Dr. Dmitri Kopeliovich

Hardening is a heat treatment process involving heating a steel above the phase transformation temperature (upper critical temperature, A₃), followed by soaking and then rapid cooling (quenching).

When steel is heated above the upper critical temperature, its structure becomes entirely austenitic.

Then the article is cooled at a rate exceeding the critical rate value.

Critical cooling rate is a function of the chemical composition and the grain size of austenite.

If the critical cooling rate is not achieved, a mixture of ferrite and cementite forms.

Depending on the cooling rate the following ferrite-cementite structures may form:

• pertlite – ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling in annealing treatment;

• sorbite- ferrite-cementite perlite-like structure with finer (than in perlite) grain structure, forming as a result of decomposition of austenite at relatively high cooling rate (cooling in air);

• trostite–fine ferrite-cementite perlite-like structure forming as a result of decomposition of austenite at high (but lower than critical) cooling rate (cooling in oil);

• bainite– very fine ferrite-cementite mixture, forming in a mechanism similar to the mechanism of martensite transformation, as a result of decomposition of austenite at high (but lower than critical) cooling rate (cooling in a quenching medium);

Cooling in water usually provides cooling rate higher than the critical value.

The structure forming as a result of quenching in water is called martensite (supersaturated solid solution of carbon in α-iron). Martensite is hard and brittle phase, having hardness varying between 500 HB to 710 HB depending on the carbon content.

The temperature interval at which the austenite-martensite transformation occurs is about 480°F - 400°F (250°C - 200°C).

Hardening temperature is the temperature to which a steel is heated before quenching.

If the hardening temperature of a hypoeutectoid steel is at least 100°F (55°C) above the upper critical temperature, quencing will result in complete austenite-martensite transformation (full hardening).

If the hypoeutectoid steel is heated to a temperature, lying between the upper critical temperature (A₃) and the lower critical temperature (A₁), quenching will result in formation of martensite with some amount of ferrite (partial hardening). This structure is softer than full-hard martensite structure.

In the case of hypereutectoid steel partial hardening results in formation of a mixture of martensite and cementite, which is harder than full-hard martensite structure.

Hardenability is the property of steel indicating the depth to which hardening effect penetrates. Hardenability depends on the chemical composition of the steel, hardening temperature, dimensions and shape of the article and other factors.

Hardenability is detrmined by the Jominy test, in which a steel bar of 1 inch in diameter and 4 inch long is heated above the upper critical point and then one end of the bar is quenched by water jet. Results of the hardness measurements conducted along the bar after quenching indicate the hardenability of the steel.

Isothermal hardening is a hardening method involving quenching in a medium (oil or molten salt) to minimize the part cracking and distortion.

There are two pricipal isothermal methods:

• Martempering is the isothermal hardening method, in which a part is quenched in a quenching medium (oil or molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium before the bainite formation.

• Austempering is the isothermal hardening method, in which a part is quenched in a quenching medium (oil or molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium after the complete bainite formation.

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• Annealing and stress relief
• Normalizing
• Case hardening
• Ion nitriding
• Tempering
• Precipitation hardening
• Cryogenic treatment of steel
• Salt bath heat treatment
• Furnaces for heat treatment
• Basic principles of heat treatment
• Iron-carbon phase diagram

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