Crystallography and morpholgy of cementite precipitates formed during rapid solidification of a ferritic stainless steel

Abstract A wedge-formed sample has been prepared for a low carbon Fe–12Cr based alloy by casting the base material into a wedge copper mould in a vacuum induction furnace. Cooling rates through the solidification temperature range were measured to be in the range from 3.7×103 to 105 K s−1 within the tip region of the wedge-formed sample (12 mm from the tip). Microstructural observations showed that a high density of carbide precipitates are formed in the delta ferrite grains within, and only within, this region. On the basis of sequential selected area electron diffraction patterns, the carbides were identified as cementite M3C (M=Fe,Cr). The cementite precipitates appeared as needle-shaped when viewed with an optical microscope. However, observations by transmission electron microscopy (TEM) revealed that the needle-shaped precipitates are actually elongated planar colonies consisting of two sets of parallel fine needles with an angle of about 70° between them. The colony planes of the planar colonies and the growth directions of the two sets of fine needles in the colonies were determined to be (100)C//{110}δ and [010]C// δ, respectively. The long dimension of the colonies was, in most cases, observed to align along the δ direction and in some cases along the δ directions on the colony plane. The orientation relationship between the cementite and the ferrite was determined to obey that of Isaichev.