Effect of quenching temperature on microstructure evolution and mechanical properties of Nb microalloyed gear steel 18CrNiMo7-6

doi:10.13251/j.issn.0254-6051.2022.02.019

Abstract

Abstract: Effect of quenching temperature on microstructure and mechanical properties of the Nb microalloyed gear steel 18CrNiMo7-6 was studied by means of Thermo-calc thermodynamic calculation software, scanning electron microscope, optical microscope, impact test and tensile test. The results show that with the increase of quenching temperature, the average austenite grain size of the Nb microalloyed gear steel increases, but remains below 20 μm, and the grain boundary stability is high. According to Thermo-calc thermodynamic calculation results, the main precipitates are Cr₇C₃, Cr₂₃C₆, NbC and AlN, among which the solution temperatures of Cr₇C₃ and Cr₂₃C₆ are relatively low (730 ℃ and 749 ℃, respectively), while the solution temperatures of NbC and AlN are relatively high (1180 ℃ and 1070 ℃, respectively). NbC and AlN are the carbonitrides that mainly pin grain boundaries and refine grains. There is a small amount of N element in NbC precipitates, and NbC tends to change to Nb(C, N) at a certain temperature. The yield strength decreases with the increase of quenching temperature, the tensile strength plateaus at 860 ℃, and the impact absorbed energy increases first and then decreases. The Nb microalloyed gear steel 18CrNiMo7-6 has a wide process design window. The optimal heat treatment process is quenching at 860 ℃ and tempering at 180 ℃, after which the tensile strength is 1455 MPa, the yield strength is 1229 MPa, the impact absorbed energy is 100 J, and the hardness is about 44 HRC.

Key words: Nb microalloying, gear steel, quenching and tempering, mechanical properties, microstructure evolution

CLC Number:

TG157.35

Hu Fangzhong, Yang Shaopeng, Jin Guozhong, Hu Naiyue, Wang Kaizhong, Yang Zhiqiang, Chen Shijie. Effect of quenching temperature on microstructure evolution and mechanical properties of Nb microalloyed gear steel 18CrNiMo7-6[J]. Heat Treatment of Metals, 2022, 47(2): 105-111.

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