Evaluation of Stationary Creep Rate in Heat-Affected Zone of Martensitic 9–12% Cr Steels

The purpose of the present study was to evaluate the contribution of distinct regions of the simulated heat-affected zone (HAZ) to the overall creep behavior of welded joints in the X20 and P91 steels. The HAZ was simulated by means of dilatometry at four peak temperatures (900, 1000, 1200, and 1350 °C) with a consequent tempering at 650 °C. Microstructure features of the four simulated HAZ regions including precipitates, prior austenite grains, and subgrains were quantified by means of electron microscopy. The quantified parameters and the measured hardness were used in three physical models for evaluation of the stationary creep rate ($\dot{\epsilon }$ at 170 MPa and 580 °C. The resulting $\dot{\epsilon }$ values fall within the range 10⁻⁸–10⁻⁷ s⁻¹, being in good agreement with the experimental data with a similar thermal history, but an order of magnitude lower than the measured values for the parent metal of the studied steels (10⁻⁷–10⁻⁶ s⁻¹). Depending on the model utilized, their output can be linearly related to hardness, subgrain size, or interparticle spacing. The model relating $\dot{\epsilon }$ to hardness was the most consistent one in prediction, being always lower for higher peak temperatures.