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 induc ed corrosion is important. A major drawback, however, of the duplex st ainless steel is the susceptibility of the ferritic phase to the 475 d egrees C embrittlement. Therefore, the use of duplex stainless steel i s limited to temperatures below 250 degrees C. In the present study th e mechanical properties of a duplex stainless steel containing 25% fer rite 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 p roperties of the duplex stainless steel are compared with those of an austenitic stainless steel. The results indicate that the fatigue prop erties of the duplex grade are little affected by annealing at 475 deg rees C fur 100 h. In addition, after the annealing the impact toughnes s still is as high as 130 J. The austenitic grade has the same nominal fatigue crack growth threshold as the duplex grade. However, the effe ctive crack growth th threshold of the duplex grade is higher than tha t of the austenitic grade. In contrast, the crack closure of the auste nitic grade is higher. A fatigue crack growth model by Majumdar and Mo rrow has been used to correlate low cycle fatigue data and fatigue cra ck growth properties. The results are rationalised in terms of microst ructural and fractographic quantities.