Dear sir, As the temperature increases, thermal motion, or entroppy ( the atoms move so violently that they are unable to line up properly and are no longer able to hold a magnet) competes with the ferromagnetic tendency for dipoles to align. When the temperature rises beyond a certain point, called the Curie temperature, there is a second-order phase transition and the system can no longer maintain a spontaneous magnetization, although it still responds paramagnetically to an external field. The Curie point, the temperature at which magnetic materials cease to behave magnetically, occurs at nearly the same temperature as the austenite transformation. This behavior is attributed to the paramagnetic nature of austenite, while both martensite and ferrite are strongly ferromagnetic (see the diagramm attached from S.L. Semiatin and D.E. Stutz, Induction Heat Treating of Steel, American Society for Metals, 1986).
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