Great question — answering as the UphillTech team:
HG31 has the sprockets phosphate coated in black. I'm not sure, but the largest ones may also be on a riveted steel spider construction, eliminating the possibility of recycling any of the unworn sprockets. Construction is not always identical in my experience, maybe it depends on the factory. HG41 is nickel plated and looks much shiner, and is generally more resistant to corrosion. The sprockets are usually individual ones pinned together and can be seperated (once the pin heads are ground off) to make custom ratios or reuse unworn sprockets. (Hardly anyone does this nowadays. However, I sometimes keep unworn/low wear 32, 34 and 36t sprockets to make cassettes with lower gear ranges than...
Full answer by Noise: https://bicycles.stackexchange.com/a/83825 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
