Characterization of the Isothermal and Thermomechanical Fatigue Behavior of a Duplex Steel Considering the Alloy Microstructure

Isothermal and thermomechanical fatigue behavior of duplex stainless steel (DSS) X2CrNiMoN22-5-3 was investigated. The aim of this work was to understand the fatigue behavior by correlation of the isothermal and thermomechanical fatigue behavior with microstructural observations. Fatigue tests at plastic-strain-amplitude of 0.2% were carried out at 20, 300 and 600 degrees C, while in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) experiments were performed between 300 and 600 degrees C. During the 20 degrees C fatigue test, a continuous softening was observed. Transmission electron microscopy examinations reveal pronounced planar slip behavior in austenite. At 300 degrees C, deformation concentrates in the ferrite, where strong interactions between CrxN and dislocations were observed that explain the pronounced cyclic hardening. DSS studied exhibits softening throughout the whole isothermal fatigue test at 600 degrees C. In ferrite, during the 600 degrees C fatigue test, the G phase, gamma ' austenite precipitated, and an unordered dislocation arrangement was observed. The stress responses of the TMF tests can be correlated to those of the isothermal fatigue tests. In IP mode, a positive mean stress resulted in premature failure. No gamma ' austenite but the formation of subgrains in the ferrite phase was observed after TMF tests. The plastic deformation of the austenite at high temperatures results in an unordered dislocation arrangement.