Mini Gas ForgeThe forge is used to heat steel. There are two basic purposes for heating, one being to shape the steel, say with a hammer and anvil or to heat treat the steel. One part of heat treating, known as hardening involves heating the steel to the appropriate temperature and then quenching it in oil or water.
Traditionally, the forge was a coal powered fire pit with some form of mechanically driven air supply to increase the temperature to get the steel hot to shape or harden. Bellows provided extra oxygen needed to burn the coal hot enough to heat steel. Similar results can be achieved very cleanly with electricity or gas. For the DIY knife maker, propane gas is effective and readily available.
In order to heat treat the carbon steel used for knifemaking, the temperature needs to get to around 800°C (1470°F), which should be cherry red colour. At this temperature iron becomes no longer magnetic. For high-performance stainless steel, we need to get to the 1000°C mark (Austenizing temperature) and hold it for 30 to 60 minutes. This is the main goal. I reckon the amount of fuel for 45 minute burn at yellow hot is in one typical camping type propane fuel bottle.
The table below, (courtesy EngineeringToolbox.com) shows the colour the steel needs to be for hardening quench.
|Temperature||Color of Heated Carbon Steel|
Also showing: Fireplace mortar (2000°F) and #10 x 2-1/4" concrete screws.
If you wish to build a similar forge, you may be able to find these things on Craig's List, Kijiji or yard sales. Good luck in your hunting.
Set the drill to hammer mode and drill the firebrick. It cuts very easily with the vibration from the hammer. I used a 3/16" bit that will accommodate the #10 concrete screws.
I put one fastener on each side. Next, I drilled the torch hole to 1/2" then enlarged it to 5/8" with a coarse round file. Firebrick cuts easily, although it's dusty. It's a good idea to wear a dust mask when drilling and cutting this brick. Be sure to mark all of the pieces as they will have to be disassembled for the mortaring.
The high temperature mortar looks and feels like mortar. It's gritty. I squeezed a bead and spread it around with some scraps of wood.
Once the mortar is in place the screws go back in to hold everything together for the setting period. I have decided the leave the screws in, but they could be taken out and the holes filled with mortar and sanded when dry.
After inserting the pipe, I marked the so it could be reinserted in the same spot again.
I drilled the torch hole to 5/8" with a step bit and elongated the hole with a round file.
A small amount of high temperature mortar at the front and back holds the pipe in place.
A very light coat of black high heat paint. (The spray can was just about out of paint.)
An option for increasing heat is to switch the fuel to propylene gas (MAP). This isn't a problem as long as the torch you are using is good for MAP gas.
A short video shows the ultimate test for a carbon steel knifemaker: getting past that non-magnetic state. Sorry about the sound quality as the swirl tip torch is loud.
This should work well for 10xx carbon steels and O1 and some similar alloys which are of interest to knifemakers.
If I have any updates on the Gas Mini Forge, I will add them here.
Update: Someone asked about the torch. I recommend this one: