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Materials Science and Engineering A 486 (2008) 112–116

Microstructure and mechanical properties of highboron white cast iron

Zhongli Liu a, Yanxiang Li a,∗, Xiang Chen a, Kaihua Hu b

a Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Department ofMechanical Engineering, Tsinghua University, Beijing 100084, China

b 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 satisfying3–5]. 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 [8–11].

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

Z. Liu et al. / Materials Science and Engineering A 486 (2008) 112–116 113

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, Fe–B,e–Mn, Fe–Cr, Fe–Si 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 Fe–Tiere added to remove oxygen and to fix nitrogen before Fe–B

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 Ø10 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 Ø8 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 (2θ)as 1◦ min−1.

. 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) 112–116

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 [12–14]. 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. 5–7 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 morphology of boride remains almostnchanged (Fig. 5). Without tempering, only a secondary precip-tation with the size of about 1 �m exists (Fig. 5). When temperedt 473 K, another secondary precipitation with the size of severalens of nanometers appears (Fig. 6); moreover, the number of thisecondary precipitation increases with the increasing temperingemperature (Fig. 7).

According to the XRD result (Fig. 8), no other phases are

ound except for M23(C,B)6 for the sample that is tempered at23 K after quenching in air. This means that these two sec-ndary precipitations have a same crystal structure. Accordingo Figs. 5–7, the secondary precipitation with a larger size forms

in air without tempering: (a) low magnification and (b) high magnification.

Z. Liu et al. / Materials Science and Engineering A 486 (2008) 112–116 115

Fig. 6. Microstructure of high boron white cast iron held at 1293 K for 2 h, quenched in air, tempered at 473 K for 1 h: (a) low magnification and (b) high magnification.

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ig. 7. Microstructure of high boron white cast iron held at 1293 K for 2 h, quenc

uring the austenitizing, and the other secondary precipitationith a smaller size forms during tempering. As we know, boron

s easy to separate by means of equilibrium or non-equilibriumtyle. Non-equilibrium segregation is a dynamic process, whichlways takes place in the course of quenching, and equilibriumegregation is a thermodynamics process, which only depends

ig. 8. X-ray diffraction pattern of high boron white cast iron after quenchingn air, and then tempered at 923 K for 1 h.

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air, tempered at 923 K for 1 h: (a) low magnification and (b) high magnification.

n the temperature [15]. Since the size of secondary precipi-ation reaches about 1 �m, it cannot form during quenching.herefore, the secondary precipitations with a larger size formsuring austenitizing, which is a process of equilibrium segrega-ion of boron. Guo et al. [16] has demonstrated that the matrix canissolve more boron in this kind of material. The dissolved boronould segregate to the grain boundary, and the number of seg-

egation increases with the increasing of temperature. However,hen the temperature reaches a threshold value, these secondaryrecipitations would be dissolved again. The secondary pre-ipitation with a smaller size obviously forms in the form ofquilibrium segregation.

.3. Mechanical properties

The mechanical properties of high boron white cast iron areisted in Table 2. The heat treatment means the samples was heldt 1293 K for 2 h, and then quenched in air, at last tempered at73 K for 1 h.

According to Table 3, the hardness of high boron white cast

ron is almost similar to high chromium white cast iron, buthe toughness is increased a lot, which is mainly due to thehanges of matrix. As shown in Fig. 9, the fracture includeshe tough fracture of matrix and brittle fracture of boride. The

116 Z. Liu et al. / Materials Science and Engineering A 486 (2008) 112–116

Fig. 9. Fractography of the sample after the test of impact toughness of high borofractography of boride.

Table 3Mechanical properties of high boron white cast iron

As-cast hardness/HRC 54.5Hardness after heat treatment/HRC 59.5Impact toughness after heat treatment (J cm−2) 12.5Fracture toughness after heat treatment (MPa m1/2) 32.1Tensile strength after heat treatment (MPa) 412

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Fig. 10. Hardenability curve of high boron white cast iron.

mprovement of the matrix toughness is the most basic reasonor the toughness increasing of high boron white cast iron.

The hardenability of high boron white cast iron is shown inig. 10. From this curve, it can be seen that high boron whiteast iron has very good hardenability, which means castings upo 120 mm size could be simply quenched in air. However, theres no study on how boron affects the hardenability.

. Conclusions

The microstructure of the high boron white cast iron com-rises a dendritic matrix and interdendritic eutectic boride M2Bn as-cast condition, where M represents Cr, Fe, or Mn. The

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n white cast iron: (a) low magnification, (b) fractography of matrix and (c)

oride morphology is much like that of carbide in high chromiumhite cast iron, but its microhardness is higher than carbide.he matrix comprises pearlite and martensite, and pearlite onlyccounts for a small percentage. After heat treatment, the matrixostly changes to martensite, but the boride morphology is

lmost unchanged. However, two kinds of secondary precipita-ions appear in the matrix. Both secondary precipitations, whichll forms by means of equilibrium segregation of boron, have ahemical formula of M23(C,B)6, but their forming processes areifferent. The hardness of high boron white cast iron is similaro that of high chromium white cast iron, but the toughness isncreased a lot, which is mainly due to the improvement of the

atrix toughness. In addition, the high boron white cast iron hasood hardenability as well.

cknowledgment

The present research was supported by the National Keyechnologies R&D Program (no. 2005BA324C).

eferences

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5085–5094.[5] M. Fiset, K. Peev, M. Radulovic, J. Mater. Sci. Lett. 12 (9) (1994) 615–617.[6] Ya.E. Gol’dshtein, V.G. Mizin, Met. Sci. Heat Treat. 30 (7–8) (1988)

479–484.[7] H. Baker, ASM Handbook: Alloy Phase Diagram [M], 10th ed., ASM

International Handbook Committee, Materials Park, OH, 1990.[8] K.D. Lakeland, US6171222B1 (January 9, 2001).[9] K.D. Lakeland, US6689315B2 (February 10, 2004).10] S. Aso, K. Ogi, J. Jpn. Inst. Met. 55 (3) (1991) 316–323.11] S. Aso, S. Goto, Y. Komatsu, et al., Int. J. Cast Met. Res. 11 (5) (1999)

285–290.12] N.F. Tisdale, Met. Prog. 41 (3) (1942) 330–331.

13] K.A. Taylor, S.S. Hansen, Metall. Trans. A 21 (6) (1990) 1697–1708.14] M. Paju, H.J. Grabke, H.P. Hougardy, Scand. J. Metall. 20 (2) (1991)

135–140.15] Y.Y. Zhu, X.L. He, et al., Acta Metall. Sinica 23 (3) (1987) A169–A175.16] C.Q. Guo, Ph.D. Thesis, The University of Queensland, Brisbane, 2002.

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