不同冲击性能低温容器用9Ni钢的微观研究

对冲击性能不同的9Ni钢试样从微观方面进行了比对分析,用扫描电子显微镜、金相显微镜、电子探针和扫描电镜电子背散射衍射(EBSD)进行组织、非金属夹杂物、奥氏体晶粒、低倍枝晶和残余奥氏体量的观察研究。研究结果表明:冲击功值低的试样断口呈脆性解理断裂,低倍枝晶形貌图显示柱状晶发达,奥氏体晶粒粗大且不均匀;而冲击功值高的试样断口呈韧性断裂,各晶区可明显区分,奥氏体晶粒细且均匀。两组试样的夹杂物评级没有明显差异,说明两组试样的冲击性能差异是由于不恰当的连铸工艺使得柱状晶过于发达,铸态枝晶间形成富镍的偏析带,镍的偏析带在随后的热加工中不能被完全消除而遗传下来,造成了钢板镍元素的微观偏析,引起了组织不均匀,并降低了9Ni钢的冲击性能。

Research on microstructure of 9Ni steel for low temperature vessel with different impact property

The microstructure of 9Ni steel samples with different impact properties was compared and analyzed. The structure, non-metallic inclusion, austenite grain, low-magnification dendritic crystals and the amount of retained austenite were observed and investigated by scanning electron microscope, metallographic microscope, electronic probe and scanning electron microscope-electron backscatter diffraction (EBSD). The research results indicated that: the fracture of sample with low impact property showed brittle cleavage fracture, the low-magnification dendritic crystal morphology figure indicated columnar crystal was developed, and the austenite grains were bulky and non-uniform. However, the fracture of sample with high impact property showed ductile fracture, the crystalline regions could be well differentiated, and the austenite grains were fine and uniform. The grade of inclusions in two groups of samples had no significant difference, indicating that the difference of impact properties between samples was due to the over-developed columnar crystal caused by improper continuous casting process. The nickel-rich segregated band was formed between as-cast dendritic crystals. The segregation band of nickel could not be fully removed in the following thermal processing and be passed down, resulting in the micro-segregation of nickel element in steel plate. As a result, the impact property of 9Ni steel was reduced due to the non-uniform structure.