Pattern welding

[Dept of PMM Hall of Steel]

Pattern welding is the practice in sword and knife making of forming a blade of several metal pieces of differing composition that are forge-welded together and twisted and manipulated to form a pattern. Often incorrectly called Damascus steel (which is produced by a different process), blades forged in this manner often display bands of slightly different coloration along their entire length. These bands can be brought out for cosmetic purposes by proper polishing or acid etching. Originally, pattern welding was used to combine steels of different carbon contents, providing a desired mix of hardness and toughness. Although modern steelmaking processes negate the need to blend different steels, pattern welded steel is still used by custom knifemakers for the cosmetic effects it produces.
History
Pattern welding was developed in both Europe and Asia as a means of averaging the properties of iron and steel. Iron is too soft and flexible for use in weaponry, while the hypereutectoid steel produced in the ancient world was too hard and brittle. The earliest known use of pattern welding in Europe is from an 8th century BCE sword found at Singen, Württemberg in Germany (Salter & Ehrenreich 1984). The Celts originally employed alternating rods of iron and steel twisted together and then welded with the use of the bloomery furnace which can bring iron and steel (carburized iron) to a welding temperature, but not melt them. In Asia, the carbon content of the iron and steel produced in the bloomery process was highly variable, and repeated folding and welding was employed to remove excess slag and impurities from the metal and to homogenize it. Two or more batches of fairly uniform steel (each batch with a different composition) were then welded together. The different steels result in blades with improved mechanical properties and a distinctive wavy pattern being observed after etching. The technique first appeared about 300 BCE, and by 500 CE was being used by the Merovingian dynasty. Through their successors, the Carolingian dynasty, the technique became common throughout Europe by about 700 CE. The period of European pattern welding, then, encompasses the Celto-Roman Iron Age through the Migration Era. Introduction of the blast furnace brought it to an end.
By the 9th century in Europe, the blast furnace became available and with it the ability to create homogenous high carbon steel. The need for pattern welding all but died out for other than cosmetic applications. During subsequent centuries the technique was slowly lost, and by 1300 there are few examples of its use. The technique survived, however, in Scandinavia, where good quality iron ores and charcoal were widely available.
It was during this same period that Damascus steel was being produced in the Middle East, and similarities in the markings led many to believe it was the same process being used. Swords made by pattern welding are sometimes said to be "Damascus blades", although the manufacture of Damascus steel is entirely different. The terms "Damascus", "Damask", "damascening", etc. are complete misnomers that have nothing at all to do with European pattern welding technology.
Pattern welding again fell from use in Europe during the 18th century, when English metalsmiths developed the puddling furnace, and then rediscovered the Indian crucible-fired steels (wootz steel) which were far superior to any mechanical methods. By the 19th century pattern welding had largely disappeared, although today it is used in custom knife making.
The technique is more commonly associated with Japan and reached a high degree of development in the 14th century. This association leads some to incorrectly believe that pattern welding originated in Japan. There are also examples of pattern welding in weaponry from the pre-colonial Philippines and Indonesia.
Techniques
The term pattern welding is applied to a broad range of techniques which pursue different goals. Most modern pattern welded steel is done for purely cosmetic reasons using different types of high carbon steel or tool steel, both of which are well suited for the given purpose. Ancient techniques were more focused on blending certain qualities of the steels used to obtain better characteristics than was possible by only using one type of steel.
Altering carbon content
Steel made with the bloomery process, used throughout much of the ancient world, was of very poor quality for making knives and swords. The bloom itself consisted of sponge iron, a porous structure of nearly pure iron filled with pockets of slag. As the slag is worked out during forging, any traces of carbon is also burned from the metal, producing in the end wrought iron, with a very low carbon content. To achieve the desired carbon content for a blade (generally around 0.7 to 1.0%) carbon must be added back into the wrought iron, generally by means of carburization. Since carburization only penetrates the surface of the iron (providing, in essence, only case hardening) the thin veneer must be mixed back into the interior to form the desired high carbon steel. This is usually done by either stacking or flattening and folding the carburized bars. This creates the characteristic thin layers seen in pattern welded steel. Additional shapes can be made by differing how the bars were stacked or folded. Viking swords were often made with twisted bars, producing spiral patterns, and today many knifemakers use steel cable to produce complex, spiral-in-spiral patterns.
Conservation of high carbon steel
Since producing high carbon steel from wrought iron was very difficult, pattern welding also served to conserve this difficult to make steel by using it only for the parts of the blades where it was needed. Many swords were made with the minimum possible amount of high carbon steel along the cutting edge, with the rest of the blade being made of low carbon steel or wrought iron. A thin strip of high carbon steel could be laminated between two layers of softer steel, or a core of soft steel could be wrapped in high carbon steel. The Japanese katana was often found with complex patterns of soft and hard steels; having 5 sections of differing hardness welded together to form the final blade was not uncommon. The end result would be a blade with a very high carbon edge (as much as 1.0%, equal to the highest carbon content found in low alloy steels in use today) and a softer spine. The very hard, but brittle, edge made the swords extremely sharp, while the spine gave the blade flexibility, so that it would bend rather than break. The problem with the laminate design was that if the brittle edge steel chipped down to the soft core (a near certainty if swords were brought into edge to edge contact), the sword was ruined; grinding out the chip would expose the soft core and result in an unsharpenable blade. Correct technique for these blades was to parry with the softer, stronger sides or back of the blade, to protect the more fragile cutting edge.
A similar technique was also employed by Scandinavian Medieval swordsmiths. The Mora knife is today manufactured with a similar technique. Today the traditional crucible steel is seldom used, but the high carbon steel is usually tool steel or stainless steel.
Decorative finish
The ancient swordmakers also exploited the aesthetic qualities of pattern welded steel. The Vikings in particular were fond of twisting bars of steel around each other, welding the bars together by hammering and then repeating the process with the resulting bars, to create complex patterns in the final steel bar. Two bars twisted in opposite directions created the common chevron pattern. Often, the centre of the blade was a core of soft steel, and the edges were solid high carbon steel, similar to the laminates of the Japanese.
While "cable Damascus", forged from high carbon multi-strand cable, is a popular item for bladesmiths to produce, some modern bladesmiths have taken pattern welding to new heights. The American Bladesmith Society's Master Smith test, for example, requires a 300 layer blade to be forged. Large numbers of layers are generally produced by folding, where a small number of layers are welded together, then the blank is cut in half, stacked, and welded again, with each operation doubling the number of layers. Starting with just two layers, eight folding operations will yield 512 layers in the blank. A blade ground from such a blank will show a grain much like an object cut from a block of wood. A layered billet of steel rods with the blade blank cut perpendicular to the layers can also produce some spectacular patterns, including mosaics or even writing. Different treatments of the steel after it is ground and polished, such as bluing, etching, or treatments with various other chemicals that react differently to the different steels used can create bright, high-contrast finishes on the steel. Some master smiths go as far as to use an electrical discharge machining mill to cut interlocking patterns out of different steels, fit them together, then weld the resulting assembly into a solid block of steel.
References
• Ian Peirce, Ewart Oakeshott (Introduction), Swords of the Viking Age, 2004, ISBN 0-85115-914-1
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