Metallographic observations on the formation and occurrence of ferrite, sigma phase and carbides in austenitic stainless steels. Part III. Electron microscopy studies

Abstract

Electron microscope techniques have been used to study the transformation characteristics of two austenitic stainless steels, an AISI Type 316 steel and an AISI Type 310 steel. In both, carbide precipitation preceded the formation of ferrite and/or sigma phase. In the Type 316 steel, ferrite nucleated at triple points and grew at the expense of the carbides formed earlier. At 800°C, the ferrite transformed in part to sigma phase after 1500 hrs. Intragranular sigma phase formed in this steel directly from the austenite after 600 hrs at 800°C. In the Type 310 steel, sigma phase precipitation followed carbide formation. The sigma phase formed at twin boundaries, along grain boundaries and intragranularly. Only occasional ferrite was detected. Initially, the carbides were uniquely oriented with the austenite matrix, { h 1 k 1 l 1} γ ‖; { h 1 k 1 l 1}M 23C 6, 〈 h 1 k 1 l 1〉 γ ‖; 〈 h 1 k 1 l 1〉 M 23C 6. This coherency was soon lost, however, and random orientations predominated. Unique orientation relationships were not detected between austenite and ferrite, austenite and sigma phase, or ferrite and sigma phase.