Predictions of long-term creep life for the family of 9–12 wt% Cr martensitic steels

Abstract

Two contemporary power-law creep methodologies for predicting long-term creep life from short-term lab-based experiments were extended to 48 different alloy compositions in the family of 9–12 wt% Cr steels. The study highlights the limitations that the data imposes on the applicability of the two methods for predicting lifetimes, while also assessing the assumptions made by both methods. It was found that a dependency exists between the creep activation energy and the test conditions when activation energy was calculated using a broader range of normalized stress states (σ/σTS) than were previously explored. This contradicts the assumptions of constant activation energy made by both contemporary methods. This raises questions about both the physical interpretation of activation energy and the reliability of the extrapolation of lifetime predictions made. The paper highlights that the modified power-laws are empirical tools and illustrates the importance of study protocol design for data collection, when short-term experiments are limited to a maximum of 6,000 h. In addition, the effects of alloying additions and tempering temperature on the creep mechanism were explored.