These three words strike terror in all who are employed, no matter what the level, in the production of aircraft and other components which are critical to safety.

What follows is a simplified overview of two related and sometimes misunderstood possible causes of Catastrophic Fracture Failure; stress resulting from manufacture and hydrogen embrittlement.

Stress is a result of flexing, partial heating, machining or grinding of a heat treated component. These stresses, if not relieved, can be a factor in component failure. This is why it is extremely important that materials which are heat treated and subject to stress accumulation must be "stress relieved". this relief of residual stress is accomplished by heating the component uniformly to a sufficient temperature to "relax" the stresses which have accumulated in the manufacturing process since the part was originally heat treated.

This stress relief operation should be accomplished after the last machining operation prior to magnetic or penetrant inspection. It must be done prior to shot peening and / or any chemical treatments such as plating. This stress relief will also help prevent failure due to Hydrogen Embrittlement

The subject nature of hydrogen embrittlement. is technical and complex; this explanation is intended for our customers who may not have a metallurgical background.

The information contained herein is considered accurate, general information on the subject of hydrogen embrittlement. as it exists in the metallurgical literature today.
Hydrogen embrittlement. is a metallurgical phenomenon that occurs in many different metals. However, high strength steel by far seems to have the highest sensitivity to embrittlement. Hence, hydrogen embrittlement. of high strength steels dominates the file records of aircraft / aerospace components which have failed over the last 40 years. therefore, we have chosen to limit the discussion in this newsletter to steels only.

Almost 75% of all the elements known to exist in the universe are metals, so it is easy to see why man has chosen to utilize metals so extensively in out civilization. One of the most important properties of metals is ductility. Ductility can be more commonly understood as the ability to deform under stress. Although this deformation or stretching under stress can sometimes cause problems in itself, it is still one of the advantages of metals compared to other structural materials such as ceramics, concrete, stone, etc.

Hydrogen embrittlement. is a metallurgical interaction between atomic hydrogen and the ferrous metallic atomic structure which inhibits the ability of the steel to deform or stretch under load. therefore, the steel becomes "brittle" under stress or load. In general terms, as the strength of the steel goes up, so does its susceptibility to hydrogen embrittlement.

The failures for which we are concerned result from very small quantities of hydrogen where traditional ductility bend tests will not detect the condition. This atomic level embrittlement. manifests itself at levels as low as 10 ppm of hydrogen. Although difficult to comprehend, numerous documented cases of embrittlement. failure with hydrogen levels this low are known. This type of embrittlement. occurs when hydrogen is concentrated or absorbed in certain areas of metallurgical instability (e.g. stress risers). This concentrating action occurs via either residual or applied stress, which tends to 'sweep' through the atomic structure, moving the infiltrated hydrogen atoms along with it. These concentrated areas of atomic hydrogen can coalesce into molecular type hydrogen, resulting in the formation of high localized partial pressures of the actual gas.

Other theories show the hydrogen to act as a grain boundary surfactant that reduces the surface film energies at the grain boundaries, promoting dislocation slip movement, and eventually micro cracks within the steel. These micro cracks tend to grow quite rapidly upon formation, since the stress intensity factor at the crack tip is astronomically high. Fracture via this type of embrittlement manifests itself by not only ductility loss, but more importantly by the actual loss, via micro cracking, of load supporting or cross sectional areas within the part. For example, a part may start out with one square inch of cross sectional area on the outside, but at time of fracture an actual load bearing area 10 to 20% lower than this may be present.

The facts are plain. The hydrogen has inhibited the metals ability to deform, and as a result the metal will break or fracture at a much lower load or stress than anticipated. It is this lower breaking strength that makes hydrogen embrittlement so detrimental in nature. Design engineers rely on the capability of metals and alloys to carry the load or stress for which they are designed. However, after the part is no longer a "blueprint" but has been manufactured, it becomes quite sensitive to the processing that takes place.

Although most of the problems with hydrogen embrittlement have occurred with aircraft / aerospace parts, the part doesn't have to "fly" in order to "die". Hundreds of human lives have been lost over the years because of hydrogen embrittlement. The effects of hydrogen on metals is serious, deadly serious!

Hydrogen can enter steel from a multitude of places, starting from the original steel making operation. Hydrogen can also enter steel from subsequent casting and forging operations. Even grinding operations after final heat treatment can induce hydrogen absorption, especially if sparking occurs in a moist environment. For the metal finisher, the two most important sources of hydrogen damage occur from acid type cleaning, and most electroplating operations.

A high percentage of hydrogen embrittlement is removed after chemical processing by baking. The normal baking temperature is between 375 and 400 degrees Fahrenheit. A lower temperature is required on components where the final tempering temperature is less than 375 degrees. The baking temperature must not exceed the final tempering temperature for obvious reasons. Most of the hydrogen is removed in the first three hours of bake time. However, the harder the material, the more bake time is needed for sufficient hydrogen removal. Up to 24 hours at temperature is required for the hardest structural steels (e.g. 300M).

A final but very important point for you, the parts manufacturer: KNOW YOUR PARTS, i.e. what is the strength / hardness level of the steel part you are machining. Don't assume it's a soft, low strength steel. Ask questions of your customer. Make sure you know any specific or unusual aspects about your parts, and, most importantly, relay this information in writing on your purchase order to your processing vendors.