Modern blacksmiths, popular in our time, knives made of Damascus steel are made by the so-called pattern welding method..
This method is, that several alternating strips of two different steels are forged forge welding in a kind of intertwined bundles, which overlap themselves several times. After forging into a flat blade of a knife or sword, it turns out about 200-400 layers of these two alternating steels.
The pattern on the surface of these knives arises because, that one layer is etched darker, than the other. The numerous intersections of these different words give bizarre patterns on the surface of the steel., whose beauty depends on the skill of the blacksmith. This patterned steel has been known since the second century AD..
The name of this Damascus steel comes from a different type of steel, which is completely different from this patterned steel, and appeared around the same time. These blades are considered real or genuine Damascus steel., because the name itself comes from them.
The real Damascus blades were named after the city of Damascus in modern Syria.. The thing is, that Western Europeans first encountered Damascus swords during the Crusades and they thought, what exactly are they made in Damascus.
Real Damascus Steel
Almost all Damascus swords, which are located in the arms departments of large modern museums, are genuine damask swords. Figure a) shows a sketch on a museum damask sword. Genuine Damascus steels were made from small ingots of very pure iron with a carbon content of about 1,5-1,7 %. Such small steel ingots were produced in large quantities in India.. This steel was called "wootz". The manufacturing method of these ingots and the mechanism for the formation of surface patterns during forging were unknown until recently..
Pattern - Genuine Damascus Steel
and) Surface drawing on an authentic damask sword 17 century
b) Longitudinal microsection from a Damascus sword, shown in figure a). IN pickling cementite particles appear dark, and the pearlite matrix is white.
Damascus patterns - carbide chains
The surface patterns of real Damascus steel are the result of carbide banding, as shown in the longitudinal section of the same sword in figure b). Microstructure shown at original magnification 90x. The dark streaks are composed of Fe cementite particles3C, which have gathered in strips as a result of the forging process. Surface patterns are due to the phenomenon of carbide banding.
Patterns from microsegregation
This banding is the result of microsegregation of carbide-forming elements, such as vanadium and manganese, which, as known, were present in the original "wootz" steel ingots. During repeated heating and cooling cycles of the forging process, carbides, which were in the original ingot, gradually dissolved and changed shape. Microsegregated vanadium and manganese atoms force carbides to move into interdendritic planes. In the process of forging, these interdendritic planes are repeatedly unfolded into the forging plane..
As seen in Figure b) the alignment of the carbide strips in the finished blade is almost perfect. They look, as if mechanically aligned, as in the case of stripes in Damascus steel, received patterned welding.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007