Forge welding is second only to material preparation in importance in the creation of quality pattern welded steel. It is not however, any form of magic.
There is a lot of information out there about how to forge weld. Some of it good, but in my opinion, much of it leaves something to be desired. This is not to say that if you follow much of this advice you will meet with complete failure, but in my experience, some of it can leave you in a position to increase your chances of imperfections, inclusions, delaminations, etc. Below I will document my forge welding process. It’s not the only way to do it, it’s not the best way to do it. It is simply the way I do it. If you ask me about these processes in a year, they will likely be different. As experience is gained, things change. I will try to keep these pages up to date with my current processes though.
In this discussion on welding, I will be referring to welding in a gas forge as this is what I use and what I have great results with. This is not to say that excellent pattern welding cannot be done in a coal or charcoal fire, it is simply not the way I do it. I have forge welded in coal, and it is a very challenging skill to master. It is also very difficult to describe without being able to actually show somebody in person. As my goal is to produce consistent steel and to help you do the same, I will stick to describing the process with a gas forge.
Forge Atmosphere
The atmosphere inside of your forge plays a critical role in welding. The atmosphere inside the forge must be a reducing atmosphere. What this means is that there must be no free oxygen inside the forge as you heat your billet. With a gas forge, the best indicator of a reducing atmosphere is the “dragon’s breath” or flame coming from the front of the forge. You should be looking for 4-5″ of orange flame exiting the forge. What this indicates is that there is some unburnt fuel the has to exit the forge in order to find enough oxygen to burn. This means that the burn within the forge is consuming all available oxygen which leave no oxygen to cause oxidation and form scale on your steel that can prevent your weld from taking.
Flux
Flux serves two purposes in forge welding. It can dissolve scale that is already on your steel, and by sealing out oxygen, can keep more scale from forming. If you have followed my procedures for steel preparation, you will not need to count on flux to remove any scale. That means that flux will serve a single purpose in your welding. It will seal out oxygen.
There are a lot of opinions on what makes a good forge welding flux. This is one place where I firmly believe that simple is best. I use 20 Mule Team Borax as my welding flux. It is widely available in grocery and drug stores, inexpensive, and a single box will last you a very long time using my methods.
You may have heard that you should add things to your flux such as flourspar, steel shavings, sand, etc. There is no need for any of these. Flourspar is supposed to make the scale dissolving properties of flux more effective. Since we’ve already established that if we clean the surfaces first, we don’t need an agressive flux. I’ve never gotten a good answer as to how steel shavings are supposed to help, but anything that adds unwanted steels that could show up in the patterns of my billet later seem like a bad idea to me. Certain sands have been traditionally used as flux, but we’ve got something better in borax, so why mess with success?
You may have also heard that you must use “anhydrous borax” or you will run into trouble. Anhydrous borax is simply borax that has had any water cooked out of it. You can make it yourself by cooking your borax in an oven at 400-500F for an hour or so. Of course, the first time your anhydrous borax come into contact with moist air, it will no longer be anhydrous. My take on this is that we will be applying our flux to 1000F steel, so any water that is in the borax will very rapidly steam off. Basically, anything at 1000F is anhydrous!
Preheating
When I speak of “preheating” a billet, I am speaking of bringing the billet to a heat that is hot enough that flux will melt, but not so hot that scale can form on your steel. In order to get your flux to melt and properly coat your billet, the steel must be at about 1000F. Translated to color, this is a dull red heat as seen in a darkened forge shop.
Place your billet in the forge, keeping it out of the direct path of your burner flame if at all possible. The cone of flame exiting your burner will not only create a hot spot , but it is also the one spot in your forge that will have free oxygen that may cause scale to form. At this point, let your billet begin to heat, occasionally rotating the billet in the forge to help it heat evenly. Pull the billet out every few minutes to get a look at how hot it is. When you begin to see some color in the steel, it’s time to flux.
Here is a place in the process where my methods differ from those of many other ‘smiths. A lot of people will tell you to just “flux the heck out of it”. I will not. I apply my flux by lightly sprinkling flux onto the hot steel like you would sprinkle flour onto a counter before kneading dough. You need a complete coating, but it does not need to be a heavy coating. A heavy coating of flux simply wastes flux, can damage the interior of your forge, and in a worst-case scenario can become trapped inside your billet and create a nasty inclusion. Initially, the flux will foam up a bit as any moisture in the flux steams out. It will then begin to melt into a clear, glassy coating on the billet. If it doesn’t, place the billet back into the forge an heat a bit more. You will be able to see the flux melting on the billet in the forge. When it begins to flow on the surface of the billet, you’ve hit the right temperature. Pull the billet back out and flux any surfaces of the billet where two layers that are to be welded come together. Generally I will flux the sides of my billets as well as the ends. There is no need to flux the top and bottom as they are single pieces of steel. Once your flux flows and forms a thin, glassy coating on the billet, you are ready to heat to welding temperature.
Heating the billet
This is a time to be patient. If you don’t wait until your billet is fully heated, your weld will not set. I took some advice from Delbert Ealy and I keep a book in the shop. I’ll sit down and read a chapter in my book, then check my billet. If it is not hot enough, I’ll sit down and read another chapter, etc. until my billet is properly heated. As your billet heats, occasionally rotate it in the forge to help it heat evenly. By sitting a reading a book, I not only avoid under heating my billet by being impatient, but I also get some good reading done!
Now that you have an excuse to enjoy a good book, you need to know what you’re looking for in the forge between chapters. The best description I have heard of what your billet should look like when it’s ready to weld is “like a melting stick of butter”. It should be at a very high orange to yellow color and you should be able to see the flux bubbling a bit and “dancing” on the surface of your billet. At this point, you should generally wait a bit longer to make certain that the center of the billet is also up to temperature. the extra wait time is determined by the size of your billet. Experience will help you to determine exactly when your billet is ready to weld.
It should be noted that you really shouldn’t stare into the forge. Your forge puts off ultraviolet as well as infrared light that can damage your eyes permanently. It’s basically the same thing as looking at an electric welding arc, but the damage happens much more slowly. Ideally, you should use protective glasses designed to filter out the UV and IR, but if you don’t have them you should only quickly glance into the forge to see your billet. I will hold my hand out to block my view of the forge and quickly move it aside and back to give me a quick glance into the forge.
Preparing to set the weld
Before I move to weld a billet, I always make sure I do a little preparation of my work area. Because heat and time are critical, I want to make certain that I know exactly what I’m going to do as well as where all my tools are. Look around your work area. Make sure nothing is between the forge and the anvil for you to trip over. Make sure the face of your anvil is clean of scale. I have a place for every tool that I need in preparation for a weld. My hammer always sits at the same spot on the anvil, facing the same way, waiting for me to pick it up and set a weld. Take a few minutes and step through your motions. Step away from the forge, turn to your anvil, pick up your hammer, and pretend to strike a blow on the anvil. Once this motion is practiced, you will have one less set of variables to slow you down and cause you to lose heat in your billet when you’re ready to weld.
Setting the weld
Once your billet is to a proper welding heat, it’s time to set your weld.
I will mention speed several times in this section, but I want to make certain that you understand that accuracy should not be compromised in the interest of speed. Practice your motions and make sure that you can strike well placed blows, quickly, and with the intended force. When you come from the forge to the anvil with your billet, you want to be able to grab your hammer on the way, lay the billet on the anvil, and set your weld without any unnecessary delay.
One very common mistake I see from ‘smiths setting forge weld is hitting too hard. Despite what you’ve been told, or what you might think, you do not need to smash the heck out of a billet to set a weld. As a matter of fact, hitting a billet really hard to set a weld can cause more harm than good. For one, you will throw more weld splatter a longer distance which creates an extra fire hazard (trust me when I say that setting something 10 feet from you on fire, while you’re performing a public demonstration, with fire fighters watching, is more than a little embarrassing 
). Additionally, if you strike your billet too hard, you can cause the layers to slip sideways rather than weld.
When setting a weld, your blows should move rapidly from the handle-end of your billet towards the far end of the billet and be struck with medium force. The way I like to think of it in order to limit how hard I strike the billet is that I’m “pressing” the layers together with my hammer, not “hitting” the billet.
After the first pass from end to end on your billet, flip the billet over and run another pass of welding blows across the other side. At this point, set down your hammer (I usually simply drop mine), pick up a wire brush, and brush the flux and scale from the sides of your billet. Then, moving quickly to avoid the formation of scale, re-flux your billet. The re-application of flux at this point is mostly for safety, just in case a weld did not set completely.
With experience, you will be able to tell as weld is set if it took or not. There is a distinct feel of a weld that has been properly set. It feels more like hitting something with a dead-blow hammer than what you are used to in forging. In addition, there is a distinct sound that a good weld makes. It’s more of a “thud” than the ordinary ring of forging. I have on many occasions heard somebody try to set a weld behind me and I’ have been able to tell them whether it set or not.
At this point, place your billet back into the forge and let it soak at a welding heat for 5 to 10 minutes. Adding this post-weld soak really does seem to have a positive effect on the quality of the weld. I have heard different theories as to why this is, but until I can state with some certainty what is going on inside the steel, I’ll stick with “this works for me, but I don’t know why”.
After the post-weld soak, I will make one more welding pass for safety and then go on to test my weld.
Testing your weld
I have two ways in which I test my welds. Both are pretty simple and can be performed one after the other.
First, after soaking the weld in the forge and making my second welding pass of hammer blows, I will rotate the billet 90 degrees so it is sitting on its side, and run a pass of firm blows on the side of the billet. If you have a weak weld, this may very well pop it open.
If make it through the first test, I will hold the billet outside of the forge for a minute and let it begin to cool. As it cools, look at all sides of the billet and see if any individual piece of the billet cools at a different rate from the rest of the billet. If you see cooling at differential rates, odds are the demarcation between those rates of cooling is a bad weld.
Fixing a bad weld
Bad welds happen. No matter how much preparation you do or how careful you are, sometimes a weld will not stick the first time. You can also open up a weld by agressive forging, or forging too cold after welding. Don’t worry, all is not lost! As soon as you notice that a weld has opened, wire brush the area and flux it. Do not pass go, do not collect $200, go straight to cleaning and fluxing. Your goal here is to prevent any scale from forming inside of the failed weld. The flux will help a bit here as well. Once you have fluxed, go back into the forge and raise that area to a welding heat, and re-set the weld.