International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 14 (2017) pp. 4675-4686 © Research India Publications. http://www.ripublication.com Microstructural Characteristics and Mechanical Properties of Heat Treated High-Cr White Cast Iron Alloys

Kh. Abdel-Aziz 1a, M. El-Shennawy 1b* and Adel A. Omar 1c

1 Taif University, Engineering College, Mechanical Engineering Department, Taif, Post Code 888, KSA. a On leave from Zagazig University, Faculty of Engineering, Materials Engineering Department, Zagazig, Egypt. b On leave from Helwan University, Faculty of Engineering, Mechanical Engineering Department, Helwan, Egypt. c On leave from Benha University, Faculty of Engineering, Mechanical Engineering Department, Benha, Egypt.

Abstract or combinations of these alloying elements in order to prevent the formation of pearlite in the microstructure [1,2]. The High Cr white cast irons are an important class of wear chromium-molybdenum white irons (class II of ASTM A532) resistant materials currently used in a variety of applications contain 11 to 23% Cr and up to 3% Mo and some Ni and that requires high wear resistance and reasonable toughness. Cu and can be supplied; either as-cast with an austenitic or The outstanding performance of these alloys is due to the austenitic-martensitic matrix, or heat treated with a presence of large amounts of chromium carbides which martensitic matrix microstructure for maximum abrasion exhibit high hardness. The size, type and morphology of these resistance and toughness. These irons provide the best carbides control the wear resistance and toughness. The combination of toughness and abrasion resistance of all white microstructural characteristics of these alloys, and irons and are used in hard rock mining equipment, slurry consequently their wear resistance, can be extensively pumps, coal grinding mills, and brick molds [3]. changed by varying the chemical composition or the solidification rate or by specific heat treatment after casting. Heat treatments for all high-Cr WCI alloys are essential to MECHANICAL PROPERTIES AND APPLICATIONS change their microstructure and therefore, to improve their OF HIGH CR IRON ALLOYS wear resistance to suit the individual application requirements. Changing in chemical composition and heat treatment carried The tensile strength of pearlitic white irons normally ranges out to this alloy related to microstructural characteristics and from about 205 MPa for high-carbon grades to about 415 MPa mechanical properties of high Cr white cast iron alloys are for low carbon grades. The tensile strength of martensitic oresented. irons with M3C carbides ranges from about 345 to 415 MPa, while high chromium irons, with their M C type carbides, Keywords: High-Cr WCI alloys, microstructural 7 3 usually have tensile strengths of 415 to 550 MPa [4]. Limited characterization, abrasive wear resistance, alloying additions, data indicate that the yield strengths of white irons are about heat treatment, mechanical properties. 90% of their tensile strengths. These data are extremely sensitive to variations in specimen alignment during testing. The elastic modulus of a white iron is considerably influenced INTRODUCTION by its carbide structure [4]. An iron with M3C eutectic The high alloy white irons are primarily used for abrasion carbides has a tensile modulus of 165 to 195 GPa, irrespective resistant applications and are readily cast in the shape needed of whether it is pearlitic or martensitic, while an iron with in machinery used for crushing, grinding and general handling M7C3 eutectic carbides has a modulus of 205 to 220 GPa. of the abrasive materials. The presence of M C eutectic 7 3 carbides in the microstructure provides high hardness needed for abrasion resistant applications. The metallic matrix MICROSTRUCTURAL CHARACTERISTICS OF HIGH supporting the carbide phase in these irons can be adjusted by CR WHITE CAST IRONS the alloy content and heat treatment to develop the proper As-cast austenitic and pearlitic microstructures balance between resistance to abrasion and toughness. All high alloy white irons contain chromium to prevent the High Cr white cast irons are ferrous alloys containing formation of graphite on solidification, stabilize the carbide chromium between 12 and 30%. It is well documented that the and to form chromium carbides which are harder than iron microstructure of all hypoeutectic high Cr white iron alloys in ones. Most of them also contain nickel, molybdenum, copper the as cast condition consists of a network of M7C3 eutectic

4675 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 14 (2017) pp. 4675-4686 © Research India Publications. http://www.ripublication.com carbides in a matrix of austenite dendrites or its hard carbides e.g. M2C and M6C [3]. Fig. 1 shows the transformation products [5-15]. The type, amount and microstructure of high Cr alloyed white cast iron in the as-cast morphology of these eutectic carbides control wear resistance state. The matrix structures can be pearlite, austenite, and and toughness [16]. There are four kinds of iron carbide martensite or some combination of these [19]. The matrix in precipitate in high alloyed white cast iron according to the alloy white cast irons in the as-cast state was primarily contents of carbon and chromium [17,18]; (Fe, C)3C, (Fe, austenite [20,21]. Austenitic structures are favored by [3]; Cr)7C3, (Fe, Cr)23C6 and (Fe, Cr)3C2. Other carbide forming faster cooling rates, high Cr/C ratio and nickel and elements e.g. molybdenum, vanadium and manganese are molybdenum. soluble in both M3C and M7C3 and may also give rise to other

(b) (a)

Figure 1: High chromium iron microstructures in the as-cast; (a) with austenitic matrix microstructure, (b) With austenitic- martensitic matrix microstructure. Both at 500X [19].

As-cast and heat treated martensitic microstructures be produced by full heat treatment. Martensitic matrix microstructures with secondary carbides of heat-treated high Martensitic structures can be obtained in as-cast, especially in Cr white irons are shown in Fig. 2 [19]. All as-cast structures heavy section castings which cool slowly in the mold. With contained patches of secondary carbides [6]. These secondary slow cooling rates, austenite stabilization is incomplete and carbides were precipitated during cooling in the baked sand partial transformation to martensite occurs. However, in these moulds and resulted in a local decrease of carbon content of alloys, martensite is mixed with large amounts of retained the matrix. Therefore, small amounts of martensite were austenite as shown in Fig. 1(b). To obtain maximum hardness present in the predominantly austenitic structures of the as- and abrasion resistance, full martensitic matrix structures must cast irons [6].

(a) (b)

Figure 2: Microstructure of heat treated martensitic matrix microstructure illustrating fine secondary M7C3 carbides for; (a) hypoeutectoid and (b) hypereutectoid high chromium irons. Both at 500X [19].

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ABRASIVE WEAR BEHAVIOR OF HIGH CR WHITE resistance increased by increasing the carbon content, and the CAST IRONS impact toughness impaired by higher carbon levels. The retained austenite contributed to work hardening under field In the abrasion wear process, the hard body may be fractured, applications [30,31]. Yongxin et al [32] reported that with the and the softer surface will be cracked and/or deformed, and increase of Cr content, fracture toughness of M B increases the material will be removed from the surface resulting in a 2 first and then decreases. measurable volume loss [2,6,22]. Bulk hardness measurements obtained for the modified HCCI alloys series Hardness of the materials increased with increasing the being studied are shown in Fig. 3. The variations in wear carbide volume fraction for the austenitic and martensitic resistance of the alloys are mainly caused by the changes in structures [25]. The abrasive wear loss decreased to a hardness resulting from the introduction of fine carbides and minimum with increasing the carbide volume fraction up to microstructure refinement [23]. The hardness of the abrasive about 30%. Above 30% volume fraction of carbides the materials has a very strong influence on relative abrasion rates abrasive wear loss increased with increasing in carbide [7]. For a harder abrasive material the abrasion resistance volume fraction. Fig. 4 shows the abrasive wear volume loss increased with the material toughness, and for a softer and hardness of the irons as a function of the volume of abrasive material the abrasion resistance increased with the massive carbides of predominantly austenitic and martensitic material hardness [24,25]. For a softer abrasive material the matrices [25]. The total percentage of carbides in the structure abrasion resistance increased with carbide volume fraction but as a function of the carbon and chromium content can be decreased significantly with a hard abrasive material [26,27]. estimated by the following equation [33]:

The as-cast structure of high Cr cast irons containing 1.7% Mo with mostly austenitic matrix achieved the hardness of 38- 45 HRC [28]. Asensio et al [29] stated that the abrasion Total % carbides = 12.3 × % C + 0.55 × % Cr – 15.2 (1)

Figure 3: Bulk hardness measurements obtained for the modified HCCI alloys series being studied [23].

The mean weight loss of the tested alloys increased with Cr white cast irons the hardness provided a good indication of increasing the volume fraction of the network eutectic the volume wear rate [35]. Sare and Arnold [36] stated that carbides [34]. The carbide morphology had a pronounced there was a poor correlation between high stress abrasion influence on the wear and fracture behavior of high chromium weight loss and material hardness for matrix produced by white irons [8]. Hypereutectic alloys are characterized by the changes in the austenitizing temperature. presence of hexagonal and primary carbides [8]. For the high

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(a (b) )

Figure 4: The abrasive wear volume loss and hardness as a function of volume of massive carbides; (a) austenitic irons, (b) martensitic irons (heat treated at 900 oC for 5 hours, air cooled) [25].

EFFECT OF CHEMICAL COMPOSITION ON toughness, the amount of eutectic carbides should be MICROSTRUCTURE AND MECHANICAL controlled with about 35% volume fraction [10], but PROPERTIES according to Gahr and Eldis [25], it should be controlled with about 30% volume fraction. But Qian and Chaochaang [34] In many previous studies [37-39], the type and form of reported that the favorable volume fraction of the eutectic eutectic carbides change from M C to M C with increasing 3 7 3 carbides in low alloy white cast irons for impact abrasion use Cr content. In unalloyed or low Cr white cast irons containing should be around 10%. The addition of hard inoculants as below 5% Cr and up to 2% C, the eutectic carbides of the mixture of Ti–B–W to a high Cr white cast iron resulted in a M C type in a continuous ledeburitic form (with hardness of 3 structure containing hard carbide particulates distributed HV= 1000) are present. At 8-10% Cr, the eutectic carbides homogeneously in microstructure [42]. The eutectic M C, rich become less continuous and fracture resistance is improved. 6 in Mo with a “fish-bone” structure, was present in the irons But above 10% Cr, the form of eutectic carbides changes to with 24-28 wt.% Cr and 3-9 wt.% Mo, together with typical the discontinuous M C type (with hardness of HV=1200- 7 3 eutectic M C [43]. At up to 10 wt.% Mo in 20 wt.% Cr (Cr/C 1800). Chromium addition strongly favours the formation of 7 3 ratio of about 7), Mo C was observed as a finely dispersed carbides during iron solidification and pearlitic matrix 2 eutectic in the final stage of solidification [43]. The additions formation during the eutectoid transformation specially in the of RE, V, Ti and B into high Cr cast iron containing 3 wt.% absence of alloying additions [40,41]. In the range from 9.5 to Mo can refine the microstructure, reduce coarse columnar 15% of chromium content, carbides type (Cr, Fe) C appears, 7 3 crystals and make the carbide smaller and uniform [44]. Fig. 5 and above 30% chromium carbides type (Cr, Fe) C solidifies 23 6 shows the impact toughness and weight loss curves of high Cr [38]. Considering the balance between the wear resistance and iron alloys with different contents of RE, respectively.

(b)

(a)

Figure 5(a) Impact toughness of the high chromium iron alloys with different contents of RE; (b) Weight loss curves for the high Cr iron alloys with different contents of RE in heat-treated conditions [44].

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Alloying element distributions in the matrix and the carbide of applications [18,48]. Their microstructures contain relatively high Cr cast iron modified RE, V, Ti, and B by the energy discontinuous networks of hard alloy eutectic carbides giving spectrum analysis are shown in Fig. 6 [44]. Imurai et al [45] improved wear resistance, fracture toughness and thermal studied the effects of Mo on microstructure of as-cast 28 wt.% fatigue resistance [49]. Fig. 7 outlines the chemical Cr-2.6 wt.% C-(0-10) wt.% Mo irons. They pointed out that compositions and the Cr/C ratios of Mo-containing, high Cr the iron with about 10 wt.% Mo was eutectic/peritectic, cast irons investigated in previous studies, plotted on the whereas the others with less Mo content were hypoeutectic. liquidus surface developed by Jackson [50] and subsequently Mo addition promoted the formation of M23C6 and M6C revised by Thorpe and Chico [51]. A high Cr cast iron with [43,45]. At 1 wt.% Mo content, multiple eutectic carbides Cr/C ratio of 7 has been studied by Ikeda et al [10] and by including M7C3, M23C6 and M6C were observed. Mo addition adding 1 wt.% Mo, M2C was found. On the other hand, increased Vickers macro-hardness of the irons from 495 to addition of 1 wt.% Mo to as-cast Cr iron containing 20 wt.% 674 HV30. Mo dissolved in austenite and increased the Cr (Cr/C ratio of 8) [52] and to Cr iron containing 16 wt.% Cr hardness by solid solution strengthening [46]. (Cr/C ratio of 6) [53] did not significantly change the structure of the alloys in their as-cast state. The addition of Wear performance of high Cr-Mo irons depends on the type, molybdenum to high chromium cast iron led to the formation morphology, amount and distribution of hard eutectic and of Mo C that crystallized in a finely dispersed form as eutectic other carbides in the microstructure, and also on the nature of 2 in the final stage of solidification [10]. By increasing the the supporting matrix [5,26,47]. Multicomponent Cr-Mo vanadium content in an alloy, the structure became finer and white cast irons with reduced C and Cr levels and additions of the volume fraction of eutectic M C and V C carbides other strong carbide-forming elements such as W, V and Nb 7 6 6 5 increased, and the morphology of eutectic colonies changed have been developed as wear resistant materials for special [54].

Figure 6: Energy spectra of the matrix and the carbide of high chromium cast iron modified with 0.8% RE, 0.4% V, 0.3% Ti, and 0.1% B: (a) matrix; (b) carbide [44].

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Niobium addition to high Cr white iron increased its wear carbides and transformation of the primary austenitic matrix resistance because of niobium formed very hard carbides to martensite. The destabilization heat treatments are usually (NbC) of hardness around HV 2400 and niobium dissolved in performed in the range of 800-1100oC, for 1-6 hrs. [59-61]. the matrix and increased the matrix microhardness from HV The matrix in the microstructure of high-Cr WCI after 693 at 0% Nb to HV 899 at 3.47% Nb [55,56]. The addition destabilization treatment is generally composed of martensite of small amount of boron modified the morphology of eutectic and retained austenite. Destabilization heat treatment of high carbides [57,58]. Therefore, boron addition resulted in an Cr cast irons normally consists of heating to a high increase in the abrasion wear resistance. temperature or austenitizing, air quenching and tempering [7]. The successful heat treatment produces austenite

destabilization by precipitation of fine secondary M7C3 EFFECT OF HEAT TREATMENT ON carbides within the austenite matrix. Class II irons containing MICROSTRUCTURE AND MECHANICAL 12 to 23% Cr are austenitized in the temperature range 950 to PROPERTIES 1010 °C [62]. It is necessary to ensure that the soaking time is sufficient to ensure a complete secondary carbide The role of the matrix surrounding the eutectic carbides is to precipitation. Typical continuous cooling transformation provide a sufficient mechanical support to prevent their (CCT) diagram illustrating the hardenability in an alloy for cracking, deformation and spalling [21]. The matrix may be class II are presented in Fig.8 [62]. Air quenching of the extensively altered through the destabilization and subcritical castings from the austenitizing temperature to below the heat treatments commonly. The main objectives of the pearlite temperature range is highly recommended. destabilization heat treatment is the precipitation of secondary

Figure 7: Chemical composition and the Cr/C ratio of Mo containing high chromium cast irons investigated in previous studies [10,47] and plotted on the Fe–C–Cr liquidus surface [50,51].

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Figure 8: Continuous-cooling transformation diagram for high Cr white cast irons (Class II) [62].

Effect of destabilization heat treatment quenching. This temperature would generally lie in the range 950-1050 oC and would depend on the particular composition High chromium white cast irons must be heat treated to of the alloy being treated. Hakan Gasan and Fatih Erturk [64] develop the full hardness and maximum wear resistance. The reported that the morphologies of secondary carbides, the destabilization (hardening) heat treatment is used with high Cr crystallographic properties of the phases, and the proper and Cr-Mo white cast irons to transform the as-cast austenitic combination of the amount of martensite, retained austenite, or pearlitic matrix structures to martensite [36,37,60-63]. and carbides were the principle parameters that affect the Better wear resistance was obtained for the alloyed heat hardness and wear behavior of the alloy. The effect of the treated iron compared with that of the as-cast and unalloyed destabilization temperatures on the hardness is shown in Fig. irons [63]. The responsible for such better wear behavior is 10 [64]. A destabilization heat treatment caused the the strengthening of the matrix by the MC carbides plus the transformation of austenite to martensite and the precipitation precipitation of secondary M C carbides and the partial 7 3 of M C -type secondary carbides, while eutectic carbides transformation of matrix from austenite to martensite after 7 3 remained unchanged [64]. Fig. 11 shows SEM micrographs heat treatment. Fig. 9 shows specific wear rate against applied of the alloys subjected to destabilization treatments at various load for the high chromium white irons. temperatures 900 and 1000oC. With increases in the treatment temperature, the precipitated carbides became coarser and less abundant and the amount of retained austenite increased. The martensitic matrix reduces wear of matrix and it in turn reduces the carbide fracture. The secondary carbides also influence the wear behavior; 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 [65]. Therefore, the austenite is more effective in this respect than martensite. The volume fraction of the eutectic carbide phase (M3C or M7C3) have an important influence on the wear resistance of white

iron alloys under low stress abrasion conditions [65]. Huang Figure 9: Specific wear rate against applied load for the and Wu [66] reported that the heat treatment of 28Cr 1Mo investigated iron alloys [63]. white iron changed the structures and properties. The austenite As the austenitizing temperatures increased, the amount of matrix destabilized when the iron was treated at a high carbon dissolved in the austenite increased [54]. Decreasing temperature because there were two secondary phases; χ and the austenitizing temperature from the optimum gave a lower M7C3II, precipitated from the matrix, and the austenite hardness due to the lower carbon martensite formed on transformed into martensite during air cooling. The higher the temperature of heat treatment the higher was the volume of

4681 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 14 (2017) pp. 4675-4686 © Research India Publications. http://www.ripublication.com precipitated secondary phases and transformed martensite The precipitation of secondary carbides during austenitising [66,67]. With increasing austenitizing temperature, the left the matrix depleted in carbon and chromium, thus raising secondary carbides in high Cr iron alloys became coarser, and the Ms temperature above room temperature and subsequent the amount of retained austenite in the denderitic areas cooling resulted in the transformation of matrix to martensite increased [53,67,68]. The optimal heat treatment process is 2 [69]. The holding time at the destabilization temperature h quenching treatment at 1000℃, followed by a subsequent 2 appeared to affect the retained austenite content of the alloy h tempering at 400 ℃ [66]. 15Cr 3Mo [20]. The short holding times with the appropriate destabilization temperature might only be required to achieve a hardened structure with the maximum hardness.

Figure 10: Effect of destabilization temperature on the hardness of a forced air 20Cr 1.7Mo 2C iron [64].

Figure 11: SEM micrographs of the alloys subjected to destabilization treatments at various temperatures of: (a) 900oC, (b) 1000oC; (d) and (e) micrographs obtained after deep etching [64].

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Effect of subcritical heat treatment 12 shows the effect of tempering on the hardness of a 20Cr 1.7Mo C iron [3]. There was a secondary hardening peak at Subcritical heat treatment (tempering) is sometimes 2 450-550 oC, perhaps through the precipitation of fine alloy performed, particularly in large heat treated martensitic carbides [3,20]. The treatments at 600oC on the destabilized castings, to reduce retained austenite contents and increase material produced a gradual drop in the hardness due to the resistance to spalling. Typical tempering temperatures range decomposition of martensite to a ferrite/carbide product. The from 480 to 550 °C and times range from 8 to 12 h. Excess effect of subcritical heat treat parameters on hardness and time or temperature results in softening and a drastic reduction retained austenite in Mo-containing high chromium cast iron in abrasion resistance [62]. Insufficient tempering results in was studied by Inthidech et al [71]. They concluded that the incomplete elimination of austenite. Fernandez and Belzunce hardness decreased gradually but the Vγ increased greatly as [70] reported that compression strengths for low and high Mo content increased in both 16 and 26 wt.% Cr cast irons in carbon white cast irons after heat treatment at 500o C was the as cast state. In the case of subcritical heat treatment, the excellent and a significant ductility was also observed. During hardness increased first and then decreased with an increase in tempering carbides were precipitated in retained austenite holding time. This phenomenon is due to a hardening caused regions which could then transform to martensite on cooling by the precipitation of secondary carbides and by martensite [3]. If a prolonged tempering time was used, then the austenite transformation from the destabilized austenite during cooling. would transform to ferrite when sufficient carbide precipitated. The structure would then consist of a tempered martensite and softer areas of ferrite-M23C6 precipitates. Fig.

(a) (b)

Figure 12: Effect of tempering on the hardness of a 20Cr 1.7Mo 2C iron; (a) the effect of temperature for holding time of 4 o hours, (b) the effect of holding time at 520 C [3].

CONCLUSIONS pearlite formation when the ratio of chromium to carbon (i.e. Cr/C) is relatively high. The main concluding remarks those could be drawn from this  The carbide morphology had a pronounced revision are: influence on the wear and fracture behavior of  Alloying additions with appropriate amounts of high chromium white irons. carbide forming elements to high Cr white cast  Molybdenum additions up to 10 wt.% Mo irons revealed marked improvements in promoted the formation of eutectic/peritectic mechanical properties and wear performance M23C6 and M6C carbides. through microstructure refinement and in situ  Destabilization heat treatment for high-Cr WCIs formation of fine new carbides. is employed to obtain the martensitic structure  The variations in wear resistance of the alloys for improving the toughness and abrasion with respect to the added carbide forming resistance of these irons. elements are mainly caused by the changes in  Subcritical (tempering) heat treatments hardness resulting from the introduction of fine following the hardening are commonly utilized carbides and microstructure refinement. to relieve the internal stresses and to control the  The addition of small amounts of molybdenum final hardness before service. (i.e. 0.5% Mo) is sufficient for suppressing

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 Better wear resistance is obtained for the alloyed Niwa, N., and Kato, O., Effect of molybdenum heat treated white cast iron compared with those addition on solidification structure, mechanical as cast and unalloyed irons. An improvement of properties and wear resistivity of high chromium cast the wear behavior was due to the matrix change irons , ISIJ International, 32 , pp.1157-1162, 1992 . from austenite to martensite with the presence of [11] Abd El-Aziz, Kh., Zohdy, Kh., Saber, D. and secondary carbides by the heat treatment. Sallam, H. E. M., Wear and corrosion behavior of  The hardness of white cast iron increased with high-Cr white cast iron alloys in different corrosive increasing the carbide volume fraction for the media, J Bio Tribo Corros 1, pp.,25, 2015. austenitic and martensitic structures. [12] Sallam, H.E.M, Abd El-Aziz Kh., Abd El-Raouf H.,  Certain level of austenite should be present in Elbanna, E.M., Flexural strength and toughness of the structure to produce the optimal abrasion austenitic stainless steel reinforced high-Cr white resistance. This required level of austenite was cast iron composite. J Mater Eng Perform 22, pp., 777, 2013. cited as at least 20-30 %. Above this level the abrasion resistance was independent of austenite [13] Sallam, H.E.M, Abd El-Aziz Kh, Abd El-Raouf H., content. Elbanna, E.M., Failure analysis and flexural behavior of high chromium white cast iron and AISI4140 Steel bimetal beams. Mater Des 52, pp., 974–980, 2013. ACKNOWLEDGEMENT [14] Powell, G. and Randle, V., The Effect of Si on the The authors would like to thank Taif University for its relation between orientation and carbide financial support in the project No. 1/734/ 2527. morphology in high chromium white irons, Journal

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