INFLUENCE OF HEAT TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF 12 % CHROMIUM STEEL WITH UNSTABLE AUSTENITE

Abstract

The structure and mechanical properties ofsteel 35Kh12G3MVFDR were investigated. It is shown that after normalization or quenching there is austenite to 35 vol. % in the steel and according to this it refers to martensitic-austenitic class. During the thermal treatment (tempering, long-term temperature keeping or isothermal quenching) austenite transforms into martensite in time not exceeding 2 hours. Martensite in 35Kh12G3MVFDR steel has great thermal stability: the first signs of its transformation into sorbitic structure are observed after 25 hours at 640 °C isothermal hardening, and its complete decomposition occurs after 50 hours. The martensite decomposition is accompanied by a reduction in the characteristics of heat resistance and hardness. Aging of quenched and tempered 35Kh12G3MVFDR steel at temperatures of 670 – 720 °C leads to decrease in hardness from 61 – 65 HRA to 55 – 60 HRA in time of 1600 – 3200 hours, the yield stress at 20 °C decreases from 1350 to 750 – 850 MPa and at 720 °C – from 310 to 160 – 230 MPa for the first 600 hours, after which the reduction of mechanical characteristics are terminated. The extent of decomposition of martensitic structure in 35Kh12G3MVFDR steel determines its creep resistance at 700 °C: the preservation of martensitic structure at short times isothermal hardening (24 hours at 640 °C) did not reduce creep strength in comparison with the state after a simple quenching and tempering (86,21 ± 9,4 and 89,26 ± 8,8 MPa, respectively), but decomposition of martensitic structure (after long-term aging at 670 °C during 1600 hours) reduces this characteristic to 63,87 ± 7,1 MPa. In contrast to martensite austenite in 35Kh12G3MVFDR steel is thermally unstable and undergoes transformation into martensite after 1-2 hours depending on heating temperature.