Shot Peening Induced Strengthening Of The Maraging Stainless Steel

The family of maraging stainless steels is widely used in various commercial manufacture applications for its high toughness,excellent corrosion resistance and simple heat treatment.Our team has made a lot of experiments on 1RK91 maraging stainless steel which developed by AB Sandvick Steel for many years.Finally,we find the best heat treatment and explore the influence of different Mo,Ti content and plasma nitriding on the microstructure and prorerties of 1RK91.At the same time,our team also has made a research on the teeth from acanthochiton rubrolineatus and believes that its hardness distribution is one of main reasons for the formation of self-sharpening:The loss of teeth in low hardness region is more than high hardness region over the feeding process.Hence,if we can apply this microstructure to the knife tool used by maraging stainless steels,the knife tool must be strong and tough,and the edge of knife tool is not easy to be blunt,which are favorable to improve the working life,efficiency and cost of knife tool.Based on this background,three different maraging stainless steels were melted,including H1 (00Cr10Ni9Mo3Cu2Ti),H2 (00Cr12Ni9Cu2Ti),1RK91 (00Cr12Ni9Mo4Cu2).Then I made them have a continous distribution of hardness gradient by shot peening.Concreat steps:Solid solution treatment at different temperature,shot peening by different type,aging treatment at 475℃ for 2 hours. The effects of the solid solution temperature, elements, shot time,distance, type and size on the microstructure and properties of three alloys were studied through direct-reading spectrometer optical microscopy,hardness testing,X-ray diffractometry(XRD),roughometer and scanning electron microsopy(SEM).At last,we found the best material and the best technology.Studies show that the best solution treatment temperature of H1 specimen is 1050℃,and H2 specimen is 950℃. They are almost composed of martensite after quenched,but 1RK91 specimen still contains a lot of retained austenite after solid solution treatment at 1100℃ for 0.5h.The surface of H1,H2,1RK91 specimens will produce large amounts of residual stress after shot peening with dislocation density increasing and grain refinement.In this process, retained austenite existing in 1RK91 can be induced to martensite by stress. At the same time,the stress field caused by shot peening will not affect the precipitation of the second phase particles in aging process and the aging process does not slack the stress too.The shot-peened layer’s longitudinal section can be divided into severe deformation layer Ⅰ、transition layer Ⅱ and matrix layer Ⅲ. With the increasement of layer depth,the vickers hardness in shot-peened layer’s longitudinal section will decrease. Through a series of experiments,I find the 1RK91 specimen is the best material for shot peening,and its best shot time is 1min.Besides,its best shot distance is 25mm, and its best shot is ZrO2 ceranics of 0.4-0.6mm.Under the optimum conditions.the hardness gradient expand further between layer Ⅰ and layer Ⅲ through aging treatment,and the hardness in layer I can be up to 580HV.In addition,shot peening treatment will increase the surface roughness of 1RK91 and cover the original scratches on the surface of 1RK91,leaving more various pits which includes lamellar tissue.The severe deformation layer I of 1RK91 can also be classified into textured martensite and nanometer globose martensite.The former produced by compression deformation, the latter is from fragmentation of martensite and induced from retained austenite by stress.