Effect of chemical banding on the local hardenability in AISI 4340 steel bar

Abstract

The microstructure of a 240mm diameter AISI 4340 mill shaft, quenched and tempered, was analyzed through its longitudinal cross section. Normally, higher cooling rates promote martensitic structure while the diffusion assisted austenite decomposition products are expected in the lower cooling rate condition. The martensitic reaction takes place inside the grains while the products diffusion starts from the prior austenite grain boundaries, nucleation and grow processes. An anomalous hardenability behavior was reported with a mixed structure containing martensite and bainite, in different proportion from the surface (higher cooling rate) to the core (lower cooling rate). This behavior was attributed to the wide banded structure. Banding due to the as cast structure segregation, is directly related to the solidification rate as well as the deformation degree due (during) to forging and rolling operations. When the deformation degree is high, the banding thickness is thin and approaches the austenite grain size. In this study the thickness of the bands embedded about four to five austenite grain sizes, in such a way that every band behaves like a particular steel composition. A semi quantitative chemical analysis was carried out through the banded structure to understand the differential hardenability behavior. The results were discussed using classical hardenability formula as well as calculated TTT diagrams for each composition.