The effect of cooling rate on the microstructures formed during solidification of ferritic steel

This article describes in detail the effect of cooling rate on the microstr ucture of a low-carbon Fe-12 pet Cr alloy. The alloy was prepared using a r elatively simple technique, i.e., rapid cooling of the melt in a copper wed ge mold. The dependence of microstructure on the cooling rate (similar to 4 0 to 10(5) W S) has been determined by X-ray diffraction (XRD), microhardne ss measurement, optical microscopy (OM), and transmission electron microsco py (TEM). It has been found that the matrix structure over a large cooling rate range is composed of columnar ferrite grains, the size of which decrea ses with increasing cooling rate. Precipitation of second phases has been o bserved at either the ferrite grain boundaries or within the ferrite grains . The former takes place along the entire wedge sample, whereas the latter characterizes a region 12 mm away from the tip of the wedge sample. The ess ential structure of the grain boundary precipitates was identified as marte nsite, which is a transformation product of austenite precipitated at high temperatures. Retained austenite was identified at the tip region as isolat ed particles (<4 <mu>m). The precipitates within the ferrite grains appeare d as planar colonies consisting of two sets of needles. The density of thes e precipitates increases with increasing the cooling rate while their size decreases. Characteristic precipitate-free zones (PFZs) at the ferrite grai n boundaries were observed and are discussed.