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Faye

Heat treat and Tempering question

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I have been hearing some contrary idea's on heat treating and tempering. I originally heard that to make a blade hard you had to get it to a white heat and cool it down really fast, like putting it in water. Then I heard from a friend that doing that made it very brittle and was a very bad thing to do. After that I checked with my dad on a few things and he said to harden a blade, get it to a white heat and quench it in oil. Happy I went about my knife making and found that my dads advice worked well. My older sister came home from school one day though, and told me I was doing my heat treating and tempering all wrong, that I had to quench it in oil from a white heat but then heat it back up to a certain temperature to "Temper" it. I went back to my dad and he couldn't tell me why heating it back up would do anything other than make it softer. I did some research and the heating it back up theory kept popping up. I have come to the conclusion through the common mention of it in reliable places, that it must hold some truth. My question is why? Why does heating it back up in an oven for a few hours make it harder and not softer?

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Check the "so you want to make a knife" threadat the top of this sub forum. 

In short, color is not what to look forin quenching point. It is pinning the grain size as much as hardness that is the goal.

And

" tempering" after hardening removes the tension and brittleness from the blade making it softer and usable.

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To add to what Vern said, you never want to take blade steel to a white heat.  Most blade steels harden from a color that appears to me (it may look a little different to you) to be a reddish orange.  There is so much information on this site alone about hardening and tempering that you could easily spend a few hours if not days reading it all.  Read Vern's pinned thread up top, then look at the "heat treatment by alloy" section.

Most blade steels do not survive a water quench, either.  What steel are you using?

Finally, a fully hardened blade is as brittle as a piece of glass and will break very easily, sometimes without any obvious reason.  As in, if left untempered overnight you may find it in three or four pieces in the morning.  Tempering, that heating it up thing, relaxes the structure a bit and makes it usable.  You do lose a couple of points of hardness, but you gain the ability to sharpen it and use it without it breaking immediately.

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Okay, that makes sense.  I did go and read the pinned thread,  and it was very helpful. 

I use old horseshoe rasps, which are already temperd to the highest possible hardness. I have only annealed one rasp before I made my knife, the rest I just work with as is and I quench in motor oil, not sure if that is an acceptable method or not it's just what I had lying around. 

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Motor oil is a no no. Wrong viscosity, it supposidly holds moisture, and on top of that it's not healthy. There are plenty of topics on quench oils here as well. Hot canola should do the trick better. 130°, or too hot to hold your finger in.

I wish you would get away from the rasps for a while and just focus on the basics. Making a knife from a rasp can add character if you know how to tastefully, and know how to heat treat, but until then you will just be fighting this uniform, deeply scarred, and unknown steel. 

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Would old leaf springs be an acceptable metal to work with? 

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Leaf springs, coil springs, lawnmower blades are all pretty fair places to start if you're going for stuff you can find for free. These are what we call mystery steels though, so you only get a general idea of the behavior of these steels. If you're just trying to practice, you can use any old metal, doesn't really matter. If, however, you want to make a good stong knife, you'll want to use something specific, like 1084 or something, and then follow the heat treat guidelines for that steel. 

 

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Alright, yeah at this point I am just looking for good metal to practice with. 

Thank you all for your help. 

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You could find brand new leaf spring cuts for cheap at your local spring shop.

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To add to whats been said already, hit scrap yards. Many will sell to you slightly above scrap rate (cheap, in comparison to buying blade steel.)

Look for things like coil springs, leaf springs, crowbars, old wrenches (do not attempt to forge chrome plated wrenches!!!) chisels, files, axles, lawnmower blades, hay tines, plow disc blades, the list goes on and on, but everything listed here is going to fall in the medium-ish carbon range and will get you started down the path for pretty cheap. 

Also try machine shops, auto repair shops, spring shops, etc. they may sell you drops and scrap for fairly cheap, or possibly even give it to you for free if youre polite and explain what youre doing with the steel. I know most shops around my area have such a pile of scrap behind the building, theyre happy to have someone reduce the size of it! 

Good luck with it all!

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