Effects of temperature and frequency on fatigue crack growth in 18% Cr ferritic stainless steel

Abstract

The fatigue crack growth behaviour of a ferritic stainless steel has been investigated as a function of test temperature, thermal exposure and frequency at intermediate growth rates. In general, fatigue crack growth rates increased with increasing temperature and in the temperature range 500-700 [deg]C growth rates were described by a kinetic process with an activation energy of 48 kJ/mole. Higher than normal growth rates at 475 [deg]C were observed and attributed to an embrittlement process which is known to occur in this temperature regime in high-chromium ferritic stainless steels. The influence of frequency on fatigue crack growth rates was examined at 500 and 655 [deg]C for a load ratio of 0.1 and over four decades of frequency. A transition from time-independent to time-dependent behaviour was observed at each temperature as frequency was lowered. The frequency at which this transition occurred was dependent on temperature. For all temperatures investigated, near threshold crack propagation occurred by a crystallographic or faceted propagation mechanism. At high crack growth rates, crack-tip plasticity was significant and propagation proceeded by a ductile striation formation process. At intermediate growth rates a mixed-mode fatigue crack growth mechanism was observed where some intergranular fracture occurred.