Hydrogen Diffusivity and Hydrogen Traps Behavior of a Tempered and Untempered Martensitic Steel

The effect of tempering temperature and tempering time on the density of hydrogen traps, hydrogen diffusivity, and microhardness in a vanadium-modified AISI 4140 martensitic steel was determined. Tempering parameters were selected to activate the second, third, and fourth tempering stages. These conditions were intended to promote specific microstructural transformations. Permeability tests were performed using the electrochemical method developed by Devanathan and Stachurski, and microhardness was measured before and after these tests. It was observed that hydrogen diffusivity is inversely proportional to microhardness, while the density of hydrogen traps is directly proportional to microhardness. The lowest hydrogen diffusivity, the highest trap density, and the highest microhardness were obtained in the as-quenched condition and the tempering at 286 °C for 0.25 h. In contrast, tempering at a temperature corresponding to the fourth tempering stage increases hydrogen diffusivity and decreases the density of hydrogen traps and microhardness. However, as the tempering time or temperature increases, the opposite occurs, which is attributed to the formation of alloy carbides. Finally, hydrogen has a softening effect for tempering temperatures corresponding to the fourth tempering stage, tempering times of 0.25 h, and in the as-quenched condition. However, with increasing tempering time, hydrogen has a hardening effect.