Study On Alloy Carbides Control In High Speed Steel

High speed steels are widely used in making high-speed cutting tools and high-load molds due to its high hardness, wear resistance and excellent red-hardness. Large amounts of alloy carbides in high speed steels guarantee its good performance, but carbides are easy to fracture under pressure causing ductile damages and even failure of materials. The key point to prepare high performance high speed steels is how to control fracture of carbides and improve toughness of materials.In the present work, great efforts have been put into the problems above. High speed steels control technology is studied considering carbides fracture factors. Carbides dimension is one of the factors, methods of increasing cooling rate and optimizing deformation process are used to refining the size of carbides. The other factor is carbides mechanical properties, laws of the influence of crystalline structure and chemical component are explored to provide scientific reference for carbides mechanical properties control. The main results obtained have been listed as follows.The influence of cooling rate on carbide size is studied on production line combining Tiangong International Co., Ltd. current situation. With the decreasing of ingot size, cooling rate increases, the as-cast structure is refined and large plate-like carbide decreases both in size and quantity, decreasing of Mo in large plate-like carbide makes it more unstable and easily spheroidized during heat treatment. Meanwhile, with the increasing of cooling rate, size and distribution of carbides become better in the deformation microstructure. More homogeneous deformation microstructure is acquired with a smaller forging ratio by increasing the cooling rate which will decrease the dependency on forging ratio and improve finished product rate.M2C decomposes into M6C and MC during heat treatment which is conductive to refine the size of carbides. It will promote the decomposition of M2C and accelerate the decomposition rate by pre-deformation before heat treatment. With the increasing of deformation, decomposition promotion effect is more evident. Defects in carbides and interface which were formed during deformation are the reason why deformation promotes decomposition. MeC, MC can nucleate on defects when M2C decomposes, it will promote the growth of decomposition product.Crystalline structure has a significant influence on the mechanical properties of carbides. Hexagonal close-packed M7C3 has a high fracture toughness(Kc,2.68MPam1/2), but hardness is low(H,22.2GPa); face-centered cubic MC possesses a higher hardness(29.7GPa) and a lower fracture toughness(1.84MPam1/2);mechanical properties of complex cubic structure M6C lay between M7C3 and M6C(23.7GPa,2.07 MPam1/2).Chemical component influents carbides mechanical properties significantly:with the increasing of Cr/Fe, ? E and Kc of M7C3 increase rapidly at first, then decrease gradually, owning the best mechanical properties when Cr/Fe=0.75-0.95.With the increasing of V/W, H of MC increases linearly, Kc decreases first then increases, acquiring the best mechanical properties when V/W=3.5-4.0. With the increasing of [W]/Fe, H of M6C increases first then decreases, reaching its maximum value when [W]/Fe=2.5-3.0; E and Kc decrease first then increase, reaching its minimum value when [W]/Fe=2.5-3.0.