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Have you ever had a firmly tightened bolt all of a sudden break on you? We understand the theory of "fatigue fracture," in which bolts repeatedly exposed to dynamic loads such as vibration eventually break. However, when a sufficiently strong bolt under stable static load suddenly breaks, "Huh? Why?" is the natural reaction. This phenomenon is known as "delayed fracture". Today, I will explain this principle and how it can be handled.
"Delayed fracture" is a phenomenon in which bolts under a constant tensile load, showing no external deformation, suddenly break after a certain amount of time.
This phenomenon is more likely to occur with greater bolt strength and cannot be detected by visual inspection, making it a bit of a hassle to deal with.
Fracture Caused by Hydrogen?
The mechanism causing "delayed fracture" is actually still not entirely understood. The bolt becomes embrittled over time due to a combination of factors, such as operating environment, material, strength, etc.
The most significant cause of delayed failure is said to be "hydrogen brittleness". The phenomenon also goes by the name of "hydrogen embrittlement". We think that during the thread machining stage or in the operating environment, hydrogen can penetrate into the bolt interior, concentrating over time at places under stress, and forming a cavity which then leads to fracture. In general, if the stress applied to the bolt exceeds 1,000 MPa, hydrogen embrittlement becomes more likely to occur. This explains what I mentioned earlier, that delayed fracture is "more likely to occur with greater bolt strength".
As hydrogen may enter during the thread machining process, delayed fracture in a single bolt means that all bolts manufactured at the same time need to be inspected or replaced.
The scale of the problem becomes apparent when you realize that you can't get away with just replacing the broken bolt.
Effects of Bolt Surface Treatment on Hydrogen Penetration
The plating process is thought to be a cause of bolt hydrogen penetration. Hydrogen generated during acid washing, as well as hydrogen generated due to electrolysis of the bath water, can make its way into the bolt during plating. There may be causes other than surface treatment: it is also known to occur more easily with bolts used in corrosive environments.
Helping Prevent Delayed Fracture
Baked treatment is effective at reducing the chance of hydrogen embrittlement, compared to plating or other surface treatments. Heat treatment also expels hydrogen that has been absorbed by the bolt. Also, it is important to keep the surface treatment time short.
Elsewhere, there are ways to manufacture bolts without generating hydrogen: replacing the processes that generate hydrogen, such as acid washing, with other methods.
NBK uses these methods to apply plating to high strength bolts, like the hydrogen embrittlement-resistant SNS-EL type... so give them a try.