WC Co cemented carbides are easy to oxidize and decompose in high temperature application, which have many problems, such as brittleness, brittle fracture, processing softening and edge breaking, etc. they are still not suitable for high speed cutting of steel, so they have great limitations. WC tic co cemented carbides are known to have wear resistance, oxidation resistance and crater wear resistance.
However, due to the fact that tic and its solid solution are much more brittle than WC, this alloy also has relatively large defects, that is, the toughness and weldability of the alloy are poor. Moreover, when the content of TiC exceeds 18%, the alloy is not only brittle, but also difficult to weld. In addition, tic can not significantly improve the high temperature performance.
TAC can not only improve the oxidation resistance of cemented carbide, but also inhibit the grain growth of WC and tic. It is a practical carbide which can improve the strength of cemented carbide without reducing the wear resistance of cemented carbide. TAC can increase the strength of cemented carbide by adding TAC into WC tic co cemented carbide The addition of TAC helps to reduce the friction coefficient, thus reducing the temperature of the tool. The alloy can bear a large impact load at the cutting temperature. The melting point of TAC is as high as 3880 ℃. The addition of TAC is very beneficial to improve the high temperature performance of the alloy. Even at 1000 ℃, it can still maintain a good hardness and strength.
Tic and TAC are insoluble in WC, while WC is soluble in tic. The solubility of WC in the continuous solid solution formed by TAC is about 70wt%. The solubility of WC in the solid solution decreases with the increase of TAC content. The properties of WC tic tac Co alloys are mainly achieved by adjusting the tic + TAC, the ratio of Ti atom number to ta atom number and the content of cobalt. When the ratio of Ti atom number to ta atom number and the content of cobalt are fixed, adjusting the content of TiC + TAC to achieve the best performance has become the focus of research.
1. The raw materials used in this experiment are: WC powder, compound carbide powder [(W, Ti, TA) C] powder and Co powder. The chemical composition and average particle size are shown in Table 1.
Table 1 Composition and average particle size of raw materials
After the powder is proportioned according to the standard table 2, it is milled and mixed on nd7-2l planetary ball mill for 34h, the mass ratio of the ball material is 5:1, the grinding medium is alcohol, the adding amount is 450ml / kg, the milling speed is 228r / min, and 2wt% paraffin is added four hours before the end of the milling. The slurry shall be screened (325 mesh), vacuum dried, screened (150 mesh) and pressed to shape after drying, the pressing pressure shall be 250Mpa, and the blank size shall be (25 × 8 × 6.5) mm. The pressed samples were sintered in vsf-223 vacuum sintering furnace at 1420 ℃ for 1H.
Table 2 composition ratio of alloy%
The three-point bending method was used to determine the bending strength of the sintered sample on sgy-50000 digital compression strength tester. The final strength data was the average value of three samples. The hardness HRA of the sample was measured on the Rockwell hardness tester. The diamond cone indenter with a load of 600N and a cone angle of 120 ° was used.
The cobalt magnetism is measured by the cobalt magnetic tester, and the coercive force is measured by the coercive force meter. After the surface of the sample is grounded into a mirror surface, the mirror surface is corroded by the equal volume mixture of 20% sodium hydroxide solution and 20% potassium cyanide solution, and then the metallurgical observation is performed on the scanning electron microscope at 4000 times. Magnetic properties magnetic properties include co magnetic com and coercive force HC. Com represents the carbon content in the alloy, HC represents the grain size of WC. According to the national standard gb3848-1983, the cobalt magnetism and coercive force of the alloy are determined, and the results are shown in Table 3. It can be seen from table 3 that the relative magnetic saturation COM / CO and coercive force HC decrease with the increase of the content of compound carbide (W, Ti, TA) C.
Table 3 test results of cobalt magnetism and coercive force of tungsten cobalt titanate
Generally speaking, the control of COM content over 85% of cobalt to ensure that the alloy does not decarburize, the COM / CO ratio in group 1 is far lower than 85%, and its HC is also abnormally high. The non-magnetic η phase (co3w3c) appears in the alloy, which belongs to the serious deodorization structure. Therefore, we will only discuss groups 2, 3 and 4:
In this experiment, the total carbon content of the 2, 3 and 4 groups of alloy is 7.18wt%, 7.61wt%, 8.04wt%, the total carbon content increases in turn, and the HC decreases in turn. The size of coercive force is related to the dispersing degree of cobalt phase and carbon content of the alloy. The higher the dispersing degree of cobalt phase is, the greater the coercive force of the alloy is. The dispersing degree of cobalt phase depends on the cobalt content and WC grain size of the alloy. When the cobalt content is determined, the finer the WC grain is, the higher the coercive force is. Therefore, HC can be used as an index to indirectly measure the WC grain size
The content of carbon affects the solid solution of tungsten in cobalt. With the increase of carbon content, the content of tungsten in cobalt phase decreases. The solid solution of tungsten in cobalt is 4wt% in carbon rich alloy and 16wt% in carbon deficient alloy. As w can inhibit the dissolution and precipitation of WC in γ phase, WC is refined and HC is high, so the total carbon content increases in turn, WC grain coarsens and HC decreases. 2.2 the hardness and bending strength test results of the influence of the micro-structure on the mechanical properties of the alloy are shown in Figure 1. The bending strength increases with the increase of the C content of the compound carbide (W, Ti, TA), while the hardness is the opposite.
Fig. 1 hardness and bending strength test results of tungsten cobalt titanate
With the decrease of C content in the compound carbides (W, Ti, TA), HC increases, that is, WC grain refinement. The hardness increases with the refinement of WC grains when the cobalt content is constant. This is because the alloy is strengthened through the grain boundary and phase boundary, and the refinement of carbide grain will increase its solubility in the bonding phase, and the hardness of γ phase will also be increased, which will lead to the increase of the hardness of the whole alloy.
However, the effect of WC grain size on fracture toughness is more complex. For the alloy with grain size smaller than sub micron, the main indentation cracks are crack (intergranular) deflection and toughness bridging, with a small amount of transgranular fracture.
As the WC particle size becomes finer, the probability of defects in the grains decreases, and the strength of the particles increases, resulting in the decrease of transgranular fracture and the increase of intergranular fracture. For the alloy with large grain size, there are only four independent slip systems in the WC crystal. With the increase of WC grain size, the deflection and bifurcation of the crack increase, resulting in the increase of fracture surface area and toughening. Therefore, it is not accurate to judge the bending strength by grain size alone, and its microstructure should also be analyzed.
The metallurgical structure of cemented carbide with four different compound carbides (W, Ti, TA) C content is shown in Figure 2. With the increase of (W, Ti, TA) C content, the shape of WC tends to be regular. Most of WC in Figure 2a are irregular long bars arranged intensively. The average grain size of WC is relatively fine, but its adjacent degree is high, which is caused by the insufficient crystallization of WC, the cobalt phase does not completely wrap WC and the thickness is uneven. And there are coarse triangular WC grains. When η phase decomposes, CO is precipitated, resulting in local co enrichment. At the same time, W and C precipitate on the surrounding WC grains to form coarse triangular WC grains. From figure 2a-2d, it can be seen that the shape, size and distribution of WC grains have obvious changes. WC grains tend to regular plate shape, the coarsening adjacency of grains decreases, and the average free path λ of bonding phase increases. In Figure 2D, WC grains are well developed, with narrow particle size distribution, low coarse adjacent degree of grains, large average free path λ of bonding phase, most of which are about 1.0 μ m plate WC, and a small amount of triangle WC around 200nm, all of which are dispersion distribution.
Fig. 2 metallographic picture of C content of different compound carbides (W, Ti, TA) in cemented carbide
The dissolution precipitation of WC occurs in the sintering process, which makes the WC with higher energy (small particles, edges and corners of particle surface, bulges and contact points) dissolve preferentially, and makes the WC dissolved in liquid phase deposit on the surface of large WC after precipitation, which makes the small WC disappear and the large WC increase, and makes the particles accumulate more tightly depending on the shape adaptation, makes the particle surface tend to be smooth, and makes the two WCS The distance between them is shortened.
In the sintering process of low cobalt alloy, with the increase of total carbon content, the amount of liquid phase and the retention time of liquid phase increase, WC dissolution precipitation process is more full, WC grains develop completely, the surface is more smooth, and the particle size distribution is more uniform. In addition, with the increase of the total carbon content of the alloy, the solid solution of W in CO decreases, and the decrease of W content in the bonding phase will improve the plasticity of the bonding phase, thus increasing the bending strength of the cemented carbide. Therefore, the bending strength increases with the increase of total carbon content.
(1) When the content of CO is constant, with the increase of compound carbide (W, Ti, TA) C content, the total carbon content of the alloy increases, HC decreases, WC grain coarsens, w solution in CO decreases, and the hardness of the alloy decreases.
(2) The metallographic structure of the alloy is closely related to the total carbon content of the alloy. The compound carbide (W, Ti, TA) C content increases, the total carbon content of the alloy increases, the WC grain adjacency decreases, the particle size distribution narrows, the average free path λ of the bonding phase increases, and the bending strength increases.
(3) The best microstructure and properties of wcta are as follows: when the total carbon content is 8.04wt%, the hardness is 91.9hra, and the bending strength is 1108mpa.