The use of magnetic Barkhausen (MBN) noise methods to detect the austenite to martensite phase transformation during cooling of steel specimens was explored, using three different configurations: conventional Barkhausen noise emission stimulated by an alternating magnetic field, the Okamura method, that measures magnetic noise emission under a fixed DC field and a novel experimental technique that measures spontaneous magnetic emission during transformation, in the absence of any external field. The phenomena associated with the phase transformation were also followed by electrical resistivity measurements and the resulting samples were characterized using optical and scanning electron microscopy. MBN measurements on a AISI D2 tool steel austenitized at 1473K and cooled to liquid nitrogen temperature exhibited a clear change near 225 K during cooling, corresponding to the Ms (martensite start ) temperature, as confirmed by resistivity measurements. Spontaneous magnetic noise emission measurements made in situ during cooling of a sample immerged in liquid nitrogen showed that individual “burst” phenomena could be detected, in a manner similar to acoustic emission (AE) measurements. This magnetic emission (ME) can thus be considered a new experimental tool for the study of martensite transformations in ferrous alloys.

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.