Research On Heat Treatment Of Powder Metallurgical High-speed Steels S390/S790 For Fine-blanking Tools

Fine-blanking is one of the key processes in the field of automobile manufacturing,and its low service time has always been a very intractable challenge for domestic companies.During past decades,a lot of research has been done to prolong the service time of fine-blanking tools,including optimization of processing and manufacturing,structure design,PVD coatings fabrication and so on.With the development of fine-blanking products towards higher strength,higher thickness and more complex shape,there is a higher requirement for the mechanical properties of PM HSSs,including compressive strength,toughness and wear resistance.But fine-blanking companies do not make the most of powder metallurgical(PM)high-speed steels(HSSs),especially the PM S390 and S790 HSS(400-600×1000 RMB/ton)used as top-end fine-blanking tools,because related research is not in-depth.The microstructure and mechanical properties of HSSs could be optimized substantially through heat treatment.Therefore,this thesis focus on PM S390 and S790 HSS,and try to obtain desired microstructure and mechanical properties through quenching treatment,tempering treatment,cryogenic treatment,plasma nitriding(PN)-physical vapor deposition(PVD)duplex treatment,which lays a strong foundation of effective use for typical PM HSS used in fine-blanking,and provides a theoretic reference for the independent fabrication of high-performance HSS in the future.(1)Firstly,the physical properties,phase constituents,phase morphology and phase structure of PM S390 HSS and the crystal orientation relationship between carbides and martensitic matrix were characterized.For physical properties,the density of hot-isostatic pressing(HIP)S390 HSS was determined to be 99.2%.Austenite transformation temperature,and martensite start transformation temperature are determined to be 836.9°C and 351°C,respectively.XRD,SEM,and TEM results confirm that phase constituents were composed of martensite,retained austenite,MC and M₆C carbide.The martensite was composed of lath martensite and twin martensite,and retained austenite was located between lath martensite.A near Baker–Nutting(B–N)orientation relationship(OR)between the MC carbide and martensitic matrix was detected,while no OR between the M₆C carbide and matrix was observed.Both SEM/EDS and CALPHAD method show MC carbide was rich in V and M₆C carbide was rich in W and Fe.A high ratio of W/Mo promotes formation of M₆C carbide,rather than M₂C carbide.(2)Secondly,the effect of the austenitizing temperature for PM S390 HSS on its microstructure and mechanical properties was explored,and the CALPHAD method was used to rationalize its microstructure evolution.As a result,the austenitizing temperature range to obtain excellent mechanical properties was determined.Based on the CALPHAD result,quantitative carbide contents varying with austenitizing temperature were determined,and the ripening behavior and ripening rate of carbides at various temperatures were analyzed.The relationship between average grain size,austenitizing temperature,and the carbide content is established.With an increment in austenitizing temperature,the average grain size grows,the hardness and compressive strength experienced an initial increase and subsequent decrease,but wear weight loss experienced an initial decrease and subsequent increase.(3)Thirdly,the influences of tempering treatment on microstructure and mechanical properties for PM S390 HSS were analyzed,including tempering temperature,holding time and tempering times.Results show that PM S390 HSS exhibits obvious secondary hardness,and prolonging time will lead to a decrease of both temperature and hardness at the peak of the secondary hardness curve.With the increment in tempering temperature and tempering time,the mechanical properties of S390 HSS decreased,which results from the softening of the martensitic matrix.The first tempering treatment promotes phase transformation from retained austenite to martensite.Triple tempering times should be chosen to obtain excellent mechanical properties.Additionally,S390 HSS exhibits strong susceptibility to the notch and the V-notch leads to a decrease of at least 60%in flexural strength,which means that tempering treatment has great influences on the V-notch-free sample,but little effects on the V-notched sample.(4)Fourthly,the phase constituents of PM S790 HSS were determined,and the influences of tempering treatment on microstructure and mechanical properties for PM S790HSS were analyzed,including tempering temperature,holding time and tempering times.Quantitative carbides contents varying with temperature were determined based CALPHAD method and equilibrium solubility product of carbides and the alloying contents dissolved in austenite and martensite were determined,which reveals the originality of red-hardness.Compared to S390 HSS,S790 HSS has a stronger intensity in XRD for retained austenite,lower carbides contents,lower Rockwell-C hardness,and wear resistance,but bigger grain size after quenching and higher flexural strength.Additionally,S790 HSS also exhibits poor susceptibility to the notch,and the V-notch leads to a decrease of at least 50%in flexural strength,which means that tempering treatment has great influences on the V-notch-free sample,but little effects on the V-notched sample.(5)Fifthly,this thesis briefly analyzed the influences of PN,PVD and PN-PVD duplex treatment on surface hardness for PM S390 HSS.Results show that the hardness slightly increased after PN treatment but greatly increased after PVD treatment.The PN-PVD duplex treatment will lead to a higher hardness than simple PVD treatment.