Preheating of tool steels for austenitizing procedures is very critical because non-uniform heating can give rise to thermal stresses being set up within the tool workpiece as it is raised up to its austenitizing temperature. This could ultimately lead to either cracking, distortion or even both.
If one is heating up the tool for austenitizing in a vacuum furnace from ambient temperature, one could heat up by both conduction and convection. This would be accomplished by evacuating the process chamber after loading and backfilling with nitrogen to a partial pressure. This means that one can now heat by conduction and convection up to a radiant temperature of approximately 1300°F. At that temperature, one would then evacuate the process chamber down to the required partial-pressure level and heat by radiation up to the appropriate austenitizing temperature.
If heating by a radiation furnace, such as a furnace that is capable of holding an atmosphere, one would preheat at a low temperature (up to 1300°F) by step heating. This could be accomplished in three stages (for example).
- Heat from ambient temperature up to 450°F, hold and equalize.
- After equalization, heat up to 850°F, hold and equalize.
- After equalization at 850°F, heat up to 1200°F, hold and equalize.
- If the steel being treated is an H-series hot-work steel or a D-series tool steel, one could transfer to the austenitizing furnace at a temperature of 1600°F, hold and equalize, followed by raising the temperature up to the appropriate austenitizing temperature to soak for the appropriate holding time.
The temperatures quoted above are not mandatory but only suggestive temperatures.
We will review tool-steel tempering in part 3.