| The pump valve body suffered the harsh conditions of high pressure cyclic load and working medium erosion during the service process,as the key component of shale gas fracturing truck,it needed good strength,toughness and corrosion resistance.The existing stainless steel pump valve body is made of Cr17Ni4 stainless steel(17-4PH stainless steel).The alloy could ensure the excellent strength,toughness,corrosion resistance,and service life.While,certain volume fraction of δ-ferrite had inevitably been introduced into the microstructure of the 17-4PH stainless steel due to the chemical composition design of addition of Cr,which seriously deteriorated the thermo-plasticity of the alloy,and it was difficult to make the 17-4PH stainless steel applicable to the large forging fields.Toward to this limitation associated to the application of 17-4PH stainless steel,a novel steel grade named Cr15Ni5 precipitation hardening stainless steel(15-5PH stainless steel)was successfully developed on the basis of 17-4PH stainless steel by increasing Ni content and reducing Cr content,and the δ-ferrite could be effectively reduced or eliminated in the final microstructure to make a good matching of strength,toughness,corrosion resistance and machinability.Wherease,there are few reports focused on the investigation of evolution of microstructure and mechanical properties for development of 15-5PH stainless steel.In present study,we focused on the effect of aging temperature on the evolution of microstructure and mechanical properties of a typical 15-5PH stainless steel.Electron backscattering diffraction technology,transmission electron microscope,X-ray diffraction and other research methods were used to explored the strengthening and toughening potential.Molecular dynamics was used to explore the interaction between the crack propagation and the typical grain boundary,which can provide experimental and theoretical reference for rational material selection of the pump valve body.The main research contents and results of the paper were as follows:(1)The effect of aging temperature on microstructure evolution of 15-5 PH stainless steel was systematically studied by means of transmission electron microscope and X-ray diffraction.Results showed that when the aging temperature was in the range of 440～610℃,there was no residual austenite in the matrix.The microstructure was the characteristics of lath martensite with high-density dislocation,and the width of martensite slat increases with the increase of aging temperature.When the aging temperature exceeds 510℃,a certain amount of nano-sized Cu-rich precipitates appear in the matrix,and the volume fraction of precipitated phase increases with the increase of aging temperature.(2)The effect of aging temperature on the mechanical properties of 15-5PH stainless steel was studied systematically.The results show that when the aging temperature is in the range of 440～540℃,the yield strength and tensile strength of the experimental steel decrease with the increase of aging temperature.When the aging temperature increases from 540℃ to 610 ℃,the changes of the yield strength and tensile strength of the experimental steel decrease slightly after increasing to a certain peak value.In the whole aging temperature range from440 ℃ to 610 ℃,the relationship between elongation and strength is inverted.(3)Based on Hall-Petch,Ashby-Orowan and other strengthening theories,combined with the microstructure,crystal orientation and substructure characterization results,the strengthening mechanism of 15-5PH stainless steel at different aging temperatures was quantitatively studied.The results show that the strengthening effects of solution strengthening and fine grain strengthening on 15-5PH stainless steel hardly change with the increase of aging temperature.Precipitation strengthening showed a trend of "from nothing to abundant,from a lot to small" with the increase of aging temperature.Dislocation strengthening firstly decreased and then increased with the increase of aging temperature,and the main strengthening mechanism of the stainless steel is variational at different aging temperatures.(4)Based on molecular dynamics theory,the toughening mechanism of 15-5PH stainless steel was investigated at atomic scale.The Fe-Cr twin-crystal model describing the typical grain boundaries of parent phase austenite(PAGB),superposition lattice grain boundary(CSL GB),martensite block grain boundary(BGB)and martensite bundle grain boundary(PGB)was constructed by Atomsk.Furthermore,Lammps calculation was used to study the interaction between typical grain boundaries and propagation cracks.The results show that the interaction between crack propagation and typical grain boundary can be divided into three stages.There is a certain correlation between stress and crack.During crack propagation,stress accumulates and releases continuously,and stress has a strong relationship with crack propagation stage.For the typical grain boundaries of 15-5PH stainless steel,PAGB grain boundaries and PGB grain boundaries have a more significant effect on inhibiting crack propagation.In order to ensure the alloy has excellent toughness and crack arrest properties,PAGB grain boundary and PGB grain boundary should be controlled mainly in the microstructure control. |
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