The effects of low temperature isothermal treatments on a quenched AISI D2 tool steel was studied using Barkhausen Noise, X-ray diffraction, dilatometry and optical and electronic microscopy. The specimens were austenitized at 1040°C and quenched in oil. The isothermal treatments involved immersion in hot oil baths at 80° or 130°C for 0.1, 0.6, 1, 3, 10 and 30 hours, except for the dilatometry, in which the specimens were submitted to a single thermal cycle for 30 hours. These thermal treatments are industrially known as “stress relief treatments”, and are used to prevent cracks and catastrophic failures during cryogenic treatments. The comparison of global and local (microscopy) measurements allows the discussion of the phenomena involved in the aging process.