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Sunday, November 21, 2010

Heat treatment oven, part I

I've been thinking of building my own heat treatment oven for my workshop to heat treat tool steels, for example drill guide bushings and cutting tools. The oven would be electric and quite small in size, maybe something like 150 x 100 mm opening and 200-300 mm deep. The structure will be quite simple, like this example from a knife forum. The maximum temperature limit will be 1300 Celsius, but I'm only going to need about 900 degrees Celsius at any given time, as I'm mainly going to use silver steel and a tool steel called Arne (W. 1.2510) or K460.

The oven will have a small door in the front that either opens to the side (lower left in the sketch) or folds down (upper left) so that the outside surface is on top to protect the insulation material in the door. In the door design that opens to the side will be a small vertical handle that pivots from the door and has a groove that engages with a small rod in the side of the oven frame to lock the door shut. The folding version would be built so that the door is forced shut by its own weight and this is accomplished by having the door closed after the pivoting point. I'm probably going to build this as a side opening version at first and upgrade some day if I rebuild the oven.

Main frame for the oven will be welded from angle iron and then primed and painted with a high temperature paint. In use the outside temperature of the oven will not rise too hot for touch, so basically any paint would do but it doesn't matter to go for the extra length in this. The frame will be covered with stainless steel sheet, probably 0.5 - 0.7 mm thick and is attached with blind rivets.

The insulation material will be type 23 insulation brick that withstands the required temperatures, is lightweight and easy to work with. The door will have a length of ceramic string around the opening to properly seal the door and thus to prevent heat loss.

Heating elements will be either bought as ready made coils from eBay or I'll buy the required amount of wire from a supplier and coil it myself, but this I'll decide when I get some approximate dimensions for the required coil size. The heating element will be made from Kanthal wire, as that is easily available and is one of the only materials to withstand around 1000 degrees Celsius easily. Wire coils will be going around the sides of the oven in grooves cut to the insulation bricks. I'm planning on about 1-2 kW of heating power, so the oven heats fast and can easily heat larger parts also.

Temperature control will be achieved by using a learning PID controller bought from eBay for about 20 EUR and this controls a solid state relay (also from eBay, 7 EUR) that delivers the power to the heating elements. Thus this setup will be quiet in operation, as there are no relays that would click and the only indication of operation is the steady temperature rise indicated by the LED display of the PID and the blinking of a LED on the solid state relay. The temperature is measured by the PID controller from a K-type thermocouple that can withstand up to about 1300 degrees Celsius.

Safety will be ensured by grounding all the metal parts, meaning the main frame and the door. The door is hinged so the electrical connection from the body to the door might not be quite good, so I'll install a small length of grounding wire across them to ensure proper grounding. The door will also have a safety switch that cuts the power to the elements if the door is opened as otherwise one could get an electric shock if the elements are touched. The oven will have feet under it so that if the outside surface for some reason heats it will not heat whatever is under the oven.

The connection wires from the elements to the controller will be heat resistant connecting wire that is insulated properly and the connections will be covered with a small box or the wires will be routed under the steel sheets to the controller box that is bolted to the side of the oven.

I'm estimating around 200 EUR for the total cost of this machine, which is very reasonable considering all the benefits I'm getting from it. Part II of this heat treatment oven will cover the building steps with hopefully lots of pictures and of course some tests to see that it really works as intended. I have access to a Rockwell C hardness meter, so I can easily check that my hardening results in the same as I intended it to be :)


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