After the plates that make up the individual billets that are used to form the core rods are prepared, the next step is to weld the blocks and draw them out to rough length. As a reminder, each of the stacks are made up of 9 layers of wrought iron / mild steel / L6 alloy, and are roughly 6" long by 1" wide and about 1 1/4" tall. Each is bound together with three loops of 'soft iron' fencing wire.
This first image shows the starting stack, the billet after welding and the rough drawn out core rod. For knives, I normally start with a stack roughly 4 inches long. My main coal forge has a good sized rectangular fire box, but even still, the six inch length of these stacks was about the longest that can be easily brought to an even welding heat.
For successful forge welding, the key is managing the fire. You can see here I have laid two lines of fire brick alongside the fire pot to further increase the size of the heat zone. The total height of the fire is roughly 12 inches, with at least four inches of burning fuel below the work being heated here. Use of a classic 'cavern' style fire is also essential to both evenly heating the metal and achieving the correct oxygen free atmosphere around it.
As soon as the stack has come up to a dull red heat, I generously flux all edges of the metal with borax. I personally use 'Twenty Mule Team' washing soda. This material still contains water in its matrix, so tends to bubble up a lot when first applied. This certainly DOES lead to more of a mess around the forge and quicker accumulation of heavy slag / clinker in the bottom of the fire pot. However, the washing borax is quite cheap (about $2 per pound) and easily available at the local grocery store. (For a longer discussion of Welding and Fluxes, see Wednesday, August 23, 2006 - 'Norse Meat Spit - Period Fluxes?'.)
This is certainly a dramatic image, if not the best illustration. Once the stack is fluxed, it is carefully brought up to an even welding heat. The stack is constantly rotated, as the lower surface in the fire is hotter than the upper. It is critical to a successful weld that the heat evenly penetrates the entire stack. A balance must be made between producing high temperatures in the forge, but at the same time not interjecting excess oxygen into the atmosphere surrounding the metal. Once the ideal temperature is reached (judged by colour and experience), the stack is quickly moved to the anvil and a series of rapid hammer strokes are worked down the surface on both sides. The pattern of these strokes not only 'tack welds' the loose plates, they also serve to sweep out the flux. This in tern floats out any debris or oxide that may have formed in between the individual plates. After 10 to 15 minutes of careful heating - the actual weld itself takes about 30 seconds! I like to do my first weld using a hand hammer, usually a 1000 gm / 2.2 pound for a combination of control and penetration.
(Sorry that I do not have an image of the actual welding. I was taking these photos myself while I worked, so they represent places in the ongoing process where I could stop for a couple of seconds to grab the camera.)
Just after the weld, while the block is still yellow hot, those binding wires should be removed. Remember these have been holding that loose stack of plates together during the initial heat to welding temperature. There is a bit of a trick to this - getting the thick stack to temperature WITHOUT burning off the much thinner wires. An alternative to using wires would be to MIG weld the plates together at the end. I'm more of a traditionalist - and the wire method is how I learned (and teach for that matter). Reguardless, the wires are now loosely welded into the flat side of the block. These are removed by grabbing the loose sides with a pair of pliers and then with a rolling motion pulling each set free. You could weld the wires into the block of course, but the way they project from the sides would make this difficult and messy. Removing the wires should be done as quickly as possible to preserve the heat in the block and reduce exposure to room oxygen (limiting scale formation).
Once the wires are removed, the block has the edges quickly wire brushed to remove old flux and any fire scale. If the hammer stokes of the first weld have been controlled and even, the sides should have remained relatively straight. The sides all round are fluxed again and the block returned to the fire. The block is once again returned to an even welding heat throughout. Careful observation during this heating will point out any areas were the initial weld may be imperfect. This will show in colour breaks through the material. Ideally there will be none of this, but if its obvious an area of the block is not welded, a second hand hammered weld may be required.
Now, I used to do my second consolidation weld using a heavier (1.5 kg / 3.5 lb) hand hammer. And then go on, again by hand, to flatten and draw out the welded billet. This process of a single weld and draw, undertaken alone, would take me roughly 2 1/2 hours. I could manage this heavy work three times over the course of two days - and then would have to take a half day off to rest. A couple of years back I invested in the first prototype of David Robertson's small air hammers. This tool, with the required air compressor, was not cheap. It does have the great advantage of not only speeding the time of drawing out, but more importantly to me (especially these days!) using MACHINE rather than MUSCLE power.
The second consolidation weld is thus taken under the air hammer. The force of the hammer ensures that the entire height of the stack is correctly welded. Starting at a welding heat also means a considerable amount of the drawing out also can occur in the same step. I work over the entire bar quickly to ensure the welds, then concentrate on drawing out using the residual orange heat.
This shows the relative size of that first weld to draw sequence. The upper stack is ready for the fire, the lower billet has been tack welded, consolidation welded with first draw. the width of both of these remains at roughly 1". You can clearly see how the thickness has been converted to length. The dark bands that show on the welded billet are the iron layers. The central bar is a core rod drawn to rough length for comparison.
I have been welding the prepared stacks two at a time. The process is to weld and run the first draw, then weld the second stack. Once both billets look pretty much as you see above, I will then finish up drawing the two to rough length. I never make it practice to attempt more than TWO billets welded as described in a single fire - or as a single work session. Both the fire and myself are too tired to perform correctly on a third weld series!
Here you can see the process of drawing the roughly 1/2 thick by 10 inch long billet out to a rough core rod. At this point I am taking each of the potential cores out to roughly 3/8 square by 24". Once all the stacks are welded, I will determine the actual length and dimension required and prepare them for the twisting process that is the hallmark of a pattern welded blade.
More to come...
Fascinating stuff. I can't wait to see the finished product.
ReplyDeleteHow did the Vikings pattern weld in the old days before powerhammers? For example, were river-powered triphammers common? Or did they just do it the old fashioned way- one guy holding the billet and one guy using a big hammer?