Heat Treatment

Copyright © 2002 by Jim Wilson

Tool steels do not become great tools with haphazard heat treatment. The thermal processing for my mortise chisels was carefully designed to achieve specific results:

This is not as easy as it might seem, because some of these objectives work against others. For example, an increase in hardness usually causes a decrease in toughness. Balancing the desired properties of the finished tool is a delicate task, requiring exacting equipment and experience.

The blade is slowly preheated to 200 degrees F below the steel's critical temperature and held there just long enough for temperature to equalize throughout the material. This slow preheat minimizes distortion, which can result if the temperature is raised too quickly, due to the non-uniform changes in density which occur near the steel's critical temperature.

After preheating, the temperature is raised to the high end of the austenitizing range, so that maximum hardness and wear resistance may ultimately be obtained. Above the critical temperature (about 1500F), the internal structure of the steel changes from ferrite to austenite. After this transformation, the blade is "soaked," or held, above the critical temperature for about 15 minutes. This allows the alloying elements and microscopic carbide particles to diffuse into the steel matrix. The soak time for these blades is a little shorter than it is in many applications because of the relatively high austenitizing temperature.

Following completion of the alloy diffusion, the steel must be cooled very quickly. This transforms the austenite into hard, brittle martensite and keeps the alloy content well distributed throughout the steel matrix. To minimize distortion and achieve a superior finish, an inert gas quench is used instead of the traditional oil quench. Until fairly recently, most of the commmon gas quenches could not cool these types of tool steels quickly enough to achieve maximum hardness. Today, however, extremely high pressure (20 bar and greater) gas quenches are available, and these can achieve the necessary cooling rates.

The quench leaves the resulting martensite very hard, but also extremely brittle and with large internal stresses. If the tool were used in this condition, the steel would shatter. Tempering relieves the stress and reduces brittleness, but higher temperatures also reduce hardness and wear resistance. Therefore, the lowest effective tempering temperature is used. Two temper cycles are performed for maximum stress relief. Each cycle includes a two hour soak, with a cool-down to ambient temperature between.

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