A critical review of impurity effects on the mechanical properties and cyclic fatigue of recycled ferritic steels

As the steel industry continues to shift towards increased use of recycled materials to reduce carbon emissions and energy consumption, understanding the impact of residual impurities on mechanical performance becomes critical. While many studies have examined how impurities affect steel's physical properties, the structural performance and cyclic fatigue life of recycled ferritic steels have not yet been comprehensively reviewed within a unified fatigue and microstructural framework. This study details the individual and interactive effects of nitrogen (N), copper (Cu), nickel (Ni), and tin (Sn) on hardness and, hence, their impact on fatigue service life of recycled ferritic steels. An existing approach to estimate the fatigue cycles to failure, considering the change in hardness due to the presence of impurities, has been used, assuming that the dominant defect-controlled fatigue crack initiation mechanisms remain comparable when residual elements are present in the steel. The change to using high-scrap steel input as feedstock for steel production may also be associated with a shift from a blast furnace (BF) to an electric arc furnace (EAF) route. A discussion on the effect of processing route (EAF compared to BF) on inclusion content and the potential effect of impurities on the inclusions is given. This paper demonstrates and discusses that while certain impurities, such as Cu and Ni, enhance strength through precipitation and solid solution mechanisms, others, when present at higher levels, such as Sn or N, may reduce fatigue resistance. This study considers their effect on bulk strength and fatigue performance, assuming that the crack initiation mechanism, whether from inclusion-induced nucleation, surface defects, or persistent slip bands, is not fundamentally altered. This assumption has been made, as currently, there is very limited data related to fatigue crack initiation mechanisms, which are identified here as an area requiring further investigation. This review, therefore, aims to bridge the gap between impurity-driven microstructural changes and cyclic fatigue performance in recycled ferritic steels. By combining microstructural properties with physics-based fatigue assessment approaches, this study provides a framework to assess whether recycled steels can safely be used in fatigue-critical applications.