Chapter Fourteen - Relevant Aspects of Carbon and Low Alloy Steel Metallurgy

Abstract

This chapter addresses carbon and low alloy steels, their composition, processing, transformation, and aging. The processing of iron from its ores, and the making of steel are immense subjects, complicated by environmental considerations and the availability of scrap. Iron ores are basically iron oxides, and the first step in the overall production process for steel occurs in a blast furnace. The charge to the blast furnace consists of iron ore, coke, and limestone. Preheated air is blown through the charge. Under steady-state operations, reactions occur. The pig iron contains about 3 to 4.5 % carbon. Although these carbon levels are excellent precursors for cast iron, they are much too high for steel. Hence, steel must be processed for carbon reduction in “steel-making” operations. A generally useful system of steel designation is the unified numbering system (UNS), which incorporates the traditional Society of Automotive Engineers (SAE) and American Iron and Steel Institute (AISI) systems. Plain carbon steel chemical specifications usually call out only four elements (other than iron): carbon, manganese, sulfur, and phosphorous. Carbon increases strength and, up to intermediate levels, increases hardenability. It is important to understand that carbon steels contain numerous additional “residual” elements in their composition. The majority of desirable steel property combinations can be developed with plain carbon steels, provided there is adequate processing capability or limited product dimensions. Alloying generally improves hardenability, or allows slower cooling to be undertaken en route to the formation of martensite. A great improvement in hardenability can be achieved with minimal alloying (a very few percent of alloying additions), and many low alloy steels or simply alloy steels, have been designed with this in mind.