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High speed steel is an extremely important tool material,accounting for 45%of the world's tool sales,including 85%of the sales of complex multi blade tools such as gear cutters and broaches.It combines excellent red hardness,wear resistance,impact resistance and heat treatability.It can be processed in the softened annealing state,and then precipitated a large number of secondary carbides through quenching and tempering heat treatment to achieve material hardening and strengthening.It has obvious advantages that cemented carbide and cermet tools do not have.
According to the preparation method,high speed steel can be divided into traditional cast forged high speed steel and powder metallurgy high speed steel.Due to the high content of alloy element and carbon element in high speed steel,casting high speed steel has some unavoidable shortcomings,such as coarse structure,composition segregation and property anisotropy.However,the appearance of powder metallurgy high speed steel fundamentally avoids the appearance of coarse carbides.With the advantages of fine and uniform carbide structure and isotropic properties,it is a milestone in the development history of high speed steel.However,China does not have the production capacity of high-performance powder metallurgy high speed steel.
The commercial powder metallurgy high speed steel appeared in the 1970s,mainly including gas atomization hot isostatic pressing,spray deposition and super solid liquid phase sintering.The process of gas atomization hot isostatic pressing(HIP)is used to produce high speed steel powder by gas atomization,and then densified steel ingots are obtained by HIP.This method can prepare fully dense,fine and uniform structure,stable quality,excellent performance and large size workpieces,but it has the disadvantages of expensive equipment,high technical threshold,large pollution and high energy consumption.Spray forming is a low cost method to produce high speed steel.The molten steel is atomized and sprayed into droplets with inert gas,and directly deposited on the lower rotating substrate under the condition of certain temperature and viscosity.After continuous deposition and solidification,the overall compact steel ingot is obtained.It has the advantages of short process,low cost and high production efficiency,but it also has the disadvantages of high energy consumption,large pollution,alloying limitation,high impurity content and non compact steel nail core.Moreover,compared with the aerosol hot isostatic pressing method,the performance is slightly poor and the quality is not stable enough.The super solid liquid phase sintering process is that the sintering temperature is in the liquid phase+austenite+carbide phase zone,and the densification is achieved by the presence of a part of the liquid phase.Its advantages include short process,low investment,low cost,high material utilization rate,being suitable for small-scale production,and being able to prepare nitrogen containing high-speed steel.However,the appearance of liquid phase promotes the distribution of bar or coarse carbides along the grain boundary,which is the worst of the three preparation methods.
The appearance of SAPP new material powder high-speed steel prepared by non atomized powder ball milling has brought exciting news to the localization of high-performance powder high-speed steel.The non atomized powder ball milling method creatively uses the non atomized ball milled mixed powder as the raw material,which can add the required alloy elements almost without restriction.Through the solid phase vacuum activated sintering method,the near net shaped billet can be prepared in one step,and then the near net shaped high-performance powder high-speed steel products can be obtained after heat treatment and a little finishing.This method does not require atomization and hot isostatic pressing equipment.The composition and performance are easy to adjust and control,which is convenient to achieve the best match between material properties and actual working conditions.At the same time,it can also meet the requirements of special applications and performance for the preparation of materials.Near net forming can greatly improve the utilization of materials,which is an optimal method for the preparation of powder high-speed steel.
