Study On The Preparation,microstructure And Performance Of Powder Metallurgy High Speed Steel

PM HSS is short form of powder metallurgy high speed steel,which is prepared by powder metallurgy method followed by heat deformation and heat treatment.Powder metallurgy technology can solve the problems of coarse carbide and severe segregation of traditional high speed steel in the smelting process,and thus greatly improve the performance.Due to its advantages of no segregation,fine grain,fine carbide,small deformation in heat treatment,uniform hardness,good toughness and wear resistance,PM HSS has been widely used as cutting tools for manufacturing difficult-to-machine materials,especially for making large broach,end mill,hob and gear shaver.To date,the production technology of PM HSS has been mature in foreign countries,mainly concentrating in a few developed countries.As a result of technical blockade to China,our country’s PM HSS products rely on import.Many researches have been committed to the research of this technology and trial production.However,it is still in the stage of experiment and trial production.In general,the problems of coarse powder particles,prior particle boundaries(PPB),ceramic inclusion,narrow sintering window and high difficulty in controlling sintering process are the key factors that restrict the performance improvement of PM HSS.Researchers in various countries aim to solve and break through these technical challenges.In this paper,to achieve the preparation of high-performance ASSAB PM30 PM HSS,powder characteristics and preparation process were studied.The research idea of powder preparation,forming and sintering,microstructure analysis,and performance test was adopted to clarify the key factors affecting the sintering density and microstructure.On this basis,a new preparation technology of HSS powder was developed,and its sintering characteristics,heat treatment process and properties were also studied.Finally,the microstructure,properties and nitrogen reaction sintering mechanism of nitride reinforced PM HSS were investigated.Based on the analysis of the morphology,particle size,compactibility and powder microstructure of gas atomized HSS powder,it can be found that the traditional HSS powder had the characteristics of coarse particle size and poor compactibility,which was suitable for employing hot isostatic pressure(HIP)to achieve sintering.In the HIP samples,PPB and nonuniform microstructure were found.Results showed that the PPB problem was mainly caused by the presence of holes on the powder particle surface during the cooling process because the gas produced at high temperature could not be discharged.The nonuniform microstructure was mainly resulted from the nonuniform powder microstructure.To solve above problems,pressureless sintering,powder sieving and ball grinding were employed.It was found that vacuum sintering was more conducive to achieving sintering densification compared with argon atmosphere sintering.Compared with powder sieving,the ball grinding treatment had better effect on microstructure optimization and improvement of powder utilization rate.Optimum sintering temperature(OST)value of sieved fine powder(<30μm)was about 1250℃ under vacuum sintering.In such case,the density was up to 99.0%.The density of PM HSS sintered at 1250℃ was 7.98g/cm3 with a relative density of 99.3%after powder being milled for 48h.Fine powder had good sintering property and was helpful to improve the microstructure of PM HSS.To obtain fine HSS powder,raw powders were prepared by water-gas atomizedprocess with an average cooling rate of 103～104K/s.The average particle size of D50 was 9.64 μm.Compared with gas-atomized powder,the weight ratio of fine powder and the microstructure homogeneity were greatly improved.However,the oxygen content was much higher.It was found that most of the harmful oxygen in powder could be eliminated through carbon deoxidation,and the oxygen content could be reduced from 2300ppm to 65ppm without forming PPB.Compared with gas-atomized powder,the OST of water-gas atomized powder was lower.After sintering,the samples had higher density and more homogeneous microstructure.Under vacuum sintering,the OST of water-gas atomized powder was 1230 ℃,and the sintering density was 7.98g/cm3 with a relative density of 99.3%.Besides,the carbides distributed uniformly with average particle size of only 1～2μm.In order to obtain the optimal mechanical properties,heat treatment was carried out on ASSAB PM30 PM HSS.Besides,0,0.3,0.6wt.%C and 0,1.0,2.0wt.%Ti were added,respectively.The microstructure and properties were analyzed.Results showed that PM HSS samples with 0.3wt.%C and 2.0wt.%Ti had the best comprehensive performance.In addition,in the field with requirement of low strength and high hardness,more carbon can be added in the PM HSS.The hardness of the sample with 0.6wt.%C reached to 68.1HRC.Besides,non-capsule HIP and forging treatment were carried out to obtain fully dense PM HSS.Relative density of 99.9%and better comprehensive performance were obtained.In such case,the bending strength,impact energy and hardness of the samples treated by non-capsule HIP and forging were 4253MPa,3698MPa,20-26J,25-30J,65.3HRC and 65.2HRC,respectively,which were comparable to the third-generation PM HSS.Fine VN reinforced PM HSS can be obtained through reaction sintering.The stability of VC and VN in the sintering process was analyzed.It was found that the Gibbs free energy of VN phase was negative and greater than that of VC phase from room temperature to 1300℃.The formation energy of VN phase(-9.44895eV)was also lower than that of VC phase(-9.08125eV),indicating that VN phase was more stable.Therefore,in the nitrogen atmosphere sintering process,nitrogen and vanadium would react to form in-situ VN strengthening phase.Due to the involvement of nitrogen in the reaction,the carbon content in the matrix was excessive and the original carbon balance was broken.To achieve the new carbon and nitrogen balance,the microstructure and properties of PM HSS with carbon content of 1.2wt.%,1.Owt.%and 0.8wt.%were studied,respectively.The results showed that when the carbon content was 1.Owt.%,the carbon and nitrogen reached the equilibrium state.Fine carbides and uniform microstructure were obtained.The carbides were mainly M6C of 1μm,while the nitrides were VN of 0.5μm.The sample had better comprehensive mechanical properties,with hardness of 65HRC,bending strength of 3011 MPa,and impact energy of 18-22J.