Modelling and Simulation of Strengthening in Complex Martensitic 9 12% Cr Steel and a Binary Fe Cu Alloy
Modern martensitic 9 12% Cr steels are alloys with excellent mechanical properties even at elevated temperatures. The high temperature strength of these materials is inevitably related to their complex microstructure in the 'as received' condition. Due to diffusional processes however, this microstructure changes during high temperature service, which leads to a decrease in strength. To identify the impact of changing microstructure on the mechanical properties of an investigated material, a new methodology to model strengthening in steel is developed and presented in this dissertation. A microstructure property link is formulated with focus on the precipitate and solid solution hardening effect. Also, additional strengthening effects, i.e. from the grain and subgrain structure, as well as dislocation strengthening are critically assessed. The developed strength module is applied on a two component Fe Cu alloy with 1.4at. Cu and a complex 11% Cr steel (COST E2) to determine the evolution of the lower yield stress or rather 0.2% proof stress of these alloys. For different heat treatments, the numerical results are compared with experimental results. The numerical simulation shows excellent agreement in the case when all operative strengthening mechanisms are duly considered. Furthermore, the role of precipitates on the creep rupture strength evolution in martensitic steels is critically discussed based on two numerical studies on the potential of heat treatment optimization to increase creep strength and precipitation reactions during service. It is shown that the evolution of precipitation parameters, i.e. size and number density of the second phase particles, significantly influences the evolution of the overall creep strength. For this, the calculated precipitate back stress is compared to the creep rupture strength of the investigated alloys. The experimental results can be reproduced in a comprehensive and consistent manner by the numerical simulations using the software MatCalc. Institute for Materials Science and Welding Graz University of Technology