The role of grain boundary ferrite evolution and thermal aging on creep cavitation of type 316H austenitic stainless steel
The role of grain boundary ferrite evolution and thermal aging on creep cavitation of type 316H austenitic stainless steel
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DOI：
10.1016/j.msea.2021.140859
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发表时间：
2021-03
期刊：
影响因子：
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通讯作者：
S. He;H. Shang;A. Fernández-Caballero;A. Warren;David A. Knowles;P. Flewitt;Tomas L Martin
中科院分区：
文献类型：
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作者：
S. He;H. Shang;A. Fernández-Caballero;A. Warren;David A. Knowles;P. Flewitt;Tomas L Martin
为了了解高温应力松弛过程中组织演变与蠕变空穴之间的相互作用，对一个先进气冷堆中服役的AISI型316 H不锈钢焊缝和热影响区（HAZ）试样进行了研究。多种技术，包括二次电子显微镜，电子背散射衍射（EBSD），透射电子显微镜（TEM）和等离子体聚焦离子束断层扫描仪被用于微观结构和蠕变腔表征。虽然在焊缝金属中没有观察到蠕变空洞，但热影响区广泛存在蠕变空洞。在随机取向的晶界处，存在蠕变空洞，并与热时效过程中形成的M23C6和铁素体沉淀物密切相关。较少的沉淀（例如，铁素体的情况下）和较少的蠕变空穴观察到在103重合点晶格边界。在服役时效过程中，在随机晶界处，M23C6的生成和长大导致γ稳定剂的局部元素贫化，促进奥氏体向铁素体的相变。用EBSD和TEM研究了铁素体和奥氏体的晶体学关系。在时效过程中形成的铁素体沉淀物经常生长到奥氏体晶粒中，这是传统的成核和生长理论所不期望的，可能是由于晶界处现有碳化物的物理约束。蠕变空洞的形成和长大与晶界上形成的M23C6和铁素体密切相关。这项研究强调了考虑热老化在加速蠕变空化中的作用的重要性。
To understand the interaction between microstructural evolution and creep cavitation during stress relaxation at an elevated temperature, an ex-service AISI type 316H stainless steel sample containing both weld metal and heat affected zone (HAZ) from an advanced gas-cooled reactor was studied. Multiple techniques that include secondary electron microscopy, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and plasma focused ion beam tomograph were used for microstructure and creep cavities characterisation. Although no creep cavities were observed in the weld metal, the HAZ was extensively creep cavitated. At randomly oriented grain boundaries, creep cavities are present and closely linked with M23C6and ferrite precipitates formed during thermal aging. Less precipitation (e.g. absence of ferrite) and less creep cavitation were observed at Σ3 coincidence site lattice boundaries. During in-service aging, at random grain boundaries, M23C6formation and growth cause the local elemental depletion of γ stabilisers and promote a phase transformation from austenite to ferrite. The crystallographic relationship between ferrite and austenite were also studied by EBSD and TEM. Ferrite precipitates formed during aging often grow into the austenite grain not expected by traditional nucleation and growth theory, likely due to physical constraints by the existing carbides at the grain boundaries. The formation and growth of creep cavities is closely associated with the M23C6and ferrite formed on grain boundaries. This study highlights the importance of considering the effect of thermal aging in accelerating creep cavitation.