Tempering in a Kiln or Heat Treating FurnaceJess Tunnicliffe
Gameco and Paragon Kilns
Gameco has been representing Paragon Kilns in Australia for nearly 10 years. Paragon Industries have been making leading kilns for nearly 75 years. In our time as a distributor, we have worked directly with their factory to ensure that Paragon knifemaking kilns are the very best kilns money can buy. We have been a part of many technological improvements in kiln design that have kept Paragon Industries as the market leaders in premium kiln design and application for knifemaking.
With their heavy-duty steel bodies and mortared fire bricks, Paragon brick kilns are built to last, easy to maintain and can be relied upon for many years of service. Our team at Gameco are trained and highly experienced in kiln maintenance and trouble shooting and we have a direct line to the Paragon factory. If you have an issue with a Paragon kiln, Gameco are here to fix it. If that means replacing the product or even a full refund, then we have the backing AND commitment to make that happen.
In recent times we have seen discussions on tempering in kilns, and there has been mention that the K type thermocouples used in kilns are not suited to low temperatures as used in tempering. This is simply not true as you will see below.
Tempering in kilns is not something we recommend as a general rule, for several reasons, however it can be done and done very accurately.
Why would you not temper in a kiln?
- Kilns are designed to get very hot, in some cases, 1150C to heat treat stainless steels. Brick kilns take a long time to reach these temperatures and once there, they take even longer to cool back down. If you force them to cool too quickly, it can be detrimental to the fire bricks and cause safety issues. Thus once heat treating your blades you will need to leave them at room temperature for many hours before you can temper. This is not best practice. Once quenched, most steels are glass hard. They are full of internal stresses and can easily crack. It is almost always recommended that you temper immediately after you quench. Many a knifemaker has left tempering for several hours or overnight only to find they now have two pieces of steel rather than one useful one. Paragon Pro Kilns cool much quicker than brick kilns.
- Kilns (Other than Paragon Pro models) have poor convection. The long thin design of kilns means that the temperature where the thermocouple is measuring is not necessarily the same at any other point in the kiln. Kilns have no fan or other means of “mixing” the air in the kiln. It can take a considerable amount of time for the temperature in the kiln to even out along its full length. Areas of heat loss, like around the door, may never fully get to the temperature indicated on the thermocouple and controller.
- The PID controllers can often over shoot temperature when aiming for lower temperatures. As supplied, PID controllers are normally calibrated for the quickest possible heating to austenitizing temperatures. PID stands for Proportional-Integral-Derivative. It sounds complicated, and their actual function is somewhat. In laymans terms, they are used to control processes that would be very difficult for a human control. They were originally developed for military uses such as the steering of a battleships. A battleship has a tremendous amount of inertia, if you tried to steer it to a set heading, the inertia of the vessel would keep it turning long after the rudder was straightened leading to inevitable over steer. The computer in a PID controller can determine how much to move the rudder and when the rudder should be straightened and over corrected to ensure that the heading is achieved. By setting the values of P, I and D the controller can hit the correct heading as quickly as possible, by allowing for and averaging overshoot and undershoot, or if overshoot is undesirable and time is no issue, they can creep up on the desired setting. For kilns at tempering temperatures we do not want any overshoot. A very short time at high temperature when tempering is detrimental. Setting the ramp speed is not correcting the reason why the kiln is over shooting.
For these reasons we recommend, either a low temperature oven, or a second kiln set up for tempering.
To set a Paragon brick kiln with a Bartlett sentinel controller up for best tempering results, the following is recommended.
Download the latest firmware for the kiln.
Under the “Adjustments” menu set the PID gain to 20 and the Dampening factor to 3 (Brick kilns only. Contact us for assistance with Pro kilns) Your kiln should no longer overshoot at the expense of some time heating. It is that simple. You can adjust these figures for your kiln making small incremental changes and recording the results.
Remember however that the temperature the kiln controller is working off is only the temperature at the thermocouple and for this reason the kiln should be left for approximately 30 minutes longer to ensure that the whole kiln has time to come up to temperature.
In short, the K type thermocouple in your kiln DOES NOT NEED REPLACING, it is perfectly suited for all temperatures between 0-1300 C.
If you are serious about your knife making, consider a Pro kiln. Paragon Pro kilns get to austenitizing temperature in approximately 15 minutes and control the kilns temperature in zones, ensuring very accurate temperatures in any location within the kiln. They can have all the benefits of more technical kilns like fluidized beds and salt baths, without the negatives.
Gameco stands by everything we sell and we have knowledge and experience to assist with anything kiln related. Please feel free to contact us with any questions.