Hydrogen in Steel
It was recognized in the early 20th century that certain internal hairline cracks in large steel forgings were related to hydrogen.5 These cracks have been termed “hydrogen flakes” and extensive research on their formation and prevention has been performed by both academia and industry.
Hydrogen is present in steel as a monatomic species with high diffusivity and low solubility in low- temperature-transformation products. The mechanism for hydrogen flake formation remains controversial, however calculations have been performed6 to show that the pressure buildup due to hydrogen with- in a steel matrix is easily high enough to exceed that which even a high-strength steel is able to withstand.
The atomic fraction of hydrogen in equilibrium with H
2 gas at pressure P (atm) is given as:
7
(Eq. 1)
where C0 is the atom fraction of hydrogen and T is in Kelvin. P must be replaced by fugacity at the pressures being considered. An approximation of the Taylor expansion can be used to estimate fugacity, f:
(Eq. 2)
Where 
is the molar volume and R is the gas constant. In order to determine the molar volume, the van der Waals equation of state for one mole of gas can be used:
(Eq. 3)
(Eq. 4)
(Eq. 5)
After determining the fugacity, the pressure and molar volume can be simultaneously solved. Fig. 3 shows the internal pressure buildup versus various amounts of hydrogen in the steel matrix at different temperatures. Fig. 4 shows the difference in relative volumes of steel, hydrogen gas, and water at standard temperature and pressure.
The important point from Figs. 3 and 4 is that even at a hydrogen content of 1 ppm, coming from a very small relative volume of water, the matrix will be unable to withstand the high internal pressure build- up at room temperature. The hydrogen present with- in the steel must be accommodated in some fashion. Hydrogen accumulates at voids and interfaces within the steel, thereby lowering the hydrogen dissolved within the matrix. Grain boundaries, dislocations, microporosity and inclusions are all potential trap- ping sites where hydrogen is able to diffuse out of the matrix and remain in these traps without detrimental flakes occurring.7 Fully dense forgings with low inclusion content are more susceptible to hydrogen flaking due to the reduced availability of trapping sites.
Hydrogen can be removed from steel forgings by subcritical diffusion annealing in order to prevent hydrogen flaking. However, the diffusion annealing practice is both time-consuming and expensive. Fig. 5 shows the required diffusion annealing time versus the forging diameter for removal of 50% of the original hydrogen content at 650°C according to Thelning’s calculation.8 The diffusion annealing time required for hydrogen removal in large cross-section forgings is prohibitively long.