....Anyway, the gear teeth are not particularly in discussion, they didn't fail, the shaft failed!.....
dascanio,
Your point is valid, but there still may be some questions about how the change in material and processing made specifically for this model MGB may have affected the weld quality.
For example, with carburizing (ie. the process used prior to the EC225) it is standard practice to mask off any surfaces that are not to be case hardened using copper plate, and then the copper plate masking is removed after carburizing. Also, since there tends to be quite a bit of quench distortion with carburizing, especially in areas with thin cross section, the carburized half of the shaft would likely be 100% machined around the weld joint surfaces after heat treatment to provide accurate fit-up for the weld joint. This post heat treat machining operation would remove any surface contamination and providing very clean material for welding.
However, with nitriding (ie. the process used for the EC225) it is common practice to nitride the entire surface of the part. While masking (tin or copper plate) can be used with nitriding, this step is commonly eliminated to save cost, and also because the nitride case can have a beneficial effect on fatigue. Another very important processing step that must be taken after nitriding is removal of the thin outer "white layer". This white layer is composed of iron nitrides, and is very brittle and full of microfractures. It can be removed through mechanical means such as grit blast, or by chemical etching. Given the presence of surface corrosion on the inside surface of the shaft, and that the shaft fractures initiated from the inside surface of the shaft rather than the more highly stressed outside surface (as one would expect for a typical reverse cyclic bending failure), it seems conceivable that the weld integrity may have been compromised by contamination left behind on the inside shaft surface after nitriding/heat treatment.
As always, just pure speculation on my part. But that's what makes these forums fun!