M2 Thermal Treatments: Quenching and Tempering

Topic

The quenching of this metal—a rapid cooling process that roughly follows Newton's law of cooling—determines, according to the Koistinen-Marburger relationship, the fraction of martensite formed as the metal passes through the martensitic transformation temperature range. This relationship describes how, as the temperature drops further below the transformation start point, more retained austenite transforms into martensite; this transformation requires no additional time at a given temperature, only that the temperature be reached. A lower minimum temperature achieved during quenching results in a higher fraction of martensite, in accordance with the Koistinen-Marburger relationship.

In addition to determining the martensite fraction, this quenching process creates a thermal gradient between the surface (which cools first) and the core (which cools later). As described by another model in this set, this thermal gradient generates residual quenching stress capable of causing the metal to crack. A more severe quench—involving faster cooling—increases the thermal gradient between the surface and the core, thereby increasing the resulting residual quenching stress, according to that same model.

Once quenched and largely transformed into martensite—yet still brittle and retaining residual quenching stress—the metal undergoes a subsequent heat treatment known as tempering. This process involves maintaining a specific tempering temperature for a set duration. The Hollomon-Jaffe parameter combines these two factors—tempering temperature and time—into a single value, quantifying their combined effect on the metal. A higher tempering temperature or a longer tempering time results in a higher Hollomon-Jaffe parameter in either case.

Consequently, given the initial quenching and tempering conditions, this Hollomon-Jaffe parameter determines the metal's final post-tempering hardness: the higher the parameter, the more the brittle martensite formed during quenching decomposes into a softer yet tougher microstructure, resulting in lower final hardness. Since this tempering process—by advancing martensite decomposition—also allows for the relaxation of some residual quenching stresses retained from rapid cooling, the subsequent heat treatment serves a dual purpose: adjusting the metal's final post-tempering hardness and alleviating, to some extent, the risk of quench cracking posed by those residual stresses.

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