Introduction
Hydrogen Embrittlement is the process by which high-strength steels become brittle and fracture following exposure to hydrogen. It is generally the result of unintentional introduction of hydrogen into susceptible metals during processing.
Hydrogen Embrittlement normally affects thin sections of high strength steel, such as chain plates, but can also present a big problem in chain pins, chain anchor bolts and chain anchor pins.
This phenomenon starts when hydrogen atoms diffuse into the metal. These hydrogen atoms combine to form hydrogen molecules which form in minuscule voids in the material. As they grow the molecules create pressure from inside the cavity to a point where the metal cracks. The loss of ductility and cracking will occur at stresses that are far below the base material’s normal tensile strength.
It is considered that Hydrogen Embrittlement affects only high strength steel that has a tensile strength circa 1000Mpa (1000 N/mm2) or above. Steel that is hardened to 32HRC (HV300) is approximately equivalent to 1000Mpa. Leaf Chain plates are approximately 44-45 HRC and leaf chain pins are around 56-57 HRC. Our chain anchor bolts are made from material which is in the range of 850Mpa and 1000 Mpa
Over the last 35 years we have had only one instance related directly to leaf chain which was concluded as being due to the cleaning method of chain links prior to zinc plating. The development of corrosion resistant leaf chain has resulted in us no longer offering zinc plated leaf chain.
Several years ago we had a problem with a batch of anchor bolts that were zinc plated following hardening above 1000Mpa inline with customers requirements, before the parts were put under any load, after investigation there was some doubt the instructions on de-embrittling process had not been completed properly. We no longer zinc plate chain anchors bolts which are hardened above 1000 Mpa and we recommended other surface finishes which do not carry the same level of risk on all
Many years ago we had a batch of zinc-plated leaf chain pin failures that was likely to be caused by Hydrogen Embrittlement. We no longer zinc plate any chain anchor pins but can offer pins which have been Mechanical Plated or Zinc flake coated. Both processes have no risk of residual hydrogen embrittlement.
Hydrogen Embrittlement in leaf chain and chain anchor bolts is rare and random in its selection of components to attack and the solutions are also normally random in nature. This means that there are not always absolute concrete answers as to why or absolute concrete solutions. For this reason we are overly cautious in our approach in order to guarantee avoiding any likelihood of this happening in the future and do not recommended a process which increases this risk
It should also be noted both Lead Acid and Nickel Cadmium batteries produce hydrogen gas during normal charging. Overcharging excessively can quickly cause batteries to produce even more hydrogen because as hydrogen builds up material degradation increases.
Reference information
As well as our own anecdotal experience we have studied the information from various known sources such as The Fastener Engineering & research Association (FERA), the heat treatment doctor and Fastenal Engineering & Design Support (FEDS).
