Influence of microstructure on fracture toughness and corrosion embrittlement of a duplex stainless steel

The influence of microstructural changes induced by means of different heat treatments on fracture toughness and sulphide stress corrosion cracking in a duplex stainless steel has been studied. Heat treatment at 825°C induced the precipitation of sigma phase after a period as short as 5 minutes. The effect of this sigma phase on fracture and corrosion behaviour is dramatic. As much as 95% of the initial toughness was lost after a period of maintenance of only 2 hours and the sulphide stress corrosion test specimens failed after a very short time. Specimens treated at 675°C for a short time exhibited a microstructure constituted by carbides but no sign of any intermetallic compound was observed. These carbides induced a loss of toughness less marked than that found in 825°C samples and the corrosion specimens passed the tests. More prolonged exposures induced the presence of intermetallic phases leading to a further decrease in toughness and the failure of the corrosion specimens. SEM examination helps to explain this behaviour.