Originally Posted by rv6ejguy
White layer formation can only happen during the manufacturing process. Nitro carburization is an intentional, surface steel hardening process used similarly to gas nitriding but at lower temperatures to minimize part distortion.
My understanding is that quench oil vapor contamination during part heating is thought to be a likely cause of white layer formation but someone with more real-world knowledge of heat treating, hardening and quenching processes could say more on the subject.
White layer formation is often hard to detect, say optically, without an actual hardness (ball) test. I know some machinists have run into the problem where high temps induced by the machining process can cause a white layer to form which kills the tool pretty fast.
It would appear to be a process control defect or oversight here to cause this issue as is usually the case where some cranks fail while the majority of them live for thousands of hours. We have seen process control issues at Lycoming also before as well as alloy recipe changes, resulting in crank failures. Nobody is immune. This stuff is all really critical to get consistently reliable cranks. A tiny change in the established and validated processes or material can cause premature failure.
I learned a bit about hardening and quenching steel decades ago from a old and wise gunsmith friend. My memory maybe isn't correct on some details here so someone with intimate knowledge of this subject, please correct any errors.
This sounds quite accurate compared to what I was taught in engineering school.
Does anyone here know at what engine hours these crankshaft failures occurred? Is there a trend or was it random high/low hours? It?s not going to make me fly if all failures were at high engine times, but I?m curious and might as well make up some time while waiting for answers from FAA/Superior...(sigh)