My take on blueing vs case hardening is that blueing is a barrier to chemical reactivity, ie, it's an oxide coating that is unreactive to further rusting, while case hardening is basically an armor plating designed to prevent physical wear from abrasion. I'd assume that the extra carbon in the hardened layer would also add some protection from chemical wear as pure carbon is also chemically unreactive. Do I have this right? Where are our real chemists here??
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