含N量对Mn18Cr18N奥氏体不锈钢的析出行为及力学性能的影响

Effect of Nitrogen Content on Precipitation Behavior and Mechanical Properties of Mn18Cr18N Austenitic Stainless Steel

采用JmatPro软件、OM、SEM和TEM等方法研究了不同含N量的Mn18Cr18N奥氏体不锈钢的析出行为及其对力学性能的影响。结果表明:析出物主要为六方结构的Cr_2N和少量M_(23)C_6,其中氮化物Cr_2N优先沿着晶界析出,随后以不连续胞状方式向奥氏体晶粒内部生长。随着N含量的增加,Cr_2N氮化物的析出变得更加敏感,当N含量为0.7%时,Cr_2N氮化物的最敏感析出温度为750℃,孕育期仅为10 min;而碳化物M_(23)C_6主要以颗粒状形式形成在奥氏体晶界上,与相邻的奥氏体晶粒保持相同的位向关系。力学性能测试结果表明,Cr_2N氮化物的析出对Mn18Cr18N奥氏体不锈钢的强度有较小的影响,但对于塑性却有强烈的恶化作用。时效后Cr_2N的析出导致伸长率和断面收缩率明显降低,伸长率从52.9%降低到27.7%,断裂模式也随着Cr_2N氮化物数量的增加从韧性断裂转变为脆性的沿晶断裂和穿晶断裂。TEM分析表明,固溶态试样在拉伸变形过程中通过滑移和孪生方式协调变形,呈现了良好的塑性变形能力。而时效后,位错通过滑移和繁殖最终堆积在Cr_2N片层之间和颗粒状M_(23)C_6周围,降低了Mn18Cr18N奥氏体不锈钢的塑性变形能力。

Mn18Cr18N austenitic stainless steel with excellent mechanical properties and corrosion resistance is widely used in nuclear industries, power plants and medicine field. However, precipitation of the second phases during hot deformation deteriorates the mechanical properties and hot formability. In order to clarify the precipitation behavior of this steel, the precipitation behavior and its influence on mechanical properties of Mn18Cr18N austenitic stainless steel with different nitrogen contents were in- vestigated by JmatPro software, OM, SEM and TEM analytical methods. The results indicate that precipi- tation phases consist of Cr2N and a few M23C6, in which Cr2N preferentially precipitates along grain bound-aries and then grows up to the interior of austenite grain by discontinuous cellular. With increasing of age- ing temperature, the precipitation of Cr2N became more sensitive. When the nitrogen content increases to 0.7%, the most sensitive precipitation temperature of Cr2N is 750 ~C with an incubation period of 10 min. However, M23C6 mainly precipitates by granular at austenitic grain boundaries and maintains cube-on- cube orientation relationship with adjacent austenite grain. The results of mechanical property test indi- cate that the precipitation of Cr2N has a negligible effect on strength and obvious deterioration on plastici- ty of Mn18Cr18N austenitic stainless steel. The precipitation of Cr2N after ageing treatment leads to re- markable decrease in elongation and reduction of area, and the elongation reduced from 52.9% to 27.7%. Meanwhile, fracture mode also transformed from ductile fracture to intergranular fracture and transgranular fracture with the increasing of Cr2N. TEM analysis shows that solution treatment sample re- veals good plastic deformation ability and coordinates deformation by slip and twinning, simultaneously. Nevertheless, dislocations slipped, propagated and eventually piled up between lamellas of Cr2N and around granular M23C6 after ageing treatment, which induce the degeneration of the plastic deformation capacity of Mn18Cr18N austenitic stainless steel.