Hydrogen embrittlement is a phenomenon that affects high-strength steels, titanium, aluminum alloys and many other high-strength metals. Hydrogen, often from pickling or plating, invades the grain structure of the metal, making it brittle and subject to sudden and catastrophic failure.
Hydrogen is the most common element in the world, and many acidic and oxidation reactions with steel will liberate hydrogen in various amounts, depending on the specific chemical reaction.
Common causes of hydrogen embrittlement can include specific atmospheric conditions, the breakdown of organic lubricants, any manufacturing process itself and heat treating. Other influences are working conditions, arc welding, atmospheric conditions and grinding in a wet environment. Parts undergoing electrochemical surface treatments are especially susceptible. Acid cleaning and electroplating at high current are the most severe, followed by electrolysis plating and conversion coatings.
Environmental hydrogen embrittlement is another form of this condition. This is generally caused by hydrogen introduced into the steel from the environment after being placed in service. In this case, the hydrogen can come from a number of external sources, as a by-product of general corrosion or as a by-product of a common reaction.
Hydrogen interacts with many metals to reduce their ductility and frequently their strength also. It enters metals in the atomic form, diffusing very rapidly even at normal temperatures because of its small size. During melting and fabrication, as well as during use, there are various ways in which metals come in contact with hydrogen and absorb it. The absorbed hydrogen may react irreversibly with oxides or carbides in some metals to produce a permanently degraded structure. It may also recombine at internal surfaces of defects of various types to form gaseous molecular hydrogen under pressures sufficiently high to form metal blisters when the recombination occurs near the outer surface. In other metals, brittle hydrides that lower the mechanical properties of the metal are formed.
Another type of embrittlement is reversible, depending on the presence of hydrogen in the metal lattice during deformation for its occurrence. Under some conditions, the failure may be delayed for long periods. A number of different mechanisms have been postulated to explain reversible embrittlement. According to some theories, hydrogen interferes with the processes of plastic deformation in metals, while it enhances the tendency for cracking according to others.
Companies supplying susceptible metal parts to the transportation and oil and gas industries are often required to perform hydrogen embrittlement testing. Wire, fasteners and pipeline steels are some of the most common parts being tested. Many in the heat-treat industry will send their parts to an outside lab to perform the testing. It is also becoming a trend among individual companies, however, to add hydrogen embrittlement test equipment to their test labs in order to meet the strict guidelines set forth in test specifications. ASTM F519-13 is the Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments. The end user may have more specific requirements, depending on the application and end-use environment.
A hydrogen embrittlement test is a simple ambient-temperature test performed on both machined specimens and finished fasteners to determine if the material is susceptible to hydrogen embrittlement. It is performed at ambient temperature for 200 hours with a stress equal to 75% of the material yield strength. While it is a relatively simple test, frames such as the ATS 2330 MAN can ensure repeatability, accuracy and reliable results on up to 48 specimens at a time. Figure 1 shows a tester holding 12 samples (3 sets of 4) of ASTM F519 1a.1 specimens. The other accepted specimen type is F519 1a.2, which is tested as one set of four. The typical specimen type for fasteners is shown in Figure 2.
Some of the beneficial characteristics of the ATS 2330 MAN are:
- Wide frame construction, which allows for a variety of environmental chambers, fixtures and other accessories while maintaining compact overall dimensions
- Convenient side operation
Unique lever-arm design
* Counterbalanced with precision ratio adjustment
* On-center loading, providing optimum strength and minimum deflection
* Rugged v-block supports for maximum linear knife-edge contact
* Four-position rotatable knife edges of high-strength, hardened tool steel designed for easy rotation or replacement of worn edges
- Precision drawhead assembly, providing loading and automatic beam leveling
- Durable vibration isolator mounts to prevent disturbance to other sensitive equipment upon specimen breakage
The test specimen is put through the same plating or coating process as the parts and then tested prior to releasing the lot to production. Because the test takes 200 hours, in-house testing provides the benefit of receiving the results sooner. This can improve the ability to decrease lead time of the parts to the end-user.
By bringing testing in house, a supplier can decrease lead time, increase quality and have more control over their products. Once the part leaves a facility for testing, it is no longer possible to control the conditions in which the specimens are kept, therefore introducing the possibility of sample failure. This can lead to false results and become costly to the supplier.
In-house solutions are available for parts and materials manufacturers at a reasonable cost with an easily attainable return on investment. A proper test stand can pay for itself in a rapid manner, depending on the testing frequency of the materials manufacturer.
For more about testing – particularly fasteners – and a discussion of hydrogen embrittlement, see what The Heat Treat Doctor had to say in a 2014 column (http://goo.gl/IbY5te).
1. Daniel H. Herring “The Heat Treat Doctor”®, Fall 2010 Wire Forming Technology International