Effect of Alloying and Heat Treatment on the Impact Resistance of Low-Nitrogen High-Boron Martensitic High-Chromium Steels

Abstract

High-chromium martensitic steels with low nitrogen and high boron contents are promising materials for the manufacture of thermal power units operating at ultra-supercritical steam parameters, which must have high creep resistance and good impact resistance. In all the studied steels, regardless of alloying and heat treatment, a lath structure with a high dislocation density is formed, which is stabilized by the M₂₃(C, B)₆, M₆C and NbX particles. The addition of rhenium together with a change in the tungsten/molybdenum and carbon contents ensures a decrease in the number density of grain boundary M₂₃(C, B)₆ particles, which allows reducing the ductile-brittle transition temperature by 15–20°C. The addition of copper leads to the formation of copper clusters/particles, which, on the contrary, increases the ductile-brittle transition temperature by 25–30°C. Increasing the quenching temperature does not affect the position of the ductile-brittle transition for low-copper steels alloyed with copper, tungsten, and molybdenum, although this shifts the Charpy curve towards lower energies due to coarsening of the prior austenite grains. For the rhenium-containing high-copper steel, increasing the quenching temperature reduces the ductile-brittle transition temperature by 5–10°C due to a decrease in the number of copper clusters/particles. The modification of alloying by increasing the content of rhenium, tungsten, and copper together with the change in heat treatment improves significantly the creep resistance, while the resistance to impact loads remains at a sufficiently high level (above 100 J × cm^-2 at room temperature), which meets the requirements for boiler materials and steam turbine blades.