Sudden brittle fractures in high strength steels resulting from hydrogen embrittlement represent a dangerous threat to industry. Not only are there the usual issues of cost such as warranty claims, but in cases of personal injury or property damage, liability points clearly and directly at the manufacturer. This is because hydrogen embrittlement is usually the result of deficient procedures in the manufacturing process.
We’ll get into the why and how of hydrogen embrittlement in the next posting in two weeks. For now though, let’s just discuss some of the characteristics of this type of failure. Perhaps you’ll recognize some of these from fractures you’ve encountered, but didn’t realize at the time that hydrogen was the cause.
Hydrogen embrittlement reduces ductility, often to the point where metals behave like ceramics. Consequently, their resistance to fatigue fracture, their fatigue strength, is significantly reduced as well. Fracture toughness, the ability of a metal to resist fracture growth when a small crack is present, is also dramatically reduced. Brittle fracture due to hydrogen embrittlement occurs without any visible distortion or other warning signs and can happen within hours of manufacture or after years in service. Hydrogen embrittlement failures have even been observed in unassembled parts in inventory, a phenomenon known as “shelf popping”.
Generally, the higher the strength of the steel, the more at risk it is to hydrogen embrittlement and the more vulnerable it is to lower levels of hydrogen. Embrittlement at levels of 10 parts per million and less are not uncommon. Some research suggests this relationship is exponential. In other words, doubling the strength of the steel, quadruples its susceptibility to hydrogen embrittlement.
Although hydrogen embrittlement occurs in many different metal alloys, high strength steel appears to be the most sensitive, is the most widely used, and accounts for the largest number of hydrogen embrittlement failures. A professionally conducted failure analysis can definitely recognize hydrogen embrittlement when present, what caused it, and how to prevent it.
In our next post we will discuss the phenomenon of hydrogen embrittlement from a metallurgical perspective – what actually occurs, on a microscopic scale that causes hydrogen embrittlement?