PLASTICITY AND FRACTURE OF AN AUSTENITIC- FERRITIC DUPLEX STEEL

The mechanical properties of the nitrogen alloyed austenitic-ferritic duplex steel X3 CrMnNiMoN 2564 are determined by the grain sizes and t he volume fractions of the phases austenite and ferrite and their geom etrical arrangement in the dual-phase microstructure. In the as-receiv ed condition the mechanical properties of the material are highly anis otropic. By means of heat treatments microstructures of varying austen ite and ferrite contents are formed. Anisotropy in the mechanical prop erties is less pronounced for alloys of high ferrite content. Using th e Hall-Petch analysis of the flow stress, the contributions of grain a nd phase boundaries to the material's strength can be separated. It is shown that the efficiency of the gamma/gamma grain boundaries and the alpha/gamma phase boundaries is influenced markedly by the partitioni ng of nitrogen between the two phases. The different deformation behav iour of austenite and ferrite causes cleavage fracture in ferrite even at low macroscopic stress levels. These cleavage cracks then grow cau sing severe plastic deformation of the surrounding austenite grains.