Effect of thermal aging on strengthening mechanisms of 12 % Cr ferritic-martensitic steel

A systematic investigation of the evolution of microstructure, mechanical properties, and strengthening mechanisms of 12 % chromium ferritic-martensitic steel 16Cr12MnWSiVNbB (EP-823‑Sh type) during long-term thermal aging (up to 5000 hrs) at temperatures of 450 – 580°C was conducted. Comprehensive electron microscopy analysis revealed quantitative changes in the structural parameters of the steel after aging compared to the initial state: a 30 % increase in ferrite grain size, a 1.3 –1.7‑fold reduction in dislocation density, coagulation of M23C6 carbides, and depletion of Mo in the solid solution. Qualitative changes include heterogeneous precipitation of Laves phase (Fe, Cr)2(Mo, W) particles after aging at 580°C for 5000 hrs. A quantitative assessment was performed to determine the contributions of individual strengthening mechanisms to the yield strength of the steel under various aging conditions. It was established that the reduction in strength properties after aging is due to weakening of dislocation and grain boundary strengthening mechanisms. The increase in yield strength after prolonged (5000 hrs) aging at 580°C is attributed to the compensatory effect of precipitation strengthening resulting from the formation of finely dispersed Laves phase particles. The obtained results are of significant importance for predicting the long-term operational durability of ferritic-martensitic steels in power engineering applications.

1. Russian Science Foundation - Project No. 24-29-00431