Temperature uniformity during any portion of tool steel heat treatment is of critical importance, particularly with vacuum heat treatment. We should be aware that when using vacuum methods of heat treatment for hardening, we rely on radiation heating and not conduction or convection heating.
What the steel can see, it will respond to. This means that if the steel being treated can see the visible radiation, it will respond and heat up accordingly. If there is any shielding (such as another component in the furnace and in front of the heating element), however, a shielding effect takes place. This means that the part nearest the heating element is heated faster than the part that it is shielding.
This could mean that there is the potential for non-uniform heating. If this occurs, there is the potential for non-uniform phases occurring within the heat-treated part. For example, non-uniform heating can give rise to the potential for areas within the tool (depending on its size) for “patches of retained austenite.” This can also affect the cooling rate, which once again can give rise to mixed phases in the quenched tool.
Further, temperature uniformity is critically important in tempering furnaces. If not, there is potential for variable hardness (due to non-uniform temperatures) within the tempering-furnace process chamber.
On a number of occasions, the writer has seen companies using furnaces for tempering without being fitted with an air-circulation fan. When the user has been challenged by the writer regarding this omission, the respons is generally “but we have thermocouples in the furnace and there are two of them.”
It should be remembered that the only temperature that the thermocouple measures is the temperature at the hot-junction end of the thermocouple. It does not, nor cannot, measure the complete temperature of the furnace process chamber. It measures only at the tip of the thermocouple and nowhere else.
Even with the inclusion of an air-circulation fan within the process-heating chamber, there is no guarantee that there will be accurate temperature uniformity. However, one has a better chance of assuring that temperature uniformity could well be within plus/minus 10°F.
Check back next week for more.