To obtain high T-c superconducting granular materials with improved mechanical properties, each parameter of the sintering processes must be optimized either from experimental studies or modelling calculations. In the present work, composite samples have been prepared using mixed powders of steel and high T-c superconducting 2223-Bi phase (T-c = 110 K). The granular composite pellets with various volume fractions Phi of steel inclusions were sintered at various temperatures (T = 790 to 873 degrees C) and with variable sintering durations (t = 1 min to 24 h). Using X-ray diffraction and levitation measurements the degradation kinetics of the initial superconducting matrices is characterized. The existence diagram of the residual Bi-2223 phase, sintered in presence of steel, is thus described in a 3 dimensional (t, Phi, T) representation. Scanning electron microscopy (SEM) analyses have been used to evidence the interphase due to complex steel-matrix chemical reactions and to characterize the chemical diffusions. To better control the degradation process in composites, a model using a simplified Fick law is proposed.