Crystallographic relations during decomposition of the ferritic phase by isothermal ageing of duplex stainless steel

In highly alloyed and duplex stainless steels the range of alloying elements leads to many different phases precipitating at higher temperatures. Duplex stainless steels consist of almost equal ratios of austenite and ferrite, and between 923 and 1273 K the ferrite begins decomposing into secondary austenite (γ2) and the σ phase. Several orientation relations between the austenitic, ferritic and σ phases have been determined by other researchers. The calculation and testing of mathematical expressions for these orientations are important for a close understanding of changes in duplex steel hardness, ductility, and other qualitative measures imposed by annealing or heat ageing. The method described in this article also offers an approach for determining parent phase orientations from inherited orientations in other metallic microstructures. When the orientation relations of adjacent grains calculated from mathematical equations and those measured by electron backscatter diffraction were compared, naturally it was found that the average orientation differs less between grains that inherit matrix structure from common parents. However, it was also found that the degree of difference depended on the variants involved in the orientations. This phenomenon can be explained by features of the microstructure and decomposition of the ferritic phase: initially the microstructure contains only primary austenite (γ1) and ferrite, then after a while it contains [beside primary (γ1) austenite] increasing amounts of secondary (γ2) austenite and the σ phase, and decreasing amounts of ferrite. The presence of two variants of austenite makes it difficult to verify parent relations for secondary (γ2) austenites.