Low-cycle fatigue of 10 % Cr steel with high boron content at room temperature

High-chromium martensitic steels are a promising material for the production of elements of boilers and steam pipelines, as well as blades and rotors of steam turbines for new coal-burning thermal generating units. The use of such materials will give an opportunity for the transition to ultra-supercritical steam parameters (temperature of 600–620 °C and pressure of 25–30 MPa), which will allow increasing the efficiency of generating units to 45 %. Modifications of the chemical composition of high-chromium steels have led to significant improvements of high-temperature properties such as 100,000 h creep strength and 1 % creep limit, while resistance to softening due to low-cycle fatigue remains understudied in this field. This work covers the study of low-cycle fatigue at room temperature with different amplitudes of deformation of martensitic high-chromium 10%Cr–3%Co–2%W–0.5%Mo–0.2%Cu–0.2%Re–0.003%N–0.01%B steel. The steel was pre-subjected to normalizing at 1050 °С followed by tempering at 770 °С. After heat treatment, the steel structure was a tempered martensitic lath structure stabilised by the particles of secondary phases of M₂₃C₆ carbides, NbX carbonitrides, and M₆C carbides. The average width of martensite laths was 380 nm, and the dislocation density was 1.4×10¹⁴ m⁻². At low-cycle fatigue, with an increase in the strain amplitude from 0.2 to 1 %, the number of cycles before failure significantly decreases, and the value of plastic deformation in the middle of the number of loading cycles significantly increases. Maximum softening (18 %) is observed at a strain amplitude of 1 % in the middle of the number of loading cycles. In general, the steel structure after low-cycle fatigue tests does not undergo significant changes: the width of the laths increases by 18 % at a strain amplitude of more than 0.3 %, while the dislocation density remains at a rather high level (about 10¹⁴ m⁻²) at all strain amplitudes.