There are probably few types of metallurgical damage that have as much misinformation swirling around as hydrogen embrittlement. The “true mechanism” by which hydrogen facilitates (usually but not always) intergranular cracks in quench or work-hardened steel is still poorly understood. Good practice to minimize the frequency of damage, even in susceptible processes, is well documented, however.
What types of steels can experience hydrogen embrittlement? The most frequent hydrogen-embrittlement situation is with hardened steel in electroplated parts. The harder the steel, the less hydrogen is needed to cause embrittlement. Very low parts per million of dissolved hydrogen atoms can cause a loss of up to 90% of the tensile strength in steels over HRC 45. However, hardness alone does not cause embrittlement. Sufficient hardness, dissolved atomic (sometimes called “nascent”) hydrogen and some level of stress must all be present simultaneously to create an embrittlement condition that causes a crack.
Once the steel is embrittled, getting rid of the hydrogen by baking or any other method may not do much good. If the embrittlement has progressed to the state where microcracks are present, the cracks won’t heal themselves even if you can remove the hydrogen. For this reason, it is critically important to bake as soon as possible after plating. Most specifications call for no more than one hour between the end of plating and the beginning of baking. Baking does not, in general, remove the hydrogen from plated steel. However, it is a good way to prevent embrittlement. Proper baking prevents the hydrogen from causing a problem.
Even steels in the Rockwell B range can suffer from hydrogen embrittlement, usually when they have been heavily cold worked and not annealed prior to plating. Again, sustained stresses in combination with the presence of hydrogen are required to cause hydrogen embrittlement. The greater the residual-stress level, the less hydrogen needed to cause a problem.
While acid and even caustic cleaning can both cause hydrogen to diffuse into the steel, it is possible for the hydrogen to diffuse back out of the steel reasonably easily. If the part is electroplated, however, there is a greater chance that the plating itself will form a barrier that prevents the hydrogen from escaping.
Next time we will conclude our discussion of baking and consider a hydrogen-induced fracture appearance.