Failure analysis and metallurgical transitions in SS 304L air pipe caused by local overheating

AISI 304 type stainless steels are widely used for structural parts in power stations due to their excellent resistance to creep and corrosion at elevated temperatures. In an incident at one of IEC's (Israel Electric Corporation) power stations, an air pipe made of AISI 304L Stainless Steel which was part of a gas conditioning system conducting hot air for mixing with natural gas, failed and cracked at two points. Failure analysis included stereo imaging, optical metallography, SEM/EDS/EBSD and magnetic tests. It was revealed that one crack occurred due to high-temperature overload and the other due to creep, showing grain coarsening and secondary phase formations respectively to an extent that is entirely atypical under standard working conditions. A follow-up investigation was conducted in order to gain further insight into the processes and conditions that led to the two crack failures in the pipe. Thermal treatments of intact specimens from an undamaged pipe area were conducted at different temperatures and times. Pipe specimens subject to heat treatments of 650 °C resembled material near the creep fracture in the field pipe. Pipe specimens subject to heat treatments of 1000 °C and 1150 °C resembled material near the high-temperature overload failure. The thermal treatment was followed by microstructural, mechanical, compositional and phase analyses.