Forge welding is one of the most fundamental techniques of blacksmithing, but one of the trickiest to master. It requires speed, precision, and practice. Practice every time you work. Make it an exercise and when you need it for a project you will have the skill. Forge welding is one of those skills it is best taught one on one, hands on. But it is possible to learn it the hard way.
One of the most common mistakes with forge welding is to hammer too hard which instead of imparting the energy required for the electrons to jump the boundaries, it shears the bonds already produced.
It is easier to weld in a coal forge than a gas forge because you have higher temperatures more easily available. To weld well, a coal fire should be clean of clinker and ash and have a large mound of coke on top to produce an insulated cavern fire. You should also have an excess of coke on hand in case you have to repeat a weld to get it to stick. You will also need a good bed of coals below the work in the tueyer.
Start with one flat bar folded back on itself and closed tight. Scrape the scale off hot with a wire brush or rasp before closing tight. Scale doesn't weld and only contaminates the weld area.
Take the steel up to a good orange heat and sprinkle with a little flux (20 Mule Team Borax sold in most grocery stores in the laundry aisle, Easy Weld, Anhydrous Borax etc.) For a small weld, 2 inches long, about half a table spoon is plenty. Sprinkle both sides at the seam.
The flux creates a low temperature glass like coating that keeps the oxygen from the steel surface. Scale (iron oxide) will not weld. If in doubt it is often wise to grind the contact surfaces to bright metal, before welding, so it is a clean as it can be.
After it is fluxed and hopefully still at a dull orange the steel is placed back in the heart of the fire, not in the direct air blast. More air means a higher temperature but also means a surplus of oxygen that will create scale, and can prevent the weld, or cause the steel to burn at the high temperatures.
Bring the steel up to a lemon yellow colour (remember not to look into the fire. Buy dydymium glasses or welders green shade 3 or 4 and get use to the colour change) and the surface of the steel is shimmering and slippery looking. Don't confuse this with the lower temperature that the flux melts at and is sliding around. This temperature is just below the point that sparks are being given off. Sparks mean the steel is burning and it is losing its integrity.
Remove the bar and run a series of light quick overlapping taps over the area to be welded. If the bar is wide you will need to run a series working from the middle out to the edges so a pocket of flux is not formed. The hammer blows should be solid but not of the same force as if you were trying to change the shape of the bar. Thicker bars require more force to get it to penetrate to the weld area.
Be careful the hot flux may spatter in all directions. Keep flammables ( paper, varsol cans, lacquers etc.) away from your welding area. If the bar is still in the yellows run the same series of taps on the other side. You may have to reheat the bar to a welding temperature again, wire brush and flux again, to work on the other side.
You only have 1.5 to 2.5 seconds to complete the weld once the steel touches the anvil. You must be fast and have everything ready. A general rule of thumb is that you have 3 chances for a weld to take. If it still doesn't stick after three attempts the surface is probably too deteriorated for the steel to weld. Grind clean or better yet start with fresh bars.
Once you think the weld is complete hot cut it and examine your results. You should see a fairly uniform bar solidly attached. If you stress the weld, along the line of the weld eg. trying to knock the piece off it will usually shear at the weld line. This is not due to a bad weld but the change in composition of the two steels at the line of contact, creating a stress concentration at this point.
The inside should be shiny and bright not dull or have pockets of scale. These are signs of a poor weld. The more that you refine the weld or forge over the area the greater the chance the electrons have to jump to the neighbouring molecules and the better the weld becomes. This of course is if the weld is strong to begin with. Once your weld is completed, start any forging of this area gently and slowly build the force up.
Refering to Pattern Welding or Damascus steel which is usually used for knives the process is repeated with layers of different types of steel.
A simple billet on the first weld could consiste of 5 layers. Mild steel, High carbon such as O1, Mild steel, O1,Mild steel, each 1/4 of an inch thick an inch wide and perhaps 4 inches long. This makes a good starter pack. The mild steel helps protect the High Carbon inside.
Forge weld this into a block, draw it out and I usually cut into 3 equal pieces. Grind flats, stack and reweld. Now we have ms,o1,ms,o1,ms,o1,ms,o1,ms,o1,ms,o1,ms,o1,ms or 15 layers. Repeat the process and now you will have 45 layers repeat again and you will have 135 layers. The most striking patterns seem to be from 90 layers to 135.
The pattern comes from twisting the bar or cutting selected sections away (such as grooving with an angle grinder). Forge to shape,heat treat, grind smooth, etch with acid (observe acid precautions). Once neutralised and washed the pattern should be clear. The different steels etch at different rates giving the pattern.
This is a brief overview of the process and there are many tricks along the way but beautiful pattern welding has been done in recent years.