Cast iron is widely utilized in the casting industry due to its excellent casting properties, cost-effectiveness, and good mechanical properties. Cast iron differs from steel by that the carbon content is much higher, usually more than 3 mass% carbon in cast iron compared to less than 2 mass% carbon in steel. Due to the high concentration of carbon in cast iron, carbon will form structures of graphite in the iron, which gives the material its characteristic properties. The graphite can be present in various forms, the two most common being long graphite flakes (in grey cast iron) and spherical graphite nodules (in spheroidal graphite iron). Cast iron components are typically used in as-cast condition, meaning control of the microstructure during solidification and cooling is of utmost importance to obtain the best possible mechanical properties.

Why is it important to study boron in cast iron?

Steel scrap comprises a large part of the iron sources for production of cast iron. The other largest sources of iron are crude iron coming directly from the blast furnace and cast iron returns from the cast iron industry. Increasing demands to the steel industry for stronger and better steels means that more trace elements are being introduced into the steels to meet these requirements. Consequently, there is a growing need to better understand how steel trace elements influence the microstructure of cast iron in order to reduce the need to mine new raw materials. While boron is frequently added as a trace element in steel to enhance hardenability, it has been found to have detrimental effects in cast iron. These effects include promotion of carbides, degeneration of nodular graphite shape, and negatively impacting the iron matrix. Hence, it is necessary to understand the mechanisms behind these effects to find potential ways to mitigate them or remove boron from the liquid iron so more boron-containing steel scrap can be used in cast iron production. 

The role of boron in low copper spheroidal graphite iron

To study the influence of boron on the microstructure of spheroidal graphite iron (SGI), an SGI alloy with low copper content was cast in cooperation with Elkem Silicon Products. The concentration of boron was varied systematically. It was observed with secondary ion mass spectrometry (SIMS) that boron primarily accumulated in two regions of the material: the last liquid to freeze and the surface layers of the spheroidal graphite particles. Carbides were detected in the material once the concentration of boron was high enough. These carbides formed in the last liquid to freeze and correlated with regions high in boron observed with the SIMS, suggesting these carbides are borocarbides. High concentrations of boron (around 500 ppm) led to a significant deterioration of graphite shape from spherical to lamellar, while lower concentrations of boron (approximately 25 ppm) caused small scale rugosities in the surface of the graphite nodules. These findings suggest boron can be incorporated in the graphite crystal structure and influence graphite growth. 

The article is available online here.