Formation Mechanism of Pearlite Colony by Multiple Orientation Relationships between Ferrite and Cementite

Abstract

To better understand the formation mechanism of lamellar pearlite, the orientation relationship (OR) between ferrite and cementite in spheroidized pearlite was analyzed using electron backscattered diffraction (EBSD) over a wide area and with high precision. Lamellar cementite was sufficiently spheroidized and coarsened, and substructures of lamellar pearlite, i.e., block and colony boundaries, were maintained even after a long period of spheroidized annealing. As a result, the cementite spheroidization facilitated the indexing of the cementite orientation in the EBSD analysis. It was consequently discovered that three different ORs-Bagaryatsky, Isaichev, and Pitsch-Petch ORs-were simultaneously established in the pearlite. Each OR deviated from the theoretical one, and the deviation was significantly larger in the Pitsch- Petch OR than in the other two. Also, the ferrite/cementite boundaries satisfying each OR were unevenly distributed, and the transition of ORs tended to correspond to colony boundaries with low-angle misorientation. From these results, the pearlite colony boundary was classified into three types: (1) the colony boundary maintaining an orientation relationship with different cementite variants, (2) the colony boundary characterized by two different ORs with low-angle misorientation, i.e., the transition between the Bagaryatsky and Isaichev ORs, and (3) the colony boundary with high-angle misorientation, i.e., the transition between the Isaichev and Pitsch-Petch ORs. In addition to the multiple variant formation, the coexistence of multiple ORs contributed to the change in lamellar alignment, with a high degree of freedom in the pearlitic transformation.