Fatigue of duplex stainless steel influence of discontinuous, spinodally decomposed ferrite

Duplex stainless steel is an alternative to austenitic stainless steel in some applications, for example, where resistance to chloride induced corrosion is important. A major drawback, however, of the duplex stainless steel is the susceptibility of the ferritic phase to the 475°C embrittlement. Therefore, the use of duplex stainless steel is limited to temperatures below 250°C. In the present study the mechanical properties of a duplex stainless steel containing 25% ferrite and 75% austenite are evaluated. The fatigue crack growth and low cycle fatigue properties are presented and the influence of a brittle, discontinuous ferritic phase is discussed. Further, the mechanical properties of the duplex stainless steel are compared with those of an austenitic stainless steel. The results indicate that the fatigue properties of the duplex grade are little affected by annealing at 475°C for 100 h. In addition, after the annealing the impact toughness still is as high as 130 J. The austenitic grade has the same nominal fatigue crack growth threshold as the duplex grade. However, the effective crack growth threshold of the duplex grade is higher than that of the austenitic grade. In contrast, the crack closure of the austenitic grade is higher. A fatigue crack growth model by Majumdar and Morrow has been used to correlate low cycle fatigue data and fatigue crack growth properties. The results are rationalised in terms of microstructural and fractographic quantities.