The ferrite-to-austenite phase transformation during water quenching of a duplex stainless steel was studied, where a duplex stainless steel was heated to 1370 degrees C (delta ferrite region) and quenched to room temperature. The microstructure consisted of coarse ferrite grains and fine needle-like austenite particles. The phase transformation mechanism appeared to be "diffusion-limited" displacive where shear was dominant, but also accompanied by prior or simultaneous diffusional elemental redistribution. A small fraction of the interfaces followed Kurdjumov-Sachs and Nishiyama-Wassermann orientation relationships (ORs) where austenite/ferrite interfaces terminated on {111}(A)//{110}(F) planes. The high undercooling associated with the fast cooling rates resulted in a considerable deviation from rational ORs. This was mostly due to the formation of intragranular austenite on Cr2N particles, which most likely caused a random OR with respect to the ferrite matrix. A detailed transmission electron microscopy (TEM) analysis revealed that the planar interphase boundaries characterised by the rational ORs typically contained one dominant set of parallel, regularly spaced dislocations. TEM analysis also showed the occurrence of small protrusions appearing on the edge/face of some austenite particles. Some of these did not leave a sub-boundary behind and formed a finger-like austenite morphology resulting from the instability mechanism. In some other cases, the protruded austenite possessed a low-angle grain boundary with the substrate austenite grain, which was the result of sympathetic nucleation of austenite on a pre-existing austenite particle.