Beware of the effects of hydrogen embrittlement!
The ductility of components particularly those manufactured from high strength material can be severely compromised by hydrogen embrittlement mechanisms causing them to fail with a short period in service.
Hydrogen Embrittlement is defined as a permanent loss of ductility in a metal or alloy caused by hydrogen in combination with local tensile stress (either externally applied or arising from internal residual stress). It is a complex process that takes on several forms with many mechanisms having been proposed.
It is generally believed that due to the small size of the hydrogen atom, nascent (atomic) hydrogen formed during corrosion and certain manufacturing processes, can enter materials such as steel alloys and certain other alloys including aluminium and titanium, where it diffuses to high energy sites such as stress concentrating features, flaws, dislocations and other areas of lattice distortion. In these areas the hydrogen combines to form molecular hydrogen (and in cases methane gas) and results in a pressure build up which, if of sufficient magnitude, can cause local microscopic deformation and cracking. This local damage typically occurring at grain boundaries results in a loss of ductility, reduced load carrying ability, localised cracking (usually sub microscopic cracks) and in certain cases brittle fracture at stress levels well below yield or even design stresses. When fracture occurs it is not continuous but occurs in a series of initiation and propagation sequences, generally along grain boundaries.
For hydrogen embrittlement to occur the material must be i) susceptible to hydrogen embrittlement, ii) in a state of local tensile stress (either applied or residual) and iii) exposed to a source of atomic hydrogen. If all these conditions are met hydrogen damage will ensue given sufficient time. As the embrittlement is diffusion controlled, it is generally delayed, with time to failure varying for any given material as a function of the material, stress, temperature, and the source/concentration of the hydrogen. Hydrogen embrittlement normally only affects high tensile carbon and alloy steel components which become more susceptible to cracking as the hardness increases above 320HV.
Embrittled components do not show obvious signs of degradation. Embrittlement can only be detected by undertaking mechanical testing and comparison of the results of such testing to control samples. The degree of degradation and susceptibility to hydrogen embrittlement can be assessed by undertaking sustained load or slow strain rate tests. These tests are defined in many standards including ASTM F606, ASTM F519, ASTM G129 and ISO 7539-7.
Unfortunately once the material has been embrittled it is not reversible!!
Next month’s tip will address the sources of hydrogen and processes that can promote failure by hydrogen embrittlement.
Published in Technical Tips by Origen Engineering Solutions on 1 November 2015