本研究針對冷鍛之304不銹鋼螺栓，以不同熱處理條件(退火、應力消除)與不同之鎖緊程度(0 N-m、30 N-m、60 N-m、80 N-m)為實驗條件，將試片浸泡於45%沸騰氯化鎂水溶液中進行腐蝕試驗。試驗後之試片，以掃描式電子顯微鏡與光學顯微鏡觀察破斷形貌及裂紋分布，且利用恆電位儀量測電化學反應，以探討不銹鋼螺栓頭部腐蝕開裂之原因。
試驗結果顯示，由於不銹鋼螺栓經冷鍛加工後，帽頭之表面形貌較粗糙，且原沃斯田體因受劇烈之加工應變，使組織轉變為抗蝕性較差之麻田散體，成為腐蝕開裂之起始點。然而裂紋擴展之形貌及方向，受組織不同而有所變化，易由滑移線與麻田散體組織處發展，形成穿晶與沿晶破壞之形貌。經應力消除熱處理後之螺栓，由於加工變形所造成之應力場，但於長時間熱能幫助下，得以重新調適而降低組織中之殘留應力，減緩腐蝕開裂之時間；若施以退火熱處理，因晶粒重新成長，已不具加工造成之殘留應力，且表面受厚實之氧化皮膜所保護，故無應力腐蝕開裂之現象。然當螺栓受較大之鎖緊扭力時，使得帽頭區承受微小壓縮應變，有效阻礙裂紋生成且抑制其擴展之速度。
综合實驗結果顯示，304不銹鋼螺栓因受嚴重加工變形，使得殘留應力存在於組織中，當處在含氯之環境下，發生穿晶與沿晶之破壞形貌且裂紋呈樹枝狀分布，此與眾多腐蝕行為中之應力腐蝕開裂之現象相符。

The effects of torques and heat treatments on the stress corrosion cracking behavior of 304SS bolt in boiling of 45% Magnesium chloride solution were investigated. SEM/EDS and potentiostate were employed to investigate the fracture features and the relationship between potential-time and potential-corrosive current under different heat treatment of bolt head, respectively.
The results showed that cracks initiated and propagated at the interface between austenite with slip lines and poor corrosion-resistance martensite zone transforming from austenite after severe cold working in the top surface of bolt head, forming both trans-granular and inter-granular fractographic. The growth of cracks was affected by the microstructures of the bolt head, the condition of heat treatments, and the tightened force. For the stress released bolts, the decrease of residual stress and readjustment of stresses fields in the bolt head after thermal treatment prolonged the nucleation time of a crack. With the recrystallization and grain growth by the annealing treatment of the bolt head, no crack could be observed under the experimental conditions in this study. For the non-heat treatments bolt with tightened force, tensile stresses at the interface between austenite and martensite zone in the top surface of bolt head transformed to compressive stresses and prohibited the forming of a crack.
By the revealed evidences, it can be concluded that the severe cold working caused the formation of complicated tensile stress field in the bolt head, and bolts were ruptured by chloride stress corrosion cracking.

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