Research On Microstructure And Properties Control Of Functionally Graded 9Cr18 Size Stainless Steel And Thixoforming Technology

Functionally graded material can meet several specific requirements during service.According to the requirement of shaft parts,the surface and inner area vary a lot for state,structure and properties,etc.The surface requires high wear resistance,while the inner area should demonstrate proper strength.A functionally graded material is required.The research is supported by National Natural Science Foundation of China(No.51175036).The experimental material is high carbon and chrominum stainless steel of 9Cr18 as well as the same size of 9Cr18Mo.Thixoforging technology is used to design and fabricate semi-solid component of steel.The research is aimed at investigating microstructure evolution and properties control theory of functionally graded 9Cr18 size steel.The compression behavior of 9Crl8 and 9Cr18Mo semi-solid billets was studied.The results showed that the compression behavior of 9Cr18 semi-solid billet was similar to that of 9Cr18Mo billet.Initially.solid skeleton broke down.Then deformation came into thixotropic stage.Liquid flow,slide of solid particles and plastic deformation of solid particles were the main deformation behavior.The stress level of 9Cr18 semi-solid billet was low at lower strain rates(0.02s-1,0.05s-1)and temperature of 1300?,and the peak stresses were 15.7MPa and 21.7MPa,respectively.The microstructure and property control during thixoforging were studied.A thixoforging set-up for semi-solid forming of steel was re-designed from a Gleeble-3500 thermal mechanical machine.9Crl8 multidiameter shaft part with full filling of the dies could be fabricated via thixoforging pocess.There existed a distinct solid/liquid boundary at the deformation region of 9Cr18 thixoforging component.The surface of component was eutectic with dendrite structure,while the inner solid particles were connetcd with rare liquid film.The microstructure demonstrated functionally graded property along the radial direction.The increase of forming temperature contributed to more free liquid and lower strain rates made deformation time sufficient for macro spearation,which was suitable for achieving material with graded microstructure.At temperature of 1340? and strain rate of 0.02s-1,the material could demonstrate better thixotropic property and the thickness of surface layer could reach up to 1000?m.The hardness at the inner austenite was about 376HV,while the hardness at the surface was about 726HV.The microstructure characterization of 9Cr18Mo thixoforging component also demonstrated functionally graded property.The research mainly analyzed the difference of wear and corrosion behavior for 9Cr18Mo thixoforging component.The microstructure evolution of 9Cr18 thixoforging component is unique on each side of the solid/liquid boundary.The alloying elements in the solid particles were the key factor in determining the microstructure evolution at the inner area.The solid austenite inside the boundary was in an over-saturated meta-stable state.Martensite transformation occurred after subsequent certain heat treatments.Tempering at 200? and 550? might help to release residual stress and the mechanical properties were improved.The solid/liquid boundary was retained after heat treatments.The hardness of surface layer remained above 720HV after heat treatments.Thixoforging component demonstrated functionally graded property with hard surface and tough center.9Cr18 thixoforging component demonstrated fine inner strength and plasticity after tempering at 550? for 2h.The compressive strength was 4680MPa and the compression ratio could reach 53.2%.The deformation behavior demonstrated four-stage hardening characterization.The formation of multiple variants of' mechanical twinning(TWIP)or ?-martensite(TRIP)played a dynamic Hall-Petch effect and the tendency of synchronous improvement of strength and plasticity was obtained.A wear resistance surface could be obtained for 9Cr18 and 9Crl8Mo thixoforging component.The high hardness and high content of Cr contributed to minimizing delamination,abrasive and oxidation wear for skin layer.The corrosion resistance of 9Crl8 and 9Cr18Mo thixoforging component was improved slightly as compared with conventional heat treatment spcimen.It will help to expand the application fields for traditional 9Cr18 and 9Cr18Mo steels by designing and fabricating a functionally graded material with an inner austenite matrix and a fine dendritic eutectic surface.