Nonclassical decomposition products of austenite in Fe-C-Cr alloys

Unusual austenite decomposition products in two Fe-0.4C alloys containing c hromium additions of 3.5 and 10 wt pet have been studied. Detailed transmis sion electron microscopy (TEM) has been carried out on partially transforme d specimens in order to determine the identities and morphologies of the ph ases and the mode of formation. The most descriptive terms for these novel products are spiky pearlite and acicular ferrite/carbide aggregates. The sp iky pearlite is distinguished by its nonnodular transformation front and by the presence of individual segments or units composed of ferrite-sheathed carbides. The acicular aggregates appear as dark-etching, macroscopic plate -shaped structures that are formed from the successive nucleation of these single ferrite/carbide subunits, which are crystallographically related to the austenite grain in which they grow, with a predominant orientation. The uniqueness of these structures has been reinforced by the detection of cus tomary pearlite in both of the alloys and by the presence of classical uppe r and lower bainites in the low-chromium alloy. It is proposed that the str uctures develop as a result of the oriented coupled growth of the individua l ferrite/carbide segments identified by the study.