FlinterNick said:
"There’s more than one way to harden a frizzen, I generally just hit with a propane torch and dip it in Track’s Surface Hardening Compound, their instructions says to quench in water, I’ve tried both water and oil, the water works better with a propane torch, oil works better with an acetylene torch. Getting the frizzen too hot (where it is non magnetic, 1800-2000 F, bright orange to yellow glow) could cause cracks. Propane torches tend to only get steel up to 1300-1500 hot."
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What Rich said is correct. The hardening will be shallow, and need to be re-done often. FYI, non-magnetic is a consistent 1414°f. If your steel is cracking when you quench at at non-magnetic, it is not the heat causing it. It would be the type of quenchant.