The effect of temperature on microstructure, hardness and toughness on the U-noted notch of heat-treated AISI D2 steel was studied in thermal cycles corresponding to Simple Austenization at 850°C, 950°C and 1100°C and Double Austenization at 850-850°C, 850- 950oC, 850-1100°C, 950-850°C, 950-950°C, 950-1100°C, 1100-850°C, 1100-950°C and 1100-1100°C. Prismatic specimens of 0.6 x 0.6 x 5.5 cm were prepared and heat treated on an L75 Series Premium Dilatometer, temperature control and cooling speeds of the thermal cycle was performed using the dilatometer software. The specimens were subsequently prepared under ASTM E-23 for the Charpy U-not test and where the hardness was measured in HV20 under ASTM E 92. Under ASTM E-93, samples were prepared for metallographic analysis and grain size was determined using optical microscopy in a first stage a representative mesh of the metal gra...