Pattern welding

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Pattern welded pocket knife

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 (generally wrought iron and cast iron), providing the desired mix of hardness and toughness needed for highly demanding tasks such as cutting through armor. 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.

1 History
2 Techniques
3 External links and resources

[edit] History

Pattern welding was originally developed in Europe; the earliest known use is for decorative elements in Roman blades. 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 etching of the different steels results in blades with improved mechanical properties and a distinctive wavy pattern. The technique first appeared in AD 100-200, and by 500 was being used by the Merovingian dynasty. Through their successors, the Carolingian dynasty, the technique became common throughout Europe by about 700.

However, during the subsequent centuries the technique was slowly lost, and by 1300 there are no examples of its use. It is assumed it was because of rapid deforestation - charcoal production became difficult - and depletion of high quality ores; uncoked coal cannot be used in bladesmithing. 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.

Pattern welding again fell from use in Europe during the 18th century, when English metalsmiths discovered 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, where it reached a high degree of development in the 14th century. Today the Japanese katana is still considered by many to be the best sword ever produced, and is so famed that the technique of folding metal to form blades is still thought by many to be Japanese in origin. There are also examples of pattern welding in weaponry from the pre-colonial Philippines.

[edit] 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.

[edit] 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 cast iron, a very brittle form of steel containing over 2% carbon by weight. The bloom is also porous, and filled with pockets of slag. As the slag is worked out during forging, the carbon is also burned from the steel, 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.

[edit] 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 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 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.

[edit] Decorative finish

An enlarged and sharpened image, better showing the woodgrain-like form of the pattern welded blade.

The ancient swordmakers also exploited the aesthetic qualities of pattern welded steel. The Vikings in particular were fond of stacking and welding bars of steel, then twisting the bars, pounding the result into a square bar, and repeating. Two bars twisted in opposite direction created the common chevron pattern. Often the center 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 done 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 2 layers, 8 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 a polished, such as bluing, etching, or treatments with various other chemicals that react differenently 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.