鍋爐主蒸汽管破損原因分析

Analysis on the Cause of Failure in the Main Steam Piping of Boilers

本文旨在分析探討鍋爐主蒸汽管發生連續破管之原因。研究方法包括外觀觀察、成分分析、截面微硬度量測、拉力試驗、衝擊試驗、OM與SEM金相分析、破斷面及裂縫SEM分析、腐蝕生成物分析及紅外線熱影像量測等。結果顯示裂縫位於內壁局部區域顏色不同處，均為周向裂縫，由內壁往外壁傳播，且為單一裂縫無分支現象，經剝開其破裂面有明顯之海灘紋現象。材質、拉力及硬度經量測分析結果無明顯之差異，酸洗前後破斷面可看到明顯疲勞紋，裂縫內部含有鐵氧化物為主之氧化層。推測此次主蒸汽管破管之主要原因為熱疲勞。其疲勞之原因經以紅外線熱影像現場量測結果，證實為取樣管較冷之蒸汽倒灌至主蒸汽管，造成局部冷區，形成熱應力所致。

The objective of this paper is to study the cause of successive failures in the main steam piping of boilers. Appearance examinations, material composition analyses, micro-hardness measurements, mechanical property tests, metallographical and micrographical analyses on fractured surfaces, corrosion product analyses, and IR thermo-graphy tests were conducted. Circumferential fissures were found on the internal surfaces of the tubing and were found propagating from the internal to the external surfaces, and these fissures did not show any sign of branching. There were some beach marks on the fractured surfaces. No significant differences in composition, tensile strength and hardness were found among the tubing samples. However, striations and oxide scales were observed on the pickled surface and in the fissures, respectively. Based upon the foregoing analyses, it was speculated that failure of the main steam piping was caused by thermal fatigue, which was confirmed by an in-situ IR thermography test. It was found in the test that the steam of lower temperature injected into the main steam piping from the sampling tube resulted in a locally low temperature area on the piping surface, which in turn led to a local thermal stress and the resultant thermal fatigue.