核电双相不锈钢超细双相组织中Sigma相析出机制及其脆化效应的研究

Duplex stainless steels with an attractive combination of mechanical properties, corrosion resistance and material cost are significantly focused to be used as suitable materials for important components in nuclear power plants. However, the susceptibilty to cracks caused by the sigma phase precipitation during hotworking limits those steel application for some heavy castings,such as pumps, valves and impellers. It can be seen that the control of the sigma embrittlement is a very important factor to resolve that problem mentioned above. A great many researchers have attached importance to avoid cracks by the suspension of sigma phase precipitation through adjusting alloy element content or heat-treatment parameters. Also, little work studied the influence of the primary microstructure characters on the kinetics of sigma phase precipitation. But, no difference of the ductility or toughness corresponding to various primary microstructure characters was considered to affect the sigma phase embrittlement effect. In this proposal, influence of the orientation difference between delta and gamma interphase boundaries on the ductility and toughness will firstly be studied in a superduplex stainless steel with micro-duplex structure. And then, mechanisms of the sigma phase precipitation and its embrittlement effect will be systematically investigated by the in-situ observation in High Temperature Confocal Laser Scanning Microscope with mechanical test instrument.The study above is expected to improve understanding the interactions among the ductility and toughness dependent on the interphase boundaries, sigma phase precipitation and its embrittlement effect, which may promote an innovation on avoiding cracks induced by the sigma phase precipitation.

由于力学性能良好、耐蚀性能优异和合金成本较低，双相不锈钢作为核电装备大型关键部件的选材，备受关注。但是，Sigma相致脆开裂，已严重限制该类钢种的实际应用。可见，避免或减轻Sigma相的脆化效应十分关键。少量研究发现，改变初始组织，能够有效延缓Sigma相析出。然而，这些工作仅单纯考察了Sigma相析出动力学的变化，忽视了由此产生的韧塑性改变及其对Sigma相脆化效应的影响。因此，本项目以核电叶轮用超级双相不锈钢超细双相组织为研究对象，主要通过共聚焦激光扫描显微镜原位、实时观察Sigma相析出和拉伸变形过程，研究相界面特征差异对材料韧塑性、Sigma相形核位置与生长方式，以及Sigma相对位错滑移、裂纹萌生与扩展的影响，系统构建超细双相组织中相界面特征、Simga相析出机制与Sigma相脆化效应之间的对应关系，从而为开发新型Sigma相致脆开裂控制技术提供理论依据。

双相不锈钢合金成本较低、综合力学性能良好、耐腐蚀性能优异，是核电装备大型关键部件选材首选。然而，双相不锈钢极易析出σ脆性相，严重影响材料的韧塑性，造成开裂报废，给核电装备关键部件质量控制提出了严峻挑战。本项目开发了核电双相不锈钢超细双相组织的制备工艺，揭示了超细双相组织中σ相的析出机制，明确了σ相的致脆开裂机理，对双相不锈钢成形过程开裂控制具有重要指导意义。主要工作与重要结果如下：.(1)超细双相组织制备工艺及其组织演化规律：以粗大铸态组织为初始组织，经1350℃高温固溶、70%冷轧大变形和1000℃等温时效工艺，获得了晶粒约1μm的超细双相组织。高温固溶保温时，粗大铸态组织中γ向δ转变、σ完全溶解，水淬后转变为大量粗大等轴δ相和在δ相晶界或晶内的少量γ相魏氏组织；冷轧大变形时，γ相魏氏组织被打碎，沿着变形方向被拉长；再经高温退火后，γ相魏氏组织发生再结晶，δ相向γ相转变和发生回复形成大量小角度晶界，最终二者均呈等轴超细组织。.(2)超细双相组织等温时效时σ相的析出机制：在850℃等温时效时，σ相优先在δ/γ相界或三叉相界处以块状形貌析出，快速向铁素体相内部生长，促使原始γ相界向δ相一侧凸起形成γ2相，从而完成δ相的共析分解。其中，σ相形核过程为界面饱和机制形核过程，σ相的析出与原始γ相界的凸起为相互交替促进，σ相的长大促进了原始γ相界向δ相一侧推移，形成新的相界面。同时，σ相又不断沿着新相界面生长，从而最终形成原始γ相凸入σ相中的形貌特征。.(3)σ相的致脆开裂机理：冲击时，裂纹在σ+γ2相内直接萌生，或者沿σ/γ相界面扩展，或者横穿σ+γ2相和δ相扩展，直至γ相内止裂，从而大幅降低裂纹的启裂功和扩展功，易于裂纹的萌生于扩展，造成冲击韧性急剧下降。拉伸形变量较小时，变形主要集中在γ相内，滑移线相互交错，位错不断在σ/γ相界面塞积，致使裂纹产生。随着变形量增大，γ相内滑移线交织密度增大，裂纹或横穿σ相，或者沿σ/γ相界面扩展，局部造成σ相碎化、剥落。