Furnace

Heat treating furnace

I built a gas fired, digitally controlled furnace to harden the swords. The design came from a conversation with Howard Clark and was modified to suit my needs. The controller is a small 1/16 DIN Microprocessor-Based Auto-tuning Control series 965 from Watlow. Omega offers similar controllers and they run around $200. These controllers allow you to establish a set point temperature and sends a signal to a gas solenoid valve turning the gas on and off as it approaches set point. I got my controller from South Eastern Heaters, contact Don Ware. The solenoid valve I use came from White-Rodgers and is in normal close position until activated by the controller. The solenoid I use is a low pressure valve and runs off a 1/2 lb regulator. The plumbing on the valve is straight forward. I use a K type thermocouple to provide input to the controller. For this system I decided that I would install a sparkplug ignition and placed sparkplugs at the two burner ports. They are constantly fired by a standard furnace transformer.

I used a 12" diameter pipe 36" long for the forge, cutting in two burner ports approximately 8" from either end. I also cut a vent in the center and welded a chimney over it. The vent helps control the atmosphere and also draws the heat to the center of the forge to equalize the heating. I lined the inside with 2400 degree 1" blanket of Inswool from A.P. Greene and coated it with a layer of high temp mortar mix. I also made a bed of soft firebrick along the bottom and cut grooves in the block to support the blades. The block also serves to break up the flame from the burners and stops it from making direct contact with the blades.

The burner system begins with a 100 cfm shaded pole blower connected to a 6" length of 1 1/4" diameter iron pipe. In the center of the pipe, drill and tap for a barbed fitting to connect the gas line. I do not use an orifice, just straight in. The pipe leads to a T and then 6" straight pipe, elbows, 4" straight pipe to a bell reducing fitting 1 1/4" to 1". I welded on pipe to the outside of the burner ports to support the burner heads and lock the burners in place with a set screw. The burner and the transformer are connected to an outlet strip which is then plugged into a surge suppressing outlet strip which powers the controller. I mounted the controller and another digital pyrometer inside a utility box.

This whole furnace can be built from off the shelf parts for under $350 and the controller also is used to run a smaller high temp salt pot that I use for my smaller blades. With the controller and the spark plug ignition, I am able to control the temperature accurately from 200F to 1900F holding a 20 degree margin.