Materials and Design 39 (2012) 303–308

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Materials and Design

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Short Communication

Effect of tungsten on microstructure and properties of high chromium cast iron

Yezhe Lv, Yufu Sun ⇑, Jingyu Zhao, Guangwen Yu, Jingjie Shen, Sumeng HuSchool of Materials Science and Engineering, Zhengzhou University, 97 Wenhua Road, Zhengzhou 450002, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 December 2011Accepted 23 February 2012Available online 7 March 2012

0261-3069/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2012.02.048

⇑ Corresponding author. Tel.: +86 371 63887502; faE-mail address: yufusun@zzu.edu.cn (Y. Sun).

In this study, effect of tungsten on microstructure and properties of high chromium cast iron was inves-tigated. The experimental results indicated that tungsten distributed uniformly in the matrix and car-bides. W carbides are composed of WC1�x, W6C2.54 and CW3 and W2C. With the increase of tungstencontent, bulk hardness and matrix microhardness both increased gradually and reached the peak at62.62HRC and 913HV, respectively. All of the tungsten-containing alloys performed better than tungsten-freealloys in impact tests and alloys containing 1.03 wt% W showed the highest impact toughness at8.23 J cm�2. Tungsten considerably improved the performance of high chromium cast iron on wearresistance and alloys containing 1.03 wt% W increased 205% compared to tungsten-free alloys. Therefore,tungsten can be used as an alloying element to increase the hardness and wear resistance without scari-fying impact toughness in high chromium cast iron. Alloys containing 1.03 wt% tungsten showed theoptimum properties.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Wear has been defined as the material removal from solid sur-face [1], which is one of the most important materials degradationphenomena and results in enormous costs to the nation frommaterials and energy consuming. This phenomenon is particularlyvital in the mining and mineral processing industries [2]. Highchromium cast iron has been considered as a kind of primarymaterials to combat wear conditions for a long history, due tothe excellent wear resistance, relative low price and easy produc-tion [2–8]. It has been long proved that the good wear resistanceof high chromium cast iron comes from the mutual interaction be-tween matrix and carbides. Generally, carbides in the microstruc-ture, depending on the type, size and distribution, provide thehardness, which is required for applications without degradation[9–11]. The matrix surrounding carbides, which could be exten-sively altered through the destabilization and sub-critical heattreatments, provides sufficient mechanical support to prevent car-bides from cracking deformation. Furthermore, it has been recog-nized that the precise combination of heat treatments coulddramatically alter the microstructure and, hence, wear properties[12].

Researchers [5,8,13–15] studied the effect of different alloyingelements, such as titanium, niobium and molybdenum, whosecarbides played an important role in improving wear resistance,on high chromium cast iron. Tungsten, which is also a strongcarbide-forming element similar to vanadium, titanium, niobiumand zirconium, was reported to be useful to improve wear

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resistance of high chromium cast iron [6]. Unfortunately, therewas little reference in these literatures about an optimal contentof tungsten.

In the current research an attempt has been made to determinethe optimal content of tungsten to maximize the wear resistancewithout sacrifice of toughness. With this aim, samples were takeninto a specialized heat treatment process. Microstructure beforeand after heat treatment was observed using Optical Microscope.X-ray diffraction was employed to describe the phase characteris-tic and the relationship between phase constitution and properties.Distribution of different elements in the alloys was measured usingEnergy Dispersive Spectrometer (EDS).

2. Experimental procedure

The research samples used in the present study were preparedin a 50 kg medium frequency induction furnace by using chargematerials of steel scrap, graphite, Fe–Cr, Fe–Mn, Fe–Si, Fe–Mo mas-ter alloys and cooper. Then the melt was rapidly superheated to1560 ± 20 �C. After holding at the temperature for 5 min, the mol-ten alloy was poured into the ladle with Fe–W master alloys andRE-Nb inoculation placed at the bottom, then transferred to Y blocksand molds. Consequently, cast bars with different contents oftungsten were obtained. Chemical analysis was conducted by spec-trometry from chilled samples obtained during casting, and the fi-nal chemical compositions are shown in Table 1.

Samples for metallography, hardness measures, impact andwear tests were cut from the cast bars, respectively. Cutting wasdone as slow as possible in order to avoid excessive overheatingwhile copious amounts of water as coolant were used. Heat treat-ments of all samples were conducted with an electrical resistance

Table 1Chemical composition analysis of samples (wt%).

Samples Fe C Cr Si Mn P S Cu Mo W

Num. 0 Bal. 3.08 24.45 0.74 1.06 0.034 0.022 1.10 1.06 0Num. 1 Bal. 2.92 24.28 1.17 0.90 0.036 0.024 1.11 1.06 1.03Num. 2 Bal. 2.62 24.18 1.19 1.19 0.034 0.025 1.17 0.85 1.95Num. 3 Bal. 2.98 24.23 0.86 1.13 0.031 0.023 1.07 0.96 2.75

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furnace. In order to study the effect of W on the microstructure andproperties, samples with different W were taken into the sameheat treatment procedure: after annealing at 950 �C for 4 h, thesamples were held at 1050 �C for 2 h, quenched in air, then tem-pered at 250 �C for 3 h and then cooled in air.

Samples for metallography were prepared in the traditionalmethod: roughed by abrasive paper and then polished using0.25 lm diamond paste. Microstructure of samples before andafter heat treatment was revealed by an OLYMPUS BH-2 OpticalMicroscope and a Philips-Quanta-2000 Scanning Electron Micros-copy (SEM) equipped with an EDS analyzer. Distributions of differ-ent elements were also observed in this stage. With the aim ofidentifying the phase constitution of the samples, X-ray diffractionanalysis was performed on a D/Max-3B XRD (X-ray diffractome-ter). The samples were scanned in a 2h range of 0–80�, usingCu-K radiation at 40 kV and 40 mA with a speed of 6� min�1. Bulkhardness of the samples was measured on the Rockwell C scale bythe mean of five measurements using HR-150 Rockwell HardnessMachine [16]. An HV-1000 microhardness tester with a load of1 kg was employed to determine the matrix microhardness ofthe samples, and 10 readings were taken on each sample in a ran-dom way [17]. Samples without notch were taken into impact testsby the mean of three measurements using a JB-30 Impact TestingMachine [18]. Finally, wear behavior of the high chromium castiron was investigated on a ML-100 wear resistance machine inambient environment [19].

3. Results and discussion

3.1. Microstructure examination

According to Liming Lu and his colleagues [20], eutectic compo-sition of high chromium cast iron could be determined with thecalculation formula: [%C] + 0.0474 � [%Cr] = 4.3. Unfortunately,the influence of silicon was not considered in this formula. In thisstudy, carbon equivalent of different samples were employed inthis formula to instead of carbon content, and the results wereshown in Table 2. According to the results in Table 2, Num. 2 sam-ple was considered as the only hypoeutectic alloy, while the restthree samples were hypereutectic alloys.

Fig. 1 shows that the as-cast microstructure of Num. 0, Num. 1and Num. 3 samples mainly consists of refined hexagonal-shapedprimary M7C3 and eutectic carbides in a matrix of austenite anda small amount of martensite. While large scales of primaryaustenitic dendrite emerge in the Num. 2 samples resulting fromthe hypoeutectic composition. As shown in Fig. 1, carbides inNum. 3 samples show a notable increase in size and a more

Table 2The [%CE] + 0.0474 � [%Cr] of samples (wt%).

Samples C Si Cr CE [%CE] + 0.0474 � [%Cr]

Num. 0 3.08 0.74 24.45 3.33 4.4893Num. 1 2.92 1.117 24.28 3.29 4.4408Num. 2 2.62 1.119 24.18 2.993 4.1386Num. 3 2.98 0.86 24.23 3.27 4.4185

irregular distribution in the matrix, compared with Num. 1, dueto the increase of tungsten content. Similar results were observedin the literature [24], which were due to the combination of tita-nium and tungsten. The microstructure of Num. 0 and Num. 1 sam-ples after heat treatment are shown in Fig. 2. It can be seen fromFig. 2 that the matrix of high chromium cast iron changed fromaustenite to lath-type martensite after heat treatment, while a lit-tle of retained austenite still existed. Furthermore, microstructureshowed a significant refinement and homogeneous due to theaddition of tungsten.

XRD results shown in Fig. 3 reveal the presence of smallamounts of martensite in the as-cast samples. The presence ofmartensite in these irons was also been widely reported in theseliteratures [21,22]. Due to the addition of chromium, molybdenumand tungsten, which absorb carbon from the austenite during eu-tectic solidification, a narrow area at the austenite region becomesempty of carbon. The lack of carbon in the austenite increases themartensite start temperature (MS), which allows austenite trans-forming to martensite during subsequent cooling down. Fig. 3 alsoshows that Cr carbides at the casting mainly comprise M23C6 andM7C3. and no obvious change in the characteristic of Cr carbideswere observed after heat treatment. WC carbides reported in liter-ature [6] were not observed in this experiment. Instead, W carbidesare composed of WC1�x, W6C2.54 and CW3 in the samples as-cast,and W2C appears after heat treatment. This phenomenon may re-sult from the higher content of W in this experiment, comparedwith the alloys in literature [6]. As shown in Fig. 4, There is noalteration in distribution of different elements before and afterheat treatment. Nearly most C, Cr and Mn distributed in the car-bides, while W showed a uniform distribution both in the matrixand carbides. Therefore, tungsten has an equal effect on the hard-ness and strength of matrix and carbides.

3.2. Microhardness, hardness and impact toughness tests

Table 3 shows that the bulk hardness increased slightly with theincreasing tungsten in the hypereutectic samples. This logical phe-nomenon was achieved due to the reinforcement of matrix by thehard well-dispersed tungsten carbides. The finer structure ob-tained by tungsten additions may have also contributed to increasebulk hardness. High chromium cast iron reported in literatures[5,8] showed a similar tendency due to the addition of titanium,another strong carbide-forming element similar to tungsten. How-ever, Num. 2 samples showed the lowest bulk hardness value butthe highest impact toughness due to the hypoeutectic composition.

Fig. 5 shows the results (average values) of microhardness, bulkhardness and toughness tests on samples after heat treatment andshows the function of tungsten content. Microhardness measure-ment was taken in the matrix regions and implied that solid solu-tion of tungsten contributed largely to the increase of matrixmicrohardness. Similar results were reported in literature [6], allthe tungsten-containing samples showed higher matrix microh-ardness than tungsten-free samples. In addition, refinement of ma-trix resulted from the addition of tungsten may also contribute tothe increase of matrix microhardness. As a result, a larger strengthof matrix could be reasonably expected.

(a) (b)

(c) (d)

Fig. 1. Microstructure of the as-cast samples with different W contents of: (a) 0%, (b) 1.03%, (c) 1.97% and (d) 2.75%.

(b)(a)

Fig. 2. Microstructure of the samples after heat treatment with different W contents of: (a) 0% and (b) 1.03%.

Y. Lv et al. / Materials and Design 39 (2012) 303–308 305

Bulk hardness also experienced a rise with the increase of tung-sten content. Namely, for the tungsten-free samples hardness is58.87HRC and increased to 62.62HRC for the samples with 2.75%tungsten. According to the results reported in literature [24], withthe combined influence of Ti and W, bulk hardness significantly in-crease attributed to the dispersedly distribution of high hardnessMC carbides inside grains and at grain boundaries. Unfortunately,single action of W was not clearly presented in this literature. In thisexperiment, more W carbides formed and precipitated dispersedlyin the matrix as a result of the increase W amount. For the same rea-son, solid solution of tungsten in the matrix may also positivelyinfluence the bulk hardness. However, such a contribution would

be relatively weaker, compared with the precipitation strengthen-ing. In addition, the finer and homogeneous structure obtained bytungsten addition could be also benefit to the bulk hardness.

In contrast, impact toughness did not show a clear trend as afunction of tungsten content. Normally a uniform increase in bulkhardness results in homologous decrease in impact toughness.However, solid solution of tungsten in the matrix dramatically in-creased the matrix microhardness and, hence, the impact tough-ness. Furthermore, impact toughness decreased regularly withthe increase of W and samples containing 1.03% W performedthe highest impact toughness value of 8.23 J cm�2. The reason forthis phenomenon is that the isolation of the excessive carbides

(b)(a)

20 30 40 50 60 700

20

40

60

80

100

120

140

160

180

20 30 40 50 60 700

20

40

60

80

100

120

140

Fig. 3. X-ray diffraction results of high chromium white iron containing 1.03% tungsten: (a) as-cast and (b) 1050 �C quenching +250 �C tempering.

Original C Cr Si Mn W (a)

Original C Cr Si Mn W (b)

Fig. 4. Distributions of elements of high chromium white iron containing 2.75% tungsten: (a) as-cast and (b) 1050 �C quenching +250 �C tempering.

Table 3Bulk hardness and impact toughness of as-cast samples.

Samples Num. 0 Num. 1 Num. 2 Num. 3

Bulk hardness (HRC) 55.75 58.75 52.47 59.08Impact toughness (J cm�2) 9.43 11.18 12.33 9.16

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precipitated in the matrix has a detrimental influence on theimpact toughness despite of the matrix strengthening by the solidsolution.

Fig. 5. Matrix microhardness, bulk hardness and impact toughness as a function ofthe tungsten content in the samples after heat treatment.

3.3. Wear performance

Results shown in Table 4 could be used to rank materialsaccording to their resistance to abrasive wear in ambient environ-ment. The data indicates that all the tungsten-containing samplesas-cast showed a decrease of weight loss, on a large scale, com-pared with the tungsten-free samples. Similar results have beenobserved and reported in a previous work and is attributed tothe solid solution of tungsten in the matrix [6].

Fig. 6 shows the typical wear surface of samples before and afterheat treatment during the wear tests. The plowing grooves on as-cast samples are deeper, than samples after heat treatment, andthe debris along the grooves are higher. The surface is gettingsmoother as the function of heat treatment and comparison ofthe wear surface indicates that the wear-resistant capacity of sam-ples after heat treatment is better than that of samples as-cast,which is in agreement with the data shown in Table 4.

It is well-know that the interaction between carbides and matrixplays an important role in protecting the materials from wear loads[23,24]. Thanks to the dispersed distribution and increasing

amounts of W carbide, which has very high thermal stability andhigh hardness, wear load on every single carbide grain declinedreasonably, and the wear resistance increased accordingly. Besides,decrease of the average degree of freedom among carbides is alsoproved to weaken the wear on the matrix [24]. Therefore, tungstennotably inhibited the wear on these samples. However, wear resis-tance of samples with 2.75% W showed a slight reduction, com-pared with the samples with 1.03% W, due to the oversizecarbides and their less uniform distribution in the matrix. A similartrend was observed on samples after heat treatment. Num. 1 sam-ples (1.03% W) showed the minimum weight loss and an enhance-ment in vast scale on wear resistance at the experimental condition,

Table 4Results of wear resistance tests.

Samples At as-cast After heat treatment

Weight loss (mg) Wear resistance (m mg�1) Weight loss (mg) Wear resistance (m mg�1)

Num. 0 8.4 5.952 8.1 6.173Num. 1 3.8 13.159 2.7 18.819Num. 2 4.6 10.870 5.2 9.615Num. 3 3.9 12.820 3.7 13.513

(b)(a)

Fig. 6. SEM micrographs of wear surface with 1.03% tungsten: (a) as-cast and (b) after heat treatment.

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approximately 205%, compared with Num. 0 (negligible W). Num. 3(2.75% W) samples also exhibited an obvious decline of weight losscompared with Num. 0, but the superfluous tungsten in these sam-ples delayed the precipitation of W carbides during the destabiliza-tion process. Therefore, transformation from austenite tomartensite was abated as a result of solid solution of large amountselements in the matrix.

4. Conclusions

The major conclusions derived from this investigation are:

(1) The as-cast microstructure of high chromium cast ironshows the typical hypereutectic microstructure and mainlyconsists of refined hexagonal-shaped primary M7C3 andeutectic carbides in a matrix of austenite and a small amountof martensite. Furthermore, carbides show a notableincrease in quantity and size and distribute more irregularlyin the matrix as the increase of tungsten.

(2) W carbides are composed with WC1�x, W6C2.54 and CW3 inas-cast samples, and W2C appears after heat treatment.Tungsten shows homogeneous distribution both in thematrix and carbides before and after heat treatment.

(3) The bulk hardness and matrix microhardness both rise withthe increasing tungsten after heat treatment and reach thepeak values of 62.62HRC and 913HV, respectively.

(4) All the tungsten-containing samples perform better than thetungsten-free sample in impact toughness and the highestimpact toughness value is obtained on the samples contain-ing 1.03% tungsten.

(5) Addition of tungsten shows a significant influence on thewear resistance. Samples containing 1.03% W improve205% on wear resistance after heat treatment, comparedwith tungsten-free alloys, and show the optimumproperties.

References

[1] Bahrami A, Mousavi Anijdan SH, et al. Effects of conventional heat treatmenton wear resistance of AISI H13 tool steel. Wear 2005;258:846–51.

[2] Tasgin Y, Kaplan M, et al. Investigation of effects of boron additives and heattreatment on carbides and phase transition of highly alloyed duplex cast iron.Mater Des 2009;30:3174–9.

[3] Chen X, Li YX. Effect of heat treatment on microstructure and mechanicalproperties of high boron white cast iron. Mater Sci Eng A 2010;528:770–5.

[4] Zhi XH, Xing JD, et al. Effect of heat treatment on microstructure andmechanical properties of a Ti-bearing hypereutectic high chromium white castiron. Mater Sci Eng A 2008;487:171–9.

[5] Bedolla-Jacuinde A, Correa R, et al. Effect of titanium on the as-castmicrostructure of a 16% chromium white iron. Mater Sci Eng A2005;398:297–308.

[6] Mousavi Anijdan SH, Bahrami A, et al. Effect of tungsten on erosion–corrosionbehavior of high chromium white cast iron. Mater Sci Eng A 2007;454–455:623–8.

[7] Zhang TC, Li DY. Effect of alloying yttrium on corrosion–erosion behavior of27Cr cast white iron in different corrosive slurries. Mater Sci Eng A2002;325:87–97.

[8] Bedolla-Jacuinde A, Correa R, et al. The effect of titanium on the wear behaviorof a 16%Cr white cast iron under pure sliding. Wear 2007;263:808–20.

[9] Tabrett CP, Sare IR. Effect of high temperature and sub-ambient treatments onthe matrix structure and abrasion resistance of a high-chromium white iron.Scripta Mater 1998;38:1747–53.

[10] Matsubara Y, Sasaguri N, et al. Solidification and abrasion wear of white castirons alloyed with 20% carbide forming elements. Wear 2001;250:502–10.

[11] Dogan ON, Hawk JA, et al. Solidification structure and abrasion resistance ofhigh chromium white irons. Metall Mater Trans A 1997;28:1315–27.

[12] Sare IR, Arnold BK. The influence of heat treatment on the high-stress abrasionresistance and fracture toughness of alloy white cast irons. Metall Mater TransA 1995;26A:1785–93.

[13] Scandian C, Boher C, et al. Effect of molybdenum and chromium contents insliding wear of high-chromium white cast iron: the relationship betweenmicrostructure and wear. Wear 2009;267:401–8.

[14] Qu YH, Xing JD, et al. Effect of cerium on the as-cast microstructure of ahypereutectic high chromium cast iron. Mater Lett 2008;62:3024–7.

[15] Zhi XH, Xing JD, et al. Effect of titanium on the as-cast microstructure ofhypereutectic high chromium cast iron. Mater Charact 2008;59:1221–6.

[16] GB/T 230.1-2009. Metallic materials-Rockwell hardness test-Part 1: Testmethod [S]. China Standard Press.

[17] GB/T 4342-1991. Metallic materials-Vickers microhardness test [S]. ChinaStandard Press.

[18] GB/T 29-2007. Metallic materials-Charpy pendulum impact test method [S].China Standard Press.

308 Y. Lv et al. / Materials and Design 39 (2012) 303–308

[19] GB/T 12444-2006. Metallic materials-Wear test method [S]. China StandardPress.

[20] Lu LM, Soda H, et al. Microstructure and mechanical properties of Fe–Cr–Ceutectic composites. Mater Sci Eng A 2003;347:214–22.

[21] Bedolla-Jacuinde A. Microstructure of vanadium, niobium, and titaniumalloyed high-chromium white cast irons. Int J Cast Met Res 2001;13:343–61.

[22] Laird II G, Powell GLF. Solidification and solid-state transformationmechanisms in Si alloyed high-chromium white cast irons. Metall MaterTrans A 1993;24:981–8.

[23] Finnie L. Erosion of surfaces by solid particles. Wear 1960;3:87–103.[24] Zhang Y, Sun YF, et al. Effect of titanium and tungsten on the structure and

properties of heat-abrasion resistant steel. Mater Sci Eng A 2008;478:214–20.