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 the volume fractions of the phases austenite and ferrite and their geometrical arrangement in the dual-phase microstructure. In the as-received condition the mechanical properties of the material are highly anisotropic. By means of heat treatments microstructures of varying austenite and ferrite contents are formed. Anisotropy in the mechanical properties is less pronounced for alloys of high ferrite content. Using the Hall-Petch analysis of the flow stress, the contributions of grain and 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 partitioning of nitrogen between the two phases. The different deformation behaviour of austenite and ferrite causes cleavage fracture in ferrite even at low macroscopic stress levels. These cleavage cracks then grow causing severe plastic deformation of the surrounding austenite grains.