Steels and Magnetism

[R.G.]

Think you have problems with steels and how they magnetize?

Check these guys:
VARIATIONS IN THE STEEL PROPERTIES AND THE EXCITATION
CHARACTERISTICS OF FERMILAB MAIN INJECTOR DIPOLES

[R.G.]

Want maximum magnetic properties out of your steel pole pieces? Anneal them after you form them.

See: You bend it, you make it less magnetic.

[Possum]

annealing....

I have a jewelry making background and am familiar with annealing stuff like silver and gold, and how to harden steel but how would you anneal say a small pole screw? Don't you have to heat to red/orange and then cool down for several hours? I tried doing one screw and quickly gave up for lack of knowing what I was doing. From what I know at this point cold rolled steel does have a brighter sounding tone which is desirable for the most part.......

[R.G.]

OK, with that background, you'll have no trouble with steel.

With steel, you have to get it over the "critical temperature" which varies with composition, but not a huge amount; it's up in the 1600-1900F range. There's a holding time, then a critical rate of cooling. Go faster than the critical rate and it hardens. The faster it cools, the more of the potential hardness for that alloy you get. If you cool it slower than X, it stays soft. There is a lot of material on the web on this.

The trick is to get a small electrically heated heat treat oven. Or make one. A heat treat oven, especially for annealing, can be pretty simple: make the inner enclosure out of high temp firebricks with routed grooves for nichrome heating coils, and cast the outside with fireclay and vermiculite about 3-4" thick. Six or eight firebricks and a cloud of dust and you're there. A pyrometer lets you read the temp, and you can figure out how fast it cools down on its own, so that's the maximum rate you can cool with the door closed.

A used electric pottery kiln is good for this too, and cheap, but is bigger and clumsier than you really need.

Don't leave it at annealing temp too long, otherwise it gets what the steel people call burned, but is really too coarse grained. But you can also rig something like this up with a dribble of argon out of a welding tank. That keeps air out and keeps the steel from scaling and rusting from the oxygen and also being decarburized as oxygen eats the carbon out of the outer layer. That last is not a big deal with low carbon steel, but with tool steel it matters.

I don't have to tell you how dangerous near-molten metals are, I'm sure...

[David Schwab]

Is this needed just after bending parts, or would any machined parts benefit as well? The most machining I do is cutting steel to size and smoothing the ends.

When I was reading about cold rolled laminated steel. I was surprised to see they have steel with the grain aligned to increase the magnetic qualities.

[Joe Gwinn]

A simple approach to annealing screws is to get a piece of black iron pipe and two iron pipe caps, fill with screws and a piece of paper, close, heat to red, allow to cool under a pile of ashes. A charcoal filre would work. The purpose of the piece of paper is to burn and use up all the oxygen in the pipe so the screws won't oxidize. Don't use too much paper, or you may case harden the screws.

[tbryanh]

A simple approach to annealing screws is to get a piece of black iron pipe and two iron pipe caps, fill with screws and a piece of paper, close, heat to red, allow to cool under a pile of ashes. A charcoal filre would work. The purpose of the piece of paper is to burn and use up all the oxygen in the pipe so the screws won't oxidize. Don't use too much paper, or you may case harden the screws.

Sounds close to a pipe bomb.

[Joe Gwinn]

Sounds close to a pipe bomb.

Certainly looks that way. But shouldn't it contain ball bearings and triacetone triperoxide?

[R.G.]

A simple approach to annealing screws is to get a piece of black iron pipe and two iron pipe caps, fill with screws and a piece of paper, close, heat to red, allow to cool under a pile of ashes. A charcoal filre would work. The purpose of the piece of paper is to burn and use up all the oxygen in the pipe so the screws won't oxidize. Don't use too much paper, or you may case harden the screws.

That works. A wood fire is better because there are more ashes to insulate the cooling, hence it's slower. You want small pieces of wood, because you don't want to burn the steel (leave it too long over the critical temp), ruins the grain structure.

Even if the outside of the steel carburizes, it will come out dead soft. You have to quench from above the critical temperature and faster than the critical cooling rate to case harden it. But it certainly sets it up with a carbon case so that if it's later heated and quenched, it will harden.

The reason I mentioned a home made heat treat oven is that I'm mulling over making one of those for my home-machining mania... er... I mean hobby...

With my penchant for knowing how much for how long, leaving a snoot full of metal inside a pipe in a fire in the back yard is just downright unsatisfying. I'm nutso about exact temp for exact time, etc. I know it as a failing inside me, and I fight it - not always successfully.

[Steve Conner]

You need to make yourself an induction heater like mine!
http://scopeboy.com/elec/induction/
http://www.scopeboy.com/elec/induction/pics2.html

I got bored of heating screws red hot with it, but now you guys are telling me that you want to heat a bunch of screws red hot? :-)

BTW, those pliers have been useless ever since: the jaws just bend when I put pressure on them.

[R.G.]

That would do it, OK! A little atmosphere control, a pyrometer and a time, and you're in.

A more standard approach is shown here:
Heat Treat Furnace