Hydrogen Trapping and Precipitation of Alloy Carbides in Molybdenum Added Steels and Vanadium Added Steels

Abstract

Martensitic steels of Fe-0.1%C-2%Mn-1.6%Mo alloy and Fe-0.1%C-2%Mn-0.2%V alloy were subjected to tempering at 873 K to investigate hydrogen trapping of Mo carbides and V carbides. We carried out the detail analysis of the alloy carbides by atomic-resolution scanning transmission electron microscopy and atom probe tomography, and the evaluation of hydrogen trapping energy by ab initio calculation. The hydrogen content of the Mo added steel tempered for 1.8 ks increases from that of the quenched Mo added steel and the hydrogen content monotonically decreases as the tempering time increases. The hydrogen content of the V added steels increases during the tempering to 7.2 ks and then keeps almost constant. Plate-shaped B1-type Mo carbide with a chemical composition of MoC0.50 is precipitated in the Mo added steel tempered for 3.6 ks. Needle-shaped HCP Mo2C is precipitated and the B1-type Mo carbide decreases in the Mo added steel tempered for 14.4 ks. Plate-shaped B1-type V carbides with a chemical composition of VC0.75 is precipitated in the V added steel tempered for 14.4 ks. We found the positive correlation between the hydrogen content and the product of the interface area and the carbon vacancy fraction of B1-type alloy carbide. The hydrogen trapping energy of the carbon vacancy at the interface between BCC-Fe and B1-type Mo carbide is higher than that of the interstitial sites in BCC-Fe. These results suggest that the main trapping site in the tempered Mo added steel is the carbon vacancy at the interface of B1-type MoC0.50, not HCP Mo2C.