Microstructurally Based Modeling of Creep Deformation and Damage in Martensitic Steels

Abstract

This chapter deals with modeling the microstructural evolution, creep deformation, and pore formation in creep-resistant martensitic 9–12% Cr steels. Apart from the stress and temperature exposure of the material, the input parameters for the models are as-received microstructure and one single-creep experiment of moderate duration. The models provide predictive results on deformation rates and microstructure degradation over a wide stress range. Due to their link to the underlying fundamental physical processes such as classical nucleation theory, Gibbs energy dissipation, climb, and glide of dislocations, etc., the models are applicable to any martensitic steel with similar microstructure to the presented case study. Note that we section the chapter into part 1: creep deformation and part 2: pore formation.

Prediction of TTR Diagrams via Physically Based Creep Simulations of Martensitic 9-12% Cr-Steels

“…The model builds on interactions of mobile dislocations (density ρm), static dislocations (density ρs), boundary dislocations (density ρb), subgrains (radius Rsbg) and precipitates and deduces the time-evolution of these constituents. The current creep strain rate 𝜀̇ (Eq.1), which is depending on the specific microstructure in each timestep, is then calculated using Orowan's law [6], which has been slightly modified [2,7]. To consider dislocation climb over precipitates, the glide velocity vg was replaced by an effective velocity veff [2].…”

“…To consider dislocation climb over precipitates, the glide velocity vg was replaced by an effective velocity veff [2]. In addition, the effect of cavitation damage was included [7].…”

“…Here, b represents the Burgers vector, M the Taylor factor and Dcav a parameter for cavitation damage. The current status of the creep model is described in book chapter [7] and the equations are explained in detail in reference [2]. It should be noted that the damage models are different in these two references.…”

“…Precipitate damage (as suggested by [4,5]) was excluded from Orowan's law, since it is already implicitly included by coarsening of precipitates. Moreover, the exponent of the strain rate in the cavitation damage law was optimized from 1 [2,5] and 0.5 [7] to 0.8 in this work to obtain a better depiction of the tertiary creep area with decreasing stresses. Furthermore, the precipitate simulations have been improved (mod.…”