Retained austenite and carbide in 55SiCr spring steel

doi:10.13251/j.issn.0254-6051.2020.05.019

Abstract

Abstract: The microstructure and phase transition point of the spring steel 55SiCr, the microstructure evolution of retained austenite and carbide during heat treatment were studied by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffractometry (XRD), thermal expansion, and Rockwell hardness tester. The results show that the retained austenite of 55SiCr spring steel is distributed in blocks on the substrate after quenching. As the tempering temperature increases, the retained austenite is reduced and distributed in a granular and film form. Carbon is enriched in retained austenite, which enhances its stability. Since Si suppresses the precipitation of carbides, the stability of retained austenite is improved. Si delays the precipitation of cementite when tempered at low temperature. When the steel is tempered at high temperature, the diffusion of carbon atoms is increased, and the precipitation of cementite is promoted to cause volume shrinkage. Carbon has enough time to diffuse, which promotes the formation of cementite and advances the formation temperature of cementite when tempered at low heating rate. Carbon does not have enough time to diffuse and inhibits the precipitation of cementite when tempered at high heating rate. When the tempering heating rate is the same, the hardness of the tested steel decreases as the tempering temperature increases. The hardness value is 51 HRC when the tempering temperature is 400 ℃ and the hardness value is 37 HRC when the tempering temperature is 650 ℃. The hardness value is 33 HRC when the heating rate is 0.1 ℃/s and the hardness value is 40 HRC when the heating rate is 200 ℃/s.

Key words: 55SiCr spring steel, retained austenite, carbide, tempering temperature, heating rate, hardness

CLC Number:

TG142

Lu Hui, Chen Yinli, Wei He, Liu Zesheng. Retained austenite and carbide in 55SiCr spring steel[J]. Heat Treatment of Metals, 2020, 45(5): 103-109.

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