Propane Forge Burning Steel?

[Eric Dennis]

I'm hoping this is the correct subforum-

 

Anyway, I've mostly used to coal, but recently I built an insulating brick propane forge. I've noticed that my steel looks a bit burned on the outer, thinner parts after repeated heats. It crinkly like it's being oxidized (I guess it is).

 

The thing is, I'v got it dialed to the point where there are dragon flames just starting to come out the opening. Shouldn't this indicate a reducing atmosphere in the forge, and thus the steel shouldn't be oxidizing? Perhaps?

 

The forge is built with a single forced air burner that is in the center of the forged and aimed directly down at the silicon carbide floor. Is excess oxygen simply hitting the steel in the "hot spot"? Any thoughts? do other people experience this?

 

Thanks,

 

 

[dablacksmith]

well if you are used to coal propane is a bit different... it will oxidize a lot more! you are probably not burning but oxidizing badly ... hard to tell with pictures on temp ... you are going to have a hotspot on this type of forge unless you do something to difuse the flame more (ribbon burner,or change forge so flame blast swirls ) . hope this helps.

[Doug Lester]

You're also going to have to coat the inside of that forge with something or the fire brick will start to break down on you. You might go to a hardware store and get some furnace patch. Thin it down with water until it's like a slightly thick paint and then brush it onto the interior.

 

Doug

[timgunn]

The idea that there is a sharp transition from Oxidizing to Reducing is one of those oversimplifications that are very useful when trying to explain broad general principles, but which can get you into deep trouble if you take them as gospel.

 

If you try to think in terms of there being a range over which the atmosphere varies between strongly Oxidizing and strongly Reducing, then factor in the position of the workpiece within the flame itself (within the flame, the Oxygen and fuel gas have not yet finished combining, so there will be more free Oxygen available to react with the steel in the early part of the flame), you may find it easier to work out what is going on.

 

If you richen up the flame a little more (more gas for the same air or less air for the same gas), there's a good chance you'll see an improvement. You may see the forge temperature drop when you do this, so may then have to increase both gas and air in the same proportion to get the forge temperature back up to where you want it.

 

There are 2 factors at play here: the temperature of the flame and the amount of flame.

 

The amount of flame is quite easily understood.

 

The temperature of the flame is dependent on the air:fuel ratio and will be highest at the stoichiometric ratio. This is the point at which all the Oxygen and all the fuel combine, leaving neither excess fuel nor excess Oxygen behind. It corresponds to the sharp transition point in the oversimplified model.

 

We do not want excess Oxygen so we are always going to run on the fuel-rich side of stoichiometric. The heat produced will therefore be limited by the amount of Oxygen present. If we richen the mixture by adding more gas, the amount of heat energy released cannot increase because there is no Oxygen for the extra gas to react with. The extra gas will absorb some of the heat energy released by the original fuel burning and this will lower the overall flame temperature.

 

We can turn up the gas-and-air flow to increase the amount of flame to compensate, but we can never get the forge temperature above the flame temperature. This means that the forge cannot be run as highly-reducing at high temperatures as it can at low temperatures.

 

Part of the skill involved in using a gas forge with variable mixture is achieving an effective compromise between gas usage and oxidation of the workpiece. In my very limited experience, gas is a whole lot easier than using solid fuel, but it still involves a bit of a learning curve.

[Joshua States]

You have already gotten some great advice on setting up, tuning, and adjusting the forge.

I can only add this little piece. On all of the forced air forges I have made and other people's I have used, the mixture is rich enough (fuel heavy) and the air flow high enough, to push the dragon's breath about 5-6 inches out the front opening. When you run a reducing fuel:air mix, that excess fuel is looking for oxygen to burn and it finds it outside the forge.

 

Tim also has a good point about the fuel and air not getting mixed enough too. Your fuel entry is not very far from your burner point.