Naturally, hardened materials wear less than the less hardened materials they rub or ride against. This is why rings should be softer than the accompanying cylinder. Can't think of any engine components that are hardened to martensite, but whatever. What was being said, is that engines are designed to ride on a cusion of oil, which is why they CAN use lead (babbit actually) and other soft materials for bearing surfaces and go for tens of thousands of miles. Roller, and ball bearings, are not designed to have a pressurized oil supply, and yes, the properties of the steels used becomes very critical indeed. In a lateral thinking vein, let us suppose that case hardened steel was used for the main bearings. How long do you think they would wear? How easily would the little bits of junk be able to be imbedded into the bearing material (very important aspect of bearing design). Most importantly, how many people would rather replace the thrashed (expensive) crank instead of the (cheap) bearings? Metallurgy is important, but wear rates are rarely the most important aspect, that is why we have lubricants.
