Hydrogen trapping and precipitation of alloy carbides in molybdenum added steels and vanadium added steels

Martensitic steels of Fe-0.1%C-2%Mn-1.6%Mo and Fe-0.1%C-2%Mn-0.2%V alloys were subjected to tempering at 873 K to investigate the hydrogen trapping of Mo and V carbides. We analyzed the alloy carbides in detail via atomic-resolution scanning transmission electron microscopy and atom probe tomography, and evaluated hydrogen trapping energy via ab initio calculations. The hydrogen content of the Mo-added steel tempered for 1.8 ks increased from that of the quenched Mo-added steel, and the hydrogen content monotonically decreased as the tempering time increased. The hydrogen content of the V-added steels increased during tempering up to 7.2 ks and then remained almost constant. A plate-shaped B1-type Mo carbide with a chemical composition of MoC_0.50 precipitated in the Mo-added steel tempered for 3.6 ks. Needle-shaped HCP Mo₂C precipitated and the B1-type Mo carbide decreased in the Mo-added steel tempered for 14.4 ks. A plate-shaped B1-type V carbide with a chemical composition of VC_0.75 precipitated in the V-added steel tempered for 14.4 ks. We found a positive correlation between the hydrogen content and the product of the interface area and the carbon vacancy fraction of the B1-type alloy carbide. The hydrogen trapping energy of the carbon vacancy at the interface between BCC-Fe and the B1-type Mo carbide was higher than that of the interstitial sites in BCC-Fe. These results suggest that the main trapping site in the tempered Mo-added steel was the carbon vacancy at the interface of B1-type MoC_0.50, not HCP Mo₂C.