Gas Mini Forge


Mini Gas Forge

The 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
(oF) (oC)
2192 1200
2012 1100
1922 1050
1796 980
1706 930
1598 870
1490 810
1400 760
1292 700
1202 650
1212 600



To keep the heat in I am going to use firebrick rated for 1093°C (2000°F). The brick size (4-1/2" x 9" x 1-1/4") is convenient enough to make a small forge suitable for heat treating small knives and can still be heated using readily available propane gas.
I collected the parts from a few places, namely Home Depot and Canadian Tire. The torch is a hose type that will let me place the propane cylinder to the side of  the forge.

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.
Be sure the torch is a "swirl flame" or similar type. This design will not work well with pencil tip torches.
Dry assemble the blocks before mortaring.

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.


With a hacksaw I cut some pieces to close in the ends. These 'caps' will help keep the heat in, yet still allow sufficient airflow to get the most out of the torch.








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.








I cut the 2-1/2" steel pipe to fit into the brick box and used a pencil to mark the hole where the torch will go through. The torch tip should enter at a slight compound angle, that is upwards and forwards or downwards and to the back. This will aid in creating a vortex inside the tube.




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.
Test burn looks okay, but I am going to make a removable cover for the back to keep the heat in.



 With a few lengths of angle iron and some 1/4" threaded rod, I fashioned a frame to hold the forge. I will be able to mount this to a stand. Working with the forge requires the opening to be conveniently positioned so one can see inside.

A very light coat of black high heat paint. (The spray can was just about out of paint.)




To test the heating and heat retention of the forge, I choose a large 1/4" thick mill file. I was able to get the end to a non-magnetic state after a few minutes of heating. This should suffice for most of the knives I want to make. Overall I am happy with the results from propane gas.

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.


Cheers!

Dan

Update: Someone asked about the torch. I recommend this one:


7 comments:

  1. Really enjoyed you site, I definitely will be back to watch the progress!

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  2. I love your site and willing to help attitude - I don't understand the 2.5 inch pipe - won't that melt - or does it just get hot and then cool off.

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    Replies
    1. Hi, that's a good question. The pipe acts a container for the heat. It will get hot, but never hot enough to melt. Steel melts around 1350°C (2500°F) and that kind of heat is not going to happen with a little propane torch.

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  3. Hello, thank you for the wealth of information you have shared on your pages!
    What is the brand and model of torch that you use here? I am looking to start making knives from 1080 and believe this type of forge is just what I need! Thanks again.

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    Replies
    1. The one I use is a generic swirl tip torch from a Canadian chain store. It has a short length of hose making it quite useful. It is a reasonable replacement for a BernzOmatic TS4000, which I wholeheartedly recommend for small forging use.

      Look for a "MAP" ready and "swirl flame" unit if you cannot locate a TS4000.

      Cheers,

      Dan

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    2. Would this design work with a slightly bigger pipe diameter that would still fit inside the brick box dimensions?
      Is the torch strong enough for the increased diameter?
      I would probably cap the back end of the pipe.
      I love your website and this forge design, I am just starting out and it gives me a great starting point without the big cash outlay required by buying everything new.
      Dan

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    3. I'd go to MAPP gas if increasing the size a little bit. Blocking heat loss is a good idea. You'll have to try with propane and see if the volume/loading have a big effect. If you don't enough jam to make the steel non-magnetic, then try MAPP.

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