冷媒產生器採樣閥304不銹鋼固定螺栓破斷分析

Failure Analysis of 304SS Sampling Valve Bolts of a Refrigerant Tank

本報告主要探討使用於某工廠冷媒產生器採樣閥固定螺栓之破損及斷裂原因。研究方法包括外觀觀察、SEM/EDS、破斷螺栓金相觀察及硬度量測。分析結果顯示，冷媒產生器於採樣及操作過程中會逸散出Cl2及SbCl3，且SbCl3長時間與空氣中的水分接觸後生成HCl，此類氯化物易累積於帽頭下緣與閥體接觸處，使得螺栓長時間暴露於腐蝕性的環境中。由於螺栓冷加工後未進行適當之熱處理，造成螺栓顯微組織不均，在長時間承受拉應力及90℃的工作溫度，由螺栓帽頭下緣硬度差距最大之界面處產生裂紋，裂紋再沿著此界面成長破壞，其裂紋經由截面金相觀察呈現樹枝狀穿晶破壞現象。綜合以上特徵，螺栓破斷應屬氯誘應力腐蝕劣化所致。

This paper was analyze the rupture mechanisms of the 304SS tightened bolts in a refrigerant tank. The research methods included visual inspection, SEM/EDS, metallographic and hardness analysis. Its result revealed that the Cl2 and SbCl3, evaporated during the sampling and operating process, react with moisture to produce HCl deposited on the surface of the bolts. Consequently, the tightened bolts were exposed to the corrosive environment for a long time. On the other hand, the bolts have heterogeneous microstructures due to its manufacture by cold work without post heat treatment. A crack initiated at the interface of the different microstructures of the bolt by long time subjected to tensile stress and 363K environment. Then, the crack propagated and went through the grain (trans-granular) at the interface region. By the revealed evidences, we concluded the tightened bolts were ruptured by chloride stress corrosion cracking.