurries. High Si cast irons are also widely used for anodes in impressed current cathodic protection systems, especially where aggressive environments such as seawater or chloride soils are encountered.
The mechanical strength and shock resistance of high Si cast irons can be improved by lowering the Si content to 12 % or slightly less. However, reducing the Si to 12 % causes a significant reduction in corrosion resistance and therefore is feasible only in applications where the loss in corrosion resistance has a minimal effect on service life or is offset by the benefit derived from the increase in strength.
High Cr cast irons containing 20 % to 35 % Cr give good service with oxidizing acids, particularly nitric, but are not resistant to reducing acids. These cast irons are also reliable for use in weak acids under oxidizing conditions, in numerous salt solutions, in organic acid solutions, and in marine or industrial atmospheres. The corrosion resistance of high Cr cast iron to nitric acid is exceptional. It resists all concentrations of this acid up to 95 % at room temperature. Its corrosion rate is less than 0.13 mm per annum at all temperatures up to the boiling point for concentrations up to 70 %. In handling nitric acid, the Cr cast irons are complementary to high Si cast irons. The former shows very good corrosion resistance to all concentrations and temperatures, except for boiling concentrated acids, while the latter give better results in stronger acid.
The low C, high Cr cast irons are satisfactory for lead, zinc, or aluminum melting pots, annealing pots, conveyor links, and other parts exposed to corrosion at high temperature. Since the corrosion resistance is imparted by Cr present in solid solution in the ferritic matrix, this element is to be present in sufficient quantity to combine with C as chromium carbide and still remain in the desired amount in the ferrite.
Chromium contents of 30 % to 33 % are common in cast irons for use under conditions of severe acid corrosion. High Cr cast irons are resistant to all concentrations of sulphurous acid at temperatures up to 80 deg C, to sulphite liquors, to hypochlorite bleaching liquors at room temperature, to cold aluminum sulphate in concentrations up to 5 %, and to some salts that hydrolyze to give acid solutions. They resist all concentrations of phosphoric acid up to 60 % at temperatures up to the boiling point and 85 % concentrations up to 80 deg C. They also have good resistance to aerated seawater and a majority of mine waters, including acidic types.
Chromium cast irons have better mechanical properties than high Si cast irons and respond readily to heat treatment when Cr and C contents are suitably balanced. TS as high as 480 N/sq mm is obtained with a hardness of 290 to 340 HB. These alloys are generally resistant to shock and can be machined. Both of these properties are improved when C content is lowered to about 1.2 %. As C is increased, machinability in the annealed condition decreases and hence cast irons with C contents of 3 % or more are to be used only when no machining is required. The maximum service temperature for high Cr cast irons is normally 810 deg C to 1100 deg C.
High Ni austenitic cast irons produced in several compositions, depending on desired properties and end use. Austenitic gray irons containing large percentages of Ni and Cu are fairly resistant to mildly oxidizing acids, including dilute to concentrated sulphuric acid at room temperature. They are also fairly resistant to hydrochloric and some phosphoric acids at slightly high temperatures. The corrosion behaviour of high Ni cast irons is similar to that of unalloyed gray cast iron in the presence of nitric acid. Although the Ni containing cast iron shows better corrosion resistance than an 18-8 stainless steel, high Si cast irons are much better for both sulphuric and hydrochloric acids under certain conditions. High Ni cast irons shows fair resistance to some organic acids (such as acetic, oleic, and stearic acids) and to red oils. Cast irons with Ni contents of 18 % or more are nearly immune to the effects of weak or strong alkalis and caustics, although subject to stress corrosion in strong hot caustics at stresses over 70 N/sq mm.
High Ni cast irons are the toughest of all cast irons containing flake graphite. Although their TS is relatively low, ranging from 140 N/sq mm to 275 N/sq mm, they have satisfactory toughness and excellent machinability. High Ni ductile cast irons, which are specially treated so that the graphite forms as nodules rather than as flakes have essentially the same corrosion resistance as high Ni gray cast irons, but have much higher strength and ductility. A similar treatment applied to high Si cast iron provides no improvement in mechanical properties. High Ni cast irons provide satisfactory corrosion resistance at higher temperatures up to around 710 deg C to 810 deg C. Above this range, high Cr cast irons are preferred.
Heat resistant cast irons
Heat resistant cast irons are basically alloys of Fe, C, and Si having high temperature properties markedly improved by the addition of certain alloying elements, singly or in combination, principally Cr, Ni, Mo, aluminum (Al),and Si in excess of 3 %. Si and Cr increase resistance to heavy scaling by forming a light surface oxide that is impervious to oxidizing atmospheres. Both the elements reduce the toughness and thermal shock resistance of the cast iron. Although Ni does not appreciably affect oxidation resistance, it increases strength and toughness at elevated temperatures by promoting an austenitic structure that is significantly stronger than ferritic structures above 540 deg C. Mo increases high temperature strength in both ferritic and austenitic cast iron alloys. Al additions are very effective in raising the equilibrium temperature (A1) and in reducing both growth and scaling, but they adversely affect mechanical properties at room temperature.
The performance of alloy gray cast irons at higher temperatures is determined by a number of related properties, such as resistance to growth and oxidation, resistance to thermal shock, response to cyclic heating, creep resistance, rupture strength, and high temperature fatigue strength.
Growth is the permanent increase in the volume that occurs in some cast irons after prolonged exposure to higher temperature or after repeated cyclic heating and cooling. It is produced by the expansion that accompanies graphitization, expansion, and contraction at the transformation temperature, combined with internal oxidation of the cast iron. Gases can penetrate the surface of hot cast iron at the graphite flakes and oxidize the graphite as well as the Fe and Si. The occurrence of fine cracks, of crazing, may accompany repeated heating and cooling through the transformation temperature ranges because of thermal and transformational stresses. Si contents of less than around 3.5 % increase the rate of growth by promoting graphitization, but Si contents of 4 % or more retard growth. Both Mn and P decrease growth by acting as carbide stabilizers.
The carbide stabilizing alloying elements, particularly Cr, effectively reduce growth in gray cast irons at 455 deg C or above. Growth is not a problem below 400 deg C, except in the presence of superheated steam, where it can occur in coarse grain cast irons at around 315 deg C. Even small amounts of Cr, Mo, and V produce marked reductions in growth at the higher temperatures.
Cr containing cast irons containing 24 % to 34 % Cr show no appreciable growth at 1100 deg C. In general, cast irons containing 20 % to 35 % Cr can be used regularly at around 1000 deg C and for short periods up to 1100 deg C with satisfactory resistance to growth and scaling.
High Ni gray and ductile cast irons are also quite resistant to higher temperature growth. In addition to resistance to growth, the austenitic gray and ductile cast irons are resistant to warpage and cracking in cyclic elevated temperature service. This resistance is attributed to the absence of phase transformation, to moderate elastic moduli, and to good mechanical properties at around 600 deg C to 760 deg C.
In addition to the internal oxidation that contributes to growth, a surface scale forms on unalloyed gray cast iron after exposure at sufficiently high temperature. The scale formed in air consists of a mixture of iron oxides. The important factor in scale formation is whether the scale, (i) is essentially adherent and protective to the base cast iron, or (ii) tends to flake and permit continued oxidation of the cast iron. Si, Cr, and Al increase the scaling resistance of cast iron by forming a light surface oxide that is impervious to oxidizing atmospheres. Unfortunately, these elements tend to reduce the toughness and the thermal shock resistance. The presence of Ni improves the scale resistance of many cast iron alloys containing Cr and, more important, increases their toughness and strength at higher temperatures.
Carbon has a somewhat damaging effect above 710 deg C as a result of the mechanism of decarburization and the evolution of carbon monoxide and carbon dioxide. When these gases are evolved at the cast iron surface, the formation of protective oxide layers is hindered, and cracks and blisters can develop in the scale.
Greater scaling rates can be tolerated in some applications, so that higher useful temperatures are possible. The presence of large amounts of Ni, or Al increase the temperature limits for the various Si-Cr cast irons. As an example, a 7 % Al cast iron has adequate scale resistance to about 900 deg C, while 16 % to 25 % Al cast irons are virtually scale free at 1100 deg C.
Measurements of short time TS, creep strength, and rupture strength provide a basis for evaluating the performance of cast irons at higher temperatures. Creep rate increases with temperature and becomes an important design factor at higher temperature applications. Creep is ordinary reported in terms of strain for a specified period of time at a given tensile stress and temperature. Since cast irons can grow at higher temperatures without the application of external stress, the measured increase in length is the sum of growth resulting from metallurgical causes and the mechanical elongation of creep.
Creep in gray cast iron is appreciably influenced by microstructure and composition. An unalloyed gray cast iron with a CE of about 4 % can usually be subjected to a tensile stress of 70 N/sq mm at 400 deg C without exceeding a creep rate of 1 % in 10,000 hours. Low alloy cast irons show even less creep under similar conditions. Ductile cast irons may sustain stresses up to 185 N/sq mm at 425 deg C without exceeding a creep rate of 1 % in 10,000 hours. Some austenitic ductile cast irons have about the same creep strength at 540 deg C as the unalloyed ductile cast irons display at 425 deg C. The short time high temperature strength of a cast iron is taken as the stress that is sufficient to break a standard tension test specimen in a short period of time at higher temperature. Often, the correlation between the high temperature strength and load carrying capacity of a cast over long periods of time is poor or nonexistent.
Because of the inadequacy of the short time tension test, creep rupture test data are used more often in the evaluation of high temperature properties. Normally creep tests are performed at stresses that do not break the specimen, rupture tests are run to failure.
Although intermediate amounts of Si increase the rate of growth in cast iron by increasing the rate of graphitization, additions of 4.5 % to 8 % Si greatly reduce both scaling and growth. Si also has the advantage of raising the transformation temperature to around 900 deg C, thus increasing the operating temperature range that may be employed without encountering a phase change. Ferritic high Si gray cast iron is rather brittle and has a low resistance to thermal shock at room temperature. However, it is superior to ordinary gray cast iron above around 260 deg C. An austenitic gray cast iron containing 5 % Si, 18 % Ni, and 2 % to 5 % Cr shows considerably better toughness and thermal shock resistance. Both the plain Si and the Ni-Cr-Si cast irons show very good resistance to scaling in air up to 810 deg C, and the Ni-Cr-Si cast iron can be successfully employed in sulphurous atmospheres. The maximum temperature for use of these cast irons is 900 deg C for the plain Si cast iron and 955 deg C for the Ni-Cr-Si iron. Si containing compositions are also available in ferritic ductile cast iron.
Cr is widely used in heat resistant cast irons because of its stabilizing influence on carbides, which deters growth, and its tendency to form a tight, protective oxide. Substantial improvement in oxidation resistance is obtained by the addition of 0.5 % to 1 % Cr for many applications up to 760 deg C. Further improvement in resistance to scaling and growth at 760 deg C without excessive loss in toughness and machinability is there for cast irons with up to 2 % Cr.
Machinable castings with considerable heat resistance are obtained with rather small additions of both Cr and Ni to cast iron. Cr additions of 15 % to 35 % are used for outstanding oxidation and growth resistance at around 980 deg C and even up to 1100 deg C in oxidizing atmospheres or in the presence of certain chemicals. The high Cr cast irons show a characteristically white structure and can be produced with fair machinability and good strength. Low Si and C contents are necessary when toughness and thermal shock resistance are required. The thermal shock resistance of these cast irons is good, but their toughness is quite limited, even when C and Si contents are low.
The austenitic cast irons containing 18 % to 36 % Ni, up to 7 % Cu, and 1.75 % to 4 % Cr are used for both heat resistant and corrosion resistant applications. This type of cast iron shows good resistance to high temperature scaling and growth up to 815 deg C in very oxidizing atmospheres, good performance in steam service up to 530 deg C, and can handle sour gases and liquids up to 400 deg C. The maximum temperature of use is 540 deg C if appreciable sulphur is present in the atmosphere. Austenitic cast iron is used at temperatures as high as 950 deg C. Austenitic cast irons have the advantage of considerably greater toughness and thermal shock resistance than the other heat resistant alloy cast irons, although their strength is rather low.
High Ni ductile cast irons are considerably stronger and tougher than the comparable gray cast irons. TS values of 400 N/sq mm to 470 N/sq mm, yield strengths of 205 N/sq mm to 275 N/sq mm, and elongations of 10 % to 40 % can be obtained in high Ni ductile cast irons.
Alloy cast irons containing 6 % to 7 % Al, 18 % to 25 % Al, or 12 % to 25 % Cr with 4 % to 16 % Al have considerably better resistance to scaling than several other alloy cast irons, including the high Si type. These irons have been little used commercially because of brittleness and poor castability.
Various corrosion resistant and heat resistant alloy cast irons can also be cast as ductile cast iron, with the graphite in the form of nodules rather than in the normal flake graphite shape characteristic of gray cast iron. When the graphite is present as nodules, the cast iron shows improved elastic behaviour, higher modulus of elasticity, a definite yield point, higher tensile strength, and improved ductility and toughness. Where service applications need these improved properties, alloy ductile cast irons are efficiently being used, although their metallurgical and production characteristics are more complex than those of comparable gray cast irons.
The most important automotive application of alloy cast irons is in brake drums and disks, for which the material is required to have a combination of high heat capacity, good thermal conductivity, and high emissivity so that it can dissipate a large amount of heat per unit volume. Also, to maintain strength and dimensional stability during cyclic heating and cooling, the material must have adequate high temperature strength and resistance to thermal shock and must resist growth due to changes in structure.
Low Si-Cr-Mo gray cast iron is generally chosen for automotive disk or drum brakes. Composition and graphite content are closely controlled to maintain adequate high temperature strength, along with the graphite size and distribution that give adequate thermal conductivity and machinability. In general, decrease of Si content from 2.5 % to 1.5 % increases thermal conductivity by about 10 %. At the same time, the CE is to be kept relatively high to ensure solidification as gray cast iron rather than as white cast iron.
Although most alloying elements decrease the thermal conductivity of gray cast iron, their effect is not as great as that of Si. Gray cast irons alloyed with Cr and Mo are having better thermal conductivity than unalloyed gray cast irons of comparable Si and C contents. The alloy gray cast irons also have more stable structures and better stress rupture properties than the unalloyed cast irons.