Page 386 - Failure Analysis Case Studies II

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Fig. 8. Schematic view of destinations for hydrogen in a metal microstructure, discussed in the text : (a) solid
solution ; (b) solute hydrogen pair; (c) dislocation atmosphere; (d) grain boundary accumulation; (e) particle
matrix interface accumulation ; (f) void containing recombined H2.

(i) Reduction in bonding energy between atoms.
(ii) Reduction in surface energy required for cracking.
(iii) Increase in pressure as atomic hydrogen forms molecular hydrogen.
Only atomic hydrogen can enter and diffuse through metal since hydrogen molecules are too
large to do this. However, molecular hydrogen can form within structural defects such as voids,
microcracks, etc., where it can remain. The behaviour of hydrogen within a steel is a function of its
solubility and diffusivity. The effect that hydrogen has on a steel is controlled by :
(i) The form of the hydrogen in the steel (molecular or atomic).
(ii) The position of hydrogen accumulation (solid solution, solute hydrogen pair, dislocations,
grain boundaries, particle matrix interface, or void), as shown in Fig. 8 [SI.
(iii) The strength and stress state of the steel. Annealed steel will tend to blister ; hardened, cold-
rolled or highly stressed steel will tend to be embrittled and crack.
Molecular hydrogen in voids and defects does not cause embrittlement. Atomic hydrogen does
not affect the elastic properties of a steel, only the plastic properties are impaired. The greatest affect
on the plastic properties is found at temperatures between - 20°C and 40T, particularly at slow
strain rates. As little as 2-2.5 ppm atomic hydrogen can lower the mechanical properties of a steel
while 5-6 ppm atomic hydrogen can be detrimental [9].
The source of hydrogen in this instance has been a corrosion reaction that has taken place on
some other steel component in the system. The pins had been heat treated into a susceptible region
and even most conditions would provide sufficient hydrogen to cause HE.
In order to prevent HE in the future, it is thus important to identify the source of hydrogen.
The hardened alloy used in these pins is less corrosion resistant than an alloy in a softer condition
(HRC 35 for example) would be. The important point, however, is that the previous chain lasted
for three years without cracking and the environment in that case was the same as that during the
failure. Therefore, the stainless steel used in the previous chain pins was not susceptible to HE.
Before recommendations for a suitable heat treatment and associated hardness can be made, more
details of the stresses involved during operation need to be furnished. In this way the optimum life
of the conveyor chain pins can be obtained.

4. CONCLUSIONS

The conveyor chain pins failed due to hydrogen embrittlement. The hydrogen came from the
corrosion of steel components in the system and not the actual pins themselves. The actual corroded