STRUCTURE AND MECHANICAL PROPERTIES OF AUSTENITIC Cr – Ni – Ti STEELS AFTER HIGH-TEMPERATURE NITRIDING

Abstract

Alloying of corrosion-resistant austenitic steels with nitrogen is widely used in production to stabilize austenite and to improve the strength and other properties of the metal. The possibility of alloying titanium-containing steels with nitrogen by introducing nitrogen into the melt is not possible, as it causes formation of the coarse defects in steel during casting and solidification of the metal (twisting of the peel, large nitride inclusions, accumulations of nitrides, etc.). The method of high-temperature gas nitriding can be alternative to liquid-phase nitriding for alloying austenitic titanium-containing chromium-nickel steels with nitrogen in order to increase their strength properties. In this work, we investigated the possibility of increasing the strength characteristics of thin-sheet austenitic corrosion-resistant Cr – Ni – Ti (Kh18N12T type) steel, containing 1.5 % and 3 % of titanium, through the use of solid-phase high-temperature nitriding. The nitriding was carried out at a temperature of 1000 – 1100 °С in an atmosphere of pure nitrogen for 5 or 8 hours. The average mass fraction of nitrogen in the samples after nitriding for 5 hours was 0.6 % and 0.7 % for the steels with 1.5 and 3 % of titanium, respectively, and after nitriding for 8 hours – 0.8 % and 0.9 %. It was shown that high-temperature nitriding followed by annealing provides a significant (by 2 – 3 times) increase in the metal strength characteristics compared with the state before nitriding, but reduces the ductility. Ductility of the steel is restored during final processing. For Kh18N12Т type steel with 1.5 % of titanium, an increase in the yield strength is obtained – by 3.3 times (from 180 to 600 MPa), strength – by 1.8 times (from 540 to 970 MPa), with a relative elongation of 28 %. An additional increase in strength properties was not found for the steel with 3 % titanium. The obtained results show the possibility of obtaining thin-sheet titanium-containing high-nitrogen steel (or products from it, for example, thin-walled pipes) by applying solid-phase high-temperature nitriding.