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Is stainless softer than mild steel? I think I have a big sheet of mild steel at my parents.
Worth it if I have the material already?
M_V-
Most stainless in sheet form generally doesn't harden in any useful way - cutting two spanners from scratch in a steel with sufficient carbon (i.e. not mild steel) will take time and effort. You might do better starting with a cheap stamped steel one that can be annealed if necessary, filed to size, then hardened again?
Quenching in water is cheap - the resultant steam is inoffensive. Expect the steel to rust soon afterwards.
Quenching in oil should leave a finish that is more likely resist corrosion - looks a bit like bluing if you take the trouble to temper it- but will fill the room with noxious fumes.
I can't imagine something with the thermal capacity of a cone spanner both vaporizing oil and being hot enough to ignite it, assuming the torch is turned off at the time...(Waits for mdcc_tester to give a more correct explanation)
MrE
Don't use stainless for spanners. Also, don't bother making your own spanners, it's more trouble than it's worth.