Creep fatigue of steels in various environments

The interaction between fatigue and creep in X6CrNi1811 and X8CrNiNb1613 has been investigated in air and in vacuum at 873 K and 973 K using smooth specimens. A new damage parameter for creep-fatigue is proposed based on the growth of small cracks. The growth rate of these cracks is measured in air and in vacuum. In addition, the influence of different hold times and strain rates in tension and compression on lifetime is examined. The results show that for a large range of hold times and strain rates, the growth of microcracks is the dominant failure mechanism for these steels. Only at high temperatures (973 K) and long hold times (45 minutes), cracks nucleate in the interior of the specimens by the linkage of grain boundary voids. In this case, the predicted lifetime deviates strongly from the measured lifetime. (IWM)