Physical and Mathematical Modelling of the Process of Formation of Gradient Metastable Modifications of Carburized Layers of Structural Steels

O. P. Cheiliakh$^{1}$, N. E. Mak-Mak$^{1}$, Y. A. Cheylyakh$^{1}$, M. A. Ryabikina$^{1}$, K. Shimizu$^{2}$

$^{1}$State Higher Education Institute ‘Pryazovskyi State Technical University’, 7 Universytets’ka Str., UA-87555 Mariupol, Ukraine

$^{2}$Muroran Institute of Technology, 27 Mizumoto-cho, 050-8585, Muroran, Hokkaido, Japan

Received: 24.07.2020; final version - 05.03.2021. Download: PDF

An algorithm for cause-and-effects relations of physical-chemical and structural factors with formation of carburized metastable layers and properties of structural steels in the process of their evolution is developed. A physical and mathematical model of varied gradient distribution of carbon and alloying elements along the depth of the carburized layer at cementation of 25KhGT and 50G steels due to the influence on martensite point $M_{\textrm{s}}$ is developed. It demonstrates the quantitative distribution of the phase and structural composition and especially metastable residual austenite ($A_{\textrm{res}}$). The degree of metastability of $A_{\textrm{res}}$ and the kinetics of its deformation-induced martensite $\gamma_{\textrm{res}} \rightarrow \alpha^{'}$ transformation at wear (DIMTW) depend on this distribution, it having a positive influence on wear resistance indices and steels service lives. The distribution of the $M_{\textrm{s}}$ point, the amount of $A_{\textrm{res}}$ and gradient change of the microstructure along the depth of the carburized layer of 50G steel after quenching from different temperatures (from 800 to 1000°С) are established theoretically and experimentally. The quadratic polynomial equations of regression of dependence of $M_{\textrm{s}}$ point, carbon content and amount of $A_{\textrm{res}}$ along the depth of the carburized layer are obtained and experimentally confirmed. In addition, the regularity of wear resistance distribution along the depth of the carburized layer of 25KhGT steel after plasma quenching at 1200–1300°С is established. The wear resistance is happened to be relatively high ($\epsilon_T$ = 6.2–5.3) until $\sim$ 0.4 mm depth, when the amount of $A_{\textrm{res}}$ is equal to 67–48%, and eventually diminishes to $\epsilon_T$ = 1.0–1.1 at 1.3–1.4 mm (when $A_{\textrm{res}}$ is absent). The developed algorithm and constructed physical and mathematical model of the process of carbon saturation of structural steels layers at carburizing with regulation of the gradient distribution of phase and structural modifications along sample thickness allow efficient applying of $A_{\textrm{res}}$ metastable states that realize $\gamma_{\textrm{res}} \rightarrow \alpha^{'}$ DIMTW due to selection of the processes of heat treatment with the aim of increasing the life time of metal wares.

**Key words:**
carburization, quenching, physical and mathematical model, austenite metastability, wear resistance.

**URL:**
https://mfint.imp.kiev.ua/en/abstract/v43/i05/0629.html

**DOI:**
https://doi.org/10.15407/mfint.43.05.0629

**PACS:**
61.50.Ks, 61.66.Dk, 64.10.+h, 64.75.-g, 68.35.Dv, 68.35.Rh

**Citation:**
O. P. Cheiliakh, N. E. Mak-Mak, Y. A. Cheylyakh, M. A. Ryabikina, and K. Shimizu, Physical and Mathematical Modelling of the Process of Formation of Gradient Metastable Modifications of Carburized Layers of Structural Steels, *Metallofiz. Noveishie Tekhnol.*, **43**, No. 5: 629—653 (2021) (in Ukrainian)

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