Delta-ferrite recovery structures in low-carbon steels
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9/11/03
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Delta-Ferrite Recovery Structures in Low-Carbon Steels R.J. DIPPENAAR and D.J. PHELAN The development of delta-ferrite recovery substructures in low-carbon steels has been observed in-situ utilizing laser scanning confocal microscopy (LSCM). Well-developed sub-boundaries with interfacial energies much smaller than that of delta-ferrite grain boundaries formed following transformation from austenite to delta-ferrite on heating. It is proposed that transformation stresses associated with the austenite to delta-ferrite phase transformation generate dislocations that subsequently recover into sub-boundaries by a process of polygonization. Experimental evidence in support of this proposal was found in a ferritic stainless steel. Thermal cycling through the high-temperature delta-ferrite/austenite/delta-ferrite phase transformation leads to the development of a well-defined recovery substructure, which, in turn, modifies the low-temperature austenite decomposition product from Widmanstätten to polygonal ferrite, with a commensurate change in hardness.
I. INTRODUCTION
ALTHOUGH it is generally conceded that the early stages of solidification and subsequent high-temperature phase transformations profoundly influence cast structure, conclusions have mostly been drawn from indirect experiments and very little work has been done on the direct observation of events. A fundamental understanding of the events occurring in the meniscus region of high-speed continuous casters, which determine the quality of the cast product, is of special interest. The delta-ferrite to austenite phase transformation occurs when the newly formed steel shell is relatively thin. The volume change and differences in thermal expansion of the phases may generate stresses, which, if the strength of this thin shell is exceeded, can lead to casting defects. Moreover, the delta-ferrite to austenite phase transformation may also play a role in the subsequent decomposition of austenite and, through this, the microstructural development on further cooling and, hence, the mechanical properties of the final product. The final alpha-ferrite grain size following decomposition from austenite in plain carbon steel is largely controlled by the grain size of the parent austenite because austenite grain boundaries, particularly grain corners, are the preferred sites for the nucleation of alpha ferrite.[1–4] A smaller austenite grain size will lead to refinement of the alpha ferrite grain size. In strip casting, and to a lesser extent in thin-slab casting, the opportunities for control of the microstructure through thermomechanical processing[5] are restricted. Therefore, the exact way in which the deltaferrite to austenite phase transformation occurs following solidification becomes increasingly important.[6] An impediment to success in previous studies has been the inability to study this high-temperature transformation directly because subsequent phase transformations mask the transformation mode. A second obstacle to a detailed study of
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