Appendix III

Metalurgical analysis

Metallographic examination. This method of surface analysis can be used to distinguish older and newer manufacturing methods and therefore identify metal produced after a general time-period. But often these time-periods are too general and vary from area to area.

In Europe , carburization of wrought iron, to produce harder steel was employed during the Iron Age. Since it was only possible to carburize a thin surface layer, the technique of forge-welding together a number of thin strips of carburized iron, to produce a piled or laminated structure, was also developed during the Iron Age (c. 1100 B.C.).

Carburization and the making of steel:

The microstructure of wrought iron, as seen in polished etched sections, consists of grains of iron, known as ferrite, crossed by fibrous inclusions of slag. Such iron is too soft for knives and needs to be hardened by heating in glowing charcoal at temperatures above 900 degrees C.

This process, known as carburization, converts the wrought iron into steel by allowing it to absorb carbon, the concentration of carbon in the iron-carbon alloy (i.e. steel) being in the range of 0.5 to 2%. The carbon combines chemically with some of the iron to form iron carbide, which results initially in a microstructure that is a mixture of ferrite and pearlite grains. With increasing concentrations of carbon (i.e. greater than 0.85 percent), the microstructure converts to a pealite and cementite mixture (Tite 1972).

Temporing

The steel produced by carburization possesses an additional important property in that it can be hardened still further by heating to a temperature above 750 degrees C in a reducing atmosphere and then quenching rapidly in cold water. This process eliminates the pearlitic structure and in its place a new phase, martensite, which is a supersaturated solution of carbon in iron and has a needle-like structure, if formed. The quenched steel, although very hard, is distinctly brittle and liable to fracture.

However, the brittleness can be reduced, with some loss of hardness, by reheating to a lower temperature (around 450 degrees). This process, referred to as tempering, converts the martensite into a new phase, sorbite, which is a fine dispersion of cementite in ferrite.

Identification of the various phases in iron-carbon alloys (i.e. ferrite, cementite, pearlite, martensite, sorbite) is possible through the examination of polished etched sections under reflected light. Consequently the processes (i.e. carbonization, quenching, tempering) used in the manufacture of iron and steel artifacts can be ascertained (Tite 1972).

KWAH’S DAGGER                          
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