高溫應力對低碳鋼熱浸鍍銘層之影響

Effect of Stress on Hot-dipped Aluminide Coatings on Mild Steels at High Temperature

本研究採用經熱浸鍍純鋁之低碳鋼，於750℃空氣氣氛施加不同應力，以光學顯微鏡觀察應力對鋁鍍層顯微結構之影響。實驗結果顯示，外加應力使鋁化層產生微量變形，加速孔洞的形成。鋁化層於750℃高溫擴散後，在相同時間承受應力之鋁化層較未承受應力之鋁化層生成較多孔洞，在長時間高溫擴散，承受應力之鋁化層厚度小於未施加應力之鋁化層。在鋁化層裂紋生成形貌上，鋁鍍層於低應力時，隨高溫擴散時間增長，裂孔成長於擴散形成的孔洞。鋁化層於較高應力時，裂口好發於初始裂紋及Fe-Al介金屬層之舌片狀形貌處，因底材變形使表面鋁化層由外而內撕開。從高應力之試片破斷處顯示，鋁化層與底材具有良好的附著性。

In this study, mild steels were coated by hot-dipping in a molten aluminum bath. The effect of varied stresses on the microstructure of the aluminide layer formed on the mild steel was investigated at 750 ℃ in static air. The results showed that the number of voids in the aluminide layer in the presence of an applied stress was greater than that in the one without any applied stress. It was also found that the thickness of the aluminide layer in the presence of the applied stress was less than that of the one without stress. This phenomenon could be attributed to the deformation of the aluminide layer caused by the applied stress, which in turn accelerated the formation of the voids. According to the cracking morphology, cracks in the aluminide layer under a low stress initiated at the voids, which formed by diffusion. In the meantime, cracks in the aluminide layer under a high stress could easily start from sites with original cracking defects and from the tongue-like Fe-Al intermetallic layer.