热加工对含氮马氏体不锈钢组织转变和性能的影响

Effects of thermal processing on the microstructural transformation and properties of nitrogen-alloyed martensitic stainless steel

采用Thermal-calc计算了含氮马氏体不锈钢20Cr13的合金相图,据此进行了关键热加工工艺参数设计。采用金相、扫描电镜、X射线衍射、高温热模拟试验、拉伸试验和硬度测试等方法,研究了高温下均热温度对高温组织转变的影响以及高温铁素体对高温塑性的影响,同时研究了退火和淬火工艺对组织和性能的影响。结果表明:铸锭中的少量δ铁素体在单相奥氏体区高温长时间均热后并未消除;δ铁素体的存在降低了马氏体不锈钢的高温塑性;在临界温度长时间退火后,组织为铁素体基体上弥散分布球状碳化物的索氏体及沿晶界呈断续分布的点状碳化物,随退火温度的提高,索氏体晶粒尺寸增大,碳化物选择性地在晶界粗化长大,并呈断续状点状分布;950~1100℃奥氏体化淬火后的组织为板条马氏体+碳化物+少量残余奥氏体。淬火温度较低时,碳化物和残余奥氏体含量较高,淬火后马氏体硬度较低,提高淬火温度,碳化物充分溶解,奥氏体中的碳含量增加,淬火后板条马氏体硬度升高。

The phase diagram of nitrogen-containing martensitic stainless steel 20Cr13 was calculated by using the Thermal-calc software,and the key thermal processing parameters were designed accordingly.Testing methods such as metallographic observation,X-ray diffraction,high temperature thermal simulation test,tensile test and hardness measurement were used to investigate the effect of soaking temperature on high-temperature microstructure transformation,the effect of high-temperature ferrite on high-temperature plasticity and the effects of annealing and quenching on microstructure and properties.The results show that a small amount ofδferrite in the cast ingot is not eliminated after high temperature soaking in the mono-phase zone for a long time,and the presence ofδferrite reduces the high-temperature plasticity.After long-term annealing at the critical temperature,the microstructure consists of sorbite with spherical carbides dispersed within the ferrite matrix and along the grain boundaries.With the increase of the annealing temperature,the sorbite grain size increases,and the carbides are preferably coarsened on the grain boundary.After quenching from austenising temperature of 950-1100℃,lath martensitic microstructure with small quantity of carbide plus retained austenite is obtained.At lower quenching temperature,higher quantity of carbides and retained austenite is obtained,and lower hardness is presented.With increasing the austenising temperature,more carbides are dissolved into austenite,the carbon content in the austenite increases accordingly,subsequently,the hardness of lath martensite increases.