Page 410 - Failure Analysis Case Studies II

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follows on hydrogen embrittlement theories and acoustic emission techniques used to detect
hydrogen activity in metals under stress.

1 .l. Hydrogen embrittlement
The effect of H has been the subject of extensive studies to understand the mechanisms of
degradation in mechanical properties of metals and alloys because industries often encounter
failure of products due to hydrogen embrittlement (HE) [3-6]. Generally atomic H, which is
absorbed initially by the metal surface, transforms to molecular H when the concentration of H is
high. Molecular H resides in voids, pores and interfaces among many other defect sites. If these
defects are not present in the vicinity of a high H area, then blisters or hair line cracks are formed
to release the high pressure. In the equilibrium situation, both atomic and molecular H will be
present inside the material at a high concentration of H. Highly tensile stressed regions will be the
most suitable sites for atomic H. During welding the H picked up, from residual H20 or damp
electrodes, diffuses to the base plate when the weld is hot and subsequently causes cold cracking.
During electroplating and pickling H can enter the lattice, but is diffused-out by a subsequent
baking treatment at 200-250°C.
Some of the important models of HE are discussed below. One of the models, the planar pressure
mechanism, predicts that the high pressure developed due to molecular H within gas pores inside
the material causes cracking [7, 81. Although this model explains the embrittlement of H charged
metal, it is not accepted as a universal mechanism for HE as it cannot explain the delayed cracking
phenomenon. A model proposed by Troiano et al. [9], suggested that the cohesive strength is
reduced due to the presence of H atoms. Hydrogen atoms diffuse easily to regions of triaxial tensile
stress, as at the tip of a crack, and assist crack propagation by reducing the cohesive strength of
the material ahead of it. Thus the crack propagates discontinuously, controlled by a critical
concentration of atomic H built up near the tip of the crack. One model based on the reduction of
surface energy, explains easy crack growth in the presence of H [lo, 111. The model proposed by
Beachem 112, 131 is based on enhancement of dislocation mobility which induces highly localised
plastic flow at very low stress levels. HE is also explained by metal hydride formation in materials
such as titanium, vanadium and zirconium (group IVa and Vb) [14]. It has also been suggested
that hydride-induced embrittlement may occur in steels containing these metals. A completely
different type of HE in the presence of H and C in plain carbon steel at high temperature and
pressure occurs due to formation of methane inside the material. The formation of methane and
the development of high pressure causes blistering which gives rise to typical failures observed in
the petroleum industry [ 151. This process produced decarburisation and is somewhat different from
low temperature HE.
The process of H cracking is the result of one or more of the micro-mechanisms such as: (i)
cleavage, (ii) intergranular decohesion, or (iii) microvoid coalescence. All the three mechanisms in
the same steel alloy, when tested at different strength levels, have been reported in the literature
[6]. Therefore, depending on materials and operational parameters various characteristic fracture
surfaces of HE will be generated.
1.2. HE and acoustic emission

Acoustic emission (AE) is the name given to the elastic waves that are generated within a material
as a consequence of deformation and fracture processes. Acquisition and analysis of these signals