Mechanical and corrosion properties of a complexly hardened maraging steel

Abstract

The results are presented of investigation of the influence of heat treat cycles on the properties of type 20NIOKhMYu maraging steels with complex hardening as the result of precipitates of carbide and intermetallide phases.In a number of works it has been shown that the effect of hardening of maraging steels increases if during tempering at least two hardening phases such as intermetallide and carbide phases are precipitated from martensite. The effectiveness of such complex hardening has been shown for type 30N12K10M6 steel (0.3% C, 11.5% Ni, 10.3% Co, 6% Mo), for which as the result of precipitation Fe2Mo intermetallide phase and Mo2C carbides it was possible to obtain a strength of cr t = 2400 N/mm 2. The method of complex strengthening as the result of precipitation of these maraging steels is not the only possible one. In particular it is possible to provide such a process in significantly lower allowed steel using type NiA1 compound as the intermetaIlide phase and types Cr23C 6 and Mo2C as the carbide phases, M. D. Perkas with his collaborators developed a theory of alloying of maraging steels [3]. On the basis of this theory as the base for development for development of such a complexly alloyed steel they selected the Fe-C-Ni-Cr-Mo-AI system. The content of the primary alloy element of maraging steels, nickel, was selected as 10% and of carbon 0.2-0.25%, which corresponds to the in high-strength constructional steels. The chromium content was selected taking into consideration the conditions of formation of Cr23C 6 carbide. The molybdenum content was selected so as to suppress the appearance of so-called 475 ° brittleness observed in tempering of high-strength steels and to decrease the tendency toward temper brittleness. The aluminum content in Fe-Ni-base steels, as shown in [3], must not be more than 2%. With a content of > 2% A1 there is a sharp increase in brittleness of the steel.Experimental investigations conducted of a larger number of heats with the chromium, molybdenum, and aluminum contents used after various variations of heat treat showed [4, 5] the validity of the ranges of variation used of the contents of the alloy elements mentioned and as the result type 20N10KhMYu Steel (0.19% C, 7.9% Ni, 4% Cr, 3.1% Mo, 1.44% A1) was proposed.Below are presented the results of investigation of the influence of different heat treats on the properties of type 20N 10KhMYu steel.As phase analysis showed, the structure of 20N10KhMYu steel in the annealed condition (annealing temperature 660-670°C, hold 12 h) contains types M23C 6 and M2C carbides and particles of type NiAI* intermetallide phase in addition to the matrix c~-phase (martensite).The basic heat treatment operation of maraging steels, austenitization, was studied in the 700-1100°C range. It was established that the temperature of the start of the ~ --, 3,-transformation A s = 780°C and the temperature of its finish Af = 840°C. It was also shown that in the 800-900°C range there is an intensely occurring process of solution in austenite of exces...