蒸氣管線銲道鹼性應力腐蝕龜裂探討

一座蒸氣產生器之蒸汽出口管線使用20年後於操作中陸續在T接管和大小頭之銲道發生洩漏，管線材質為A106碳鋼，本文針對大小頭洩漏處取樣進行破損分析，色括金相分析、破斷面SEM和EDS觀察以及裂縫附近沉積物之化學組成和pH值分析，來探討銲道洩漏機制和肇因，並提出改善方案；分析結果顯示蒸汽管線銲道洩漏機制為鹼性應力腐蝕龜裂，肇因水質管控不佳以及根部銲道存在高銳角角隅導致鹼濃縮析出，構成高鹼性環境，高銳角角隅處形成高應力集中效應是應力主要來源，因而導致裂紋的發生和成長，根部粗晶熱影響區費德曼組織以及銲道針狀麻田散鐵組織促進裂紋成長，主要改善和預防方案包括改善和監控鍋爐水pH值以及焊接時應避免根部銲道突出太高、銲蝕或不同壁厚對接錯位角隅等銲道缺陷所導致的應力集中效應，不同壁厚對接較厚壁管線內部應車削出1：3之斜邊，來消除角隅問題，此外銲接後的熱處理可有效釋放殘留應力，降低鹼性應力腐蝕破裂。

關鍵字

碳鋼；銲道；鹼性應力腐蝕龜裂

並列摘要

A leakage occurred at the weld of T joint and reducer at the outlet piping of a steam generator after twenty years operation. The piping material was A106 carbon steel. In this paper, failure analysis of the reducer was conducted through metallographic analysis, SEM observation, EDS analysis of fracture surface, XRD chemical composition and pH value analysis of white deposit to investigate the leakage mechanism and root cause. Finally, the methods of mitigation and prevention were discussed. Analysis results showed that the leakage mechanism of piping was alkaline stress corrosion crack. The pool control of water and a preferred settlement of caustic soda due to corner at root pass weld resulted a high local caustic concentration. The concentrated stress formed at the corner at root pass due to an acute angle to promote the initiation and growth of cracks. This presence of widmenstaten structure with coarse grains at the heat affected zone of root pass weld and needle type of martensitic welded areas enhanced the growth of cracks. The methods of mitigation and prevention include control and monitoring of BFW pH value, reduce the presence of the weld protrusion from the root pass, undercut and corner at the weld of different wall thickness. A 1:3 taper at inner wall can reduce the corner at the weld of different wall thickness. Post weld heat treatment also can effectively reduce the weld reside stress to mitigate the alkaline stress corrosion crack.

並列關鍵字

Carbon Steel ； Weld ； Alkaline Stress Corrosion Crack

國際替代計量

蒸氣管線銲道鹼性應力腐蝕龜裂探討

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