I am a begining smith, but I have been welding for a few years. I have a shallow pan forge I bought, and have been using.

I  want to build a bigger forge with coal storage, and have seen plenty of good designs, but my question is, if I build the fire pot out of 1/4" mild steel, why doesn't the heat from the coal melt, and burn through the fire pot?

The bottom of a firepot doesn't burn out because the sweet spot (where melting temperatures can occur) of the fire occurs a few inches above the bottom. Since the coal uses an updraft to achieve decent forging temperatures, a lot of the heat is moved away from the bottom. Possibly the tray also helps act as a heat sink? 

 

It may eventually oxidize through, but this should take place much slower than with thinner metals, and shouldn't actually melt. I did lose a loose screen over the tuyere one time when it got tilted on end by my coal rake.

 

At least that's what I think. Someone with more expertise can and probably will explain it better. My firepot is homemade refractory funneling in over a brake disc, and it takes a couple of years to slag away the portland cement in it so that I have to patch or replace it. In my experience from making casting furnaces, this refractory recipe would start to slag at anything much above 2300 degrees, or brass casting temperatures.  

My firepot has been glowing red to nearly orange with the very hot burning stoker coal I used, a little scale but no major problems. My pot is 1/4" steel.

Actually the melting point of mild steel is higher than the melting point of cast iron

The 55 forge is metal from a 55 gallon drum, thin 16-14 ga at best and I suspect mild steel. Once there is an inch of ash in the bottom for insulation the bottom is surprisingly cool. All the fire is above the ash layer.

 

I have seen a cast fire pot glowing above red when a newbie used electric blown air and poured the heat and coal to the forge. But burning that much coal at full power, for that long, I would expect it to get hot. LOL  And yes he had to back away from the fire as the radient heat was intense and he burned most of the metal he put in the fire.

Welcome aboard, glad to have you.

 

1/4" will be just fine for quite a while. Of course being a newcomer to the craft mistakes are the norm. SO, make the fire pot a drop in affair, just put a rim around the top so it sits in a fitted hole in the forge table. If you make a basic template you'll be able to duplicate the firepot easily.

 

Frosty the Lucky.

The 55 forge is metal from a 55 gallon drum, thin 16-14 ga at best and I suspect mild steel. Once there is an inch of ash in the bottom for insulation the bottom is surprisingly cool. All the fire is above the ash layer.

 

I have seen a cast fire pot glowing above red when a newbie used electric blown air and poured the heat and coal to the forge. But burning that much coal at full power, for that long, I would expect it to get hot. LOL  And yes he had to back away from the fire as the radient heat was intense and he burned most of the metal he put in the fire.

The 55 forge is metal from a 55 gallon drum, thin 16-14 ga at best and I suspect mild steel. Once there is an inch of ash in the bottom for insulation the bottom is surprisingly cool. All the fire is above the ash layer.

 

I have seen a cast fire pot glowing above red when a newbie used electric blown air and poured the heat and coal to the forge. But burning that much coal at full power, for that long, I would expect it to get hot. LOL  And yes he had to back away from the fire as the radient heat was intense and he burned most of the metal he put in the fire.

Heat rises so even though the metal is in contact with the coals, the main heat goes upward. Also note the fact that you've got fresh air constantly coming in right around the bottom which helps keep things cool. 1/4- inch is not ideal for firepot material. 1/2-inch or thicker s good. However, 1/4-inch plate WILL work!!!!! I built and used a small 1/4-inch plate firepot for several years, and although it is in rough shape now, it can still be used. It has been kept outside for it's entire life of four years.