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Hydrogen trapping and de-trapping behavior was investigated for steels with and without V. The de-trapping of hydrogen is very slow while the trapping presumably proceeds rapidly for steels containing VC precipitates. The activation energy for de-trapping is in the range of 33 to 35 kJ/mol. The trapped-hydrogen content and diffusible-hydrogen content in the steady state increase with increasing hydrogen entry rate into the steel. The density of hydrogen trapping sites decides the maximum trapped-hydrogen content; 9 ppm for 1 % V steel tempered at peak secondary hardening temperature. Analysis of hydrogen embrittlement cracking tests in terms of hydrogen contents such as the critical hydrogen content should be performed on the specimens with uniform hydrogen distribution and must consider the nature of hydrogen whether it is trapped or diffusible.

The hardenability change by an addition of B to 0.15%C-Ti added steel was investigated considering the effect of Mo in order to understand one of the peculiar characteristics that austenitizing at higher temperatures reduces the hardenability effect of B. Hardenability monotonically increases with increasing B content up to the optimum B content without the effect of Mo. The optimum B content increases with increasing Mo; 6 ppm, 9 ppm, 13 ppm for 0 % Mo, 0.25 % Mo, 0.50 % Mo. The addition of Mo retards the precipitation of M 23 (CB) 6 and thus more B in solution that contributes to hardenability can exist along grain boundaries by Mo. The optimum B content is reduced with increasing temperatures because much more B concentrates along grain boundaries through non-equilibrium segregation mechanism during cooling from an elevated temperature and thus precipitation of M 23 (CB) 6 easily occurs.

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