Analysis of ferrite nucleation on single crystal oxide to understand acicular ferrite formation in steel welds

摘要

To obtain finer microstructures in heat affected zone of steel welds, it is well known that Bl-type compounds are effective as a nucleating agent for the austenite-to-ferrite transformation in steel. In this study, the nucleation behavior of ferrite from a B1 -type compound was analyzed using single crystal Fe alloy particles formed on low-index planes of single crystal MgO substrate. Thin film of pure iron or Fe-Ni alloys with various Ni concentrations was deposited on the MgO substrate, then heated to obtain spheroidized, submicron single crystal austenite particles, and cooled down to cause the ferrite transformation in the particles. It was confirmed by EBSD that each particle was single crystal at both austenite and ferrite stages, implying that only one nucleation of ferrite occurred at the austenite/MgO interface. It was shown that ferrites having Baker-Nutting (B-N) orientation relationship with MgO form preferably on (100) MgO. It was also revealed by combining XRD measurement that the number fraction of ferrites having B-N orientation relationship with the (100) MgO decreased as the transformation temperature decreased due to the increase of the cooling rate or Ni concentration in alloy.