Sometimes evaluation of fracture surfaces in the scanning electron microscope reveals features that do not easily fall into any of the classical crack categories. Usually, this has something to do with the environment and/or the microstructure. If the environment is aggressive enough to cause oxidation or some other composition change to a slow-growing crack surface, then the crack features may not be revealed at all. Instead, you may see the topography of the oxide instead. Sometimes this is “nothing to write home about,” or, in other words, barely worth taking a photo. However, sometimes these oxidized surfaces can be quite beautiful. Figures 1 and 2 show a low-magnification overview and a higher-magnification detail of some oxide “spheroids” that were found on a piece of oxidized copper.
Other times, the image is revealing the actual metallic surface, but with features that are not typically seen together. Figure 3 shows one of these unusual fracture surfaces. The steel was a high-carbon grade used for some spring clips that had been subjected to an austempering process. Springs are generally used at high stress levels. That is, the normal operating stress is often somewhere over half of the yield strength. This spring clip was intended to hold something in place. Thus, it was assembled into the end product, and found broken, along with several of its “friends” from the same manufacturing lot a few days later. The delayed fracture and sustained high stress immediately call to mind the potential that this may be a hydrogen embrittlement fracture. In fact, my records revealed that I had diagnosed a likely hydrogen embrittlement problem with these same parts a few years ago. At that time, the client took my advice and changed the coating to a low hydrogen material. They also decided to test every lot of incoming parts in a fixture designed to stress the parts in a similar way to the service condition. So why was this problem recurring? Or was something different going on?
Well, it was determined that the supplier was changed for some unrelated reason, and, being unaware of the historical hydrogen problem, decided to electrolytically coat the part with zinc, despite the print requirement for a specific, low-hydrogen coating. In other words, the parts were now being coated with a process that is known to cause problems in high-hardness parts subject to high sustained loads. (Note that a previous column discussed hydrogen embrittlement.) So we still have reason to suspect that the problem is due to hydrogen embrittlement.
Figure 3 is marked “IG” in the areas that do look like classical intergranular cracks. This is consistent with classical hydrogen embrittlement. However, there are plenty of features that look more like cleavage, and these are marked “C.” (Intergranular and cleavage brittle microscale features are explained in Part 1 and Part 2 of this Microfractography series.) In Figure 4, we zoom in on a colony of mostly cleavage features. However, even here we see some small patches of microvoids, the DUCTILE microscale feature. The interesting thing is that the cleavage features are so small. The original magnification of this image was 1,580x. Figure 5 shows the nital-etched metallographic coupon, with the micron marker bars matched (the magnifications of Figures 4 and 5 are similar).
Note the bright-etching areas in Figure 5, highlighted with red arrows, are of similar size to some of the ridges in the SEM photo of Figure 4. The ridges in the SEM view seem overall to be somewhat longer than the longest of the white-etching features. It is possible that the cleavage features are being created within ferritic-bainite areas of the austempered microstructure. But why cleavage in a sustained load situation? Usually, cleavage is associated with sudden loads. Maybe this fine-grained material LOCALLY experienced a sudden loading within the ferritic-bainite regions when the intergranular areas separated due to the hydrogen.
At least that was the best answer I could come up with for the allotted budget!