Why preheat? Is there such a thing as preheating? During preheating you are still heating it. So, what is preheating? Preheating, as part of the hardening process, is the act of heating to equalize the temperature in order to stabilize the material.
Taking a tool directly from room temperature to the austenizing temperature will cause uneven stress and strain. It is this stress and strain that causes many future problems during the heat treating process or while in operation. Simple small round symmetrical thin-wall tools will heat evenly from room temperature to the austenizing temperature. There aren’t many tools that fit this description. Most tools are complex and asymmetrical. These tools will not heat evenly. Thick sections will heat to the austenizing temperature much more slowly than the thin sections.
If a tool is not evenly heated to the austenizing temperature, then the inner portion of the tool will not properly austenize. Preheating allows the tool to reach an equalized temperature prior to any additional heating above the critical temperature.
For the complete heat treating procedure, including tempering charts, tips and tool steel data sheets, see our "Tool Steel Simplified" book.
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