Application of the Hydrogen-Permeation Method to the Study of Carbide Precipitation Kinetics in a Low-Carbon Martensite

The kinetics of carbide precipitation during the tempering of low-carbon martensite at 453K was studied by means of the hydrogen-permeation method. The hydrogen diffusivity obtained by this technique attained a minimum value in the quenched specimens and increased with increasing tempering time to reach a constant value between 3 and 6h. This change in the hydrogen diffusivity is attributed to modifications in hydrogen trapping due to carbide precipitation, and the kinetics of precipitation are described by a Johnson-Mehl or Avrami equation such that the precipitated carbide fraction is given by f(t) = 1 − exp(−0.06t0.36). Hydrogen-trapping modifications are explained in terms of the generation of a plastic zone when a local volume change occurs.