Continuous cooling transformation curves of undercooled austenite of medium carbon Cr-Mn-Si low alloy steel

doi:10.13251/j.issn.0254-6051.2024.12.024

Abstract

Abstract: Thermal expansion curves of a medium carbon Cr-Mn-Si low alloy steel under different cooling rates were determined by using Formast-FⅡ dilatometer, and the continuous cooling transformation (CCT) curve of the undercooled austenite of the tested steel was plotted in combination with the observation of the metallographic microstructure. The effect of cooling rate on the microhardness and the onset temperature of the undercooled austenite phase transformation of the tested steel was investigated, and the microstructure and properties of the medium carbon Cr-Mn-Si low alloy steel large forgings at different positions after heat treatment were analyzed. The results show that Ac₃=855 ℃, Ac₁=770 ℃, Ms=375 ℃ and Mf=200 ℃ for the medium carbon Cr-Mn-Si low alloy steel. When the cooling rate is less than or equal to 0.28 ℃/s, the ferrite and pearlite transformation mainly occurs in the high temperature zone, and the bainite transformation occurs in the middle temperature zone. When the cooling rate is 0.28-0.83 ℃/s, the bainite transformation mainly occurs in the middle temperature zone, and the martensite transformation occurs in the low temperature zone. When the cooling rate is greater than or equal to 1.66 ℃/s, only the martensite transformation occurs. With the increase of cooling rate, the microhardness of the tested steel firstly rises linearly and then increases slowly, and the phase transformation start temperature firstly decreases sharply and then basically remains stable. The edge of heat-treated medium carbon Cr-Mb-Si low alloy steel large forgings microstracture is martensite and bainite complex, the positions at R/2 and the core microstracture are bainite, the tensile strength (1467.0 MPa) and hardness (475 HV5) values at the edge position are the highest, indicating that appropriate amount of martensite can significantly improve the strength of bainite structure while maintaining good plasticity.

Key words: medium carbon Cr-Mn-Si low alloy steel, CCT curve, cooling rate, microhardness, phase transition temperature

CLC Number:

TG161

Gao Xin, Ning Jing, Su Jie, Gao Qi, Jiang Qingwei. Continuous cooling transformation curves of undercooled austenite of medium carbon Cr-Mn-Si low alloy steel[J]. Heat Treatment of Metals, 2024, 49(12): 142-147.

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