Creep strength evaluation, design, and life management of Cr-Mo-V rotor steel using stress relaxation testing

The traditional approach to high temperature design involves long time creep testing, and attempts to incorporate microstructural evolution in the test measurements. A recently developed alternative approach aims to exclude these changes in a short time high precision test. Also, the traditional approach uses a single test to evaluate both creep strength (e.g. minimum creep rate) and fracture resistance (the time to rupture). Design for Performance recognises that different tests are necessary to measure these two properties. For creep strength, a stress vs. creep rate response is determined from a stress relaxation test (SRT), and for fracture resistance a constant displacement rate test (CDR) of a notched tensile specimen is performed at a temperature where the part is most vulnerable to fracture. In this paper we will show a comparison of creep strength measurements for a Cr-Mo-V steel using the two methods. Similarities and difference will be noted, and ways to use the SRT data directly in alloy optimisation, design analysis and life assessment will be presented. Included in this study are effects of temperature and strain, prior thermal exposure, evaluation of repeatability and path dependence (deformation history effects).