A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 2 1 also subsequent processing using forging hearths. In other words, these primary and secondary processes could, and did, result in harder regions (owing, as we now know, to higher carbon contents) in blooms and subsequent products. Early ironsmiths were able to distinguish these harder regions, adapt thermo-mechanical treatments for producing medium and high-carbon steels, and selectively use these materials for the production of tools and weapons. This involved forge-welding and shaping selected portions of blooms, followed by grinding and polishing. An early example, a sword from the Hittite New Empire (1400 to 1180 B.C.) is shown in Fig. 4[13]. STEEL DEVELOPMENTS IN ANTIQUITY Since the discovery of steels much attention has been paid to weapons, culminating in high quality allsteel swords, made by empirically controlling their carburization levels, microstructures, and overall chemistry. These controls were exercised by heat treatments in reducing and/or oxidizing environments, and by thermo-mechanical working, including quenching and tempering. This means that steels have been produced intentionally from iron blooms, with varying but generally increasing success, for over 3000 years. Figure 5 schematically represents an intermediate stage from Roman times, when processing developments allowed various types of swords to be manufactured[14]. The long-term success of obtaining steel products from bloomery iron may be attributed to its workability, owing to its generally low carbon content. In turn this meant that smelters empirically favored production conditions yielding low-carbon blooms. However, during the latter part of the 1st century B.C. and into the first and second millennia A.D., the production of crucible steels was developed in India, several central Asian lands and Anatolia. These steels were typically derived from melting wrought iron with charcoal, but other additions such as wood and cast iron were sometimes included. The resulting ultrahigh-carbon (UHC) steels (having about 1.0 to 2.3 wt% C), though more difficult to work, were used to make heavy duty tools as well as weapons (swords and knives). In particular, UHC steels were used to make so-called Damascene steel swordblades, renownedfortheirsurface patterns. These patterns were achieved by complex thermo-mechanical forging and are due to the alignment of iron carbides into bands. The iron carbides and their distributions in the UHC steels resulted from solid-state precipitation during thermo-mechanical working. Figure 6 shows two artifacts, one a knife from Central Anatolia[15] and the other an ornamental plaque from Iran[16]. The corresponding microstructural images show the typical bands (a) (b) (c) (d) (a) (b) (c) Fig. 4 — (a) Hittite fire-welded iron and steel sword with cast-on bronze hilt; (b, c) etched crosssections of a sample, showing layers with differing carbon contents and stringers of elongated slag inclusions (arrowed). Adapted from Yalçin[13]. Fig. 3 — Heavily corroded objects made from bloomery iron: (a) axe fragments, Western Anatolia, Archaic Period[10]; (b) spearhead, Northern Iran, Iron Age III[11]; (c) etched and (d) unetched images of the corresponding microstructures, showing single phase (ferrite) metal and Si-Fe-Al slag inclusions. These microstructures are representative of worked bloomery iron, with locally varying slag contents and, as shown by the etched sample, non-uniform grain sizes.
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