Heat Treatment Effect on Microstructure, Hardness and Wear ...

CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol. 28,aNo. 1,a2015

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DOI: 10.3901/CJME.2014.0620.116, available online at www.springerlink.com; www.cjmenet.com; www.cjmenet.com.cn

Heat Treatment Effect on Microstructure, Hardness and Wear Resistance of Cr26 White Cast Iron

ZHOU Shaoping1, *, SHEN Yehui1, ZHANG Hao2, and CHEN Dequan2

1 Key Laboratory of Pressure Systems and Safety of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China

2 Shanghai First Fluid Machinery, Co., Ltd, Shanghai 201709, China

Received February 27, 2014; revised June 4, 2014; accepted June 20, 2014

Abstract: High chromium cast iron(HCCI) is taken as material of coal water slurry pump impeller, but it is susceptible to produce

serious abrasive wear and erosion wear because of souring of hard coal particles. The research on optimization of heat treatments to

improve abrasive wear properties of HCCI is insufficient, so effect of heat treatments on the microstructure, hardness, toughness, and

wear resistance of Cr26 HCCI is investigated to determine the optimal heat treatment process for HCCI. A series of heat treatments are

employed. The microstructures of HCCI specimens are examined by using optical microscopy and scanning electron microscopy. The

hardness and impact fracture toughness of as-cast and heat treated specimens are measured. The wear tests are assessed by a Type M200

ring-on block wear tester. The results show the following: With increase of the quenching temperature from 950 ℃ to 1050 ℃, the

hardness of Cr26 HCCI increased to a certain value, kept for a time and then decreased. The optimal heat treatment process is 2 h

quenching treatment at 1000 ℃, followed by a subsequent 2 h tempering at 400 ℃. The hardness of HCCI is related to the precipitation

and redissolution of secondary carbides in the process of heat treatment. The subsequent tempering treatment would result in a slight

decrease of hardness but increase of toughness. The wear resistance is much related to the “supporting” effect of the matrix and the

“protective” effect of the hard carbide embedded in the matrix, and the wear resistance is further dependent on the hardness and the

toughness of the matrix. This research can provide an important insight on developing an optimized heat treatment method to improve

the wear resistance of HCCI.

Keywords: Cr26 white cast iron, heat treatment, microstructure, hardness, wear resistance

1 Introduction

High chromium cast irons(HCCIs) ferrous based alloys containing 11–30 wt.% chromium and 1.8–3.6 wt.% carbon, with molybdenum, manganese, copper, and nickel and some other alloying elements. HCCIs have long been considered as the third-generation wear-resistant materials for the wear-affected components operated under extreme conditions, such as facilities in the slurry pumping systems in the oil sands handling, mineral processing, coal and cement industries due to their excellent abrasion resistance imparted by the hard alloy eutectic carbides present in the microstructure. Generally, it was believed that the microstructure of HCCIs has an important influence on their properties. For the past decade, many researchers have been done to determine the effects of chemical composition and microstructure of HCCIs on their properties, such as hardness, impact toughness, and wear or erosion resistance.

* Corresponding author. E-mail: [email protected] Supported by National Hi-tech Research and Development Program of

China(863 Program, Grant No. 2013BAF01B01) © Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2015

These researches can be classified into two categories: 1) to find the structure-property relation and 2) to optimize the composition of the microstructure by heat treatment in order to further improve the hardness and wear resistance.

In the first category of research, the effect of addition of alloying elements on the microstructure, phase composition and wear resistance of HCCIs was examined For instance, ZHANG, et al[1], found that by alloying yttrium to the cast white iron can considerably improve the performance of the alloy cast white iron, and further that 1.0 wt.% yttrium is the optimum quantity. They further suggested that the role of yttrium be mainly attributed to its positive effect on the corrosion resistance of the cast iron due to the enhanced surface passive film. MOUSAVI ANIJDAN, et al[2], investigated the effect of alloying tungsten into high chromium cast iron. They found that tungsten can effectively improve the hardness of the matrix and wear resistance of the high chromium cast iron. CHUNG, et al[3], investigated the effect of alloying titanium addition on the microstructure and wear resistance of hypereutectic high chromium white cast iron. The results showed that a fine eutectic microstructure with dispersed TiC carbide was achieved when 2 wt.% Ti was added. The eutectic structure

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led to the highest hardness and wear resistance. SCANDIAN, et al[4], investigated the effect of molybdenum and chromium contents on the relationship between the microstructure and sliding wear of high chromium white cast iron, the results showed that the microstructure of the matrix displays a strong influence on sliding wear resistance.

In the second category of research, the focus is on the understanding of the change of the microstructure. SARE, et al[5], TABRETT, et al[6], studied the effect of heat treatment on the abrasion resistance of a family of commercial-grade alloy white cast iron composites. It was found that standard austenitizing treatments led to a substantial increase in hardness, a marked decrease in the retained austenite content in the matrix, and a significant improvement in gouging abrasion resistance. The gouging abrasion resistance tended to decline with increasing austenitizing temperature, although the changes in hardness and retained austenite content varied, depending on alloy composition. YUAN, et al[7–8], ZHU, et al[9], investigated the effects of quenching temperature, quenching soaking time, tempering temperature on the microstructure, hardness and impact toughness of Cr26 HCCI by using the orthogonal test and determined the optimum heat treatment. CARPENTER, et al[10–11], studied 26.6% chromium white iron microstructure with XRD and electron microscope. It was shown that primary and eutectic austenite revealed a slight difference in lattice parameter and secondary carbides precipitated in the microstructural matrix of the white iron were of the form (Fe,Cr)23C6 and had a cubic crystal structure. INTHIDECH, et al[12], studied the effect of alloying elements(Ni, Cu, Mo, V) on heat treatment behavior of hypoeutectic high chromium cast iron, and established the relationship between the amount of alloying elements and hardness as well as the volume fraction of retained austenite after heat treatment. AI, et al[13], found that the quenching treatment with long soaking time can lead to the dissolution of secondary carbide, resulting in the improvement of wear resistance. ZEYTIN, et al[14], studied the effects of boron addition and heat treatment on mechanical properties of white cast iron. Optimum mechanical properties were obtained by lower destabilization temperatures; however, increasing temperature reduced the wear resistance and hardness. More recently, ALBERTIN, et al[15], developed a methodology by combining the empirical knowledge gained from the production of high Cr cast iron castings with the use of computational thermodynamics to guide the alloy and heat treatment selection to meet the requirements of industrial applications. XU, et al[16], studied the effect of carbide on crack behavior of roll (HCCI and HSS) under the condition of cold rolling. It indicated that the crack initiation and propagation of roll was related to the fine structure of carbide, interface structure of carbide and matrix, and the carbide morphology. The wear test results showed that the relative wear resistance of HSS roll was

above 4 times of the HCCI roll. ZHENG, et al[17], investigated the precipitation of grain boundary carbides and the evolvement of chromium depleted zone in isothermal heat treatment of 690 alloys. It pointed out that the optimum isothermal heat treatment is 715 ℃/10–20 h, and the formation of chromium depleted zone was jointly decided by the precipitation of carbide behavior and atomic migration.

The goal of this research is to address the effect of different heat treatments on the microstructure, hardness, impact toughness, and wear resistance of Cr26 HCCI, i.e., KmTBCr26. The heat treatments were divided into two groups. The first group included quenching heat treatments at the destabilization temperature in the range from 950 ℃ to 1050 ℃. In the second group, the tempering treatments at different temperatures were carried out after quenching treatment at 1000 ℃.

2 Experimental Procedures

The high chromium white iron, Cr26, was cast from

1500 ℃ as 80 mm diameter balls into a chilled mold. The chemical composition was given in Table 1. The specimens with dimensions of 12 mm´12 mm´19 mm were used for heat treatment and wear tests, and the specimens were cut from the cast bars, respectively. Cutting was done as slow as possible in order to avoid excessive overheating.

Table 1. Mass fraction (wt.%) of alloy examined

Alloy class C Si Mn P S

Cr26

2.55 0.62 1.76 0.03 0.03

Cr Ni Mo V Fe

27.1 0.29 0.53 0.09 Balance

Different heat treatments were performed in electrical resistance furnace. The procedures of heat treatments for different specimens were summarized in Table 2. For all specimens, the heating rate was kept to be 5 ℃/min. All specimens were annealed at 650 ℃ for 2 h firstly and then heated to different quenching temperatures(QTs). The cooling was air cooling. Totally, eight different heat treatments were used. These treatments can be divided into two groups. The first group included quenching heat treatments at different QTs for 2 h. For instance, for the specimen S1, the heat treatment procedure included annealing at 650 ℃ for 2 h, quenching at 950 ℃ for 2 h, and then cooling down to room temperature in air. In the second group, treated at different tempering temperatures (TTs) for 2 h were carried out after quenching treatment at 1000 ℃. For instance, the specimen denoted as S31 in Table 3, the heat treatment procedures were listed as follow, annealing at 650 ℃ for 2 h, quenching at 1000 ℃ for 2 h, cooling down to ambient temperature in air, then tempering at 200 ℃ for 2 h and cooling down to room temperature in air.

ZHOU Shaoping, et al: Heat Treatment Effect on Microstructure, Hardness and Wear Resistance of Cr26 White Cast Iron

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Table 2. Hardness of Cr26 high chromium cast iron

with different quenching temperature(HRC)

Specimen

code

Quenching

temperatures

T/℃

Average

hardness

H/HRC

Range of

hardness

H/HRC

S1 950 59.7 58.9–60.1

S2 980 61.0 60.2–61.5

S3 1000 61.0 60.5–61.8

S4 1020 61.0 60.2–61.7

S5 1050 59.6 58.2–60.9

Table 3. Hardness of Cr26 high chromium cast iron

with different tempering temperature (HRC)

Specimen

code

Tempering

temperatures

T/℃

Average

hardness

H/HRC

Range of

hardness

H/HRC

S31 200 57.9 56.5–58.8

S32 400 58.9 58.2–59.2

S33 550 48.1 48.1–48.3

Samples for microstructural analyses were prepared by

the following procedure: milled using abrasive paper, polished using a 0.25 µm diamond paste and then etched with 4% Nital. The microstructure of the samples before and after heat treatment was observed by JSM-6360LV scanning electron microscopy(SEM) equipped with an energy dispersive spectrometer(EDS) apparatus. A mixture of secondary electron(SE) and back scattered electron(BSE) modes was used to obtain digital SEM images with the best possible resolution and contrast to investigate different types of microstructural phase in greater detail. Crystal structures were determined by means of a Philips Xʼpert diffractometer set-up with a copper K-alpha X-ray source.

The bulky hardness of the samples was measured on the Rockwell C scale by using HR-150DT Rockwell Hardness Machine with a total load of 1471N. For individual samples, five indentations were randomly produced on the surface to get representative values of hardness. Indentations were spaced sufficiently far apart so that the indentation behavior was not affected by the adjacent indentations. For some samples treated after tempering treatment, with notch were taken into impact tests by the mean of three measurements using a JB-30 Impact Testing Machine [18].

The abrasive wear test was carried out on a Type M200 ring-on block wear tester. Schematic diagram of wear test was shown in Fig. 1. The test was performed at a constant speed of 120 r/min(equal to a liner velocity of 31.0 cm/s)under a load of 98 N. The silicon carbide ring was used as friction pair. For individual specimen, the total testing time was kept to be 15 h. After every 3 h, i.e., the sliding distance of 3.336 km, the mass loss of the sample was measured by TG328A photoelectric balance with accuracy of 0.01 mg. The wear rate was defined as mass loss per unit sliding distance(mg/km). The worn surface morphology of the sample was observed by SEM.

Fig. 1. Schematic diagram of wear test

3 Results

3.1 Effect of heat treatment on microstructure Effect of quenching treatments at different temperatures

on the microstrcture of Cr26 HCCI is illustrated with Fig. 2. In as-cast, the austenite matrix with eutectic carbides can be found in the microstrcture, as seen in Fig. 2(a). After quenching treatment, the matrix structure changed from austenite to martensite and precipitation of secondary carbides can be seen clearly. Generally, when the specimen was heated to and held at the austenitizing temperature, the austenitic matrix would transform to martensite during the cooling process. However, the content and distrubution of martensite matrix and secondary carbide precipitation were dependent on the quenching temperature and soaking time. At low quenching temperature, e.g., 950 ℃, the secondary carbides precipitated from austenitic matrix, leading to the reduction of the carbon and alloying elements and a low content of carbon martensites, as seen in Fig. 2(b). When the quenching temperature is 1000 ℃ , different phases including eutectic carbide, martensite and secondary carbide distributed uniformly, as seen in Fig. 2(d). With further increasing the quenching temperature, the dissolution of secondary carbides can be found, leading to the increment of carbon and alloying elements in austenitic matrix. In such a case, the stability of austenite increased and the temperature point from austenite to martensite, MS, reduced. Hence, the coarse martensite and a large number of residual austenite can be observed in the microstructure, as seen in Fig. 2.

The tempering treatment after quenching also has an important influence on the microstructure of Cr26 HCCI, as seen in Fig. 3. By comparing with the microstructure of Cr26 HCCI quenching at 1000 ℃, it can be seen that the eutectic carbides and secondary carbides precipitated from martensite matrix after tempering treatments. However, when the termperature of tempering treatment is low, i.e., 200 ℃, the precipitated eutectic carbides distributed more densely, as seen in Fig. 3(a). This may be due to the fact that the motion of Cr and C atoms was slow at low


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tempering temperature and collection of atoms was difficut. With increasing the tempering temperature to 400 ℃, the elongated eutectic carbides can be observed, as seen in Fig. 3(b). Moreover, the secondary carbide clusters were generated in the martensite matrix. When the tempering

temperature was extremely high, i.e., 550 ℃, the dissolution of elongated eutectic carbides would lead to the size reduction of eutectic carbides. In such a case, the dispersive distribution of eutectic carbides was also observed, as seen in Fig. 3(c).

Fig. 2. Micrographs of Cr26 HCCI as-cast and quenching at different temperatures(´2000)

The tempering treatment after quenching also has an important influence on the microstructure of Cr26 HCCI, as seen in Fig. 3. By comparing with the microstructure of Cr26 HCCI quenching at 1000 ℃, it can be seen that the eutectic carbides and secondary carbides precipitated from martensite matrix after tempering treatments. However, when the termperature of tempering treatment is low, i.e., 200 ℃, the precipitated eutectic carbides distributed more

densely, as seen in Fig. 3(a). This may be due to the fact that the motion of Cr and C atoms was slow at low tempering temperature and collection of atoms was difficut. With increasing the tempering temperature to 400 ℃, the elongated eutectic carbides can be observed, as seen in Fig. 3(b). Moreover, the secondary carbide clusters were generated in the martensite matrix. When the tempering temperature was extremely high, i.e., 550 ℃, the dissolution


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of elongated eutectic carbides would lead to the size reduction of eutectic carbides. In such a case, the dispersive distribution of eutectic carbides was also observed, as seen in Fig. 3(c).

Fig. 3. Micrographs of Cr26 HCCI tempering at different temperatures after quenching at 1000 ℃(´2000)

3.2 Phase composition

The X-ray diffraction patterns of as-cast Cr26 white cast iron, and the irons after different heat treatments are shown in Fig. 4(a). It can be seen from the figure that the crystalline phases of Cr26 consist mainly of retained austenite, martensite, eutectic carbide, and secondary carbide phases. The peaks located at about 45° corresponding to austenite/martensite can be seen. The

XRD diffraction spectrum of specimen after quenching treatment after 1000 ℃ shows the highest peaks corresponding to austenite/martensite. Effect of tempering treatment on the X-ray diffraction patterns of Cr26 white cast iron is shown in Fig. 4(b). It can be seen from the figure that the peak located at about 45º corresponding to austenite/martensite is almost not changed after the tempering treatment.

Fig. 4. XRD diffraction spectrum of specimen after different heat treatments

The chemical contents within the eutectic carbides of

as-cast Cr26 white cast iron and heat treated iron after tempering treatment were summarized in Table 4. It can be seen from the table that after quenching treatment and subsequent tempering treatment, the Cr content in eutectic carbide increases apparently while the contents of Si and Fe decreases. Through stoichiomtries, the composition of eutectic carbides in as-cast was determined as Fe5.6Cr1.4C3, while it was changed to be Fe2Cr5C3 after the heat treatments.

3.3 Effect of heat treatment on hardness and impact toughness

Fig. 5 and Fig. 6 respectively show the hardness of the irons after different quenching treatments and tempering


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treatments. The hardness of HCCI in as-cast is 50.6 HRC. From Fig. 5, it can be seen that the quenching treatment can improve the hardness of Cr26 white cast iron. However, with increasing the quenching temperature, the hardness increases, keeps at constant value, and then decreases. The tempering treatment following the quenching treatment at 1000 ℃ has an important influence on the hardness of iron, as seen in Fig. 6. With increasing the tempering temperature, the hardness increases, reaches a local maximum and then decreases. By comparing with the hardness of the iron quenched at 1000 ℃, the subsequent tempering treatment leads to a slight decrease of hardness.

Table 4. chemical contents (wt.%) of eutectic

carbide tested by EDS

Specimen code

Chemical contents(wt.%) of eutectic carbide

C Si Cr Mn Ni V Fe

A 0.92 0.82 18.08 1.72 0.68 Balance

B 2.44 0.24 67.47 1.74 Balance

C 1.98 66.28 1.60 0.37 Balance

Fig. 5. Hardness of the irons after different quenching treatments

Fig. 6. Hardness of the irons after different tempering treatments

The impact toughness of as-cast iron and iron after two

different heat treatments were listed in Table 5. By comparing with as-cast iron, the quenching treatment at 1000 ℃ for 2 h reduces the impact toughness, while the

subsequent tempering treatment of 400 ℃ for 2 h after the quenching treatment increases the impact toughness.

Table 5. Hardness and toughness of wear test specimens

Specimen code Hardness

H/HRC

Toughness

Ak /(J·cm–2)

A 50.6 1.96

B 61.0 1.24

C 60.0 2.12

(A: As-cast B: 1000 ℃(QTs) C: 1000 ℃(QT)+400 ℃(TTs))

3.4 Effect of heat treatment on wear resistance

The wear rate of the HCCI samples, averaged over two repeats, is shown in Fig. 7 as a function of sliding distance. It can be seen in Fig. 7 that, at a given sliding distance, the wear rate of as-cast iron is high. After the quenching heat treatment at 1000 ℃, the wear rate decreases, indicating that the quenching treatment would increase the wear resistance of HCCI. Moreover, the subsequent tempering treatment at 400 ℃ leads to the further improvement of the wear resistance.

Fig. 7. Wear rate versus sliding time for Cr26 HCCI.

A: As-cast B: 1000 ℃(QTs) C: 1000 ℃(QTs)+400 ℃(TTs)

4 Discussion

It is known from the literature that the change of

hardness and wear resistance of high chromium cast irons is largely dependent on their microstructures[19]. The present study reveals the significant contribution to the change from the heat treatment. In particular, a phenomenon can be recapped here: when the quenching treatment temperature is lower than 980 ℃, the hardness after the heat treatment is about less than 60 HRC. This may be due to the fact that the quenching temperature is lower than the austenitizing temperature and in this case, the sufficient precipitation of secondary carbide fails to reach, as seen in Figs. 2(a) and 2(b). However, when the quenching treatment temperature further increases, say higher than 1020 ℃ , the hardness after the quenching treatment gradually decreases. This may be due to the fact that the secondary carbides redissolves at high temperatures,


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as seen in Fig. 2(f) and Fig. 5. The similar phenomenon was also observed by HOU, et al[20]. Therefore, the hardness after the heat treatment is contributed by the factors including the temperature in heat treatment, microstructure, and soaking time.

The wear resistance is also contributed by the synergistic effect of microstructure, hardness and toughness[21]. By comparing with as-cast, the hardness and wear resistance of the HCCI after the quenching treatment at 1000 ℃are relatively high. The main reason for the increase of wear resistance was due to the increase in hardness after the quenching treatment which is further attributed to the transformation of austenitic matrix into martensite and to the precipitation of secondary carbides during the quenching treatment process. This theory seems to be in agreement with the view in literature that the carbide is a significant contributor to the improved abrasion resistance [5, 13, 22].

Fig. 8. shows the local worn surface morphologies of different Cr26 specimens after the wear test. It can be seen from the figure that for as-cast Cr26 iron, numerous abrasive pits and fatigue spalling appear at the worn surface, as seen in Fig. 8(a). In such a case, the wear rate is relatviely high, as seen in Fig. 7. For the iron with the quenching treatment at 1000 ℃, the worn surface becomes smooth and some micro-cutting can be found, as seen in Fig. 8(b). By comparing with as-cast specimen, the wear rate is relatively low, as seen in Fig. 7. The improvement of the wear resistance of the quenching treated specimen was due to the increase of hardness and precipitatation of secondary carbides within a martensite matrix, as seen in Fig. 2. However, the amount of micro-cutting is related to the “supporting” effect of matrix and the “protective” effect of hard carbide embedded in the matrix[23]. For the specimen subject to the quenching and subsequent tempering treatments, no obvious abrasive pits and micro- cutting can be observed at the worn surface. However, a few small cracks can be found, as seen in Fig. 8(c). This specimen exhibits the highest wear resistance. By comparing with the specimen of the quenching treatment only, the specimen of the quenching and subsequent tempering treatments shows slightly low hardness and high toughness. Hence, the improvement of the wear resistance of specimen of the quenching and subsequent tempering treatments may be due to the increase of toughness. It is noted that similar results were also obtained by TABRETT[22], which gives a strong support to the theory here.

5 Conclusions

(1) For Cr26 white cast iron with a specific chemical composition, the optimal heat treatment process is 2 h quenching treatment at 1000 ℃, followed by the subsequent 2 h tempering at 400 ℃.

(2) The distribution and amount of secondary carbides are the underlying mechanisms, and they depend on the

quenching temperature. The subsequent tempering treatment following the quenching treatment at 1000 ℃ will enhance the impact toughness.

(3) Within a certain range, wear resistance is positively correlated with the hardness. The wear resistance depending on the “supporting” effect of matrix and the “protective” effect of hard carbide embedded in the matrix, is jointly decided by the hardness and the toughness of matrix.

Fig. 8. Local worn surface morphologies of different Cr26 specimens after wear tests ´2000

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Biographical notes ZHOU Shaoping, born in 1966, is currently a professor and a MS candidate supervisor at Key Laboratory of Pressurized System and Safety of Ministry of Education, East China University of Science and Technology. She received her PhD degree from East China University of Science and Technology, China, in 2009. Her research interests include fluid machinery and engineering, structure healthy monitoring and performance analysis of material. Tel.: +86-21-64252461; E-mail address: [email protected] SHEN Yehui, born in 1985, is currently master candidate at Key Laboratory of Pressurized System and Safety of Ministry of Education, East China University of Science and Technology. His research interests include wear resistance of material. E-mail: [email protected] ZHANG Hao, born in 1981, is currently an engineer in Shanghai First Fluid Machinery Co., Ltd, China. He received his bachelor degree from University of Shanghai for Science and Technology. His research interests include pump design and performance analysis of material. E-mail: [email protected] CHEN Dequan, born in 1971, is currently a senior engineer in Shanghai First Fluid Machinery Co., Ltd, China. He received his bachelor degree from East China University of Science and Technology. His research interests include pump design and performance analysis of material. E-mail: [email protected]

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