Environmental Engineering Reference
In-Depth Information
Most often, the fracture occurs at sustained loads below the yield strength of
the material. The fracture time increases as the load is decreased. The applied
stress versus the fracture time plots resemble similar plots obtained in SCC or
liquid metal embrittlement. Like the fatigue S-N curve, a threshold stress exists
below which HSC does not occur. This threshold stress is a function of the
strength level of the steel and the specific hydrogen-bearing environment. Gener-
ally, the threshold stress decreases as the yield strength and tensile strength of
the material increase. Lower strength alloys below a minimum tensile strength
are not usually affected. In ferrous alloys, HSC is generally restricted to those
alloys having a hardness of 22 HRC or greater. The sensitivity increases with
the increase in strength level, as shown in Fig. 8.3.
A predominant feature of HSC is that the occurrence of the fracture is delayed,
which implies that hydrogen diffusion in the metal lattice is important for the
building up of sufficient hydrogen concentration at the regions of triaxial stresses
for crack nucleation or at the crack tip for its propagation. The susceptibility to
cracking, therefore, depends on the kinetic factors, such as hydrogen gas pressure
and temperature, which influence the diffusion process. Increasing the hydrogen
pressure reduces the threshold stress intensity for crack preparation and increases
Figure 8.3
Ductility versus hydrogen content for quenched and tempered steels at vari-
ous strength levels. Figures in parentheses indicate the ultimate tensile strength in MPa [21].
