Study On Microstructure And Properties Adjustment Of PH13-8Mo Martensitic Precipitation-hardening Stainless Steel

In this paper,PH13-8Mo martensitic precipitation-hardened stainless steel is taken as the research object.Based on the traditional heat treatment process,process experiments are carried out on the three variable parameters of aging temperature,aging time and cooling mode after solution.Mechanical property testing,a variety of characterization methods,measurement methods are used.The relationship among heat treatment process,microstructure and properties of PH13-8Mo steel was revealed.Firstly,the effect of aging temperature on microstructure and properties of PH13-8Mo steel was studied.The results show that when the aging temperature is 510～560℃,the strength decreases slightly with the aging temperature increasing,while the impact toughness increases significantly.For the following reasons: Firstly,with the increase of recovery degree of martensitic matrix,the decrease of dislocation density in matrix,the increase of high-angle grain boundary and the decrease of effective grain size all have favorable effects on toughness.Secondly,the increase of precipitated phase spacing of Ni Al contributes to the improvement of toughness by increasing the crack propagation energy.Finally,the presence of thin-film reversed austenite also contributes to the increase in toughness.At the late aging stage at 560℃,the toughening mechanism changes from matrix toughening mode to matrix and contravariant austenite compound toughening mode.Meanwhile,the nanoscale Ni Al precipitates distributed in the matrix have a great influence on the yield strength during the whole aging process.Secondly,the effect of aging time on microstructure and properties of PH13-8Mo steel was studied.The results show that when the aging temperature is 540℃,the strength decreases gradually with the aging time,while the toughness increases first and then stabilizes.When the aging time is less than 16 h,the decrease of dislocation density in matrix,the decrease of effective grain size,the increase of Ni Al precipitated phase spacing,the increase of contravariant austenite content and TRIP effect are the reasons for the increase of impact toughness.After 16 h,the precipitate coarse-grained phase weakens the gain effect of other factors on the toughness,and the impact toughness remains in a stable state.The coarsening of precipitated phase and the increase of reversed austenite are the main reasons for the strength decrease.In addition,due to the high aging temperature,the precipitate is prone to coarsening,and its ripening process conforms to LSW diffusion control theory.Finally,the effect of cooling mode after solution on microstructure and properties of PH13-8Mo martensitic steel was studied.The results show that thanks to the good hardenability of PH13-8Mo steel,martensite structure can be obtained at different cooling rates.However,the degree of supersaturation of martensite matrix is different at different cooling rates,which affects the effect of solution strengthening and aging strengthening,and is finally reflected in the mechanical properties: With the decrease of cooling rate,the hardness after solution decreases gradually,the strength decreases first and then becomes stable after aging,while the impact toughness increases first and then becomes stable.The changes of residual austenite,reversed austenite and high Angle grain boundary also have certain influence on the properties of the material,while The Block size is less affected by the cooling rate.