研究了s相对核电一回路主管道Z3CN20.09M不锈钢冲击韧性的影响, 利用原位拉伸、显微硬度、断口形貌等手段分析了s相的脆化机理. 结果表明, s相显著降低一回路主管道不锈钢的冲击韧性, 时效处理Z3CN20.09M不锈钢中以s相为主的由铁素体共析分解生成的(s+g₂)结构的硬度远高于奥氏体基体, 两者变形协调性差, (s+g₂)结构阻碍位错滑移, 提高材料强度, 同时降低塑性; (s+g₂)结构内部存在大量高能量s/g₂ 和a/s/g₂非共格界面, 变形时应力在此处集中, 成为潜在裂纹源, 易萌生裂纹. 高应变速率下, 裂纹迅速在其内部产生、扩展是材料韧性降低、变脆的本质原因.

Cast austenite stainless steel (CASS) possesses excellent mechanical properties, good workability and high resistance to localized corrosion in chloride environments due to the dual phase microstructure in which the island a-ferrite phase distributes in the g-austenite matrix. So they are widely used in the primary coolant pipes of nuclear power plants. However, undesirable s phase can precipitate in these steels when they are welded or heat treated and it severely decreases the toughness of stainless steels. Although some works have been done to investigate the effect of s phase on mechanical properties of CASS, the mechanism of embrittlement was still lacking. In this work, the effect of s phase on toughness of Z3CN20.09M CASS was investigated, and the embrittlement mechanism of s phase in CASS was discussed by using in situ tensile test, microhardness technology and fracture analysis. It was found that the impact energy of specimens aged at 750 ℃ decreased severely due to the presence of s phases. The (s+g₂) structure formed by the eutectoid decomposition of a phase is very hard and its hardness is much higher than that of austenite. This makes the deformation between (s+g₂) structure and austenite incoordinate in aged specimens. The precipitation of s phase brought more s/g₂ and a/s/g₂ high energy non-coherent boundaries. These boundaries hindered dislocation movements and brought stress concentrations. So cracks initiated at the s/g₂ or a/s/g₂ boundaries preferentially and propagated rapidly when the aged specimen bearded impact stress. The much potential cracking sites (s/g₂ and a/s/g₂ boundaries) in the (s+g₂) structure is the main reason of embrittlement of aged Z3CN20.09M CASS with low toughness.