Investigation On High Pressure Martensitic Transformation Of 20# Steel

Martensitic transformation is of great scientific and technologic importance,since steels can be strengthened by a factor of 10 via martensitic transformation.However,such strengthening is strongly dependent on carbon content,and the traditional low carbon steels commonly form dislocation lath martensite showing insignificant hardening by a factor of2-3.Adding more carbon and other alloying elements may enhance the hardening,but leading to unexpected rising in production cost and the waste of valuable earth resources.Therefore,how to strengthen low carbon steels without relying on alloying elements is of great significance.This paper aims to use pressure,a thermodynamic variable independent of chemical composition,to monitor the microstructure of low carbon martensite,to enhance its strengthening effect and to reveal the underlying mechanism.We choose 20#steel as the experimental material to perform thermal cycle at different pressures(1—6 GPa)in a hexahedral multi-anvil pressure facility.The microstructures together with the orientation relationship between martensite and austenite of the final products were characterized by using optical microscope,X-ray diffraction,scanning electron microscope and transmission electron microscope.The strengthening was evaluated by a Vickers hardness tester.Detailed comparison between the water quenched samples under atmospheric pressure was made so as to reveal the pressure effect.The following conclusions were obtained:(1)The 1—6 GPa high-pressure samples have undergone complete martensitic transformation,resulting in twinned lath martensite.Resemble to that of the traditional dislocation lath martensite induced by water quenching under atmospheric pressure,twinned lath martensite(M)has a typical Packet-block-lath multi-stage structure and follows the classic K-S relationship with prior-austenite(A):{1 1 1}_A//{0 1 1}_M,<1 1 0>_A//<1 1 1>_M.However,this lath martensite contains a large number of laths that are twin related with respect to the adjacent laths,namely,mirror symmetrical about{1 1 2}but with{0 1 1}boundary.The is significantly different from the traditional{1 1 2}<1 1 1>twin,where the symmetric plane and the boundary are both{1 1 2}.(2)The thickness of martensitic lath decreases with the increase of pressure.The average lath thickness of 1—6 GPa high-pressure sample is 0.111μm(1 GPa),0.031μm(2GPa),0.029μm(3 GPa),0.017μm(4 GPa),0.014μm(5 GPa)and 0.015μm(6 GPa),respectively.(3)Pressure promotes the selection of twinned martensiticvariants.With the increase of pressure,the content of variant pairs with small misorientation decreases,whereas those that are twin-related increase.(4)High pressure enhanced the hardening by martensitic transformation.The hardness of 1—6 GPa samples exceeds that(400 HV)of water-quenched sample under atmospheric pressure,reaching 510 HV(1 GPa),550 HV(2 GPa),600HV(3 GPa),650 HV(4 GPa),700 HV(5 GPa)and 700 HV(6 GPa),respectively.Hardening ceases increasing as the pressure exceeds 5 GPa.