Determination of the dispersion hardening ability of a new die steel with controlled austenitic transformation

The chemical composition of the stamping steel for high operating temperatures (above 700 °C) with controlled austenitic transformation during operation (CATO) was adjusted to implement its hardening by the dispersion hardening (aging) mechanism. The base steel was chosen as 3Х3Н8М7Ф, in which quenching and subsequent aging did not lead to such hardening. It was taken into account that in order to implement dispersion hardening, CATO steels should have a predominantly austenitic (rather than martensitic) structure in the hardened state, which was not provided for the base steel. As a result of changes in the content of Mn, Ni and C in the base steel, it was determined that the required conditions are met by CATO steel of grade 4Х3Н3Г6М7Ф (95 % austenite after quenching with a predominantly ferrite base in the annealed state). The experiments have established the ability of this steel to harden with ageing. Strength growth was determined by high-temperature tensile tests immediately after aging (without intermediate cooling of samples to room temperature), which is a feature of testing steels with CATO. The highest strength growth (compared to the quenched state) is provided by heat treatment in the following mode: quenching 1150 °C, 2 hours, oil and subsequent aging 725 °C, 2 hours (at a test temperature of 750 °C, s0,2 increases to 674 MPa, sВ to 697 MPa). This hardening is due to the release of dispersed particles of the Laves phase of Fe2Mo and carbide of type VC during aging. In the aged state, the steel retains its austenitic structure at high temperatures, and when cooled below 200 °C, it undergoes a γ → α transformation according to martensitic kinetics and acquires a hardness of 49 HRC. The achieved high-temperature (700...900 °C) strength characteristics of 4Х3Н3Г6М7Ф steel are twice as high as those of the high-temperature die steel 5Х3В3МФС (DI23) and are not inferior to the heat-resistant alloy ХН35ВТЮ (EI787). This makes it possible to effectively use it instead of commercially available heat-resistant martensitic die steels at operating temperatures above 700 °C. Keywords: die steels CATO, alloying, quenching, austenitic structure, dispersion hardening, high-temperature strength.