304不銹鋼油壓管破損原因分析

Fracture Analysis of a SUS304 Hydraulic Pipe

某工場的油壓管採用SUS304無縫鋼管，輸入管設計壓力250 kg/cm2，使用較厚（S160）管壁；輸出管設計壓力5 kg/cm2，使用較薄（S40）管壁。二者管內物皆為中油的46AWS液壓油，操作溫度40 ~ 60℃；二者管外環境會受到輻射熱與水氣的影響，估計管外溫度約80~ 150℃。常在使用約2年後，即有管壁龜裂漏油，請中鋼腐蝕測試中心分析其龜裂原因。試驗方法分別為外觀觀察、成分分析、附著物及生成物分析、材質金相與破斷面分析。分析結果顯示輸入管與輸出管皆為S與Cl腐蝕因子所引起的應力腐蝕破壞，起於管外壁蝕孔，受管內油壓壓力的作用下，逐步由管外進展到管內而造成洩漏。該工作環境中含有S與Cl腐蝕因子並不適合使用300系不銹鋼，建議使用2205雙相不銹鋼或鎳基合金（Ni > 30%），或該油壓管先作防蝕帶包覆再作隔熱包覆與蒙皮，阻絕外界輻射熱、水氣與S與Cl腐蝕因子的影響。

A workshop's hydraulic pipe was made of SUS304 seamless pipe. The input pipe had a thicker wall (S160), while the output pipe had a thinner wall (S40). The working pressures for the input and output pipes were 250 and 5 kg/cm2, respectively. The fluids in the two pipes were 46AWS hydraulic oil from CPC, which was operated in the temperature range of 40 ~ 60 ℃. The external walls of the two pipes were exposed to the environment suffering from the radiating heat and moisture at temperature around 80 ~ 150 ℃. The oil leakage from the cracks of the pipes was founded after 2 years operation. China steel corrosion testing center was requested to analyze the cracking mechanism for the pipes. Testing methods include appearance observation, compositional analysis, attachments and corrosion products analysis, metallurgical and fracture surface observations. The analysis results showed that the cracks of the input and the output pipes were due to the stress corrosion cracking, which initiated from the pitting of the external surface of pipes caused by S and Cl corrosion factor. The inner pressure of the pipes caused the pipes to crack and propagate from the external to inner wall gradually. The service condition of the failure pipes in this study, containing S and the Cl corrosion factor, is not suitable for 300 series steel pipes. 2205 duplex stainless steel or inconel superalloy (Ni > 30%) are suggested to be used as the pipe materials. The alternative method for corrosion prevention is to pack the pipes with anticorrosion tape as the inner layer and with insulate coverage as the outer layer to keep from radiating heat, moisture and S and Cl influences.