Study On The Tempering Microstructure And Performance Of High Speed Steel Containing Cobalt

High speed steel is one of the basic raw material in machinery manufacturing industry, which is widely used in the important field of high speed cutting tool, high load precision mould, air temperature bearing etc. With the development of modern machinery manufacturing industry, cutting speed and accuracy are improved continuously; higher requirements are put forward on high temperature properties of high speed steel materials especially hot hardness. When non-carbide forming elements of Co is added to high speed steel, the high speed steel red hardness is improved, so high speed steel contained Co are developed. At the moment, the effects of Co on red hardness of high speed steel are still unclear, so it’s unable to find the ideal alloy elements instead of Co with scientific theories. This paper used high performance high speed steel contained Co as the researching object, to study the microstructure and alloy carbide evolution rules during tempering of HSS with Co contained.The purpose is to clarify the mechanism of Co on hot hardness of HSS and provide theoretical supports for the optimization design of high performance high speed steel, and develop excellent performance steels.The research results show that the tempering process of HSS has the phenomenon of secondary hardening. At 550℃, it reaches its peak hardness.As temperature rising, the hardness decreased. The variation of hardness is related to the secondary carbides, after tempering at 550℃, there are lots of secondary carbides Precipitating, which includes disc-like MC carbides and strike-like M2C carbides. M represent alloy elements such as W、 Cr、Mo、V and so on. MC carbides are FCC structure and M2C carbides are HCP structure. MC have Backer-Nutting relation with matrix, that’s (100) MC (100) α, M2C have Pitch-Schrader relation with matrix, that’s (0001) M2C(011) α. MC and M2C have coherent or semi-coherent orientation relationship with matrix, which have the precipitation strengthening effect and improving the hardness of high speed steel. With tempering temperature increasing, MC and M2C carbides grow up, and carbides become non-coherent with matrix. As result, strengthening effect of precipitation disappears and hardness decreased, but the orientation relationship with the matrix invariant. The MC carbides do not have structure transition during tempering, but M2C transform to M23C6 and M6C.Co improves the peak hardness of secondary hardness, and decreases the falling rate of hardness. What’s more, Co promotes the precipitation of carbides and increases carbide precipitation quantities, so that increasing the hardness. Co can affect carbide’s components at peak hardness:W, Mo content reduced slightly, Fe, Cr content increased, which is conducive to reducing the carbide nucleation barrier, promoting the precipitation of carbide. At the same heat treatment conditions, Co can reduce MC and M2C alloy carbide sizes, maintain coherent relationship between carbide and matrix hardness, improve the hardness and hot hardness of high speed steel.The research results show that during over tempering, hardness decrease sharply at the early stage, and decrease slowly later. At the early stage of tempering, MC and M2C grow rapidly, with a loss of coherent relationship between carbides and matrix, resulting in the decrease in hardness. During the process of growing up, alloy element contents in MC, M2C and the matrix changed, alloy elements exist redistribution phenomenon which would affect the growth process of carbide. Although Co isn’t involved in the formation of carbides, but would affect the alloy redistribute of different phases, and then affect the growth process of alloy carbides. At the early stage of tempering, Co promoted Fe element enrich in carbide, which is conducive to reducing the carbide nucleation barrier and promoting the precipitation of carbide; at later stage, Co promotes Cr; V elements enrich in the carbide, which can reduce the growth rate of carbide, inhibition of carbide coarsening.The reason why Co can increase the hot hardness of HSS is:on one hand, Co can promote carbide precipitation, improve the carbide precipitation quantities; on the other hand, Co would affect the alloy distribution behavior during tempering, reduce the growth rate of carbide, and slow down the hardness decrease trend. Therefore, if we want to choose the best elements insteaded of Co, we need to consider the effect of carbide precipitation and carbide grew up, and chooses the elements which not only promote the carbide precipitation, but also suppress carbide’s growth.