Research On Surface Low Alloy Hss By Plasma Surface Metallurgy W,MO,C,Alloying Process

High speed steel (HSS) is a very important tool steel in machining industry. The global output of HSS is 1/4 million tons in recent years, and about 40,000 tons comes from China. The producing of HSS is a quite sophisticated process due to the addition of multiple alloy elements and their high contents, so it has always been expensive but very important alloy steel. HSS has high wear-resistance and heat-durability, its hardness reaches HRC63 ~ 67 after heat treatment and keeps at above HRC60 at 550-600.These features of HSS make it a very important material for producing cutting tools, dies and those parts requiring high wear performance or working at high temperature.Majority of the HSS is produced by the traditional metallurgy process, or ingot-casting and forging rolls. Due to the high alloy content, high dimension of ingot and low cooling rate, the coarse ledeburite eutectic segregation will be formed within the ingot inevitably. The segregation is harmful to the mechanical property of the steel and also makes the forging and rolling process more difficult..The severe composition and structure segregation could be eliminated by annealing and quenching process, but the elimination of the non-homogeneous distribution of the coarse ledeburite is impossible through any kind of heat treatment process, and it is more damaging to the property of the steel.. Since the ledeburite eutectic carbides are formed during the crystallization process, they are not soluble to the austenite even at high temperature of 1300C, resulting the lack of alloy elements soluble to the austenite, and hence the decrease of hardness, wear-resistance and heat-durability.The general solution to the problem caused by the ledeburite eutectic carbides is a forging operation to break up the carbides. High forging ratio is beneficial to reduce the segregation, but it is almost impossible to eliminate the non-homogeneous distribution through forging process. Investigation shows that for average low alloy steel, deformation of 3-4 times is enough to obtain uniformed microstructure which could meet the need for working condition. While for HSS, with deformation lower than 5 times, the netshaped carbides still exist along the direction of deformation, it starts to break up at 7 times of deformation, and the netshape turns to continuous wide strip. At 30 times of deformation, the wide strips change to narrow strips and remain almost unchanged at further higher deformation rate. The hard and brittle eutectic carbide always exists after very high rate of forging, so the extent of segregation of the carbides is a very important criterion to judge the quality of the HSS..According to statistics, the main alloying elements in HSS, tungsten andmolybdenum, will be exhausted on earth in 40-60 years, even if considering the potential ore, the time span is no more than 100 years. Saving the valuable elements is very important and can also reduce the cost to produce the steel. Toward this end, HSS with lower alloy content are developing in different countries. Since 1970's, with the enlarged shortage in alloy elements reserve, high performance HSS with low alloy content has drawn more and more attention around the world. Until now, a series of products has been developed, such as SW3S2 in Poland, D950 in Sweden, VacoDyne in U. S., 301/D101/D106 in China, etc. The alloy content in low alloying HSS usually is close to the matrix alloy content in traditional HSS, and the whole alloy content is reduced to about 1/2-1/3 of traditional HSS. And to ensure comparable main properties, usually Si, N, Re are added. The main feature of the low alloying HSS compared with traditional HSS is: higher bending strength and toughness; a little higher or similar secondary hardening ability and red-hardness under 600C; lower red-hardness and high temperature hardness above 600C, a result of reduced W/Mo contents.To further save the alloy elements, surface metallurgy HSS has been developed in recent years. By applying surface alloying process on surface of cheap steels ,alloy...