A508-Alloy 52與316L-Alloy 52異質金屬銲件之高溫水環境應力腐蝕特性研究

Abstract

在核能電廠中，異質金屬銲接為連接反應器壓力槽出入水口管嘴（低合金鋼）與冷卻水管件（不銹鋼或鎳基合金）常用之接合製程。為求降低異質材料間的冶金、機械及物性差異，目前銲接製程多使用鎳基合金作為填料。本研究主要針對A508-Alloy 52與316L-Alloy 52之異質金屬銲件，評估銲道、熱影響區及接合界面等區域之應力腐蝕特性，試驗方式則採用預置缺口的圓棒拉伸試片，在高溫水環境下以定速率拉伸（CERT）試驗進行之。此外，銲件顯微組織觀察、傳統氬銲與回火銲珠製程之施銲比較、以及鎳基填料Alloy 52與52M之熱裂敏感性評估等試驗，亦為本研究之探討要點。 實驗結果得知，A508-Alloy 52銲件熱影響區為變韌鐵與肥粒鐵之混合組織。單道次銲件熱影響區以晶粒粗大之變韌鐵為主；多道次銲件熱影響區因受到後續銲道回火影響，有發生晶粒細化、肥粒鐵含量增加與硬度下降等現象。A508-Alloy 52銲件接合界面處可觀察到麻田散鐵及Type II boundary。欲降低A508熱影響區之硬度，可藉銲後熱處理或回火銲珠製程來達成，其中後者對於無法進行長時間熱處理之大型組件具有便利性，適合應用在核電廠的銲補施工。316L-Alloy 52銲件熱影響區並未發生顯微組織改變，熱處理後銲道亦無敏化現象，唯銲道過渡區在施銲時易發生熱裂現象，但可藉由降低銲接熱輸入量，或在施銲鎳基填料前先以309L覆銲緩衝層等方式，降低其熱裂敏感性。在填料選擇上，Alloy 52M對於抑制銲後熱裂以及延性降低破裂之效果皆較Alloy 52為佳，因此在核電廠未來的銲補施工上，應可取代現行之Alloy 52填料。 CERT試驗結果證實，缺口試片可確實評估銲件特定區域之SCC特性。A508-Alloy 52銲道區域（包含接合界面）受SCC之影響主要反映在試片的延性損失，其抗SCC能力強弱依序為：銲道未稀釋區域＞銲道過渡區＞銲件接合界面處。觀察試片破斷面亦可獲得相同趨勢，即抗SCC能力較差者具有較大範圍的脆性區域。A508/Alloy 52接合界面處之顯微組織不連續性，以及沿Type II boundary破裂之現象，均為造成SCC劣化的主因。A508熱影響區受SCC之影響則反映在試片的缺口拉伸強度損失。使用較低電流進行多道次銲接，可細化A508熱影響區晶粒，並改善其抗SCC性質。316L- Alloy 52銲件之接合界面與母材測試結果顯示，其拉伸強度及抗SCC能力皆遜於Alloy 52銲道。

The welding of dissimilar metals is widely used for joining low alloy to stainless steels at several locations such as pipe and nozzle joints in nuclear reactor pressure vessels. Nickel- based alloys, such as Alloy 52, are often applied in dissimilar metal welds (DMWs) as filler metals to reduce differences in physical, metallurgical and mechanical properties between the involved materials. In this study, the susceptibility to the stress corrosion cracking (SCC) of the A508-Alloy 52 and 316L-Alloy 52 welds in high-temperature water were evaluated by using constant extension rate tensile (CERT) test with notched specimens. Furthermore, the TIG welding process, filler metal selection (Alloys 52 and 52M) and microstructure of the DMWs were also investigated. Experimental results indicated that the heat-affected zone (HAZ) of the A508 side of DMWs consisted of a mixture of bainite and ferrite. For single-pass welds, the grain size in the HAZ of A508 side was considerably large and the structure was mainly bainite. However, the HAZ comprised mostly ferrite and fine grains in the multi-pass welds as a result of multiple weld thermal cycles during the process. The use of temper bead technique eliminated the need of post-weld heat treatment (PWHT) and lowered the HAZ hardness of the A508 side. Such a technique is particularly suitable for field repair and renovation. In the weld metal, martensite and Type II boundaries were observed in the transition zone adjacent to the weld interface of Alloy 52/A508, which may cause corrosion related failures in service. On the other hand, the HAZ of the 316L side showed neither microstructural change nor sensitization after a PWHT at 621°C/24 h. The transition region of the Alloy 52/316L weld metal was susceptible to hot cracking which could be reduced by lowering heat input in welding. Moreover, the weld overlay of a 309L buffer layer prior to deposit Alloy 52 on the 316L substrate significantly reduced the hot cracking susceptibility and could tolerate the substrate with higher contents of S and P. The results of varestraint tests demonstrated that Alloy 52M had better hot and ductility-dip cracking resistances than Alloy 52. As a result, it is recommended that Alloy 52 should be replaced by Alloy 52M for welding A508-316L and that kind of components in the nuclear power plant system. The results of the constant extension rate tensile (CERT) tests in 300°C water revealed that the notched round-bar specimen with a circumferential notch at various locations of the DMWs was useful in evaluating the SCC behavior of a narrow region in the welds. In the weld metal of A508-Alloy 52 welds, the relative susceptibility to SCC in terms of the ductility loss in increasing order of severity was as follows: the undiluted weld metal, the transition zone and the weld interface. SEM fractographic observations were consistent with the SCC results, i.e., an increased ductility loss or SCC susceptibility was associated with more brittle fractures. Apparently, the presence of Type II boundaries caused intergranular cracking and significantly reduced the SCC resistance of the weld in 300°C water. The structural discontinuity at the interface also increased the SCC susceptibility of the weld interface specimen.The test results of the A508 HAZ specimens clearly indicated that the lower welding current was beneficial to the SCC resistance,which was in terms of the loss of notched tensile strength, in the HAZ of A508 steel. In multi-pass welds, the use of a low heat input resulted in a better SCC resistance than that of a high heat input due to the existence of a more refined microstructure in the HAZ. Additionally, Alloy 52 weld metal also revealed better SCC resistance than either the 316L base metal or the weld interface of Alloy 52/316L.