Laminated steel blade

A laminated steel blade or piled steel is a knife, sword, or other tool blade made out of layers of differing types of steel, rather than a single homogeneous alloy. The earliest steel blades were laminated out of necessity, due to the early bloomery method of smelting iron, which made production of steel expensive and inconsistent. Laminated steel offered both a way to average out the properties of the steel, as well as a way to restrict high carbon steel to the areas that needed it most. Laminated steel blades are still produced today for specialized applications, where different requirements at different points in the blade are met by use of different alloys, forged together into a single blade.

Piled steel developed out of the necessarily complex process of making blades that were both hard and tough from the erratic and unsuitable output from early iron smelting in bloomeries. The bloomery does not generate temperatures high enough to melt iron and steel, but instead reduces the iron oxide ore into particles of pure iron, which then weld into a mass of sponge iron, consisting of lumps of impurities in a matrix of relatively pure iron, with a carbon content of around 0.06%. The bloom must then be heated and hammered to work out the impurities, resulting in the relatively soft wrought iron.

Iron is too soft to make a good cutting edge; a good edge requires the addition of carbon to make steel. By heating thin iron rods in a carbon-rich forge, carbon could be added to the surface, making a thin layer of steel on the surface through a process called carburization (see also case hardening). From the beginning of the Iron Age, around 1200 BC, piled steel was the only way to get good steel. Obtaining the right level of carbon was an art, and was very important to the finished product. Too much carbon, or too many of the wrong trace elements, and the resulting steel becomes too hard and brittle, which can result in a catastrophic failure of a sword; too little carbon and the sword will not hold an edge. The ideal sword is one with a hard, sharp edge, and tough enough to bend, but not to shatter.

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