Abstract
Understanding how microstructural factors govern hydrogen (H) diffusion and trapping is essential for mitigating HE susceptibility in advanced high-strength steels. This study quantifies the influence of film-like retained austenite (RA) and η-carbides on H diffusion and trapping in martensitic medium-carbon direct-quenched and partitioned (DQ&P) steel with tailored silicon (Si) alloying. In L-Si (0.25 wt.% Si) alloy, the microstructure comprises mainly of lath-martensite, 11% film-like RA, and cementite. In H-Si (1.5 wt.% Si) alloy, cementite precipitation is inhibited, resulting in a lath-martensitic microstructure containing 15% film-like RA and η-carbides. To isolate the effects of η-carbides, H-Si was tempered at 450 °C for 1 hour (H-Si-T) to transform η-carbides into cementite. H diffusivity was assessed using electrochemical H permeation, H concentration by melt extraction, and H trapping behaviour by thermal desorption spectroscopy (TDS). H diffusion coefficients were excessively high (~ 10⁻⁷ cm²/s) to be associated with RA, implying that H percolates around RA films. Melt extraction and TDS revealed lower H uptake in H-Si compared to L-Si, despite its higher RA fraction, confirming that RA is ineffectively charged under the applied conditions. Both steels exhibited H desorption below 300 °C, evidencing only weak trapping without the high-temperature peaks typically associated with RA. TDS peak analysis highlighted a systematic shift of the max peak temperature to higher values in H-Si relative to L-Si. Upon tempering (H-Si-T), the max peak temperature shifted to lower temperatures, demonstrating that η-carbides, although weak traps, possess higher activation energies than other weak sites such as dislocations, grain boundaries, and cementite. Collectively, these results show that film-like RA acts as a diffusion barrier rather than an H trap, while η-carbides provide weak trapping with comparatively elevated activation energies.