Study the Microstructure and Mechanical Properties of High Chromium White Cast Iron (HCWCI) under Different Martempering Quenching Mediums

This study aims to find the effect of hydroxide mixture as a quenching medium in martempering heat treatment on microstructure and mechanical properties of high chromium white cast iron. This mixture is cheaper and more available than the ordinary nitrate mixture in Iraqi markets. High chromium white cast iron is used in mining, crushing and cement plants as mill liners and it is subjected to extreme conditions of wear and impact that cause failure, reduction in life and raise the cost of repairing. Hence it is important to improve its mechanical properties. In this study, two types of quenching mediums were used:(50% NaOH: 50% KOH ) mixture and (50% NaNO3 + 50 % KNO3) mixture. The specimens were austenitized at 950°C for 1 hr then the first group was quenched in nitrate mixture, and the other was quenched in hydroxide mixture both at about 350°C for (1/2, 2,4,6,8) hr. The results showed an increase in hardness and decrease in toughness for both mixtures, and the higher hardness value was found for both of the mixtures at martempering temperature 350°C for 4hr quenching time. Keyword: Hardness, High Chromium White Cast Iron, Martempering, microstructure, quenching mediums. How to cite this article: A.H. Ataiwi and Z.A. Betti, “Study the Microstructure and Mechanical Properties of High Chromium White Cast Iron (HCWCI) under Different Martempering Quenching Mediums,” Engineering and Technology Journal, Vol. 37, Part A, No. 04, pp. 112-119, 2019.


Introduction
High chromium white cast irons represent a class of materials commonly used for materials handling, crushing and grinding applications in the minerals and mining industries due to their excellent wear resistance [1,2]. After heat treatment, High chromium white cast irons microstructures 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. Microstructures in the matrix are affected by three factors: thermal cycle, chemical composition and the initial state of alloy (as-cast or annealed). This will, in turn, affects the hardness and wear resistance of the material. In general, at the end of the solidification process, the microstructure of high-chromium white cast iron is composed of a primary phase (austenite dendrite) and a eutectic compound (M7C3) [3]. The reference [4] studied the influence of variables such as holding temperatures and times during aus tempering of High Chromium White Cast Iron (HCWCI). It is found that the best performance of alloy against the abrasion was at austempering at 450 ºC with 6 hours holding time. The reference [5] studied the effects of soft annealing and hardening and tempering on the hardness and microstructure of high chromium cast iron. Their results showed that a proper selection of parameters of heat treatment could control the hardness and composition of phase microstructure of chromium cast iron. Hardening has positive effects on the hardness of castings, but tempering and soft annealing enhance ductile properties. The reference [6] observed the effect of changing the chemical composition and heattreatments on the microstructure and mechanical properties of high chromium white cast iron. The results revealed marked improvements in mechanical properties and wear performance by adding carbide forming elements .on the other hand, the Destabilization heat treatment employed to obtain the martensitic structure for improving the toughness and abrasion resistance of these irons. In addition, the subcritical (tempering) heat treatments following the hardening are commonly utilized to relieve the internal stresses and to control the final hardness before service. The reference [7] studied the effects of heat-treatment on microstructure and mechanical properties of high chromium white cast iron alloyed with titanium. The austenitiztion heat treatment with temperatures of 980°C and also 1150°C for 1 hour followed by tempering at 260°C for Two hours have been accomplished, and the effect of these processes on (wear resistance/impact toughness) combination was reported. The results clarify that the maximum tensile strengths were achieved for the (1.31%Ti) irons; also, maximum impact toughness was obtained for the irons without Tiaddition. The wear resistance results were higher for the samples austenitized at (980°C) in comparison to the irons treated at (1150°C). For both treatments, optimum wear resistance was obtained with (1.3% Ti). The current study aims to find cheaper and more available quenching medium than the ordinary nitrate mixture and compare its effect on microstructure and mechanical properties.

Materials and Methods
The metal sample of the category (A-532 Class ІІІ Type A) is used. The sample is obtained from (Kufa Cement Plant) mill liners. The chemical composition of the sample is investigated via (AMETEK, SPECTRO MAXx) at (State Company for Inspection and Engineering Rehabilitation (S.I.E.R) at Industry and Minerals Ministry, Iraq). The chemical composition of the sample used in this work is shown in the following table (see Table 1). The heat treatment used here is Annealing, quenching and martempering with two types of quenching mediums: (50% NaOH: 50% KOH) mixture and (50% NaNO3 + 50 % KNO3) mixture with differing quenching intervals. All heat treatment processes are illustrated in Figure  1-A. The procedure of martempering heat treatment is illustrated in Figure 1-B. The samples were prepared to be examined under an optical microscope by polishing using (320 up to 2000) grit papers and using 1μm polycrystalline diamond paste. The specimens were etched using 0.5 g copper (II) chloride in a solution of 11 ml absolute methanol, 11 ml distilled water and 11 ml concentrated HCl (Kalling's reagent) [8]. The specimens were examined under a scanning electron microscope and X ray diffraction spectroscopy. Impact toughness of the specimens was tested using Charpy impact tester(direct readings), and the hardness was tested using Vickers macro hardness tester.

Results and Discussion
The results showed that the different heat treatment affects the microstructure and mechanical properties of the specimens in different ways. The results obtained were compared with the as-received specimens, and they showed an increase in hardness of quenched and martempered specimens whereas there was a decrement in hardness values of annealed specimens. The influence of temperature, time can explain this different behavior, the severity of quenching, microstructure type, and second phase particles type in the heat-treated specimens. The hardness values depend on the amount of precipitate of secondary fine carbides and also on the formed martensite [4]. On the other hand, the impact test showed a decrease in toughness of quenched and martempered specimens which is also found by [9] also decrease in the annealed ones as illustrated in Table 2.

I-Vickers macrohardness and impact values
From Table 2, it is observed that the highest hardness values were obtained at martempered specimens for (4 hr + quenching) in both molten hydroxide and nitrate mixtures. While the highest impact value was observed in asreceived and martempered specimens for (6 hr + quenching) in nitrate mixture.

III. Optical microscope images
From Figure 3 it is observed that the microstructures of the specimens martempered at the hydroxide and nitrate mixtures approximately the same. These microstructures consist of (austenitic, ferritic, marensitic) matrix (darker areas) with eutectic and secondary carbides (lighter areas) that are dispersed in these matrixes in approximarely the same shapes and patterns for samples shown in Figure 3(A, B, C, D1  Generally, the fracture has brittle nature in all the samples, but its path is different from one to the other according to the original direction of grain growth during solidification of the original casting.