Yet another lockpick instructable

Step 9 Tempering the picks

If you try and use the picks after they have been hardened they will probably snap in locks and create a great deal of unwanted fuss, this step will temper them and make them more springy.

This is probably the hardest step, but with a bit of care and practice you should be able to do it really quickly.

Turn the blow torch on at a very low setting and slowly bring the pick near the flame, as the pick heats it should start to turn blue, move the pick around trying to get it all to turn blue without heating any part of it overly, and definitely making sure none of it glows.
Once most of it has turned blue move it out the flame and let it cool in the air.

If this step goes wrong go back to step 7.

The pick is now tempered and once cooled should be fairly springy and when flexed should return to its original shape without staying bent of snapping. To be honest i have absolutely no idea what happens to the structure of the metal to give it this property, feel free to comment if you do, i would rather like to know.

Comments

[TwistedButSane]

Could this process work with a gas stove top?

[samando]

Unfortunately there's no easy way to explain tempering, but I'll try... When you soften (anneal) the steel, you were right in saying the grains grow larger. However it also has to do with the structure of the carbon and iron molecules in the steel. There are three basic structures to worry about at this point - ferrite, cementite and pearlite. Ferrite is more or less pure iron, and cementite is like really brittle cast iron. Pearlite is made of alternating bands of ferrite and cementite, kind of stripy. For the type of steel used in hacksaw blades, when it's in the soft state it's mostly big fat pearlite grains bordered by cementite. When you heat it above 721 degrees celsius all the carbon dissolves into the ferrite, so you're left with what's called austenite. This is where there's a cube shape of iron atoms, one on each corner, with a carbon atom in the middle. Now this can't exist below 721 degrees. When it's hot, the iron atoms spread further apart (you know how things expand when they're heated), which is how the carbon atom fits into the cube. When it's cold it doesn't fit. If you cool it slowly the carbon can escape, and it'll end up back how we started, the large pearlite grains and cementite. If you cool it quickly though, by dunking it in water, the carbon is trapped. This means the iron cubes are bent out of shape around the carbon atom. On a larger scale, this makes the crystal structure look really spiky and jagged. This is called martensite. Martensite is very hard, but very brittle. Pretty useless for something like a lockpick. What you're doing by tempering is allowing some of the carbon to escape the cubes and form back into pearlite, which is much stronger, though softer, than martensite. So you have pearlite for strength and martensite for hardness. Hope that was understandable, sorry to ramble on!

[xerxesx20]

It's not a bad explanation at all for such a tricky (to say the least!) subject to explain.

Nicely done.

[cirano]

Detailed explanation for those among us who are a little more curious. Thanks!

[samando]

.......... actually, i'm sad to say that that was my brother, i'll see if he can comment

[vestie]

Too... Much... Late at Night. Need to sleep and reread to understand. (thanks for the explanation).

[Jonny Katana]

I used to be very into knifemaking, and this is a very, very solid explanation :)

[}{itch]

That was fantastic explanation! thank you.

[wheatstone]

I would take the last step and heat it and lay it in a pile of sugar... that would give it a nice case hardening. I use this method on nuts with relief grooves to chase a set of new threads on whatever.