Puddling: A new look at an old process

Abstract

Throughout most of the 19th century, the process of choice in the west for producing wrought iron and steel was "puddling" of cast iron. A major development of the process, "wet puddling" became widespread commercially beginning in 1830. That process is examined quantitatively using modern thermodynamic, solidification and rheologic understanding. The 'clearing' and 'boiling' steps are considered in detail. In the 'clearing' step, the silicon is oxidized (by iron oxide) resulting in bath heating. Subsequent carbon removal from the melt, prior to initiation of solid iron solidification results first in the 'low boil', with simultaneous melt cooling. Then, as decarburization continues, solidification of iron begins, and with it, the 'high boil', with vigorous gas evolution and significant temperature increase. The importance of the iron solidification in promoting the decarburization reaction has not, in our opinion been fully understood heretofore. The lower the carbon content at the start of the boil, the higher the final melt temperature, and the lower the carbon content in the fully solid iron. During the high boil the four phase slurry of slag, liquid iron, solid iron and gas is thixotropic in nature, readily agglomerates, and possesses a low viscosity up to fractions solid of 0.2-0.3, with that viscosity rising rapidly at higher fractions solid. Comparison of the model presented with observations from the actual puddling process is made. The qualitative and quantitative model presented agrees in general outline with the historical record and serves to strengthen our respect for those master puddlers who could control such a complex process with little other than their senses to guide them. © 2009 ISIJ.