The aim of this study was to determine the optimum conditions of the DL EPR test in order to evaluate the highest values of the degree of sensitization (DOS) of a forged austenitic stainless steel type 316L evaluated by the ratio Ir/Ia or Qr/Qa in %. The criteria of sensitization to the IGC corresponds to Ir/Ia > 1% and Qr/Qa > 1%. A model using a full factorial design has been established, and the selected factors were the sweep rate (dE/dt), sulphuric acid concentration (H2SO4), ammonium thiocyanate concentration (NH4SCN), and temperature in °C. A first order model is obtained by using a 24 full factorial design. An experimental test carried out using a 24 factorial design indicated that all the factors and their interactions have a positive effect on the response Ir/Ia. Furthermore, the highest value detected of Ir/Ia was found to be 65.6%. On the other hand, a microstructural study based on an optical microscope and a scanning transmission electron microscope (STEM) has been carried out for annealed and aged samples (550°C–80000 h). The profiles of the chromium, nickel, and molybdenum concentration established by X-ray microanalysis in STEM confirm a decrease of the content of these elements leading to the formation of Cr-depleted zones responsible for the sensitization to the IGC.