Cryogenic study of the magnetic and thermal stability of retained austenite in nanostructured bainite.

ABSTRACT

First magnetic characterization of a recently developed generation of carbide free bainitic steels, known as Nanobain, has been performed. Stability of its retained austenite at cryogenic temperatures has been studied by means of X-ray diffraction, microscopy, dilatometry and magnetic measurements. Two morphologies for this phase (blocky-type and film-type) appear in a different proportion depending on the chemical composition and the applied thermal treatment. Inhibition of the martensitic transformation, when decreasing the temperature down to -271°C, has been observed in those microstructures with higher proportion of film-type austenite. The paramagnetic state of austenite at room temperature seems to lead to different magnetic behaviors (ferromagnetic, antiferromagnetic) at cryogenic temperatures (TC or TN being around -23°C in all the studied samples), depending on the proportion of such morphological features. Furthermore, irreversibility with temperature on the evolution of such magnetic behaviors has been observed for all the studied bainitic structures and is proposed to be due to a magnetic proximity effect.