U-notched impact toughness and internal friction curves were measured for steels 0·4C–2Mn, 0·4C–2Mn–Mo, and 0·4C–Cr, 0·4C–Cr–Ni, 0·4C–Cr–Ni–Mo after different heat treatments. It was found that in these steels temper brittleness always corresponds with the decrease in Köster peak height. The mechanism of the phenomenon has been suggested as being the result of an aging process of α solid solution with precipitation of Fe3C(N) particles causing ‘dead pinning’ of dislocations. The effect of molybdenum in diminishing the susceptibility to temper embrittlement may be considered as being due to its high affinity to C,N atoms by which the precipitation processes of the α phase are obstructed.
