高溫純水中316L不銹鋼與52合金異材銲件之應力腐蝕龜裂行為研究

An Investigation into SCC of 316L SS - Alloy52 Dissimilar Metal Welds in Simulated BWR Environments

數十年來在輕水式反應器中，材料的應力腐蝕龜裂為嚴重的劣化機制。諸多反應器運轉經驗發現，氧化性環境、殘餘應力和敏感性材料會造成BWR結構組件之應力腐蝕龜裂。尤其對結構組件中異材銲接處的應力腐蝕而言，銲材表面的殘餘應力和成份之改變對材料的抗腐蝕性與可靠度會產生相當大的衝擊。過去常藉由調整材料熱處理和水化學環境，期望能達到腐蝕抑制的效果。
為了探討敏化316L不銹鋼、316L SS及52合金異材銲件中的應力腐蝕龜裂，本研究在高溫含氧的水環境中執行一系列的慢速拉伸試驗。在執行慢速拉伸試驗前，試樣經過固溶處理與銲後熱處理，此外亦測量銲道與鄰近區域之微硬度與殘餘應力。在慢速拉伸試驗之後，使用掃描式電子顯微鏡觀察材料破裂面之形貌。研究結果顯示，固溶熱處理可有效改善材料的機械表現；銲後熱處理本意為消除材料表現的殘留應力以抑制SCC，但依本實驗目前的熱處理條件顯示，破斷面仍有穿晶應力腐蝕龜裂的發生，機械表現未有顯著改善，仍需進一步的研究以尋找適當之熱處理條件。

Stress corrosion cracking (SCC) has been a serious degradation mechanism of structural materials in light water reactors for decades. Considerable reactor experience has shown that an oxidizing environment, residual stress and susceptible materials combine to produce SCC in structural components of boiling water reactors (BWR). Especially for the cases of SCC in dissimilar metal welds, changes of the residual stress and chemical composition of metal welds in BWR have a huge impact on the corrosion resistance and reliability of structural materials. Through the alteration of water environment and the process of heat treatment for materials, the effect of corrosion mitigation is expected to be achievable.
In order to explore the mechanism of SCC in sensitized 316L SS and dissimilar metal welds of 316L SS and Alloy 52, a series of slow strain rate tensile (SSRT) tests in high temperature oxygenated pure water were arranged and conducted in this study. Prior to the SSRT tests, samples were prepared and underwent various pretreatments, including solution annealing and post-weld heat treatment (PWHT). Micro-hardness and residual stresses were profiled adjacent to and in the welded joints of the welded samples. After the SSRT tests, morphologies of the fractured surfaces of the samples were also characterized by using the scanning electron microscope.