Microstructure, Hardness, and Tensile Properties of Vacuum Carburizing Gear Steel

Wu Chen,Xiaofei He,Wenchao Yu,Kefu Yao,Maoqiu Wang

doi:10.3390/met11020300

Abstract

We investigated the effects of the austenitizing temperature on the microstructure, hardness, and tensile properties of case-carburized steel after vacuum carburization at 930 °C and then re-austenitization at 820–900 °C followed by oil quenching and tempering. The results show that fractures occurred early with the increase in the austenitizing temperature, although all the carburized specimens showed a similar case hardness of 800 HV0.2 and case depth of 1.2 mm. The highest fracture stress of 1919 MPa was obtained for the experimental steel when the austenitizing temperature was 840 °C due to its fine microstructure and relatively high percentage of retained austenite transformed into martensite during the tensile tests. We also found that the stress–strain behavior of case-carburized specimens could be described by the area-weighted curves of the carburized case and the core in combination. The strain hardening exponent was about 0.4 and did not vary with the increase in the austenitizing temperature. We concluded that the optimum austenitizing temperature was around 840 °C for the experimental steel.

Highlights

As vacuum carburizing might result in coarse grain sizes when a high carburizing temperature above 930 ◦ C is used, re-austenitizing and quenching are required instead of direct quenching after carburizing
The carburized case consisted of plate martensite, carbides, and retained austenite, which are typical for high-carbon steels
If we look carefully at the hardness variation in the near-surface region, we can find a slightly higher hardness of the specimen austenitized at 820 ◦ C in comparison with the specimens austenitized at other temperatures

Summary

Introduction

Gears are typically carburized to have very high hardness and wear resistance in the case and excellent toughness in the core. Re-austenitizing and quenching after vacuum carburizing at around 930 ◦ C are used in certain cases, in particular for heavy-duty gears. The effect of the re-austenitizing temperature on the microstructure and mechanical properties of vacuum-carburized specimens has not been fully clarified, and the optimum re-austenitizing temperature should be determined for particular situations. Due to the complex microstructure in the carburized case [1], which is typically composed of martensite, retained austenite, and carbides, the mechanical properties of vacuum-carburized specimens can vary largely and be influenced by many factors [3,4,5]. The mechanical properties of carburized specimens are usually characterized in terms of the hardness and fatigue resistance.

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Conclusion