Surface matters: Decarburising wootz crucible steel ingots

Abstract

Wootz, the Indian crucible steel, is a hypereutectoid iron–carbon alloy and famous for its outstanding qualities. Due to the paucity of archaeological and historical ingot finds and conservative sampling strategies, discussions of the homogeneity of such ingots and the microstructural representativeness of samples have remained generic and assumptive. Thus two major shortcomings in the study of crucible steel ingots include the determination of their absolute carbon content and its relative distribution across the ingots. The recent discovery of a large hoard of wootz ingots from Telangana (Jaikishan et al. 2021) offered a unique opportunity to study their microstructure and determine their carbon content.

Reports based on traditional metallography suggest a wide carbon range, from 1 to 2 wt% carbon, for similar ingots (Scott 2013). Recent work based on image analysis (Desai and Rehren 2023) offered narrower carbon estimates (about 1.8 wt%) for several of the recently discovered ingots, with some variation in concentration towards the edge of the samples. As a collaborative effort to determine absolute carbon values and potential uneven distribution of the carbon in the Telangana ingots, traditional metallography was coupled with laser-induced breakdown spectroscopy (LIBS). Beyond documenting the microstructure across several ingots, the study provides macrostructural evidence of rim decarburisation, which we believe to be intentional. This study presents the micro- and macrostructure of two of the hypereutectoid Telangana ingots, highlighting the skill of the craftsmen in decarburising the outer surfaces of their ingots, potentially for ease of subsequent forging.