Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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My Blade Won't Stay Sharp!
Q. I've made a multi card tool made of mild steel but the blade won't stay sharp, why? and how can I fix the problem?
Garth jeffery-Perth, Wa, Australia
2002
A. Hi Garth. I hate being the bearer of bad news, but it won't stay sharp for the same reason that a blade made of butter or soft wood wouldn't: the material you have chosen is a soft material. Maybe--but it's a big maybe depending on whether it's really 'mild' steel--you can harden it simply by heating it red hot and then quenching it in oil. The rapid temperature drop can, with the right metal, harden it.
I am no metallurgist but I have made curved sewing needles several times by: heating them red hot, allowing them to cool slowly on the kitchen stove burner, bending them to the shape I want because this has softened them, then reheating them, and dropping them into water.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002
2002
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!
Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2002
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.
Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
2002
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.
Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2002
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!
Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
Indeed it does, Freeman. People are welcome to make a curved sewing needle the way I mentioned; it's educational and maybe even practical thing at times. But if you heat a pin rather than a needle and drop it in the cold water it will not get hard because it doesn't have sufficient carbon to be hardened that way.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002
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