The welded joints of reduced activation ferritic/martensitic (RAFM) steel should exhibit superior toughness to ensure that structural components are not prone to fracture due to irradiation embrittlement under long-term neutron irradiation conditions. Friction stir welding (FSW) of RAFM steel was performed, considering the potential advantages of obtaining welded joints with a good balance of strength and toughness. This work systematically investigated the effect of hierarchical martensitic microstructural evolution on the impact toughness of friction stir welded (FSWed) RAFM steel in the as-welded condition and post-weld tempering treatment at 760 °C. Prior austenite reconstruction based on the electron backscatter diffraction (EBSD) technology and precipitate analysis according to the transmission electron microscopy (TEM) data were conducted to study the microstructural evolution comprehensively. Tempering treatment reduces the average hardness of the SZ from 446 HV to 269 HV, but it remains 39 HV higher than that of the base material (BM). The tensile specimens containing whole regions of the welded joint all fracture in the BM. The ultimate tensile strength (UTS) of the tensile specimens containing only the SZ reaches 1033.7 MPa and 799.0 MPa in the as-weld and tempered at 760 °C conditions, respectively, which is greater than the 689.3 MPa of the BM. The FSWed joint of RAFM steel in the as-welded condition has acceptable toughness, and its ductile-brittle transition temperature (DBTT) corresponding to 68 J impact absorbed energy reaches −74.2 °C. Post-weld tempering treatment at 760 °C further reduces the DBTT to −110.7 °C, slightly higher than the −114.8 °C of the BM. The refined grain size is the crucial microstructure that determines the FSWed joint of RAFM steel has better toughness and lower DBTT than fusion welded joints.

中文翻译：

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分级马氏体显微组织对搅拌摩擦焊低活化铁素体/马氏体钢韧脆转变行为的影响

低活化铁素体/马氏体（RAFM）钢的焊接接头应表现出优异的韧性，以确保结构部件在长期中子辐照条件下不易因辐照脆化而发生断裂。考虑到获得强度和韧性良好平衡的焊接接头的潜在优势，对 RAFM 钢进行了搅拌摩擦焊 (FSW)。这项工作系统地研究了分级马氏体微观结构演变对搅拌摩擦焊 (FSWed) RAFM 钢在焊态和 760 °C 焊后回火处理下冲击韧性的影响。基于电子背散射衍射（EBSD）技术的预先奥氏体重构和根据透射电子显微镜（TEM）数据的沉淀物分析，以全面研究微观结构的演变。回火处理将 SZ 的平均硬度从 446 HV 降低至 269 HV，但仍比母材 (BM) 高 39 HV。包含焊接接头整个区域的拉伸试样均在 BM 中断裂。仅含SZ的拉伸试样在焊态和760℃回火条件下的极限拉伸强度（UTS）分别达到1033.7 MPa和799.0 MPa，大于BM的689.3 MPa。RAFM钢的FSWed接头在焊接状态下具有良好的韧性，68 J冲击吸收能对应的韧脆转变温度（DBTT）达到-74.2 ℃。760℃的焊后回火处理进一步将DBTT降低至-110.7℃，略高于BM的-114.8℃。细化的晶粒尺寸是决定RAFM钢FSWed接头比熔焊接头具有更好的韧性和更低的DBTT的关键显微组织。