If you want to forge metal, you need a forge to heat the metal  to bright orange. That seems obvious. And if  you can throw some money at it and buy an actual forge, that will not be a problem. Otoh, if you are on a budget, it is not so obvious. However, building a DIY forge is not complex.

There are 2 common ways to make a forge: gas powered or coal powered. I prefer coal for the simple reason that it is more traditional, and much safer. It's not that I am afraid of gas. I cook on a gas powered furnace. But still, I would not build something gas powered myself.

A coal or charcoal powered forge is simple in principle. You have a firepit in which the coals burn. The firepit is insulated to keep the heat inside, and there is a way for air to blow through the fire to feed oxygen to the burning coals.

I decided to build a forge out of my old BBQ. The first job was to build a firepit in a way that would keep the heat inside. There are various ways to do this. I chose the cheap way. I got the idea from Dan O' Connor. Make a paste of cheap kitty litter, ashes and water. Use this to coat the bottom of the BBQ with 2 or 3 inches. Then put a styrofoam or plastic box in the middle, and spread the paste around it to build a cavity of 6 inches deep.

For the mix I used 3 or 4 parts kitty litter, 2 parts ashes, 1 part refractory cement, and one lovely assistant. I forgot to add straw for internal strength. This is not a dramatic mistake, but it will make the end result a bit more prone to cracking.

After a lot of mixing and playing with mud and gooey paste, I ended up with this. The hole in the middle is where the air will come out of, and the square shape is where the diffusor will fit. Diffusor is just an expensive word for a metal plate with holes in it to allow the air to blow through the coals.

In the front view you can see the airholes in the pedestal. The airholes were already there for the BBQ. That is where I will fit the connector for the 4 inch pipe in that will blow air through the fire.

One comment: kitty litter absorbs an enormous amount of water. This makes sense of course. In order to make it easier to mix, add water to the kitty litter before you mix it with anything else. And another thing: because of all the water, this thing is heavy. So don't plan on carrying it by yourself if you have to carry it far.

It will also take quite some time for it to dry out. In fact, it took so long that I decided to help matters along by firing it up as soon as the exterior was no longer wet. The firepit shrank a bit, which you can see, and it cracked too. That doesn't matter though.

To feed the air I used a simple blowdryer. Hardly ideal, but the main goal was to get a hot fire burning, and to check (with some steel) if the fire would get hot enough. The test was a success. There was one problem. I used BBQ cocos briquettes. They got hot enough, but as soon as the fuel was exhausted, they completely pulverized and left the entire pit smothered. Briquettes are held together by clay and compounds, and these don't burn. They remain, which makes briquettes unsuitable for forging. Charcoal and coal are much better.

For moving a lot of air in a controlled manner, a hairdryer doesn't cut it of course. So I decided to use a strong industrial fan that I've had for 15 years. I got it for free at the time, and I decided to keep it for when I knew what to do with it. It is ideal for this purpose.

A fan by itself is not that useful of course. I could have connected it directly to my forge, but that would have been impractical. For one, my forge remains outside. The electric fan would not survive the winter conditions. And second, I wanted it to be portable so that I could also use if for other purposes, like powering a shop VAC (more on that later). So I decided to build it into a box, made from scrap wood. I never throw away cut-offs. The small ones are for the kids to play with, the larger ones are save for when I do something like this.

Of course, my lovely assistant is helping me out  by sawing planks.

 The base for the fan was a plank with a 4" hole. The grey square is made from foam, and is to ensure that there are no air leaks between the fan and the output.

Next step is to mount the fan and the sides. Internally I added a strain relief for the electric cable.

Halfway done. The fan is now completely encapsulated. I closed the entire space between the fan and the sides with PU foam to keep the back separate from the place where the air gets sucked in. For simple use as a blowing fan it would not matter if there were leaks or not. However, I also want to use this to power a shop VAC, and in that case it is important that there is no loss of pressure between the input and the exhaust.

I made the hole to the backside big enough to allow free circulation of air for the little fan that cools the coils of the motor. I also covered the hole with a double layer of garden wire fence to keep leaves, garbage and little kids fingers from accidentally touching the wiring within.

The front side of the box. The flange is a standard 4" flange for connecting flexible air piping. I mounted wire here as well, mainly to prevent horrible accidents involving curious kids. Inside, the fan blades could be spinning at thousands of RPM. Anything touching them at that point will be turned into hamburger meat.

The dimmer switch is used for regulating the speed of the fan. The way I built it is a bit funny, admittedly. I bought a simple wall socket dimmer to see if it would work. Not all dimmers work with inductive loads. After I proved it would work, I decided to try the dimmer switch that was at that point still mounted in a wall switch. But that turned out not to work with inductive loads. It would have been ideal, because then, the outside would have only been a knob.

Since I didn't want to risk buying yet another switch that might or might not work, I decided to mount a simple wall socket, and insert the dimmer in the socket, with a cable leading back to the fan. I know it looks a bit silly, but this way I didn't have to buy anything new. A minor note for the electricians among you: yes, I know that this is a crummy solution. A fan like this should be powered by a frequency convertor, not by a dimmer switch which chops the sine wave. Although this is horribly inefficient, the fact that the power is reduced means the motor starts missing cycles, and settles on a speed in which the small bursts of power work like the repetitive pushing of a flywheel. Since the fan does not draw significant currents, the electrical pollution remains insignificant. A frequency convertor costs +200$, a cheap dimmer switch only 10$.

The side of the box is the air intake. Like the exhaust, the hole is protected by wiring. The 5" flange has a rubber ring that will allow me to hook it up to the shop VAC system without much problems.

Finally, everything could be put together. The flexible piping fit snugly over the ridges of the flanges, and the fan gave a nice controlled and adjustable air flow.

Not quite the fires of Mordor, but more than sufficient for forging knives and smaller things. With the fan set a bit higher, the flames were hot enough to turn iron rebar way past red hot into the yellow - white. The first time I used it, I used it with a low airflow to turn 2 blades nice orange for heat treating. The white knob on the left side is a strong magnet that is used to determine the point where the steel turns non-magnetic. That is the point where I quench the steel.

As you can see, my quenching setup is equally unimpressive. A bucket of water for quenching carbon steel, and a container of maize oil (the cheapest I could find) for quenching oil hardening steels.

The end result is a forging setup that is made completely from scraps and repurposed materials. It is simple to make, and cheap. All you need is a bit of time to put everything together.