Cast Iron: A Historical and Green Material Worthy of Continuous Research

Throughout history, cast iron has been unique amongst metallic materials. No other metal can boast such a long history, together with such a wide diversity of variants, properties, and applications. Arguably, no other material can claim to have such complexity. While the cast iron foundry produces myriad components, researchers and engineers have humbly ensured the continued development of this sophisticated material. We control this process not with furnaces and wirefeeders, but with knowledge. This knowledge enables the creation of a material with a unique combination of design flexibility, mechanical properties, wear resistance, recyclability, low life cycle energy consumption, and low cost. And it will be with the continued pursuit of understanding and knowledge that tomorrow’s researchers and engineers will ensure the continued growth of new material variants, with improved material properties and new applications that make the world a better place. Cast iron: thousands of years of development and progress behind us; thousands of fascinating mysteries and opportunities ahead of us.

Cast iron; History; Market share

Cast iron is an easy-to-shape material whose properties have evolved over the years in line with improvements in the technical and scientific fields. As of 2018, the various forms of cast iron represented 70% of the 110 million tons of total metal cast per year worldwide (10% for cast steel, 20% for aluminum and other alloys) (Census of Word Casting Production, 2019). Cast iron is a low-cost recyclable material with relatively low levels of pollution when compared to its present-day competitors. This is schematically illustrated in Figure 1, where so-called gray cast iron is compared with cast steels and aluminum alloys, in terms of price per MPa of yield strength vs. embodied energy (Figure 1a), and CO₂ footprint (Figure 1b). The latter two terms refer to energy used and CO₂ emitted, respectively, for the primary production, casting, and recycling of 1 kg of alloy.

Long before the dollar was established as a universal term of reference, and before aluminum had even been thought of, cast iron was already attractive for use in several applications in agriculture, domestic applications, and decoration. Cast iron is, in fact, a historic material that first appeared during the Iron Age, when the temperatures in furnaces became high enough for the processing of iron ore. It is therefore of first interest to summarize the evolution of cast iron materials, since its first inception up to the modern era, which we will do in the section to follow. As with other materials, over the last two centuries, several significant steps have been taken in the processing of cast iron, in casting technology, and in the cast iron itself. These are covered in the following sections.

Nowadays, cast iron consists of a family of materials, as depicted in Figure 2. Two main branches can be defined depending on the carbon-rich phase, which can be either cementite and other carbides, or graphite. Alloys within this former branch, also called white cast iron due to the color of their rupture surface, have high wear properties and good heat and corrosion resistance when alloyed but tend to be brittle. This branch, however, is a minor part of the cast iron family and most of the current production consists of gray (or graphitic) cast irons, in which the carbon-rich phase is graphite, giving a dark coloring to the rupture surfaces. The vast majority of these irons are based on Fe-C-Si alloys, and thus, can also be called silicon cast iron. This group of irons will be the focus of this paper. Ni-resist graphitic cast irons are heat and corrosion resistant, while very high-Si alloys are corrosion resistant. Behind the sorting in Figure 2 is a continuous evolution of cast iron alloys and their processing, as described in the section "Main Steps".