Sulphide stress cracking of a valve stem of duplex stainless steel

The present paper describes a failure investigation of a double disc valve stem made of duplex stainless steel 329 (UNS S32900). The valve stem which operated in moist H2S environment at 128 °C in a heavy water plant had undergone cracking during operation. The microstructure of the stem material showed precipitates of σ-phase at ferrite/austenite boundaries and elongated oxide inclusions. The formation of σ-phase had occurred during the manufacturing stage of the stem. The crack propagation was more favourable in the ferrite phase and along the ferrite/austenite boundaries where precipitates of σ-phase were formed. It was concluded that the valve stem failed by sulphide stress cracking due to hydrogen embrittlement of ferrite phase. Recommendations to avoid such failures are also suggested.