What is Hydrogen Embrittlement?
Hydrogen embrittlement by definition is: A condition of low ductility in metals resulting from the absorption of hydrogen. The mechanism begins with hydrogen atoms diffusing through the metal. When these hydrogen atoms re-combine in minuscule voids of the metal matrix to hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength, up to where it can crack open, in which case it would be called Hydrogen Induced Cracking (HIC). High-strength and low-alloy steels, aluminum, and titanium alloys are most susceptible.
For the fastener industry standard practice should be: never electroplate anything with hardness above HRC32. This eliminates the possibility of introducing hydrogen to the hardened fastener. Alternative coatings to prevent corrosion should be used, such as e-coat or zinc flake.
In most cases, hydrogen embrittlement is a delayed failure and usually occurs in the first 24-48 hours after tension is placed on a joint.
Standard Methods of Testing
The most common way of testing fasteners for hydrogen embrittlement is the “wedge test” per ASTM B839: Standard Test Method for Residual Embrittlement in Metallic Coated, Externally Threaded Articles, Fasteners, and Rod-Inclined Wedge Method. This is a very basic but effective test method for threaded fastener that creates pressure under one side of the head of the fastener. The fasteners are installed into the wedge shaped block and left to sit for 24 hours. After 24 hours the fasteners are check for any cracks under the head or if the heads have fallen off. The fasteners are re-torqued and left to sit another 24 hours. During the re-torqueing process failures can occur do to nature of how hydrogen embrittlement operates.
The American Society for Testing and Materials (ASTM), an international organization dedicated to developing technical standards, has published two other standard methods of testing for this particular purpose, ASTM F1459 and ASTM G142.
ASTM F1459, the first of the two standard methods of testing, utilizes a diaphragm loaded with differential pressure. By exposing metallic materials to high pressure doses of hydrogen gas, this particular testing method offers a quantitative estimate of the metal’s susceptibility to embrittlement.
ASTM G142, the other standard method of testing, employs a cylindrical tensile metal, conducting the tests in an enclosure pressurized with hydrogen. Once completed, this particular method is useful in diagnosing the testing effects on material composition, processing and heat treatment, in addition to the obvious determination of loss of strength and ductility when under the testing conditions.
Have you used either of these standard testing methods in your own practices? Comment below and share your thoughts or experiences with us! For more information about hydrogen embrittlement, contact Bossard at ProvenProductivity@bossard.com..
Standard Methods for Testing Hydrogen Embrittlement by Bossard
