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Hydrogen Embrittlement of Chain Link

A failed chain link was analyzed. It had failed while supporting a 42,000 lb. counterweight for an emergency Taintor gate. Under normal conditions the chain is submerged in a freshwater lake and is constantly supporting the weight of the 42,000 lb counterweight. The Taintor gate (and chain) is tested once a year to ensure it can operate properly. The Taintor gate was not being operated at the time the chain failed. The sample was about 6” long x 3.6” wide x 1.1” thick.  The diameter of the center holes was about 1”.  

The appearance of the rust on the surface was typical of exposure in freshwater. One standard nominal 0.5” dia. x 2” length specimen was prepared for tensile testing. The results are tabulated in the table below. The sample met the tensile requirements for SAE 1330 with some discrepancy in % Elongation. The reduced ductility points to hydrogen embrittlement. These high strength low alloy steels are highly susceptible to hydrogen embrittlement.

The fracture surface was ultrasonically cleaned to remove rust for fractography

This is the center of the failure area.  The forging had some porosity inside, which could have weakened it at that point, especially as that pore appears to have been very close to the surface.

The fracture surface was ultrasonically cleaned to remove rust for fractography

The chain link was corroded.  A cross-section showed intergranular cavities that were present to some depth in the material. These cavities appear to have been due to hydrogen damage. The chain link failed due to corrosion resulting in hydrogen embrittlement.  The significant porosity observed could weaken the steel. The microstructure in the base metal was tempered martensite, typical for SAE 1330 steel. Atomic hydrogen may generate during corrosion reaction and diffuse into the steel since the size of the atomic hydrogen atom is relatively very small. These atomic hydrogen atoms combine in the voids of the metal to form hydrogen molecules that exerts pressure in the cavity. These pressures are high enough that the material loses its ductility and catastrophic failures may occur. High-strength, low-alloy steels are very susceptible to this condition. 

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