| 24Mn18Cr3Ni0.62N high nitrogen austenitic stainless steels were studied in this paper. Tensile properties of a new type austenitic stainless steel 24Mn-18Cr-3Ni-0.62N at room temperature were investigated. During tensile the austenitic stainless steel has not obvious yield point elongation. Tensile property parameters are:σ0.2=525MPa,σb=890MPa,δ=41 %,φ=57% .It possesses very high strength and excellent plasticity. Slip bands formed were coarse and dense, regular triangle deformation microstructure can be founded if optical microscopy section is parallel or approximately parallel to (111) γplane.The fracture is toughness fracture. The inclusions are one of crack headstream, and purifying materials can further enhance tensile strength of materials.At the same time, tension-tesion fatigue properties of high nitrogen austenitic stainless steel (24Mn18Cr3Ni0.62N )under different loads were investigated in this paper. Fatigue samples under different loadings were tested at 233K,293K and 423K,then the respective fatigue life and S-N curve was obtained.In this paper,the fracture morphology under 398MPa,477 MPa,588 Mpa and 796MPa were emphatically analysed,and the fracture mechanism was given under preliminary analysis. Observed the deformation structure of fatigue sample, it was found that 24Mnl8Cr3Ni0.62N austenitic stainless steel has a single austenite phase,where exists many twins and stacking faults.Numerous dislocations glided by shear stress will produce slip linesAt the initial stage,there is only one sliding systerm playing the leading role and only one sliding band can be seen obviously,other directional sliding bands can not be seen distinctly.During the developing process of the deformation,a new directional sliding band can be seen.With the deformation increasing,the sliding lines are gradually broadening and the step-shaped image is made.Fatigue initial area, crack propagation area and fatigue fracture area can be seen from observation of macroscopic fracture morphology. And observation from microscopic fracture morphology showed that there were fatigue striations, river pattern,cleavage steps,intergranular cracks,fishbone-shaped morphology,Wallner lines and the secondary cracks in fatigue crack propagation area, and a lot of dimples in fatigue fracture area. Based on the previous results and macroscopic fracture morphology,the mechanism of fatigue failure mechanism is mixed mode fracture by cleavage fracture and aggregation microporous fracture.Tension-tension fatigue property of high nitrogen steel is studied by S-N curve in this paper.With the loading stress the ordinate and corresponding fatigue life the abscissa, the S-N curve had been drew,which showed the stepladder-like trend downward.If Nf < 2×105, fatigue property of high nitrogen steel at 233K is superior to the fatigue properties at 293K and 423K; If 2×105< Nf <1×107, fatigue property at room temperature is best.The corresponding platform of S-N curve was approximately stess : 405MPa at 233K,378MPa at 293K, and 382MPa at 423K .Under this stess,fatigue failure can still be observed in this kind of new austenitic stainless steel,which has no traditional fatigue strength and infinit life,but condition fatigue strength.Using the results of tensile test at room temperature, prediction of the fatigue life of high nitrogen steel was done. Randomly distributed models of micro-plastic strains were used to calculate the micro-plastic deformation produced at each cycle of the steel under high cycle fatigue test. The micro-plastic strains are determined by combining the true fracture ductility of the steel under test. For 24Mn18Cr3Ni0. 62N high nitrogen steel, the errors of the fatigue life calculated with the proposed method are less than 20% compared with that determined by test.It is proved that the estimation of the high-cycle fatigue life is feasible,which is playing a guiding role for the fatigue life prediction of high nitrogen steel.
|
PDF Full Text Request