D0I:10.13374/j.issn1001-053x.1989.06.030 第11老第6期 北京科技大学学报 Vol,11 No.6 1989年11月 Journal of Uniersity of Science and Technology eeijing Nov,1989 Heat Treatment of WC-Co Hardinetals and Transformation Temperature of Cobalt Phase' Lai Heyi(锁和怡)，Yang Jinhui(杨金诨)* ABSTRACT:The Present work has found that the transverse rupture stren. gth of WC-Co hardmetals can be improved by queuching heat treatment.The increment of transverse rupture strength (ATRS)was dependent on the cobalt content of hardmeta'.The higher the cobalt content of hardmetal is,the more the increment of transverse rupture strength is.The main reas)n is that the trensformation cf face centered cubic cobalt stabilized at bigh temperature to hexagonal close packed cobalt can be depressed by quenching.The transformati- on temperature of hexagonal close packed cobalt binder phase was determined by differential thermal analysis.It was found that the transformation tempera- ture increases with increase of cobalt content of hardmetal.The reason is that the cobalt binder phase of high cobalt hardmetal contains higher tungsten conte- nt than that of low cobalt hardmetal after quenching. KEY WORDS:heat treatment,hardmetal,cobalt,transverse rupture strength In recent years the scientists and producers pay much attention to improve the mechanical properties of WC-Co hardmetals by heat treatmentt1,2).The reason why the properties cf WC-Co hardmetals can be improved by heat trea- 、 tment is that the face centered cubic(fcc)cobalt stabilized at high temperature can be completely or partly kept at room temperatuae by quenching.In order to reasonably select the quenching temperature of WC-Co hardmetals the trans- formation tempetature of hexagonal close packed (hep)cobalt phase to face cen- tered cubic cobalt phase must be determined.The present work has determined the transformation temperature of three kinds of WC-Co hardmetals by differe- ntial thermal analysis.The differences of the transformation temperatures bet- ween three kinds of WC-Co hardmetals have been discussed.In addition,the Manuscript Received August 15,1989 ..Dept.of Materials Science and Engineering 575
第 卷第 期 年 月 北 京 科 技 大 学 学 报 。 盯 只手 武 爪 一 良 击 椒 叭 协 止 万 少 赖 和 怡 ， 厂 夕 ‘ 杨 金 辉 、 、 ‘ 里 从厂 一 手 丛 〕 、 ， 一 · 士。 卜 卜 叭尸 · 里 。 一 认 ， 五 ， ， 、 、 火 一 一 ， 〕 一 汉 士 · 一 飞 。 一 ， 一 一 · ， ， ， · DOI ：10．13374／j ．issn1001－053x．1989．06．030
mechanical properties and phases of WC-Co hardmetals which were quenched also have been studied. 1 Experimental Method (1)Raw Materials Total carbon content of WC powder is 6.01%and free carbon content of WC powder is 0.03%. Total cobalt content of cobalt powder is 98.86%. (2)Size of Specimens The size of specimens which were used for testing transverse rupture stren- gth (TRS),hardness (HRA),density (D),chemical analysis and phase analysis etc.was5mm×5mm×30mm. (3)Composition of Hardmetal Specimens The composition of hardmetal specimens was shown in Table 1. Table 1.Composition of hardmetals Composition,wt% Grade WC Co YG8 92 8 YG15 85 15 YG20 80 20 2 Experimental Results 2.1 The Transformation Temperature of Cobalt Binder Phase during Heating The differential thermal analysis (DTA)curves of WC-Co hardmetals were given in Fig.1. 庆 742℃ 821C (a)YG8 (b)YG15 (o)YG20/ Fig.1 The Differential Thermal Analysis (DTA)curves of WC-Co hardmetals 576
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From Fig.1,it has been found that the transformation temperatures of hep cobalt binder phase to fcc cobalt binder phase during heating are as follows:YG8- 742°C,YG15-770°C and YG20-821°C. 2.2 The Influence of Heat Treatment on the Properties of WC-Co Hardmetals The mechanical properties and density of WC-Co hardmetals after sintering and heat treatment are shown in Table 2. Table 2 Properties of WC-Co hardmetals Grade Conditions TRS(MPa)HRA D(g/cm3)ATRS(MPa) YG8 as-sintered 1985 89.8 14.64 YG8 quenched in oil from 1000C 2017 89.6 14.65 32 YG8 quenched and tempered at 2052 89.7 14.67 67 600°Cfor2h YG15 as-sintered 2923 85.6 14.03 YG15 quenched in oil from 1250C 3092 85.5 14.07 169 YG20 as-sintered 2951 83.5 13.45 YG20 quenched in oil from 1250C 3238 83.7 13.50 287 2.3 The Phases of WC-Co Hardmetals X-ray diffraction analysis was carried out on the specimens of as-sintered and quenched hardmetals.The results are shown in Table 3. Table 3 The phases of WC-Co hardmetals Grade Conditions Phases Found YG8 as-sintered WC,WC1-x,e-Co YG8 quenched in oil from 1000C WC,Coe WeC,Co3W YG15 as-sintered WC1_x,Co:W,e-Co YG15 quenched in oil from 1250C WC1_x,Co3 Wa C,Coe WeC,e-Co,a-Co YG20 as-sintered WC,WC1_x,CosW,8-Co YG20 quenched in oil from 1250C WC,WC1-x,Coe WeC,a-Co,e-Co a-Co=fcc Co,:-Co=hcp Co, 3 Discussion From Fig.1,it was found that the transformation temperatute of hcp co- balt binder phase to fcc cobalt binder phase increases with the increase of cobalt content of hardmetal and is much higher than the transformation temperature (421C)of pure cobalt.The factors which have influence on the transformation 577
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temperature of ccbalt binder phase must be sludied. The temperature of allotrepic transformaticn of crbalt depends criticaily on purity and the rate cf temperature change.The cebalt binder phase of hardme- tal usually dissves tungsten and carbon by which the transformation temperature of ccbalt binder phase will be changed.It hes already found that carbon does not successfully stabilize fcc cobalt binder phese at all temperatures as tungsten which is usually in cobalt solid solution counteracts the effect of dissolved carben by stabilizing hcp cobalt binder phaset3l.According to cobalt-tungsten phase diagram,the more the tungsten content in cobalt solid solutien is,the higher the transformation temperature ist1. Tungsten content of cobalt binder phase of as-sintered hardmetal depends on the follow ing two factors:sintering temperature and cooling rate.In terms of WC-Co phase diagram the solubility of WC depends on the sintering temperature. The liquid cobalt phase dissolves more WC at highsi ntering temperature than that at low sintering temperature.Tungsten content in cobait binder phase can be hept mcre by quick cooling to room temperature than tht by slow cooling.If the cooling rate is the same,cobalt binder phase of 'as-sintered YG8 has more tungsten than that of YG15 and YG20 because the sintering temperature of YG8 is the highest.This was approved by present study.The composition of cobalt phase cf YG15 and YG20 was analyzed by energy spectrum and the results were given in Table 4. Table 4.Composition of cobalt binder phase cf YG15 and YG20 Composition,wt% Grade Conditions Co W YG15 as-sintered 74.28 25.72 YG15 quenched in oil from 1250C 69.60 30.40 YG20 as-sintered 81.86 18.34 YG20 quenched in oil from 1250C 63.71 36.26 It should be very reasonable to justify that the transformation of cobalt bi- nder phase of YG15 should be higher than that of YG20.The present experim- ental resuits in Fig.I was contrary t)above inference.What is the reason? It is wellknow n that there are many f:.ctors w hich influence on the transforma- tion temperature of cobalt phase for example:impurities in hardmetal,grain size of cobalt phase etc..In parlicular,the grain size affects the stability of two allotropes of cobalt and fine grain size is favourable for fcc cobaltt31.In pre- sent study it w as f.und that during rebealing the tungsten content in cobalt phase 578
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will increase.The increment of tungsten of low cobalt hardmetal is lower than that of high cobalt hardmetal.From the results in Table 4,it can be found that before quenching tungsten content in cobalt binder phase of YG15 and YG20 is 25.72%and 18.34%respectively and after quenching that of YG15 and YG20 is 30.40%and 36.26%respectively.That is why the transformation temperature of hcp cobalt binder phase to fcc cobalt binder phase increases with the increase of cobalt ccntent of hardmetal. Frcn tne experimental results of Table 2,it can be found that the transver- se rupture strength of WC-Co hardmetal can be improved by quenching and tem- pering.The increment of transverse rupture strength was dependent on the coba- It content of hardmetal.After quenching the increments of TRS of YG8,YG15 and YG20 were 32,169 and 287 MPa respectively.The higher the cobalt con- tent of hardmetal is,the more the increment of transverse rupture strength is. The increment of TRS of YG8 after quenching and tempering was up to 67 MPa. The properties of hardmetal depend on their compcsition,WC grain size, industrial process etc..The properties of hardmetals which have same cobalt content mainly depend on the composition and structure of cobalt phase and gra- in size of WC.The improvement of properties of WC-Co hardmetal by heat treatment mainly belonged with the change of composition and structure of cobalt phase,although the grain size and mophology of WC also had a little change. WC-Co hardmetals have higher transverse rupture strength and toughness if co- balt phase has more ductile fcc cobalt.The fcc cobalt phase stabilized at high temperature can completely or partly be kept at room temperature by quenching as shown in Table 3.That is,the reason why the transverse rupture strength and toughness can increase by heat treatment.This was discussed in detail in cur early published papert11. In addition tungsten and carbon dissolve in cobalt phase.The solubility of tungsten in cobalt phase is appreciably greater at hign temperature than that at room temperature so that tungsten content in cobalt phase at high temperature can be frozen by quenching as shown in Table 4.The stacking fault energy of fcc cobalt phase increases with increase of its tungsten content.Tungsten content in fec cobalt phase not only increases the solid solution hardening but also increases the stacking fault energy and decreases the width of the stacking fault.Therefore the martensite transformation of fcc cobalt phase to hep cobalt phase can be depressed by dissolved tungsten.For example cobalt binder phase of as-sintered YG20 was hep structure and after quenching it became fcc and hep structure as shown in Table 3. 4 Conclusion (1)During heating the transformation temperatures of hcp cobalt binder pha- 579
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