本研究針對商用 304不銹鋼螺栓，施以不同鎖緊程度(0、30、60、80 N-m)之扭力後，置於 45%沸騰氯化鎂水溶液進行應力腐蝕試驗。實驗後之試片，以掃描式電子顯微鏡與光學顯微鏡觀察帽頭區破斷之形貌及內部裂紋成長方向，另配合恆電位儀量測表面之動電位極化曲線，據此探討螺栓帽頭部應力腐蝕破裂之行為。分析結果顯示：冷鍛製螺栓之帽頭區，在製造時由於受到嚴重加工變形且表面具有相當程度之缺陷，易成為裂紋之起始點。當裂紋開始生成時，其分布形態會受施加扭力之大小與顯微組織之不同而有所影響；前者為阻礙裂紋成長之主要因素，後者則改變破壞形貌。此外，若對於螺栓施加適當之扭力時，將造成帽頭區具有微壓縮變形之效應，而此效應可抑制裂紋之起始與減緩擴展之速度。

The effects of torques on the stress corrosion cracking behavior of SS304 bolt in boiling of 45% Magnesium chloride solution were investigated. The research methods included visual inspection, SEM/EDS, metallographic and high temperature electrochemical test. Its result revealed that the crack initiation sites always occur on the coarse surface of the bolt and strain induced martensite microstructure. The effects of different torques and microstructure changed the cracks propagation. Furthermore, suitable tightened force on the top surface of bolt head has micro-compressive deformation effect, and this effect will slow down the rate of crack formation.