Increase The Toughness Of Steels After Quenching Process

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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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