Evolution of iron carbides during tempering of low-alloy tool steel studied with polarized small angle neutron scattering, electron microscopy and atom probe

The magnetic scattering of iron carbides in low-alloy tool steel was investigated ex-situ by polarized small angle neutron scattering measurements after tempering the steel at 550 degrees C and 600 degrees C. Magnetic features could be detected in the as-quenched sample resulting in a negative interference term, believed to be either 0-Fe3C, i-Fe2C, or e-Fe2-3C. During tempering the evolution of cementite could be studied by the variation of the interference term and in y-ratio, which is the ratio of the magnetic to nuclear scattering length density contrast. From scanning transmission electron microscopy (STEM) and atom probe tomography, it is evident that cementite (0-Fe3C) is present directly when reaching the tempering temperature of either 550 degrees C or 600 degrees C. At longer tempering times, cementite gets enriched with substitutional elements like chromium and manganese, forming an enriched shell on the cementite particles. STEM and energy dispersive x-ray spectrometry show that the chemical composition of small cementite particles approaches that of Cr-rich M7C3 carbides after 24 h at 600 degrees C. It is also seen that small non-magnetic particles precipitate during tempering and these correspond well with molybdenum and vanadium-rich carbides.