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[quote]QUOTE: Originally posted by oltmannd Well, not quite. Case hardening (carburizing, induction hardening, forging, etc) all increase the overall strength of the part as well. In fact, on loco crankshafts, that's exactly why they're hardened. EMD and GE have been using hardened cranks for years and years. Hard to believe the new 6000 HP engine crank isn't hardened- must have VERY large bearing diameter! [/quote] While I agree that work hardened steel, such as happens in forging and extrusion, does increase strength, that is not the case with temperature hardened steel, whether induction, radiation or convection. That embrittles the metal. The case hardening usually put on cranks, valves, camshafts and the like is done for wear resistance. If you manage to wear through the protective layer the underlying metal wears very quickly. If you look at a micrograph of the surface and of the underlying metal the difference in the crystal structure is obvious. I've worked with far to many improperly hardened shafts, both where they were over hardened and we snapped them during high torque operation and with under hardened shafts where the seals fretted the metal away in short order. I do not understand why the designers would not want to protect the crank by hardening. The extra weight would be minimal and the benefits are considerable.
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