The ice-water phase transition after an ultrafast temperature jump is studied in HDO:D2O (15 M) ice with use of 2-color IR spectroscopy. The OH-stretching vibration is applied for rapid heating of the sample and for fast and sensitive probing of local temperature and structure. For energy depositions beyond the limit of superheating (330 +/- 10 K) partial melting in two steps is observed and assigned to (i) catastrophic melting within the thermalization time of the excited ice lattice of 5 +/- 2 ps and (ii) secondary melting with a time constant of 33 +/- 5 ps that is assigned to interfacial melting at the generated phase boundaries. The latter process is found to consume energy amounts in agreement with the latent heat of melting and is accompanied by an accelerated temperature and pressure decrease of the residual ice component.