INVESTIGATION OF THE EFFECT OF HEAT TREATMENT MODE ON THE MICROSTRUCTURE AND HARDNESS OF PIPES MADE OF CHROME-NICKEL-MOLYBDENUM STEEL WITH HIGH-RESISTANT AUSTENITE

Abstract

The results of a study of the microstructure and hardness of seamless hot-formed pipes made of martensit-ic chromium-nickel-molybdenum steel are presented. Structural hypoeutectoid alloy steels containing chromium, nickel, and molybdenum, while highly hardenable, exhibit increased hardness in the hot-formed state. Martensite formed in the seamless pipe metal after hot deformation and cooling in still air compli-cates machining operations and necessitates intermediate heat treatment. Structural transformations oc-curring during high-temperature tempering and isothermal annealing of pipe specimens are studied. Based on the experimental data, a regression equation is derived describing the effect of holding time during high-temperature tempering of pipe specimens made of the studied steel on their hardness. It was established that the most effective spheroidization of the steel structure and, as a result, a reduction in its hardness, oc-curs during isothermal annealing at a temperature of 710 °С, followed by holding near the Al temperature. It was found that during isothermal annealing, the total heat treatment time is reduced by more than 5 times, relative to the total high-temperature tempering time required to reduce the hardness to the required values. The obtained experimental data showed that reducing the annealing temperature from 740 °С to 710 °С allows for an increase in the statistical stability of hardness after heat treatment due to an increase in the homogeneity of the microstructure, which is caused by a higher proportion of undissolved carbides, which have a nucleating effect on the formation of granular pearlite near the Al temperature. Based on the results of the study, some recommendations are given for selecting optimal heating temperatures and isothermal holding for heat treatment of products made of chromium-nickel-molybdenum steels.