Effect of rapid tempering and cementite morphology on hydrogen diffusion and trapping in a medium-carbon advanced high-strength steel

Microstructural changes during the tempering of carbon steels can influence their interaction with hydrogen (H) in several manners, and novel heat treatments could aid in reducing susceptibility to hydrogen embrittlement. Here, the effect of rapid tempering (RT) on H diffusion and trapping was investigated in a direct-quenched (DQ) medium-carbon steel and its tempered variants, T420 (RT at 420 °C) and T720 (RT at 720 °C). RT reduces RA fraction, dislocation density, microstrain, and low-angle grain boundary surface area while increasing the cementite fraction and modifying its morphology, resulting in mainly rod-like cementite in T420, and a higher fraction of globular cementite in T720. Electrochemical H permeation and thermal desorption spectroscopy were used to evaluate H diffusion coefficients (D) and trapping behaviour. DQ and T420 exhibit similar D, while T720 shows significantly faster diffusion, with an inverse correlation to H content. Cementite acts as a weak H trap, showing increased trapping with increasing interface area for rod-like cementite.