White Cast Iron

White Cast Iron

The term cast iron refers to those iron (Fe)-carbon (C)-silicon (Si) alloys which contain 1.8 % to 4 % C and normally 0.5 % to 3 % Si. Cast iron is an important engineering material with a number of advantages, mainly good castability and machinability and moderate mechanical properties. Cast irons can be divided into different groups, based on composition and metallurgical structure namely (i) gray cast iron, (ii) ductile cast iron, (iii) white cast iron, (iv) malleable cast iron, (v) compacted graphite iron and, and (vi) alloy cast iron.

The transition from gray cast iron to white cast iron arises from the nucleation and growth competition between the stable graphite (gray) and meta-stable cementite (white) eutectics. The transition from graphite to cementite eutectic during solidification is also related to the so-called ‘chill’ of cast iron.

White cast irons have traditionally been used for wear resistance applications. The carbides in the micro-structure depending on their type, morphology, and volume fraction, provide the hardness needed for crushing materials without degradation. The supporting matrix structure can be controlled by alloying elements and / or heat treatment for developing pearlitic, bainitic, austenitic, or martensitic structures to provide the most cost-effective balance between abrasive wear resistance and toughness.

White cast irons are hypo-eutectic alloys in which the C remains dissolved in the carbide phases without decomposing into graphite during solidification. Because of the hard carbides, they have high abrasion resistance needed for the applications in mining, milling, earth-handling, and manufacturing industries. In order to improve further the wear resistance, these cast irons are normally alloyed with strong carbide forming elements such as tungsten (W), manganese (Mn), molybdenum (Mo), and chromium (Cr) etc.

White cast iron contains 1.8 % C to 3.6 % C, 0.5 % Si to 1.9 % Si and 1 % Mn to 2 % Mn. A fast-cooling rate prevents the precipitation of C as graphite. Instead, the C, which is in solution in the liquid iron, forms iron carbide (Fe3C), also called cementite. White cast iron does not have the easy castability associated with other cast irons since its solidification temperature is normally higher, and it solidifies with C in its combined form as iron carbide. By chilling gray or ductile irons on the outside and letting it cool slowly inside, it is possible to produce parts with a hard surface of white cast iron with a ductile core (chilled cast).

White cast iron is the only member of cast iron family in which C is present only as carbide. Because of the absence of graphite, it has a light appearance. The presence of different carbides makes white cast irons extremely hard and abrasion resistant, but also very brittle. The micro-structure of white cast iron contains massive cementite (white) and pearlite. White cast iron derives its name from the white, crystalline crack surface observed when a casting fractures. Majority of the white cast irons contain less than 4.3 % C, with low Si contents for preventing the precipitation of C as graphite. White cast iron cannot be machined easily.

White cast irons dates back to 1917, when the first patent was filed for a high chromium white cast iron. This cast iron has been extensively used in the mining and mineral processing industries and has contained 25 % to 30 % Cr, 1.5 % to 3 % C, and 0 % to 3 % Si in a Fe-base. During 1920-1930, alloyed white cast irons have been developed for getting improved mechanical properties like ultimate tensile strength, yield strength, and impact strength etc. In the 1960s Abex Corporation (USA) undertook an exhaustive series of alloying experiments for developing optimum abrasion resistance in high Cr white cast irons. The corporation investigated Ni (nickel), Mo, Mn, Si and Cr as alloying elements and developed a patented alloy ‘Paraboly’. However, original high Cr white cast iron has been widely used

White cast iron has a high compressive strength and alloyed varieties have a good retention of strength and hardness at higher temperatures. Because of its large masses of carbides, especially when alloyed, white cast iron has very good resistance against wear and abrasion. It is used for shot-blasting and extrusion nozzles, certain types of drawing dies, rolling mill rolls, crushers, pulverizers, ball mill liners and handling of abrasive materials such as minerals and ores, both dry and as slurries.

White cast iron is normally considered not weldable. The absence of any ductility which can accommodate welding-induced stresses in the base metal and heat affected zone adjacent (HAZ) to the weld, results in cracks during cooling after welding.

White cast iron can be divided into three classes namely (i) normal white cast iron which contains C, Si, Mn, P (phosphorus) and S (sulphur) with no other alloying elements, (ii) low-alloy white cast iron in which the total mass fraction of alloying elements is less than 5 %, and (iii) high-alloy white cast iron in which the total mass fraction of alloying elements is more than 5 %. These three classes of white cast irons have similar crystallization rules and structures. The as-cast structures contain a large quantity of carbides. These irons are wear-resistant because of their high hardness and find wide applications for abrasion-resistant components.

ASTM (American Society for Testing and Materials) specifications A532 covers the composition and hardness of white cast iron grades used for abrasion-resistance applications. Several castings are ordered as per these specifications. However, a large number of castings are produced with modifications to composition for specific applications. The chemical compositions have several classes of white cast iron alloys, where Class I are the Ni – Cr white cast irons, while Class II and Class III are high Cr white cast iron alloys.

High-alloy white cast irons conforming to ASTM A532 standard can be classified in three main classes. These are Ni-Cr, Cr-Mo, and high Cr white cast irons. Ni-Cr white cast irons contain 3.3 % Ni to 5 % Ni and 1 % Cr to 11 % Cr, Cr-Mo cast irons contain up to 3 % Mo and 12 % Cr to 23 % Cr, and high chromium white cast irons contain 23 % Cr to 30 % Cr. However, it is a common practice to name hypo-eutectic cast iron alloys based on the ternary Fe-Cr-C system with compositions between 11 % Cr to 30 % Cr and 1.8 % C to 3.6 % C as high Cr white cast irons.

The International Organization for Standardization (ISO) standard ISO 21988 defines the grades of abrasion-resistant white cast irons. It specifies the grades in terms of chemical composition and hardness. The types of abrasion-resistant white cast irons covered are unalloyed or low alloy white cast irons, Ni-Cr white cast irons, and high Cr white cast irons.

The European standard EN 12513 defines the grades of abrasion resistant white cast irons. It specifies the grades in terms of chemical composition, and hardness. The types of abrasion resistant white cast irons covered by this standard are (i) unalloyed or low alloy cast irons, (ii) Ni-Cr white cast irons covering two general types namely 4 % Ni – 2 % Cr white cast irons, and 9 % Cr -5 % Ni white cast irons, and (iii) high Cr white cast irons covering four ranges of chromium content namely 11 % or less Cr, 14 % or less Cr, 18 % or less Cr, and 23 % or less Cr.

Normal white cast iron – Normal white cast iron is the oldest type of cast iron, produced especially for resistance against abrasion. It has been in production for centuries. This family of cast irons has several names, and foundry-men refers to these irons as pearlitic, indefinite chill iron, mottled white iron, chill iron, or simply white iron. In all cases, chilling the iron against a cold conductive surface and / or material forms the abrasion-resistant carbide network. Part of the carbon is in the form of graphite and another part is in the form of carbides. It is used in the production of items operating under conditions of dry friction (brake shoes) and wear-resistant parts such as rollers, paper-making and flour-milling shafts etc.

Normal white cast irons are unalloyed cast iron with low C and Si content such that the structure is hard brittle iron carbide with no free graphite. These irons are limited in applications because of their lack of impact resistance and the difficulty in maintaining the structure in thicker sections. In some cases, the castings are designed and produced to have a white structure in certain areas and a gray or flake structure elsewhere to improve toughness. Normal white cast iron, without any alloying elements, is used mainly in the industry for the applications of (i) abrasion resistant components without especially high wear-resistant casting requirements, and (ii) white cast iron for the production of malleable castings.

The composition characteristics for abrasion resistant components are high C and low Si contents, so as to increase the quantity of carbides for improving the wear resistance. However, the chemical composition for white cast iron for making malleable iron castings contain higher Si and lower C content for accelerating graphitization during the annealing process and improving the morphology of the resulted graphite.

The formation of carbides as opposed to graphite is brought by the ambivalent nature of the Fe-C system, where the C can be precipitated either as graphite (stable phase) or as Fe3C (meta-stable phase). The former results in the well-known family of gray, nodular, compacted graphite, or malleable irons and the latter in low-alloy, abrasion-resistant irons or white irons. The meta-stable reaction in the Fe-C system occurs because of the rapid cooling and typically needs that the liquid iron is solidified against a cold, conducting surface or ‘chill’. As the iron rapidly cools, it undergoes a eutectic solidification reaction, forming gamma-iron and Fe3C and / or alpha-iron. Another name for this eutectic mixture of gamma-iron + Fe3C is ledeburite. The micro-structure of white cast iron consists of pearlite and ledeburite, a eutectic of pearlite, converted from austenite, and cementite. Fig 1 shows micro-structure of white cast iron.

Fig 1 Micro-structure of white cast iron

Low-alloy white cast iron – Low-alloy white cast iron occurs when alloying element(s) are deliberately added, but their total mass fraction is less than 5 %. The functions of alloying elements are to increase the micro-hardness of carbides, strengthen the matrix, and further improve the wear resistance. Alloying elements normally used include Cr, Ni, Mo, Cu (copper), V (vanadium), Ti (titanium), and B (boron). Normally, for low-alloy white cast iron, the Si content is lower (normally Si = 0.4 – 1.2 %) for ensuring that a ‘white’ structure is achieved. In this case, the range of C content is wider and is normally C = 2.4 – 3.6 %. Low-alloy white cast irons are used mainly for abrasion resistant castings.

There are different metallographic morphologies of eutectic cementite which can be observed in hypo-eutectic low-alloy white cast irons, these are (i) ledeburite, (ii) network-like, and (iii) plate-like. The plate-like modification normally forms under large under-cooling conditions or with high contents of P, Si, Te (tellurium), Sb (antimony), and rare-earth elements (RAE). Hence, the ledeburite and net-work-like modifications are the ones most generally observed in morphologies of low-alloy white cast irons. Ledeburite and net-work-like modifications are produced in the form of massive and continuous carbides (playing the role of matrix) which can break-up since dispersed particles help in the tightening of these cast iron alloys.

High-alloy white cast iron – The high alloy white cast irons are mainly used for abrasion resistant applications and are readily cast in the shape needed in equipments used for crushing, grinding, and general handling of the abrasive materials. The presence of M7C3 eutectic carbides in the micro-structure provides high hardness needed for abrasion resistant applications. The metallic matrix supporting the carbide phase in these cast irons can be adjusted by the alloy content and heat treatment for developing the proper balance between resistance to abrasion and toughness.

High alloy white cast irons have typical compositions of 15 % Cr – 3 % Mo and 23 % Cr to 28 % Cr and have a superior combination of abrasion resistance and toughness. In some cases, these cast irons can be used as cast, but are normally hardened for developing the optimum properties. Some of these cast irons can also be machined after annealing and then hardened to produce a machined abrasion resistant part.

All high alloy white irons contain Cr for preventing the formation of graphite on solidification, stabilize the carbide, and to form chromium carbide (Cr3C2) which is harder than iron carbide (Fe3C). Majority of these irons also contain Ni, Mo, Cu, or combinations of these alloying elements in order to prevent the formation of pearlite in the micro-structure. The Cr-Mo white cast irons (Class II of ASTM A532) contain 11 % Cr to 23 % Cr and up to 3 % Mo and some Ni and Cu and can be produced either as-cast with an austenitic or austenitic-martensitic matrix, or heat treated with a martensitic matrix micro-structure for maximum abrasion resistance and toughness. These cast irons provide the best combination of toughness and abrasion resistance of all white cast irons and are used in hard rock mining equipment, slurry pumps, coal grinding mills, and brick moulds.

It is most desirable that the designer, metallurgist, and foundry-operator cooperate for specifying the composition, heat treatments, and foundry practice for developing the most suitable cast iron alloy and casting design for a specific application. As per the type of alloying elements used, high-alloyed white cast iron can be sub-divided into three types namely (i) the Ni-Cr white cast irons, (ii) the Cr-Mo white cast irons, and (iii) the high Cr white cast irons.

Ni-Cr white cast irons – These white cast irons contain both Ni and Cr. The Ni-Cr white cast irons are low-Cr alloy cast irons which contain 3 % Ni to 5 % Ni and 1 % Cr to 4 % Cr, with one alloy modification which contains 7 % Cr to 11 % Cr. The trade name is Ni-Hard types 1 to 4 which normally identifies them. Cr at lower concentrations (less than 2 % Cr to 3 % Cr) has little or no effect on hardenability, since majority of Cr is tied up in the carbides.

Ni-Cr white cast irons are also known as martensitic white cast irons and these are used in large quantities in mining operations, such as ball mill liners and grinding balls. Ni is the primary alloying element since at levels of 3 % to 5 %, it is effective in suppressing the transformation of the austenitic matrix to pearlite, hence ensuring that a hard martensitic structure (normally containing considerable quantities of retained austenite) develops upon cooling in the mould. Cr is added in these cast iron alloys, at levels from 1.4 % to 4 %, for ensuring that the liquid iron solidifies as carbidic (M3C-type), for counter-acting the graphitization effect of Ni.

Abrasion resistant structures containing eutectic mixtures of austenite and carbides can be achieved in thin and thick section sizes independent of the use of chills. It is possible to get traces of graphite in thicker sections or when higher levels of C and Si are used. Barring these conditions, the dominant micro-structure of Ni-Hard white cast irons is one composed of a ferrous matrix surrounded by hard metal carbides.

The presence of 3 % Ni to 5 % Ni allows pro-eutectic austenite to reach the martensite start (Ms) temperature unhindered by the formation of pearlite. No transformation is perfect and as-cast Ni-Hard cast iron micro-structure contains a mixture of austenite and martensite. If the casting is of variable thickness, then thicker sections can contain traces of pearlite. Hence, it is obvious that it is quite difficult to make predictions about the wear performance of the casting, which is based on initial chemistry, with little or no knowledge about dimension, or thermal history.

For applications needing a high degree of strength, hardness, and wear resistance, Ni-Hard cast irons are among the effective materials available. Ni-Hard iron castings have shown outstanding in variety of severe applications including work rolls for hot steel rolling mills. High Cr white cast irons and high-speed steel type alloy are also widely used in steel plant, and Ni-Hard iron is normally used in finishing stands. The optimum composition of Ni-Cr white cast iron alloy depends on the mechanical properties needed for the service conditions and the dimensions and weight of the casting. The Ni-Cr white cast irons have proven to be very cost-effective materials and are also used for crushing and grinding equipments.

The predominant characteristics of Ni-Hard irons are that their high strength and toughness can be achieved when heat-treated at relatively low temperatures. Low temperatures for heat treatment are favourable for large castings which are not suitable for heat treatment at high temperatures and are prone to cracking. Of all the abrasion resistant cast irons, Ni-Hard irons are produced in large tonnage for a variety of mineral processing industries. The low cost of Ni-Hard iron is because of its low alloy content, its ability to be cast into a variety of shapes, and its high hardness in the as-cast condition. Its high hardness is what clearly separates it from pearlitic abrasion resistant cast irons. High hardness results from the formation of martensite against pearlite in the as-cast condition. This metallurgical shift is the result of the high Ni content in the Ni-Hard iron.

In Class I Type A, the castings need maximum abrasion resistance in applications such as ash-pipes, slurry pumps, roll heads, muller tires, coke crusher segments, and classifiers etc. Type B is desired for applications needing more strength and exerting moderate impacts, such as crusher plates, crusher concaves, and pulverizing pegs. Class I Type D, Ni-Hard Type 4, has a higher level of strength and toughness and is hence used for the more severe applications which justify its added alloying costs. It is normally used for pumps volutes handling abrasive slurries, coal pulverizer table segment, and tires.

The Class I Type C alloy (Ni-Hard 3) is specially designed for the production of grinding balls. This grade is both sand cast and chill cast, chill casting has the advantage of lower alloying cost, more important, provides a 15 % to 30 % improvement for 8 hours at 260 deg C to 315 deg C. There are two general types containing 4 % Ni – 2 % Cr, and 6 % Ni – 8 % Cr. Both have a structure of iron and Cr-carbides in a matrix of martensite and bainite, but the higher alloy content materials have a type of carbide which is dis-continuous and confers higher impact and corrosion resistance, i.e., M7C3 type of carbide. These cast irons can be used as cast, but heat treatment improves the hardness and resistance to surface cracking and spalling.

Cr-Mo white cast irons – These white cast irons are for abrasion resistance application. The Cr-Mo white cast irons (Class II of ASTM A532) contain 11 % Cr to 23 % Cr, and up to 3 % Mo and are frequently alloyed with Ni or Cu. These cast irons can be produced either as cast with an austenitic or austenitic-martensitic matrix, or heat-treated with a martensitic matrix micro-structure for maximum abrasion resistance and toughness. These cast irons are normally considered the hardest of all grades of white cast iron. Compared to the low-alloy Ni-Cr white cast irons, the eutectic carbides are harder and can be heat treated for achieving castings of higher hardness. Mo, as well as Ni and Cu when needed, is added for preventing pearlite and to ensure maximum hardness.

High Cr white cast irons – High Cr white cast irons represent a class of materials normally used for materials handling, crushing and grinding applications in the minerals and mining industries because of their very good wear resistance. After heat treatment, high Cr white cast irons micro-structures are comprised of eutectic carbides and a metallic matrix comprised of secondary chromium carbides, martensite, austenite and in some cases ferrite. During solidification, the eutectic carbides are formed, and they do not undergo a further transformation, but the matrix can be altered. Micro-structures in the matrix are affected by three factors namely thermal cycle, chemical composition, and the initial state of alloy (as-cast or annealed). This, in turn, affects the hardness and wear resistance of the material. In general, at the end of the solidification process, the micro-structure of high-Cr white cast iron is composed of a primary phase (austenite dendrite) and a eutectic compound (M7C3).

During the solidification of high Cr white cast irons, primary austenite dendrites form, followed by a eutectic mixture of austenite and M7C3 carbides or one of its transformation products. The high quantity of Cr in these alloys favour the formation of carbides (type M7C3 in between 9.5 % Cr to 15 % Cr and M23C6 above 30 % Cr) and a pearlitic matrix in the absence of alloying additions. Ni, Cu, and Mn are normally added for improving the hardenability and prevent pearlite formation. Likewise, Mo is added for increasing the hardenability, but it also leads to the formation of other hard carbides apart from the M7C3. The quantity, type, size, shape and distribution of these carbides determine abrasion resistance of the white cast iron.

High Cr white cast irons undergo several solidification reactions and a number of different solid-state transformation reactions on cooling to room temperature, and during reheating to higher temperature below solidus temperatures. Hence, a number of different phases form in high Cr white cast irons which have influence on the mechanical properties and service life of the material. The cast irons under this heading have the highest Cr-content within the high-alloy white cast iron family. High Cr gives these irons good wear resistance, corrosion resistance, impact toughness, and hardenability. The resistance to corrosion and abrasive wear, and wear at higher temperature are also remarkably improved. Class I and Class II of high Cr white cast irons are superior in abrasion resistance and are used effectively in impellers, volutes, impeller blade, liners for short blasting equipment, and refiner disks in pulp refiners.

The high chromium white cast irons (Class III of ASTM A532), are normally called 25 % Cr cast irons, containing 23 % to 28 % Cr with up to 1.5 % Mo. For preventing pearlite and attaining maximum hardness, Mo is added in all but the lightest-cast sections. Alloying with Ni and Cu up to 1% is also practiced. Although the maximum attainable hardness is not as high as in the Class II Cr-Mo white cast irons, these cast iron alloys are selected when resistance to corrosion is needed. In several applications, they withstand heavy-impact loading, such as impact hammers, roller segments and ring segments in coal grinding mills, feed-end lifter bars and mill liners in ball mills for hard-rock mining, pulverizer rolls, and rolling mill rolls.

In an acidic medium, white cast iron with 28 % Cr has much better wear resistance and high temperature oxidation resistance than a white cast iron with 15 % Cr. The C content of this white cast iron can vary between 2 % to 3.3 %. Increasing the Cr content and reducing the C content can improve its corrosion and abrasion resistance. Cr26 high Cr white cast iron castings are used mainly after quenching and tempering, but can also be used as-cast. White cast Irons for corrosion resistance are alloys with improved resistance to corrosion, for applications such as pumps for handling fly ash, are produced with higher Cr content (26 % to 28 %) and low C-content (1.6 % to 2 %). These white cast irons provide the maximum Cr content in the matrix. The addition of 2 % Mo is desired for improving resistance to chloride containing environment. Full austenitic matrix structures provide the best resistance to corrosion, but some reduction in abrasion resistance is to be expected. Castings are normally supplied in the as-cast conditions.

Because of castability and cost, high Cr white cast iron castings are frequently being used for complex and intricate parts in high-temperature applications at considerable savings compared to stainless steel. These cast irons grades are alloyed with 12 % Cr to 39 % Cr at temperatures up to 1,040 deg C for scaling resistance. Cr causes the formation of an adherent, complex, Cr rich oxide film at high temperatures. The high Cr white cast irons designated for use at higher temperatures fall into one of three categories, depending on the matrix structures. These are (i) martensitic cast irons alloyed with 12 % Cr to 28 % Cr, (ii) ferritic cast irons alloyed with 30 % Cr to 34 % Cr, and (ii) austenitic cast irons which contain 15 % to 30 % Cr as well as 10 % Ni to 15 % Ni for stabilizing the austenite phase.

The C content of high Cr white cast iron alloys range from 1 % to 2 %. The choice of an exact composition is critical for the prevention of sigma-phase formation at intermediate temperatures and at the same time avoiding the ferrite-to-austenite transformation during thermal cycling, which leads to distortion and cracking. Typical applications include recuperator tubes, breaker bars and trays in sintering furnaces, grates, burner nozzles, and other furnace parts, glass moulds, and valves seats for combustion engines.

Mechanical properties and applications of high Cr white cast iron – The tensile strength of pearlitic white irons normally ranges from around 205 MPa for high-C grades to around 415 MPa for low C grades. The tensile strength of martensitic white cast irons with M3C carbides ranges from around 345 MPa to 415 MPa, while high Cr cast irons, with their M7C3 type carbides, normally have tensile strengths of 415 MPa to 550 MPa. Limited data indicate that the yield strengths of white cast irons are around 90 % of their tensile strengths. These data are extremely sensitive to variations in sample alignment during testing. 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 GPa to 195 GPa, irrespective of whether it is pearlitic or martensitic, while a cast iron with M7C3 eutectic carbides has a modulus of 205 GPa to 220 GPa.

Micro-structural characteristics of high Cr white cast iron – High Cr white cast irons are ferrous alloys containing chromium between 12 % and 30 %. The micro-structure of all hypo-eutectic high Cr white iron alloys in the as cast condition consists of a network of M7C3 eutectic carbides in a matrix of austenite dendrites or its transformation products. The type, quantity, and morphology of these eutectic carbides control wear resistance and toughness. There are four kinds of iron carbide precipitate in high Cr white cast iron as per the contents of C and Cr. These are (Fe, C)3C, (Fe, Cr)7C3, (Fe, Cr)23C6 and (Fe, Cr)3C2. Other carbide forming elements e.g., Mo, V, and Mn are soluble in both M3C and M7C3 and can also give rise to other hard carbides e.g., M2C and M6C. Fig 2 shows the micro-structure of high Cr alloyed white cast iron in the as-cast condition. The matrix structures can be pearlite, austenite, and martensite or some combination of these. The matrix in alloy white cast irons in the as-cast condition is mainly austenite. Austenitic structures are favoured by faster cooling rates, high Cr/C ratio, and Ni and Mo.

Fig 2 High Cr white cast iron micro-structures in as cast condition

Martensitic structures can be achieved in as-cast, especially in heavy section castings which cool slowly in the mould. With slow cooling rates, austenite stabilization is incomplete and partial transformation to martensite occurs. However, in these cast iron alloys, martensite is mixed with large quantities of retained austenite as shown in Fig 2b. For getting maximum hardness and abrasion resistance, full martensitic matrix structures are to be produced by full heat treatment. Martensitic matrix micro-structures with secondary carbides of heat-treated high Cr white irons are shown in Fig 3. All as-cast structures contain patches of secondary carbides. These secondary carbides are precipitated during cooling in the baked sand moulds and results in a local decrease of C content of the matrix. Hence, small quantities of martensite are present in the pre-dominantly austenitic structures of the as-cast irons

Fig 3 Micro-structure of heat-treated martensitic matrix showing fine secondary M7C3 carbides

Abrasive wear behaviour of high Cr white cast iron – In the abrasion wear process, the hard body can be fractured, and the softer surface is cracked and / or deformed, and the material is removed from the surface resulting in a measurable volume loss. The variations in wear resistance of the alloys are mainly caused by the changes in hardness resulting from the introduction of fine carbides and micro-structure refinement. The hardness of the abrasive materials has a very strong influence on the relative abrasion rates.

For a harder abrasive material, the abrasion resistance increases with the material toughness, and for a softer abrasive material, the abrasion resistance increases with the material hardness. For a softer abrasive material, the abrasion resistance increases with carbide volume fraction but decreases considerably with a hard abrasive material. The as-cast structure of high Cr white cast irons containing 1.7 % Mo with mostly austenitic matrix achieve the hardness of 38 HRC (hardness Rockwell C scale) to 45 HRC. The abrasion resistance increases by increasing the C content, and the impact toughness impairs by higher C levels. The retained austenite contributes to work hardening under field applications.

Hardness of the materials increases with increasing the carbide volume fraction for the austenitic and martensitic structures. The abrasive wear loss decreases to a minimum with increasing the carbide volume fraction up to around 30 %. Above 30 % volume fraction of carbides, the abrasive wear loss increases with increasing in carbide volume fraction. The total percentage of carbides in the structure as a function of the C and Cr content can be estimated by the equation ‘total % carbides = 12.3 × % C + 0.55 × % Cr – 15.2.

The carbide morphology has a pronounced influence on the wear and fracture behaviour of high Cr white cast irons. Hyper-eutectic alloys are characterized by the presence of hexagonal and primary carbides. For the high Cr white cast irons, the hardness provides a good indication of the volume wear rate. There is a poor correlation between high stress abrasion weight loss and material hardness for matrix produced by changes in the austenitizing temperature.

Effect of chemical composition on micro-structure and mechanical properties – The type and form of eutectic carbides change from M3C to M7C3 with increasing Cr content. In unalloyed or low Cr white cast irons containing below 5 % Cr and up to 2 % C, the eutectic carbides of the M3C type in a continuous ledeburitic form (with hardness of Vickers hardness HV = 1,000) are present. At 8 % to 10 % Cr, the eutectic carbides become less continuous and fracture resistance is improved. But above 10 % Cr, the form of eutectic carbides changes to the discontinuous M7C3 type (with hardness of HV = 1,200 to 1,800).

Cr addition strongly favours the formation of carbides during liquid iron solidification and pearlitic matrix formation during the eutectoid transformation specially in the absence of alloying additions. In the range from 9.5 % to 15 % of Cr content, carbides type (Cr, Fe)7C3 appears, and above 30 % Cr carbides type (Cr, Fe)23C6 solidifies. Considering the balance between the wear resistance and toughness, the quantity of eutectic carbides is to be controlled with around 30 % to 35 % volume fraction. The favourable volume fraction of the eutectic carbides in low alloy white cast irons for impact abrasion is to be around 10 %. The addition of hard inoculants as mixture of Ti-B-W to a high Cr white cast iron results in a structure containing hard carbide particulates distributed homogeneously in micro-structure.

The eutectic M6C, rich in Mo with a ‘fish-bone’ structure, is present in the white cast irons with 24 % to 28 % Cr and 9 % Mo, together with typical eutectic M7C3. At up to 10 % Mo and 20 % Cr (Cr/C ratio of around 7), Mo2C is observed as a finely dispersed eutectic in the final stage of solidification. The additions of RAE, V, Ti, and B into high Cr cast iron containing 3 % Mo can refine the micro-structure, reduce coarse columnar crystals, and make the carbide smaller and uniform.

Niobium (Nb) addition to high Cr white cast iron increases its wear resistance since Nb forms very hard carbides (NbC) of hardness around HV 2,400 and Nb dissolves in the matrix and increases the matrix micro-hardness from HV 693 at 0 % Nb to HV 899 at 3.47 % Nb. The addition of small quantity of B modifies the morphology of eutectic carbides. Hence, B addition results into an increase in the abrasion wear resistance.

Effect of heat treatment on micro-structure and mechanical properties – The role of the matrix surrounding the eutectic carbides is for providing a sufficient mechanical support for preventing their cracking, deformation, and spalling. Normally, the matrix can be extensively altered through the destabilization and sub-critical heat treatments. The main objective of the destabilization heat treatment is the precipitation of secondary carbides and transformation of the primary austenitic matrix to martensite.

The destabilization heat treatments are normally done in the range of 800 deg C to 1,100 deg C, for 1 hour to 6 hours. The matrix in the micro-structure of high Cr white cast iron after destabilization treatment is normally composed of martensite and retained austenite. Destabilization heat treatment of high Cr white cast irons normally consists of heating to a high temperature or austenitizing, air quenching, and tempering. The successful heat treatment produces austenite destabilization by precipitation of fine secondary M7C3 carbides within the austenite matrix. Class II high Cr white cast irons containing 12 % to 23 % Cr are austenitized in the temperature range 950 deg C to 1,010 deg C. It is necessary for ensuring that the soaking time is sufficient to ensure a complete secondary carbide precipitation. Air quenching of the castings from the austenitizing temperature to below the pearlite temperature range is highly desired.

Effect of destabilization heat treatment – High Cr white cast irons is to be heat treated for developing the full hardness and maximum wear resistance. The destabilization (hardening) heat treatment is used with high Cr and Cr-Mo white cast irons for transforming the as-cast austenitic or pearlitic matrix structures to martensite. Better wear resistance is achieved for the alloyed heat-treated cast iron compared with that of the as-cast and unalloyed irons. The responsible for such better wear behaviour is the strengthening of the matrix by the MC carbides plus the precipitation of secondary M7C3 carbides and the partial transformation of matrix from austenite to martensite after heat treatment.

As the austenitizing temperatures increases, the quantity of C dissolved in the austenite increases. Decreasing the austenitizing temperature from the optimum gives a lower hardness because of the lower C martensite formed on quenching. This temperature normally lies in the range 950 deg C to 1,050 deg C and depends on the particular composition of the high Cr white cast iron being treated. The morphologies of secondary carbides, the crystallographic properties of the phases, and the proper combination of the quantity of martensite, retained austenite, and carbides are the principal parameters which affect the hardness and wear behaviour of the high Cr white cast iron.

During the destabilization treatments at different temperatures ranging from 900 deg C to 1,000 deg C, it is seen that with increases in the treatment temperature, the precipitated carbides become coarser and less abundant and the quantity of retained austenite increases. The martensitic matrix reduces wear of matrix and it in turn reduces the carbide fracture. The secondary carbides also influence the wear behaviour. They strengthen the martensitic matrix which in turn increases the mechanical support to the carbide phase. Dynamic fracture toughness of white cast irons in both as cast and heat-treated conditions is determined mainly by the properties of the matrix. Hence, the austenite is more effective in this respect than martensite. The volume fraction of the eutectic carbide phase (M3C or M7C3) has an important influence on the wear resistance of white cast iron under low stress abrasion conditions.

The higher is the temperature of heat treatment the higher is the volume of precipitated secondary phases and transformed martensite. With increasing austenitizing temperature, the secondary carbides in high Cr white cast iron become coarser, and the quantity of retained austenite in the dendritic areas increases. The optimal heat treatment process is 2 hours quenching treatment at 1,000 deg, followed by a subsequent 2 hours tempering at 400 deg C. The precipitation of secondary carbides during austenitizing leave the matrix depleted in C and Cr, hence raising the Ms temperature above room temperature and subsequent cooling results in the transformation of matrix to martensite. The holding time at the destabilization temperature appears to affect the retained austenite content. The short holding time with the appropriate destabilization temperature can only be needed for achieving a hardened structure with the maximum hardness.

Effect of sub-critical heat treatment – Sub-critical heat treatment (tempering) is sometimes done, particularly in large heat-treated martensitic white cast iron castings, for reducing retained austenite contents and increasing the resistance to spalling. Typical tempering temperatures range from 480 deg C to 550 deg C and time ranges from 8 hours to 12 hours. Excess time or temperature results in softening and a drastic reduction in abrasion resistance. Insufficient tempering results in incomplete elimination of austenite.

The treatments at 600 deg C on the destabilized material produces a gradual drop in the hardness because of the decomposition of martensite to a ferrite / carbide. The effect of sub-critical heat-treatment parameters on hardness and retained austenite in Mo-containing high Cr cast iron shows that the hardness decreases gradually but the gamma phase volume increases greatly as Mo content increases in both 16 % Cr and 26 % Cr white cast irons in the as cast state. In the case of sub-critical heat treatment, the hardness increases first and then decreases with an increase in holding time. This phenomenon is because of a hardening caused by the precipitation of secondary carbides and by martensite transformation from the destabilized austenite during cooling.

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