Regularities of Structural-Phase Transformations and Hardness Reduction of Alloyed Tool Steel During Isothermal Treatment
DOI:
https://doi.org/10.32515/2664-262X.2026.14(45).172-184Keywords:
die steel, isothermal heat treatment, bainite, carbide coarsening, hardness, structural evolutionAbstract
The aim of the study is to establish the regularities of structural evolution and to determine the mechanisms of hardness reduction in high-alloy die steel during multistage isothermal heat treatment. Special attention is paid to the influence of temperature–time parameters on the formation of a structurally stable state with reduced hardness and improved machinability.
The material under study was high-alloy Cr–Ni–Mo–V die steel 4Cr4N5M4V2. A comprehensive experimental approach was applied, including metallographic analysis, microhardness and hardness measurements, and scanning electron microscopy. It was established that isothermal holding in the bainitic temperature range (603–723 K) leads to the decomposition of supercooled austenite with the formation of a bainitic structure, which retains a relatively high hardness level. Subsequent heating in the temperature range 823–853 K promotes carbide coarsening and redistribution of alloying elements, resulting in the formation of a ferrite–carbide structure. A quantitative relationship between the volume fraction of the bainitic component and hardness was determined. The obtained results reveal a nonlinear dependence of hardness on temperature during post-bainitic heating.
It was found that the minimum hardness (32 HRC) is achieved after double heating at 823–853 K with a total holding time of 40 h, which ensures the formation of a stable ferrite–carbide structure with a minimal amount of bainite. The optimal heat treatment regime includes austenitization, isothermal holding in the bainitic range, and subsequent double-stage heating. The results can be used to optimize heat treatment processes and improve the technological properties of die steels.
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Copyright (c) 2026 Olexandr Kuzyk, Mykola Bosyi, Viktor Slon, Serhii Shmatko, Volodymyr Kropivnyi, Dmytro Atroshchenko
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