. Short Fatigue Crack Behavior in Duplex Stainless Steels

M.C. MARINELLI; S. HEREÑÚ

Recent Trends in Fatigue Design

Nova science

Año: 2014; p. 161 - 196

The excellent combination of mechanical properties and corrosion resistance of duplex stainless steel is obtained from balanced amount of ferrite (α) and austenite (γ) in the microstructure. However, this grade of steel embrittles when exposed in the temperature range of 280?500ºC limiting its application to temperatures below 280ºC. In order to evaluate the effects that the ?475° embrittlement? produces on the fatigue life during low- cycle and high-cycle fatigue, fatigue tests were conducted on a standard duplex stainless steel in two different heat treatment conditions (homogenized and embrittled). Transmission (TEM) and scanning electron microscopy (SEM) in combination with automated electron back-scattered diffraction (EBSD) techniques were carried out to analyze the surface damage as well as the initiation and propagation of fatigue cracks. These studies have revealed that during high cycle fatigue, the fatigue limit of the embrittled samples is substantially larger than that of the conventional samples at 107 cycles in the homogenized condition. Otherwise, during low-cycle fatigue, the fatigue life becomes similar or shorter depending on the strain amplitude range. Moreover, analytical electron microscopy revealed that under low-cycle fatigue loading conditions almost all the ferrite and the austenite grains showed plasticity, while under high-cycle fatigue loading conditions, slip band formation was limited to the softer austenite grains and a low plastic activity is observed in the ferrite. Once being formed, the bands generate high stress concentrations, where they impinge the α-γ phase boundaries, eventually, leading to the crack initiation. This is discussed by applying a numerical simulation approach which takes into account the real two-phase microstructure and its elastic/plastic anisotropy. This model, validated by experimental data, permits to describe quantitatively the propagation behaviour of microstructural short fatigue cracks in the homogenized and embrittled conditions.