Reply To: Brittle Plough plane blades
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That’s not case hardening. Somebody ruined the temper by overheating the tip while grinding and probably burned off surface carbon, making it softer. Your multiple attempts at sharpening probably ground enough steel to get past the ruined tip which had carbon in the steel burned off. . It likely had little to do with the sharpening angle, but if you had success, keep doing it. The steeper angle gets you past the burned part sooner.
Case hardening is the controlled introduction of carbon INTO a thin surface layer of the steel. The modern method is by heating the steel in a high carbon gas environment. The “case” is the thin high carbon layer formed over the softer core.
The ancients knew about case hardening, usually accomplishing it by heating iron in charcoal dust or any high carbon concoction including dead animals and meat, or even urine.
They didn’t understand the chemistry, which made conversion to steel a the stuff of magic dwarves and such. Ill confess I think of Wayland or Hephestius every time I see a Smith do it at a forge