Friday, April 16, 2010

Some Layered Steel Billets

One of my current commissions is for a heavy hunting knife, using the pattern welding technique.
Note to readers that I use 'pattern welding' to specify the Northern European method of twisted rods, the 'archaeological definition'. Contemporary knifemakers typically use the term to refer to a flat stack which has been modified by punching or cutting, then flattening. These methods are normally is used to produce regular, geometric patterns on the finished surface.

There have been a number of past articles posted here on Hammered Out Bits on this topic (search : 'pattern weld' ) . There is also a discussion on the Wareham Forge Bladesmithing page.

(The click out images here are all direct scans of the billets, at life size.)

The Northern European method appears to have been developed specifically for the manufacture of swords, although some knives do exist using the technique. The finest example of the method is found in the sword from the Sutton Hoo burial, which dates to roughly 625 AD. It is suggested that blade was forged in Denmark. Pattern welded swords are relatively common through the Viking Age, but the technique falls out of use into the beginning of the Medieval period (very roughly : post 1000 AD).

Historic pattern welded blades typically have the twisted rods with layer counts at either 7 or 9. Although some writers attempt to make this mystical, the reason is purely functional. When forge welding, it is important that all the component pieces are brought to the same temperature. You want your stack to be as wide as it is high. In practical terms, the ideal size for this stack is between 1 to 1 1/2 inches wide and tall. If you forge out the component plates, they are likely to be in between 1/8 to 3/16 thick. Simple addition shows the most likely number of plates in the stack is thus going to range between 7 to perhaps 11. (So much for the 'mystic 9 for O∂in'!)

When making any layered steel object, the measurement of complexity is the number of WELDS - not the number of LAYERS. Remember that layer count increases geometrically. Four welds of a flat stack, drawn to four pieces is this 9 x 4 x 4 x 4 = 576 layers. (Add just one more weld and the count is over 2300 - so much for the mythic Japanese sword.)

Over the years, my own experience has shown the best way to combine the visual impact of the layered and twisted rods with a highly functional tool.
I find the most interesting layered effect for the twisted rods is found with counts in the range of 50, so on the second weld series. (Historic PW swords typically stop at the first weld, at 7 or 9 layers. This is because the gross effect of combining hard for spring and soft for shock absorbing, was desired.)
Remember that the layering combines both hard and SOFT alloys. The net effect is LOWERING the average carbon content, and that unevenly between individual layers. For that reason, I form the decorative back separately from the functional cutting edge. For the edge I take two layered billets and weld them to a core made of high carbon steel. This hard layer then creates the cutting edge of the finished blade.

Because of the method, I count the layers of each twisted rod, then add the total layers in the edge stack. Typically the process involves a total of four welding series:
1) Initial stack
2) Draw for either 4 pieces and weld (or if three pieces make a third draw and weld here)
- 2/3 is drawn and twisted / 1/3 is flattened
3) weld the two flat pieces to a carbon core
4) stack the twisted rods and blade block and weld
You can see that this requires more welds (and more difficult techniques) than a simple fold and weld method - at the cost of in effect LOWERING the layer count.

I will typically use four different alloys in the stacks:
I - Antique Wrought Iron - produces a 'rope like' texture (usually 3/16)
H - High Carbon Steel - produces black lines (usually 3/16)
L - L6 Alloy - produces bright lines (usually 1/32)
M- Mild Steel - produces a medium grey colour, also acts as a buffer between the different consistencies of the other metals (usually 1/8)

Image above shows the first billet I created for the project. It is composed of a total of 205 layers. That is two twisted rods on the back, then two flat layer blocks applied to a carbon steel core for the edge. The starting stack was : M/I/M/L/H/L/M/I/M . Four working sessions to produce (so basically four separate half day sessions - for me, anyways)

Now, although this is NOT the point of this article, Check the top image in the set below. The commission required the blade have an integral guard. I carefully hot slit the billet at the mid way point to provide the required material. And then realized I had pulled the guard off the WRONG SIDE. In effect, destroyed the week's work! Too much occupying what is left of my mind...

Start over again!

The bottom most billet here is the replacement for the screw up of my first attempt.
The starting stack here was a bit different than the first : M/H/L/M/I/M/I/M/L/H/M . It is similar in overall construction to the first (two twisted rods plus two flat layers with carbon core). I had also increased the overall length of the starting stack to 5 1/2 inches. This increased the finished billet size by about 20 %. The total layer count here is 265. You can see how increasing the starting amount of both wrought iron and carbon steel have increased the contrast in the herring bone pattern. For the finished blade forging, most likely the left side as shown here will be the point side.

The middle billet was a spare that I also worked up , 'just in case'.
I'm not sure of the exact starting composition, but given its appearance under the quick surface etch, I suspect there was no wrought iron in the mix. With the layer count as a flat stack at 213, I worked the surface of both sides over with a very sharp cross peen. The natural alignment of the billet and hammer resulted in a series of sharply defined diagonal strokes, the direction repeated on the opposite side. I then took the angle grinder and flattened the bar to remove the impressions . This effectively cuts through the raised portions, and creates the irregular diagonal pattern seen here. The finished billet was then drawn and cut in half and welded to a full carbon steel core. The end result is a 427 layer billet rendered in a more standard 'damascus' style. (This billet available to be forged into a custom blade, should any readers be interested.)

Expect to see some images of the finished knife in about a week.

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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