Effect of Heat Treatment on Structure and Properties of WC-Co Hardmetals

D0I:10.13374/j.issn1001-053x.1991.s2.006 塘13老第5(1)期 北京科技大学学报 vot,13 No.5(1) 1991年9月 Jouraal of University of Science and Technology Beijing Sept.1991 Effect of Heat Treatment on Structure and Properties of WC-Co Hardmetals Yang Jinhui◆Lai Hoyi ABSTRACT:Heat treatment changes the structure of the cobalt binder phase and the properties of the WC-Co hardmetals.The higher the cobalt content in the alloy is,the more effective the heat treatment is.The present work has found that the transformation of a-Co to e-Co can be depressed by quenching the specimens in oil from 1000C and hence the transverse rupture strength of these specimens can be increased.During tempering the quenched specimens, new precipitated phases which have dispersion hardening effect on the cobalt binder phase have been observed. KEY WORDS:heat treatment,cobalt binder phase,transformation In recent years the scientists and producers pay much attention to develop new grades of hardmetals and try to improve the properties of the traditional or basic grades by heat treatment and etc(-s).The present work attempts to control the structure of cobalt binder phase and to improve the properties of hardmetals by heat treatment.We have found that the transverse rupturi stre- ngth (TRS)of WC-25%Co and WC-20%Co increased to 122%and 111%respe- ctively by quenching.The TRS of WC-20%Co alloy iacreased to 126%by quenching and tempering.The influence of quenching temperature on the struc- ture and properties of WC-25%Co alloy and the influence of tempering tempe- rature on the hardness of WC-10%Co alloy were also studied. 1 Experimental Method (1)Raw Materials 1391一06一13收稿 ·矿业研究所(Institute of Mining and Mineral Engineering) ··材料科学与工程系(Department of Materials Science and Engincering 23

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Total carbon content of WC powder is 6.01%and free carbon content is 0.3%. +↓ The cobalt content of cobalt powder is 98,86%. (2)The size of the specimens which were used for test transverse rupture strength (TRS),hardness (HRA)and density was 5mm x 5mmx 30mm, 2 Experimen'tal Results 2.1 Influence of Heat Treatment on the Properties of Hardmetals with Different Content of Cobalt The sintered hardmetals,the properties of which are shown in Table 1, were heated at 1000c for 30 minutes and then queached in oil.., Table 1 Influence of heat treatment on the properties of hardmetals with different content of cobalt Composition Heat treatment Density Hardness TRS △TRS of alloys (g/cm3) (HRA) (N/mm3) (N/im3) WC-10%Co as-sintered 14.49 88.0 1454 WC-10%C0 quenchea 14.47 87.6 1454 0 WC-15%Co as-sintered 13.76 85.3 -1677 WC-15%Co quenched 13.77 85.3 1706 29 WC-20%Co as-siatered 13,39 83.0 1758 一 WC-20%Co quenched 13.39 83.1 1927 169 WC-25%Co as-sintered 13.03 81.8 1822 WC-25%Co queached. 13,01 82.2 ,2164 342 21 The increment of transverse rupture strength (ATRS)after quenching was increase with the content of cobalt in hardmetals as shown.in Table 1. 2.2 The Influence of Quenching Temperature on the Properties of WC-25% Co Alloy The specimens of WC-25%Co alloy,the properties of which are shown in Table 2,were heated at different temperatures (700-1100C)for.30,minutes and then quenched in oil. The specimens of WC-25%Co alloy heated at 1000C for 30 miutes and quenched in oil have got the highest TRS. 2,3 The Properties of WC-20%Co and WC-25%Co Hardmetals The properties of WC-20%Co and WC-25%Co hardmetals after tempering at 400C for 17 hours are shown in Table 3.. 30

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Table 2 The influece of quenching temperature on the properties of WC-25%Co alloy Quenching temperature Density Hardness TRS (℃) (g/cm3) (HRA) (N/mm2) 700 13.012 83.4 204.5 750 13.019 83.9 218.0 800 13,029 82.3 214.9 850 13,056 82.2 214.6 900 13.989 81.6 199.0 950 13.063 83.1 238,2 1000 13,052 83,1 242.0 1050 13.031 82.1 208.1 1100 13.009 82.2 216.4 Table 3 The,properties of WC-20%Co and WC-25%Co alloys after quenching and tempering Composition Heat treatment Density Hardness TRS of alloys (g/cm3) (HRA) (N/mm3) WC-20%Co as-sintered 13.39 83.0 175.8 WC-20%Co quenched and tempered 13.45 83.1 193.8 WC-25%Co as-sintered 13.03 81.8 182 WC-25%Co quenched and tempered 13,03 84.0 223.7 2,4 The Influence of Tempering Temperature on the Hardness of WC-10%Co Alloy The influence of tempering temperature on the hardness of WC-10%Co alloy was shown in Table 4.The specimens were heated at 1000C for 30 minutes and quenched in oil and then tempered at different temperatures,400- 900℃. Table 4 The influence of tempering temperature on the hardness of WC-10%Co alloy No. Tempering temperature Haraness (℃) HV60 400 1253 500 1350 600 1314 4 700 1287 800 1332 900 1332 2.5 The Results of Magnetic Examination and X-ray Analysis The influence of heat treatment on the coercive force Hc and magnetic 3

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energy product of WC-20%Co alloy was shown in Table 5. Table 5 The influence of heat treatment on the magnetic Properties of WC-20%Co alloy Heat treatmen Coercive force Max.energy product Hc (Oe) BHms (G.Oe) as-sintered 28.5 152 quenched in oil from 1000c 24.4 130 qucached as above and tempered 26.0 136 at700℃for17 hours X-ray diffraction was carried out on the specimens of quenched or unqu- enched WC-15%Co and WC-20%Co alloys and tempered WC-20%Co alloy at different temperatures.The results are shown in Table 6. Table 6 The results of X-ray diffraction analysis on the specimens of WC-15%Co and WC-20%Co alloys Alloy Hcat treatment Phases found WC-15%Co as-sintered WC,W2C,WC1-,e-Co WC-15%Co queached in oil from 1000c WC,W2C,WC1-,a-Co,:-Co WC-20%C0 as-sintered WC,W2C,WC1-x,e-Co WC-20%C0 quenched in oil from 1000c WC,WaC,WC1-,a-Co,:-Co.. WC-20%Co quenched and tempered at WC,W2C,WC1-x,Co,WoC4 500℃for17 hours WC-20%Co quenched and tempered at WC,W2C,WC1-:,Co3W3C 600℃for17 hours WC-20%Co quenched and-tempered at WC,W2C,WC1-,a-Co,e-Co 700℃for17 hours In order to determine the relative amount or ratio of a-Co to e-Co in the quenched and unquenched specimens of WC-Co hardmetals,the peak values of a-Co and e-Co in X-ray diffraction patterns have been compared,For cali- Table 7 The ratio of a-Co to e-Co of WC-Co hardmetals before and after heat treatment Alloy Heat treatment a-Co:e-Co WC-15%Co as-sintered 0:10 WC-15%Co quenched in oil from 1000C 5：5 WC-20%Co as-sintered 0:10 WC-20%Co quenched in oil from 1000c 7：3 WC-20%Co quenched in oil frem 1000c 6:4 and then tempered at 700c 32

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bration pure a-Co and pure e-Co were used,The ratio of a-Co to e-Co of WC-Co hardmetals before and after heat treatment was listed in Table 7. The scanning electron microscope picture of WC-25%Co alloy quenched from 1000C and tempered at 400C for 17 hours was shown in Fig.1 which revealed that precipitated phas's could be found in cobalt binder phase. Fig.1 SEM photograph pf WC-25%Co alloy quenched from 1000℃and tempered at400℃for17 hours 3 Discussion It is as noted above from Table 1 that the transverse rupture strength of WC-Co hardmetals will increase by heat treatment,quenching and tempering. The properties of hardmetals depend on their composition,WC grain size, industrial process and ete.The properties of hardmetals which have the same content of cobalt binder phase mainly depend upon the composition and stru- cture of cobalt binder phase and WC grain size.In present work the specimens made from the same alloy,which had the same composition and same grain size,were used.The WC grain size was unchanged during quenching or tempe- ring.The reason why the propertics of hardmetals were chanyed by heat treatment is that there are some changes which have happened in cobalt binder phase.That is why the tratisverse rupture strength increases with the coblt content in hardmetals.The higher the cobalt content is,the more effe- ctive,the heat treatment is. The cobalt is an allotropic clement and has two crystal structures,face center cubic.(fcc),a-Co and hexagonal close packed (hcp),e-Co.The allo- tropic transformation temperature of pure cobalt is about 420C.The allotro- pic transformation temperature of cobalt binder phase in WC-Co alloys is rai- sed to about 720C by dissolve of tungsten and carbon in it during sintering. The cobalt in WC-Co alloys is a binding phase which makes the brittle tung- sten carbide grains closed together and the alloys had strength and'toughness, 33

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