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.
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 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.
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 BH points. Following sub-zero treatment, the castings are almost always tempered at deg C to 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 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.
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. 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 deg C to 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.
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 deg C and deg C. The rate of precipitation depends on both time and temperature. As the austenite cools further, through the range of deg C to deg C, it tends to transform to pearlite. 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 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.
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. 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 HB for the low C grade and HB for the high C grade. 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 HV to HV Vickers hardness depending on retained austenite content. 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 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. 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 to GPa, irrespective of whether it is pearlitic or martensitic, while a cast iron with M7C3 eutectic carbides has a modulus of to GPa. The density of white cast irons ranges from 7.
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. 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.
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 The high Si cast irons are also very resistant to nitric acid. Increasing the Si content to The 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. 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. Not to be confused with drilling, it When researching some of the different ways rubber parts are made, you may come across compression We use cookies to improve your experience.
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ASM International. Publication date:. Book Chapter. Gundlach Richard B. Climax Research Services. Douglas V. The correspondence between commercial and microstructural classification, as well as the final processing stage in obtaining common cast irons, is given in Fig. A classification of the main types of special cast irons is shown in Fig. Basic microstructures and processing for obtaining common commercial cast irons. Enter a phrase to search for:. Search by.
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