Failure of a martensitic stainless steel pipe weld in a fossil fuel power plant

摘要

A main steam pipe of a newly commissioned boiler ruptured within a few hours of being put into service. Subsequent inspection revealed root cracking of several other main steam pipe joints. The failure investigation covered every aspect of welding, heating techniques and NDT inspection methods. X-20 is a martensitic grade of stainless steel having approx 0.2 percent carbon, which makes it highly prone to cracking in hardened condition. X-20 weld joints are given complex cycles of pre-heating and post-weld heat treatment to avoid cracking and to ensure that correct mechanical properties are achieved. The resistance-heating method was found to be inadequate for post-weld heat treatment of X-20 joints. As a result weld joints contained very hard and brittle martensite which facilitated propagation of cracks. The use of induction heating instead of resistance heating ensures the mechanical properties are within specified limits. It was demonstrated that double tempering heat treatment would give acceptable mechanical properties. Micro-cracks and fissures in the welds were identified as hydrogen induced cracking. Lowering the soaking temperature below l00 deg C in order to avoid double tempering is not advisable since it increases the risk of hydrogen induced cracking. It was concluded from failure investigation that every aspect of welding requires strict adherence to proven practices and there is no room for any experimentation. Electrode baking, pre-heating, post-weld heat treatment and ultrasonic inspection are critically important. Remedial actions were taken to ensure that the remaining joints were safe.