New insights into the threshold behavior of a 9–12% Cr ferritic/martensitic steel – Separation and quantification of crack closure mechanisms

Abstract The energy transition and the associated increase in the share of regenerative energies require flexible power plant operation to compensate fluctuations in residual load. As a result of the decreasing dwell times at constant high temperatures, the significance of fatigue damage gains in importance. At present, existing design codes are of limited validity for this new loading scenario. This necessitates the development of adapted design codes based on damage tolerance concepts. For this reason the frequency dependent threshold behavior of the ferritic/martensitic steel X20CrMoV12-1 was investigated in a temperature range from 300 °C to 600 °C. In order to exploit the maximum lifetime of a component without compromising safety, detailed knowledge on active crack closing mechanisms is of elementary importance. Therefore, a general experimental approach for separation and quantification of crack closure mechanisms and their impact on crack propagation threshold values was developed. By this methodology, both the influence of temperature and corrosion on the threshold can be separated and quantified despite superposition of several mechanisms. The only application criterion of this procedure is SR ≪ 1, because roughness induced crack closure has to be negligible small. If roughness induced crack closure cannot be neglected, the methodology developed in this study has to be modified by models which quantify the influence of roughness induced crack closure.