高氮高钼合金连铸表面裂纹原因分析

Analysis of surface cracking mechanism of high nitrogen and molybdenum alloy continuous casting

S31254是一种超级奥氏体不锈钢,含有大量的Cr、Ni、Mo、N和Cu等合金元素,具有优异的抗腐蚀性能和综合力学性能。当前,S31254合金连铸主要问题是表面裂纹。通过对S31254合金的材料特性、热模拟断裂机制、凝固原位观察研究,利用金相显微镜和扫描电镜系统分析了S31254裂纹缺陷成因。研究结果表明,S31254合金中有大量分布于铸态组织晶界的析出相,析出相包括σ相、χ相、Laves相和氮化物等,析出相特征是富Cr和Mo、贫Ni。当温度升高至1 240℃左右时,所有的析出相熔入到奥氏体相中,最后熔化的析出相为σ相。随着S31254合金析出相沿晶界析出,铸态晶界脆化,铸坯试样断裂机制演变为沿晶的脆性断裂,这降低了合金的高温塑性,增大了发生裂纹的倾向性。

S31254 is a super austenitic stainless steel(SASS),which contains a large amount of alloying elements such as Cr,Ni,Mo,N and Cu,and has excellent corrosion resistance and comprehensive mechanical properties.At present,the main problem of S31254 alloy continuous casting is surface crack.By studying the material properties,thermal simulation fracture mechanism and in-situ observation of solidification of S31254 alloy,the cause of crack defect of S31254 was analyzed by metallographic microscope and scanning electron microscope.The results show that there are a large number of precipitates in the as-cast grain boundary of S31254 alloy.The precipitates include Sigma phase(σ),Chi phase(χ),Laves-phase and nitrides etc.The precipitates are characterized by rich Cr and Mo and poor Ni.When the temperature rises to about 1 240℃,all precipitates melt into austenite phase,and the final molten precipitates are σ phase.As the precipitated phase of S31254 alloy precipitates along the grain boundary,the as-cast grain boundary becomes brittle,and the fracture mechanism of the cast billet specimen changes into the brittle fracture along the grain boundary,which reduces the high temperature plasticity of the alloy and increases the tendency to crack.