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    Materials Science and Engineering A 486 (2008) 112116

    Microstructure and mechanical properties of highboron white cast iron

    Zhongli Liu a, Yanxiang Li a,, Xiang Chen a, Kaihua Hu ba Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Department of

    Mechanical Engineering, Tsinghua University, Beijing 100084, Chinab Zhedong Precision Casting Co. Ltd., Ningbo 315137, China

    Received 31 May 2007; received in revised form 21 August 2007; accepted 2 October 2007

    bstract

    In this paper, high boron white cast iron, a new kind of wear-resistant white cast iron was developed, and its microstructure and mechanicalroperties were studied. The results indicate that the high boron white cast iron comprises a dendritic matrix and an interdendritic eutectic boriden as-cast condition. The distribution of eutectic boride with a chemical formula of M2B (M represents Cr, Fe or Mn) and with a microhardnessf HV2010 is much like that of carbide in high chromium white cast iron. The matrix includes martensite and a small amount of pearlite. Afteruenching in air, the matrix changes to martensite, but the morphology of boride remains almost unchanged. In the course of austenitizing, aecondary precipitation with the size of about 1 m appears, but when tempered at different temperature, another secondary precipitation with theize of several tens of nanometers is found. Both secondary precipitations, which all forms by means of equilibrium segregation of boron, have a

    hemical formula of M23(C,B)6. Compared with high chromium white cast iron, the hardness of high boron white cast iron is almost similar, buthe toughness is increased a lot, which attributes to the change of matrix from high carbon martensite in the high chromium white cast iron to lowarbon martensite in the high boron white cast iron. Moreover, the high boron white cast iron has a good hardenability. 2007 Elsevier B.V. All rights reserved.

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    eywords: White cast iron; Boride; Wear-resistant material

    . Introduction

    In the course of the development of white cast iron, themprovement of toughness is a concerned problem all the time.he invention of high chromium white cast iron was considered areakthrough, as its toughness was increased a lot compared withlain white cast iron and Ni-hard white cast iron, which attributeso the improvement of carbide morphology [1,2]. However, theigh chromium white cast iron is still a kind of brittle mate-ial that cannot meet the demand of serious work conditions.

    any works have been done to improve the toughness of highoron white cast iron further, but the results are not satisfying35]. The reason is that it is difficult to change the characteris-

    ics of matrix. Since the matrix causes the toughness problem,he increase of the high chromium white cast iron depends onhe improvement of the toughness of the matrix. Therefore, it

    Corresponding author. Tel.: +86 10 62773640; fax: +86 10 62773637.E-mail address: yanxiang@tsinghua.edu.cn (Y. Li).

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    921-5093/$ see front matter 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.10.017

    s a crucial problem to get a strengthening and tough matrix forigh chromium white cast iron, which seems impossible. Theevelopment of white cast iron needs another breakthrough.

    High boron white cast iron is a new kind of wear-resistantaterial, which takes boride as strengthening phase and solves

    he problem of low toughness of matrix. The basic thought ofesigning high boron white cast iron is that boride is used toeplace carbide in high chromium white cast iron, and at theame time the carbon content in high chromium white cast iron isecreased to a low level to get a strengthening and tough matrix.his thought is feasible, as on the one hand, boride has higherardness than carbide [6], which can be taken as strengtheninghase; on the other hand, the solubility of boron in iron is veryow (below 973 K, the solubility is less than 0.0004% [7]), which

    akes the formation of boride possible when boron is added inhe iron melt. This kind of material appears at the end of 1980s,

    y far, there have been some patents and papers reported [811].

    This paper intends to study the basic characteristics of highoron white cast iron and to provide some information for furtheresearch.

    mailto:yanxiang@tsinghua.edu.cndx.doi.org/10.1016/j.msea.2007.10.017

  • Z. Liu et al. / Materials Science and Engineering A 486 (2008) 112116 113

    Table 1The chemical composition of tested high boron white cast iron (wt.%)

    B 1.62C 0.32Si 0.46Mn 0.61Cr 10.85CTA

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    . Experimental procedure

    The chemical composition of the tested high boron white castron is presented in Table 1.

    The high boron white cast iron was melted in a 100 kg capac-ty medium-frequency coreless induction furnace with SiO2ining, with charge materials of steel scrap, graphite, FeB,eMn, FeCr, FeSi master alloys and Cu. As boron is anctive element, oxygen and nitrogen in the melt should be effec-ively removed to ensure the yield of boron. Al wire and FeTiere added to remove oxygen and to fix nitrogen before FeB

    lloy was added in. The melt was superheated to 1550 C, andhen was poured into Y blocks made by investment casting.

    All samples were cut from the lower part of the Y blocks.he sample surfaces were removed by 3 mm to eliminate anyxidized layer prior to the hardness measurement. The heatreatment of samples was carried out in an electrical resistanceurnace. The samples were held at 1293 K for 2 h, quenched inir, and then tempered at 473 K for 1 h. After the heat treatment,he samples were machined to 20 mm 20 mm 110 mm.mpact tests were done using a 150-J capacity machine at roomemperature. The impact toughness values reported are the aver-ges of three tests. Hardness and microhardness were tested on aockwell hardness machine and NMt-3 machine, respectively.ive readings were taken on each sample and the average of them

    s reported. The tensile strength was tested on an AG-100KNEesting machine, and the size of samples is 10 mm 130 mm.he fracture toughness was tested on a MS New 810 testingachine, and the size of samples is 20 mm 40 mm 140 mm.

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    Fig. 1. As-cast microstructure of high boron white cast iro

    Fig. 2. X-ray diffraction pattern of as-cast high boron white cast iron.

    ilatometer test was carried out on a Gleebe-1500 testingachine, and the size of sample is 8 mm 120 mm. After

    tched with 10 vol.% HNO3 + 3 vol.% HCl + 10 vol.% saturatedeCl3 + 77 vol.% ethanol solution, the microstructures of theamples were examined with a Neophot 32 optical microscopeOM) and a FEI Quanta 200 FEG scanning electron microscopeSEM) equipped with energy dispersive X-ray spectrometerEDX). X-ray diffraction (XRD) analysis was performed on a/max-RB X-ray diffractometer to determine the boride type

    fter heat treatments. The specimens were scanned using Cu radiation at 40 kV and 300 mA. The scanning speed (2)as 1 min1.

    . Results and discussion

    .1. As-cast microstructure

    In as-cast condition, high boron white cast iron comprises

    dendritic matrix and an interdendritic eutectic compound

    Fig. 1). The eutectic compound has a chemical formula of M2Bccording to XRD picture (Fig. 2), where M represents Fe, Cr orn in terms of the EDX of boride (Fig. 3). The boride morphol-

    n: (a) OM micrography and (b) SEM micrography.

  • 114 Z. Liu et al. / Materials Science and Engineering A 486 (2008) 112116

    Fig. 3. EDX spectra of boride in high boron white cast iron.

    Table 2The composition of martensite and pearlite (wt.%)

    Phase C Cr Mn Si Cu

    MP

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    artensite 0.29 6.23 0.59 0.48 0.22earlite 0.45 7.01 0.60 0.54 0.24

    gy of high boron white cast iron is much like that of carbide inigh chromium white cast iron, but the microhardness of borideeaches HV2010 that is higher than that of carbide.

    The matrix is made up of martensite and a small amount ofearlite, and the pearlite is mainly distributed in the center ofatrix. The compositions of martensite and pearlite detected byDX are listed in Table 2.

    From Table 2, the main differences between the compositionf martensite and that of pearlite are carbon content. As we know,oron can greatly increase the hardenability of steels with a littlemount, but this effect is decreased quickly with the increase ofarbon content [1214]. Though most of the boron forms boride,here should be some boron dissolved in matrix and thereby

    ffect the matrix. Since carbon content in pearlite is higher thanhat in martensite, the region that changes to pearlite has a lowerardenability than the region that changes to martensite.

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    ig. 5. Microstructure of high boron white cast iron held at 1293 K for 2 h, quenched

    ig. 4. Dilatometer curve for the high boron white cast iron started from as-casttate.

    .2. Microstructure after heat treatment

    In order to find adequate austenitizing temperature, the phaseransformation point was tested, and the dilatometer curve washown in Fig. 4.

    According to Fig. 4, the phase transformation of auteniteegins at about 1123 K and ends at about 1173 K. Therefore,he austenitizing temperature is decided at 1293 K, which canssure the whole austenization of high boron white cast iron.

    Figs. 57 demonstrate the microstructures of high boronhite cast iron with no tempering, tempered at 473 and 923 K

    fter quenching in air. After quenching, the matrix is changedo martensite, but the mo