Increase The Toughness Of Steels After Quenching Process

1976 Words8 Pages
Tempering is an important heat treating process, which is used to increase the toughness of steels after quenching process. Hollomon-Jaffe equation H=T(c+log⁡t ) is widely used to predict hardness with different tempering temperature and tempering time. T is the tempering temperature, t is the tempering time and c is the constant for the specific steel. The physical explanation of c constant was not clear. In this paper, the hardness and c/a ratio of tempered martensite for AISI 4140 were measured by X-Ray Diffraction and Rietveld refinement. The result reveals that the c/a ratio decreases instantly within the first 60s tempering time. After analysis based on the experiment results and derivation of Hollomon-Jaffe equation, c, the parameter in Hollomon-Jaffe equation, is found to stand for the slope of carbon-hardness curve.

Key words: Tempering, martensite, X-ray diffraction, Hollomon-Jaffe equation

1. Introduction
All steels that are hardened are subjected to a subcritical heat treatment referred to as tempering . Tempering improves the toughness of as-quenched martensite (body center tetragonal) but lowers strength and hardness 1, . The microstructure transformation is very complex: Stage1: The formation of transition carbides and lowering of the carbon content of the martensite to 0.25% (100 to 250℃) Stage2: The transformation of retained austenite to ferrite and cementite (200 to 300℃) Stage3: The replacement of transition carbides and low-temperature
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