Page 419 - Failure Analysis Case Studies II

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5. Discussion
The present material has undergone nearly 80% cold working resulting in a fine and elongated
pearlitic microstructure. The strength level is more than 2000 MPa. Therefore this material has a
tendency to absorb H and retain the same at the interfaces of the pearlitic microstructures. The
cold working by way of the drawing operation also leads to a residual stress pattern where high
tensile stress exists in the central region. It is well known that H has a tendency to stay in a high
tensile stress region. Hydrogen evolved during pickling operations (Fe + HCI + FeCl, + [HI) is
adsorbed by the wire rod. In industrial practice the ingress of H is retarded by employing a proper
inhibitor in the pickling solution. In spite of the inhibitor, any trace of hydrogen that may have
remained in the products would then be removed by a suitable baking operation. If any one of the
above processes is ineffective, retention of H even of the order of 1 ppm would render the product
prone to hydrogen related failure [24].
An attempt is made to explain, by decohesion model, as to how even a low level of H can cause
brittle fracture, by invoking Griffith's theory [22].

where of is the fracture stress necessary to cause the propagation of an elliptical crack of length 2c;
E is the Young's modulus; ys is the surface energy. When H is absorbed, it decreases the bond
strength and the surface energy [I 11. From eqn (1) it can be seen that the fracture strength is
reduced appreciably due to the reduction of surface energy. This reduction in fracture strength can
also be expressed as a function of H concentration at the root of a pre-existing crack [25]:

Cr'--lJN = pc2
where cH is the fracture strength of material containing H; c is the concentration of H; p and II are
appropriate constants which can be determined by experiments.
As this material is heavily cold worked, a number of defects, primarily dislocations, interfaces,
globular shaped cementite, pores, microcracks etc. are generated and these defects are the most
potential sites for H entrapment. In the presence of any external stress, the microcrack tips act as
stress raisers and would further attract H from the surrounding region. When this H concentration
reaches a critical limit, the fracture stress is reduced drastically and the crack extends to the limit
of accumulated H before it stops. Again, H diffuses more readily to the fresh crack tip and assists
in further propagation of the crack. The data obtained from the AET during delayed cracking
tests confirms the general understanding, as explained above, of the way the cracks propagate
under stress, below the yield stress, in presence of H.
It is interesting to note that baking for half an hour has improved the ductility and yield strength
of the material and reduced the cracking tendency as evidenced in the fractograph (Fig. 7). The
early yielding in the as-received sample, as compared to the baked sample, can be explained by H
assisted plasticity. As H reduces the cohesive strength, there will be a corresponding reduction in
resistance to localised dislocation mobility in the as-received sample as compared to the H free
sample (baked sample). It has been reported that hydrogen can diffuse out even at room tem-
perature if the materials are left in fresh air for a lengthy period. However, H trapped in the lattice