Determination of step aging effective modes of a stamped alloy for hot deformation

Abstract

Using the method of mathematical planning of the experiment, the influence of two-stage (low-temperature + high-temperature) aging on the hardness and high-temperature (750 °C) impact strength of a stamped alloy ХН35ВТЮ (EK39) for hot pressing of metals was investigated. The need to increase the high-temperature impact strength of the alloy is a significant decrease (failure) with increasing test temperature from room temperature to 700… 750 °C. To isolate the particles of the reinforcing γ′-phase of the Ni3(Al,Ti) type during aging, the alloy was previously hardened in oil from a temperature of 1150 °C. Determination of the most effective modes of two-stage aging was performed using a small replica of the orthogonal plan of the second order type 24-1. The first (low-temperature) stage corresponds to the beginning of spinodal decay of supersaturated solid solution at the stage of formation of Guinness-Preston zones, and the second (high-temperature) - to the formation of particles of stable γ′-phase type Ni3(Ti,Al). Temperature and aging time on each step varied. According to the planning matrix, regression equations were calculated, and additional experiments were performed to determine the modes of step aging, which provide the alloy with a higher level of selected properties compared to the recommended single aging (780 °C, 10 hours). According to the obtained regression equations, the change in the aging temperature at the second stage of aging does not affect the hardness of the alloy within its variation (750 °C–800 °C). The effect on impact strength of changes in temperature (650 °C–700 °C) and holding time (2–6 hours) at the first stage of aging is also insignificant. According to the results of processing and analysis of experimental data, it is established that the increase of high-temperature impact strength from 38 to 120 J/cm2 (while maintaining hardness and high-temperature strength at the same level) is achieved after step aging 700 °C, 20 hours + 750 °C, 2 hours. The increase in hardness from 29 to 33 HRC with a simultaneous increase in impact strength to 55 J/cm2 provides step aging of 725 °C, 10 hours + 775 °C, 6 hours. According to the results of electron microscopic study of the microstructure, it was found that the achieved level of impact strength after step aging is provided by increasing the dispersion of spherical particles of the γ′-phase of the type Ni3(Al,Ti). The increase in hardness is due to a more uniform volume distribution and a smaller cross-sectional scatter of γ′-phase particles. Keywords: aging, dispersion hardening, impact strength, microstructure, stamping tool.