{"id":1857,"date":"2019-05-22T02:48:24","date_gmt":"2019-05-22T02:48:24","guid":{"rendered":"http:\/\/www.meetyoucarbide.com\/single-post-heat-treatment-of-tungsten-carbide-products\/"},"modified":"2020-05-04T13:12:03","modified_gmt":"2020-05-04T13:12:03","slug":"heat-treatment-of-tungsten-carbide-products","status":"publish","type":"post","link":"https:\/\/www.meetyoucarbide.com\/heat-treatment-of-tungsten-carbide-products\/","title":{"rendered":"Heat Treatment of Tungsten Carbide Products"},"content":{"rendered":"
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Cemented carbide(hardmetal) is a general term for alloys composed of carbides, nitrides, borides, or silicides of high melting point metals (W, Mo, Ti, V, Ta, etc.). Divided into two major categories of casting and sintering. The cast alloy has high brittleness and low toughness, and has little practical application value. Widely used are sintered alloys, which are generally sintered from tungsten carbide or titanium carbide and cobalt powder and have high hardness, wear resistance and hot hardness. Mainly used to manufacture high-speed cutting and processing of hard materials, in recent years, the use of carbide in the mold industry is also increasing, so it is of practical significance to discuss and study the hard alloy heat treatment.<\/div>\n

1. Features of Cemented Carbide<\/h2>\n
Carbide is made by the method of powder metallurgy from the refractory metal hard compound and the metal bonding phase. The commonly used hard compounds are carbides. As the hard alloy for cutting tools, commonly used WC, TiC , TaC, NbC, etc., the binder is Co, and the strength of the cemented carbide mainly depends on the content of Co. Because the carbide in the cemented carbide has a high melting point (such as a melting point of 3140\u00b0 C. of Ti C), a high hardness (such as a hardness of 3200 HV of TiC), a good chemical stability, and a good thermal stability, the hardness and wear resistance thereof are high. Sex and chemical stability are much higher than high-speed tool steels.<\/div>\n
The commonly used cemented carbide hard phase is mainly WC, which has good wear resistance. Although some carbides have similar hardness as WC, they do not have the same wear resistance. WC has a higher yield strength (6000 MPa), so it is more resistant to plastic deformation. WC’s thermal conductivity is also good, and thermal conductivity is an important performance index of the tooling. WC has a lower coefficient of thermal expansion, about 1\/3 of that of steel; its modulus of elasticity is 3 times that of steel, and its compressive strength is also higher than that of steel. In addition, WC has good resistance to corrosion and oxidation at room temperature, good electrical resistance, and high bending strength.<\/div>\n

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Fig.1 The quasi-equilibrium diagram of WC-Co alloy<\/div>\n

2. Heat treatment and alloy organization<\/h2>\n
It has been studied on the bonding phases of WC-Co alloys with different C\/W ratios of 5% to 35% WC. The conclusions are drawn as follows: \u03b3-phase or (\u03b3+WC) phases are generated in the alloy at slow cooling; When there are (\u03b3+\u03b7) phases appear. However, since the (\u03b3+\u03b7) phase is unstable, the (\u03b3+\u03b7) phase will transform into a stable (\u03b3+WC) phase after annealing. According to the test results, the quasi-equilibrium phase diagram shown in Fig. 1 is drawn (the solid line is the phase diagram of the stable system, and the dashed line is the local phase diagram illustrating the \u03b7 characteristics of the quasi-stable phase).<\/div>\n
The annealing (slow cooling) of the typical cemented carbide depends mainly on the carbon content: when C\/W>1, the free carbon precipitates on the WC-Co phase boundary; when the C\/W<1, the microstructure of the alloy has In both cases: One is in the three-phase region (WC + \u03b3 + \u03b7). It is inevitable that the \u03b7 phase appears after the alloy is slowly cooled. If such a large amount of \u03b7 phase exists in the cementitious phase, branched crystal grains appear, and the small grains are unevenly distributed; if there is a large grain of \u03b7 phase, the grains are separated by a long distance, so there is information that the \u03b7 phase is Higher temperatures have begun to form.<\/div>\n
In the other case, when the alloy is in the two-phase (WC+\u03b3) region, the W alloy will be precipitated as Co3W from the bonding phase after the low-carbon alloy is annealed. The reaction process can be expressed by the following formula. Co Face-centered cubic \u2192 Co Face-centered cubic + Co3W Therefore, this low-carbon two-phase WC-Co alloy will be transformed into a three-phase (WC + \u03b3 + CoW) structure after annealing. Figure 2 shows the dissolution curves of W for two-phase WC-Co alloys at different annealing temperatures. The curve is the critical temperature curve for two-phase alloys transformed into three-phase (WC+\u03b3+CoW) alloys: above the curve temperature Annealing results in a two-phase microstructure alloy; annealing at temperatures below the curve yields a three-phase structure containing Co3W.<\/div>\n

3. Effect of heat treatment process on mechanical properties of hardness alloy<\/h2>\n
(1) Effect on Strength Since WC has different solid solubility at different temperatures in Co, it provides the possibility of precipitation hardening of the binder phase by solid solution temperature quenching and subsequent aging. Quenching can inhibit the precipitation of WC and the homotropy transition of Co (Co dense hexagonal, Co face centered cubic). It has been reported that the strength of the alloy containing 40% cobalt can be increased by about 10% after quenching, but the strength of the alloy containing 10% cobalt is reduced after quenching. Considering that the amount of cobalt contained in cemented carbides commonly used in engineering is generally 10% to 37%, the effect of heat treatment on the alloy strength is very small. So someone dared to assert that quenching is not a way to increase strength for W-Co alloys. Annealing also causes a decrease in the strength of the alloy, as shown in Tables 1 and 3. The properties of tungsten carbide vary with the amount of Co contained and the thickness of the grains, as shown in Figure 4.<\/div>\n

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Fig. 2 The solid solubility curve of tungsten in WC-10%Co two-phase alloy<\/div>\n

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Fig.3 Effect of annealing at 800\u00b0C on the flexural strength of WC-10%Co content<\/div>\n