Study On Fatigue Properties Of A New Type Austenitic Stainless Steel

24Mn-13Cr-1Ni-0.44N austenitic stainless steel was studied in this paper. Fatigue properties of this new kind of austenitic stainless steel was studied by rotary bending fatigue test at room temperature. Mechanical properties of this material isσ_0.2=430MPa,σ_b=790MPa,δ=40 %,(?)=55 %. It possesses very high strength and excellent plasticity. Fatigue lifes were got by appling different loadings. Fitting correlation of fatigue curve was analyzed by making use of different median life values at different loadings. Through analysis, It was found that the relationship between S and lgN was linear. lgN=9.4151-0.008S was got by method of least squares. That was e^0.01842SN=10^9.4151. Variance of regression equation was analyzed. When the significance level a was 5%, the result indicated that the regression equation was meaningful. S-N curve was drawed by regression aquation. The fatigue limiting was 341MPa. The relationship between fatigue limiting and static strength was congruent.Observed the deformation structure of fatigue sample, it was found that the saturation dislocation structure were intersected planar dislocations. The reasion was that dislocations hard to cross slip because of fault energy was low by high nitrogen content. During the process of fatigue deformation, twinnings and dislocations slip all took part in deformation.Through XRD analysis of sample whenσ_max was 404MPa, Nf was 1.792×10⁶, we could not found martensite, which confirmed that the structure stabilization of this material was good at the room temperature.SEM was used to investigate the fractutral characterizations. Whenσ_max was 347MPa ,we found feature of quasi-cleavage fractutral, and this feature was very obvious. The fatigue marking was found only at area adjacent transient rapture area. As the loading grows, quasi-feature of cleavage fractutral could not found easily, but the fatigue marking was very obvious. Whenσ_max was 393MPa, the fracture was made up by undistinguishable small planes and fatigue marking.Fracture mechanism was analyzed after fracture analysis. Many inclusions were formed because of metallurgical quality of this material was not good. Through analysis, it was found that all of the fatigue fractures were formed at inclusions. So improving metallurgical quality, fatigue nature could be increased more. When loading was low, feature of quasi-cleavage fractutral was very obvious. fractures expanded along {111}_γplanes and intersected dislocations were found at small fracture planes. The reason was that small cracks were expanded along slipping planes which amost parallel shear stress early in the cycles. As expansion of fractures, fracture plane was vertical to loading direction. During the stress of repeated pulling and press, many dislocations and twinnings were formed. Dislocations were hard to slip due to the interactivity of nitrogen atoms and dislocations. Many dislocations induced fractures were generated along slip plane. As long as fractures expanded along {111}_γ, secondary slip of dislocations were hard to proceed., which enlarged the positive stress of fracture tip, when the positive stress acted at one {111}_γplane was large than the bond strength, breakdown occurred, so the cleavage planes were formed. As quasi-cleavage small planes expanded, the planes between themselives become more and more small. During the process of fractures expansion, plastic deformation occurred in interconnecting areas of fractures, which leaded to laceration at last, so the tear ridge was formed. With expansion of fractures, true stress become larger, so theâ–³K become more and more large, whenâ–³K reached to a given value, the fatigue marking was formed by cross slip of dislocations at fractures tip. Whenσ_max was 393MPa, the corresponding planes of cracks mutual were fictioned by this large loading, so undistinguishable planes were formed.