| Martensitic precipitation hardening stainless steel is prone to thermal aging embrittlement when it is used in medium and low temperature environment for a long time,which leads to the increase of brittleness of stainless steel and easy to fracture,and brings great potential safety hazards to the safe operation and service life of equipment.Therefore,it is of great practical significance to study the thermal aging embrittlement of martensitic precipitation hardening stainless steel in the medium and low temperature environment.In this paper,the Fe-32Cr-4Ni-4Cu model alloy was designed with reference to the chemical composition of 17-4PH stainless steel.After thermal aging of the model alloy at 475℃,500℃and 540℃for 100h to 1000h,the microstructure under different thermal aging conditions was characterized and analyzed by OM,SEM,XRD and TEM,and the precipitation and amplitude modulated decomposition mechanism of nano Cu rich phase in the matrix were studied.The microhardness,impact toughness and tensile properties of the model alloy under different heat aging conditions were tested,and the fracture morphology was observed.The effects of precipitates and amplitude modulated decomposition on the mechanical properties of the model alloy under different heat aging conditions and the fracture mechanism of the alloy were studied.The observation and analysis of the microstructure showed that during the thermal aging process,the location of dislocation and high interface energy is more conducive to the nucleation and precipitation of the rich Cu phase,in addition,the rich Cu phase will also nucleate in the areas where Ni elements are enriched.TEM observation shows that with the increase of aging time,the shape of Cu rich phase precipitated from the matrix changes from spherical to ellipsoidal or short rod,and the size gradually becomes larger.The crystal structure of the model alloy changes from BCC structure to stable FCC structure through 9R or 2H structure.When the heat aging treatment is carried out at475℃,the ferrite in the matrix will undergo spinodal decomposition and form a lamellar structure with black and white phases.With the extension of thermal aging time,spinodal decomposition will change from grain boundary to intragranular,and the more fully the spinodal decomposition is carried out,the more uniform the distribution of decomposition products will be.During the heat aging process,the matrix martensite structure will undergo reverse transformation,and fine and continuous inversion austenite will be formed in the high Ni zone of the martensite matrix.When the heat aging temperature is within the range of500℃～540℃,there will be inverted austenite in the matrix,and the content of inverted austenite in the matrix will decrease when the temperature is raised.This is because the inverted austenite produced when the temperature is too high is unstable,and the martensite structure will be regenerated and become a new strip boundary after the redissolution,which refines the martensite structure to a certain extent and improves the hardness of the model alloy.The hardening behavior of model alloy under different heat aging conditions was studied by microhardness test.During the thermal aging process,the hardness of the model alloy changes with the thermal aging temperature and duration,which is mainly caused by the nano Cu rich phase precipitated from the model alloy matrix during the thermal aging process and the Cr rich phase produced by the spinodal decompositionα′the influence of phase and inversion austenite content.Oscillographic impact test and room temperature tensile test show that the fracture mode of model alloy changes from original ductile fracture to brittle fracture with the increase of heat aging temperature.When the temperature is higher than 500℃,the fracture mode is ductile fracture again.When the aging time is extended from 100h to1000h,the tensile strength of the model alloy increases first and then decreases,while the change trends of impact absorption energy A_k and elongation after fracture are opposite.Which indicates that the strength of the alloy has decreased during the thermal aging process,but its plasticity is getting better,and the longer the thermal aging time is,the better the plasticity of the model alloy is. |
PDF Full Text Request