Abstract
This case study investigates the effect of microstructural banding on hardenability in a large diameter (240 mm) AISI 4340 steel mill shaft that exhibited mixed microstructures (bainite and martensite) in both surface and core regions after quenching and tempering. Longitudinal evaluation of the bar revealed a coarse banding structure with band thickness corresponding to approximately four to five austenite grain sizes. While the hardenability multiplying effect of alloying elements is well documented, this study demonstrates a surprising phenomenon where microstructural banding significantly influenced hardenability across the entire cross-section of the water-quenched bar. Chemical analysis confirmed that compositional variations associated with the banding were responsible for the differential hardenability observed, resulting in alternating martensitic and bainitic structures from surface to core despite the severe water quenching conditions. These findings highlight the importance of controlling segregation and banding in large-section alloy steels to achieve consistent microstructural properties throughout critical components.
