My Blade Won't Stay Sharp!

A. You're spot on, Ted. I am a metallurgist - you can't harden mild steel by heat treatment. You can surface-carburise it and then heat treat it so that the carburised surface case hardens, but below that layer - less than a millimetre thick - there is still soft mild steel.

That's why it's called mild steel. It doesn't much matter what you do to it with heat, you won't (accidentally or otherwise) harden it, not will you significantly soften it. Its characteristics are about as mild as you can get!

A. Hi Garth,

Perhaps if you heated it and then RAPIDLY cooled it down, wouldn't then the carbon in the mild steel go to the surface and you'd get a harder outside veneer ... using water to rapid cool. Using oil might not be as fast and would tend, perhaps, to help the annealing process and not make it as brittle... nor as hard.

But as Ted said, use the right material in the first place!

And as Bill said, 'tis only a temporary measure because the next time you tried to sharpen it, it wouldn't stay sharp.

Lastly, perhaps you go a buy a nice sharp knife and butcher that up to serve as your card cutter.

A. Hi Freeman,

No, the carbon already in the mild steel will stay evenly distributed through the steel, and the material will not harden even if you have a thin section and quench it in water.

The carburising that I mentioned is a process where the material is heated to around 900 °C in the presence of special powders or gases that cause extra carbon to diffuse from the powder or gas into the surface of the steel. Penetration of perhaps a millimetre is usual. THEN, when the item is quenched, the high carbon surface layer will come up harder. But if the case-carburised item is subsequently heated, the extra carbon near the surface will gradually diffuse into the bulk of the metal leaving you with mild steel that has a slightly higher carbon content than it originally had, and it has gone back to an un-hardenable situation.

A. Hi Ted,

I sure do appreciate you getting back to me re surface hardening of mild steel.

I was taught that 'iron' can only be made into 'steel' if certain percentages of some ingredients are vastly lowered (Phosphorous and sulfur for example) to around 0.06 BUT that the carbon content HAD to be under 0.50 percent.

Hence if a 'steel' is also denuded of carbon, you can't shock cool it and make it harder. Certainly case hardening is viable for people with the equipment but not for Mr. average Handyman. Hence what Ted Mooney says agrees with me ... that 'steel' of his had some good ole Carbon in it!

Cheers!