The Mechanical Stability Of Reversed Austenite In 0013Cr4NiMo Steel And Its Effect On Mechanical Properties

Super martensitic stainless steel has high strength,good weldability and corrosion resistance,and is widely used in hydraulic turbine components,valve bodies,oil and gas pipelines.With the extreme service environment,new requirements are put forward for the strength and toughness of the material,reversed austenite plays an important role in the control of strength and toughness of the material.In this paper,0013Cr4 NiMo steel was used as the experimental material to optimize the design of the cycle heat treatment process system,analyze the nucleation and distribution of reversed austenite from the perspective of microstructure refinement,and explore the mechanical stability of contravariant austenite and its influence on the mechanical properties of the material by using the crystal plastic finite element method.The main research work of this paper is as follows:(1)Combined with the characteristics of thermal cycle structure and recrystallization,the process of cyclic quenching and tempering heat treatment is designed.The results show that the average equivalent circular diameter of prior austenite grain is reduced by 3μm,the nucleation position and quantity of reversed austenite are increased,and the reversed austenite volume fraction reaches 11.3% at room temperature after cyclic quenching and double tempering.The mechanism of the preferential nucleation of reversed austenite at high-angle grain boundaries is explained by using the two-spherical-cap model.(2)The deformation induced phase transformation behavior of reversed austenite during tensile process is simulated by a mesoscopic crystal plastic finite element model.The effects of reversed austenite shape size,orientation relationship and distribution position on its mechanical stability are analyzed by simple models.The transformation behavior of reversed austenite is calculated based on the typical characteristics of reversed austenite.It is found that the film reversed austenite located at the martensite packet boundary and the martensite block boundary is the first to undergo deformation-induced martensitic transformation than the block reversed austenite distributed in martensitic lath.When the material engineering strain reaches the maximum,the phase transformation of the block reversed austenite almost completely takes place,while part of the film reversed austenite remains stable.(3)Comparing and analyzing the tensile properties of the samples with different heat treatment processes,it is found that under the same tempering temperature,the tensile strength of the thermal cyclic heat treatment sample is about 3% lower than that of the conventional quenching sample,but its fracture surface shrinkage is increased by about 2%～3%.Combined with the results of crystal plastic finite element simulation,the mechanism of transformation induced by reversed austenite deformation to enhance plasticity and toughness and the effect of reversed austenite in two different distribution positions on stress and strain distribution were analyzed.(4)Comparing and analyzing the impact properties of the samples of different heat treatment processes,it is found that the tempering samples after thermal cyclic heat treatment were better than those treated by conventional quenching in impact toughness.After cyclic quenching,the impact energy of the primary tempering sample is 264.5J at-100℃,which is 20% higher than that of the conventional quenching.Based on the Yoffe diagram,the mechanism of improving the toughness of low temperature by grain refinement was explained,and the toughening mechanism of reversed austenite was listed.The microscopic mechanism of stress relaxation induced by deformation-induced martensitic transformation to improve fracture toughness was analyzed by crystal plastic finite element simulation.