鎳鉻系不鏽鋼具有優良抗氧化與抗腐蝕性質，為廣泛使用之商用不鏽鋼，但其具有一特殊的敏化現象如下：經過焊接的不鏽鋼，於熱影響區會發生粒間腐蝕的情形，其原因為Cr23C6碳化物析出在晶界處，使晶界附近的基地鉻含量降低，讓原本在晶界附近連續的氧化鉻保護膜變得不再連續，容易發生粒間腐蝕，稱之為敏化現象。本論文針對304不鏽鋼焊接熱影響區、304不鏽鋼、與額外加0.2 wt.%碳之304不鏽鋼進行研究。發現304不鏽鋼焊接熱影響區有大量BCC麻田散體相的存在。於304不鏽鋼做1100°C的固溶處理，發現約2 m寬的BCC麻田散體晶粒出現於沃斯田體的晶界處。並且對304不鏽鋼與加碳的304不鏽鋼進行在500°C至1000°C恆溫處理後，發現Cr23C6碳化物析出量較多的溫度約落於900°C至800°C之間，並觀察到較多BCC麻田散體晶粒出現，推測麻田散體相的出現與鉻含量的減少有關。這解釋了敏化現象除了要考慮貧鉻區的沃斯田體相，可能需要將BCC麻田散體晶粒出現，降低不鏽鋼抗腐蝕性能納入考量。

This paper focuses on the phase changes of the heat-affected zone of 304 stainless steel after welding, and the heat treatment of 304 stainless steel and 304 stainless steel with 0.2 wt.% carbon. It is generally believed the reason for the sensitization of 304 stainless steel is that Cr23C6 carbide precipitates at grain boundaries, which reduces the chromium near grain boundaries. The chromium oxide protective film that was originally continuous at grain boundaries will become discontinuous, thus intergranular corrosion is more likely to occur. However, after analyzing the constituent phases of the heat-affected zone of 304 stainless steel during welding, a large number of BCC martensite phases was found. Afterward, the 304 stainless steel underwent solution treatment at 1100°C for one hour and then water-quenched to room temperature. It was found that in addition to the expected austenite phase, there was also the appearance of BCC martensite phase. After heat treatment of 304 stainless steel and 304 stainless steel with 0.2wt% carbon at 1000°C to 500°C for 20 hours, more BCC martensite phases was found at the aging temperature with more precipitation of Cr23C6 (between 850°C to 750°C). It can be speculated that the appearance of martensite phases is related to the reduction of chromium, which happens to be the temperature range where sensitization occurs. This shows that in addition to the austenite phase in the chromium-poor area, the influence of BCC martensite may need to take into consideration on the corrosion resistance of 304 stainless steel.

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