Features of formation of austenite grains in 12 % Cr heat-resistant ferritic-martensitic steels

Abstract

Ferritic-martensitic heat-resistant high-chromium steels (FMHS) with chromium content of 11 – 12 % are quenched to martensite from temperatures of 1050 – 1100 °С. Possible undesirable consequences of heating to such high temperatures are an increase in the size of austenite grains, increase in the amount of delta ferrite in the final structure, and a decrease in mechanical characteristics. In this work, the change of all these factors during heating of FHMS to quenching temperatures in the range of 950 – 1250 °С was studied. Ratios of the contents of martensite (its amount was identified with the proportion of austenite before quenching) and high-temperature delta ferrite on metallographic sections were analyzed. It was found that behavior of structure of the studied FHMS upon heating to temperatures of 1150 °С and above depends on the steels structural class. In steels whose structure at room temperature consists of martensite and delta ferrite, or in which delta ferrite begins to form at heating temperatures of 1200 °С and higher, size of austenite grain decreases with increasing temperature in the range of 1200 – 1250 °С, and the amount of delta ferrite – increases. Such structural transformations can be associated with features of the phase equilibrium diagrams of steels of this class. Such structural transformations can be associated with a change in the position and (or) inclination of boundaries of the high-temperature region of coexistence of austenite and delta-ferrite in the phase equilibrium diagrams of FHMS at a change in heating temperature in this range. Compression tests at 20 °С of 15Cr12Mn3SiMoW2VB steel samples after heat treatment with heating to temperatures for hardening 1000 – 1250 °С showed that formation of an additional amount of delta ferrite at temperatures above 1200 °С is a stronger factor than the refinement of austenite grains. This causes a decrease in yield strength of the samples quenched from these temperatures followed by high tempering.