I have a lot of respect for Marc and Miller and their background. I have but one college level metallurgy course and a similar background to Miller's in shops plus some years reading met-lab results for airplane parts.
What I have noted is that modern low alloy steels of medium carbon content (4130,4140,4340, etc) have been successfully "case hardened" (generic term) or "carbo-nitrided" while maintaining a moderate core hardness. I ran across it on one of our critical airplane parts designed in the mid 60s and made until recently. The process used carried a proprietary name that escapes me at the moment. But, I do believe it is done and done on critical parts.
