Carbide distribution and high-temperature fracture mechanism of high nitrogen stainless bearing steel

The high temperature fracture behavior of high nitrogen stainless bearing steel was studied by high temperature tensile test. The distribution characteristics of carbide in 170 ℃ and 470 ℃ tempered steel were investigated. The tensile fracture, microstructure evolution and carbide distribution law were analyzed. The results show that with the tempering temperature increasing from 170 ℃ to 470 ℃, the carbide larger than 0.8 μm in high nitrogen steel increases significantly, M₂₃C₆ strengthening increment increases by 2.59 MPa, and solution strengthening increment decreases by 118.82 MPa. The tensile strength of 470 ℃ tempering steel decreases at room temperature, and the tensile fracture behavior of the steel at room temperature is quasi-cleavage and a small amount of tear dimms. When the tensile temperature is increased to 300 ℃, the fracture surface of sample is equiaxed dimple, and the size of fracture source carbide of the 170 ℃ and 470 ℃ tempering samples is 2.8-3.6 μm and 5.5-6.7 μm, respectively. The tensile fracture at 450 ℃ is characterized by plastic-pore dimples, and the size of fracture source carbide at 170 ℃ and 470 ℃ is 2.7-3.4 μm and 5.8-6.4 μm, respectively. When the tensile temperature increases from 300 ℃ to 450 ℃, the effect of solution strengthening and dislocation strengthening is weakened, the interatomic bonding energy of metal decreases, the discontinuous stress distribution between carbide and matrix increases the deformation incompatibility, and the carbides bear higher stress then fracture occurs. Under the action of pure heat, the percentage of carbides of 0.5-0.8 μm increased. Under the action of thermodynamic coupling, dislocation proliferation caused by stress in steel provides channels for carbon diffusion, and 0.2-0.8 μm carbides are nucleated at grain boundaries and dislocation lines. The crack propagates rapidly along the maximum shear direction at angle of 45° with the tensile direction and then fractures. Finally, serrated fracture is formed. The increase of small carbides hinders dislocation slip and results in reduced plasticity. The large size carbides in steel distribute unevenly among the carbides to form large deformation plastic holes and increase the plasticity of steel.