某汽電公司執行蒸汽管膨脹接頭(expansion joint)更新與銲道檢測時，於原管端的銲道附近位置，經目視觀察發現表面有明顯的裂紋產生，然於銲道處及膨脹接頭母材端則沒有發現。為了釐清膨脹接頭表面裂紋發生之原因，故進行破損分析。由橫截面金相觀察得知，發生表面裂紋的原管端與正常的膨脹接頭母材中，均包含肥粒與波來鐵兩種組織。於膨脹接頭母材中，兩者呈現帶狀(band)分佈，具有特定的優選方向且互相交錯平行於管壁方向，為一軋延結構；而於原管端發生?縫處，兩種組織的分佈並無明顯優選方向，呈現類似等軸晶之金相組織，且其晶粒尺寸明顯較大。另外，於原管端材料之管外及管內處，可觀察到發生嚴重的脫碳現象。而由管內腐蝕生成物之XRD 分析中，亦可發現FeO高溫相的存在。由上述結果顯示，原管端材料曾經歷不當的高溫熱履歷(>570℃)，然此蒸汽管之操作溫度僅為 180~190℃，加上EDS mapping 結果顯示裂縫兩側出現寬大的氧化層，於上述正常使用溫度下應不易生成，因此，推測原管端材料的表面裂縫應於使用前即已存在，材料遭受高溫熱履歷導致晶粒粗大化與材料表層發生脫碳的現象，加上不當的外應力(如水淬)，而造成表面裂紋的發生。

In the process of expansion joint replacement and welds examination of steam tubes, the obvious cracks were observed on the surface in the vicinities of the welds near the joined tube, however, not in the welds and matrix. In order to realize why the cracks reveal, the failed materials were thus analyzed. Two kinds of metallurgical structures, including ferrite and pearlite, are found from the cross section observation for above two specimens. However, they are quite different in the microstructure. For the expansion joint, two structures present alternately in the specific orientation and parallel with longitude direction of the tube showing band rolled structure. The equi-axial crystal with lager grain size is demonstrated on the jointed tube instead. Otherwise, the severe decarburization is depicted in the outer and inner side of the joined tube. From the XRD analysis results, FeO, only formed in the high temperature (＞ 570℃), is found on the corrosion product from tube inside. It expresses that the joined tube had ever gone through high temperature treatment. However, the operation temperature of the tube is in the range from 180 to 190℃. Under the temperature, the wide oxide layer existed in the vicinity of the cracks should not be found. Therefore, it suggested that the cracks on the surface of the joined tube had existed before operation which is related to the combination of the unsuitable high temperature treatment and applied stress.