摘要
本文以24Mn18Cr3Ni0.62N高氮奥氏体不锈钢为研究对象,研究了24Mn18Cr3Ni0.62N在室温下的拉伸性能,该新型奥氏体不锈钢拉伸时没有明显的屈服平台,力学性能为:σ_(0.2)=525MPa,σ_b=890MPa,δ=41%,φ=57%。24Mn18Cr3Ni0.62N高氮钢具有很高的强度和优良的塑性,拉伸时形成的滑移带粗大、密集,若金相截面和某(111)_γ面平行或接近平行,还可发现具有正三角形的形变组织,拉伸断口为韧性断口,块状夹杂物为裂纹发源地之一,净化材料可以进一步提高材料的抗拉强度。
同时,本文重点研究了24Mn18Cr3Ni0.62N高氮奥氏体不锈钢的拉-拉疲劳性能。分别在233K、293K和423K三个温度下、不同载荷水平下对试样进行疲劳试验,以获得高氮钢的疲劳寿命,并绘制S-N曲线。
本文重点分析了σ_(max)为398MPa、477 MPa、588 Mpa和796 MPa时的断口形貌,初步探究其疲劳断裂机理。对疲劳试样断口附近的形变组织进行观察,可知24Mn18Cr3Ni0.62N高氮奥氏体不锈钢室温下为单一的奥氏体组织,存在很多孪晶和层错。在切应力作用下,众多位错发生滑移会产生滑移线。滑移初期只有一个方向的滑移系统占主导地位,只能看到一个方向的滑移带比较明显,其他方向的滑移带只是隐约可见。形变发展的过程中,可以看到出现新方向的滑移带。随着变形量的加大,滑移线逐渐变宽,出现“台阶”现象。
宏观断口上可观察到疲劳源区、裂纹扩展区和瞬断区。对断口进行微观分析,在裂纹扩展区可观察到疲劳条带、河流花样、解理台阶、沿晶裂纹,瓦纳线以及二次裂纹。在瞬断区可观察到许多韧窝。同时,对疲劳裂纹萌生的机制和扩展机理作了初步探索。疲劳断裂的微观机理可能是解理断裂和微孔聚集断裂的混合型断裂。
利用S-N曲线,研究了高氮钢的拉-拉疲劳性能。用载荷作纵坐标,对应的疲劳寿命做横坐标绘制S-N曲线,得出的S-N曲线呈阶梯下降趋势。利用S-N曲线比较三个温度下高氮钢的疲劳性能,可知当疲劳循环周次N_f<2×10~5周次时,233K温度下高氮钢24Mn18Cr3Ni0.62N的疲劳性能优于293K和423K时的。当2×10~5 结合常温拉伸试验的结果,对高氮钢的疲劳寿命进行预测。由实验测出的高氮不锈钢的拉伸应力-应变曲线,可确定材料微元屈服极限服从对数正态分布。经计算与分析,24Mn18Cr3Ni0.62N高氮不锈钢疲劳寿命估算值与实测值吻合较好,误差<20%,初步证明本文中提出的估算高周疲劳寿命的方法是可行的,对高氮不锈钢的疲劳寿命的预测起到一定指导意义。
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:σ_(0.2)=525MPa,σ_(b)=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 N_f < 2×10~5, fatigue property of high nitrogen steel at 233K is superior to the fatigue properties at 293K and 423K; If 2×10~5< N_f <1×10~7, 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.
引文
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