Understanding Hydrogen Embrittlement
Fastener under stress can fail well below their theoretical strength without warning because of hydrogen embrittlement. Hydrogen embrittlement is generally associated with high strength hardened fasteners manufactured from carbon and alloy steels with a hardness above HRC 40 although fasteners between HRC 35 and HRC 40 also have the potential to be susceptible to hydrogen embrittlement to a lesser degree.
Internal hydrogen embrittlement
Is the most common form of hydrogen embrittlement and results when atomic hydrogen is absorbed into the fastener during the acid cleaning and / or in the electroplating process. If the hydrogen remains in the steel, it can migrate to areas of high stress and built up causing small microcracks, which enlarge rapidly under load, causing the fastener to suddenly fracture.
To reduce the risk of hydrogen embrittlement, a strick baking procedure called ‘’relief-baking’’ soon after plating should be put in place which either removes the hydrogen, or diffuses it throughout the base metal. All high strength steel fasteners used in critical joints, having a hardness of HRC 35 and above or a tensile strength of 145000psi or 1000 MPa need to be ‘’relief baked’’ between 374-428F or 190 -230C between 4-24 hours depending on the actual hardness/tensile strength of the fastener base metal.
Environment hydrogen embrittlement
Is another form ofhydrogen embrittlement whereby atomic hydrogen is absorbed into the steel from the environment after exposure to externally applied stress. The hydrogen comes from a number of external sources including a by-product of general corrosion, or as a bi-related hydrogen embrittlement phenomenon, but by far the most dangerous. The potential for hydrogen embrittlement affecting fasteners below HRC 40 increases if the fastener is acting as the cathode in a galvanic couple.
Tinplating is usually carried out to enhance the ‘’solderability’’ as well as the electrical conductivity of fasteners, particularly in the electronics industry. A ‘’tin whisker’’ is a single crystal of tin which grows spontaneously from the tin plat surface and can end up being a few millimeters long. As ‘’tin whiskers’’ grow, they can cause electrical shorting or can flake off, causing damage to electrical circuits. The cause of this phenomenon has not yet been fully understood.