Home Annealing of Hardened Drill Rod?

[Andy Birko]

I have a little project I'm doing that's basically going to add a gear box to a standard Zither tuning hammer. This project will require cutting and drilling some holes in the shaft of the hammer. So I ordered a hammer and when it arrived, I tested it with a file which left no marks and so discovered that the hammer is made from hardened tool steel - probably some sort of drill rod.

The question is, does anyone know of a way to anneal tool steel at home?

The websites I found state that tool steel needs to be brought up to about 1600 degrees F for a long time and cooled very slowly to fully anneal it. Anyone know of any tricks to do this some other way?

This is a pretty standard hammer, shaft is about 3/8" thick. It's not really practical to make my own out of some other sort of steel because the "star" that fits over the pin is very precisely made and I think it's just beyond my scope of abilities to make something like that.

[Andy Birko]

Looks like home ovens get up to about 900F for self cleaning and we need to run a cycle anyway. Any metallurgists think that will do anything?

[Alan Carruth]

'Tempering' steel is an annealing process. You might be able to soften it enough using a torch, without too much fancy equipment.

Remove the shaft of the hammer from the handle, if you have not already. Polish up the shaft, or, at least, get back to bare steel if it's plated. Heat the handle end in the torch. You'll see the surface of the steel start to change color as it builds up a think oxide layer. The color of this layer depends on the temperature; it will start out pale yellow, go through red and brown, to blue and black. When you see the proper color (ad I can't tell you what that would be) at the end, just slowly move the torch flame up and chase that band of color up the shaft as far as you want. Lay it down someplace that's out of air currents, and allow it to cool, and try it with a file. If it's not soft enough, polish it up again and go for a darker color.

Alan Carruth / Luthier

[David King]

Annealing is easy, I keep the heat on it all the way to dull red glow and gradually pull it away. Extra thermal mass is helpful in slowing down the cooling so you could slip a drill collar over the outside maybe but you will need a bigger torch. Cover it with aluminum foil to reflect the heat back but don't let it touch. Repeated heating will eventually burn the carbon out at the surface and it won't re-harden as well after that but that carbon can be replaced with Casenit or equivalent.
You can also just buy a carbide drill on ebay and ignore the hardness. I've seen people drill hardened steel with a masonry bit if it doesn't need to look pretty. Grind a notch or flat to get the drill started.

[Bob Menzel]

I just came across something at McMaster-Carr while researching something at work and thought it might be helpful. I was unable to post a direct link to the web page so you'll need to enter "about alloy steel" in their search bar, and then scroll down that page for "View information about the physical, chemical, and mechanical properties of steel, as well as heat-treating data for tool steel."

[Andy Birko]

Unfortunately, the McMaster-Carr article states what I already knew which is that it takes a temp of about 1500F with a slow cool down to fully anneal tool steel. That might not be totally necessary though so I think I'm going to try torch or more likely the oven cleaning cycle first and see how things go.

[Steve Senseney]

FIrst, try to not over heat the socket where you want the steel to be hard.

Second, this is not really too hard to do. You may need more than a propane torch to get the shaft hot enough to drill. This means using a MAP gas torch or Oxy acetylene.

If you go to a machine shop in your neighborhood, any one there with a torch will be able to handle this for you in about a minute.

The process of hardening steel and tempering is somewhat exacting. Getting the temp high enough, and cooling quickly enough with inducing too much brittleness.

The process of softening the steel is not very exacting. I agree that you should get it to a reddened color (best seen in subdued light), and then let this cool down over a few minutes by taking the torch further back from the metal and letting it cool.

To minimize the heat on the socket end, you need to keep it covered with a wet rag or the equivalent.

[Paul Rhoney]

I've been wondering about this myself, cause I've been using O1 tool steel for my truss rods and was never sure if I was supposed to heat-harden them before insertion or if they were OK as-is.

[Steve Senseney]

One other thought--I am not sure what your design entails. You might be able to cut off the head, and either weld, brass solder, or silver solder onto the gear.

[Tom Sommerville]

Paul,

Don't harden truss rods, hardening makes the steel brittle.

Not sure it's necessary for Andy's contraption either.

I've softened old sawzall blades with Map Gas while cooking on the grill. After the steaks cooked, I heat the steel with the torch and then toss it in the coals. Next morning I do whatever shaping I planned heat it red hot and quench in vegatable oil. Smells like a fish-fry. Tempering follows. as Al relates above.

[Andy Birko]

Tuning hammers are hardened because they have an 8-point socket that goes on 4 point tuning pegs all the time. Hardening just makes the socket last longer which also lowers the likelihood of rounding over the tuning peg. It's not essential that they're hardened but it is a nice to have. I might do a re-harden once I cut and drill the shaft but like you, I don't think it's essential for this project.

I had thought of welding etc. but I think that for this project, drilling and pinning will be the simplest solution. The MAP gas/charcoal thing sounds like the best idea so far.

[Bob Menzel]

A stupid thought: maybe there's a drill bit that is harder than the steel you're trying to drill that would work and avoid annealing and retempering?

I just bought a couple of end mills from Kodiak Cutting Tools and tried to shorten them using 2 different types of Dremel cut-off wheels. I didn't want to over heat them and kill their temper, so I didn't bear down on them for any length of time. I hardly made a dent in them and gave up. Perhaps Kodiak has a bit that would work for you, or offer some advice.

[Andy Birko]

um....no way

[Bob Menzel]

I know, it's not done like that in the real world...do your cutting, milling, drilling; and then temper. Like I said, 'a stupid thought'...but that's one of the reasons I come here: to become less stupid.<BG>

[Andy Birko]

Just to put a fine point on it, mill, drill harden then temper. Tempering is actually removing some of the hardness to add some toughness back into the tool.

The process I'm looking at is called annealing which is basically removing any hardening. The process of heating it to a dull red with a torch and then throwing it in the BBQ sounds like a very simple way to anneal the piece I'm going to work with. Hardening is as simple as heating with to a dull red and then quenching in either water or oil (I'll probably use water since I don't feel like making a mess or starting a fire).

There are of course more sophisticated methods of doing it that give better results but for this prototype these old school methods should be o.k..

Drilling or even grinding hardened stuff is tough business (as you found out).

[Mike Sandor]

Andy, do you just need to drill a specific size hole in the hammer? If so, you can in fact buy carbide spade drill bits. These will drill through high speed cutting tools. Your really must use a rigid drill press or better yet a milling machine as you need to hold the drill bit very ridge and control the spindle feed. Light but very even pressure. Cutting oil is a must. You will not get good results using a hand held drill motor.

[Paul Rhoney]

Paul,

Don't harden truss rods, hardening makes the steel brittle.

Thanks very much for that answer, Tom.

[David King]

What size hole are we talking about? Drilling hardened steel with carbide requires slow speed and heavy pressure. The drills are extremely brittle so any misalignment or side pressure will usually shatter the drill. Masonry drills are much more resistant but aren't efficient cutters.

[Nicholas Blanton]

I agree, you won't get great results from drilling with a carbide masonry bit. It might work in theory, but I have never gotten it to work in practice, unless the piece being drilled was quite thin. And you create so much friction that the part you're drilling typically gets very hot.

The regular T-tuning hammers seem to be made of 01 tool steel, and they seem somewhere around Rockwell 45, pretty hard.. Drive the pin out, and you get the wood handle off, right? Not one of the little tiny cheap L- shaped tuning hammers? Don't know how you'd get the plastic molded-on handle off one of those.
If the tuning hammer is made from a tool steel tougher than simple 01 high carbon steel, it will be more difficult - maybe much more difficult- to get it to soften; you would then want to heat it to a gray- to just before it turns red- and you'd have to repeat the process.

It might be possible to heat the shaft to a dull red and let it cool on its own, like people here are saying, but it's small enough that might not anneal it completely because it will cool too fast. I like burying small parts in wood ashes; it takes quite a while for anything to cool down buried in wood ashes, and wood ashes are cheap.

[Andy Birko]

Was firing up the barbie yesterday for dinner so I annealed the hammer. I didn't get MAP gas so I just heated it up to red with propane but soon learned that it was a superfluous step: after dropping it in the coals, it got much hotter than it did with the torch.

You can see in the first pic shortly after dropping it in the BBQ. Next pic is after it was in there for about 5 to 10 minutes. Final pic shows a saw cut on the part (not where I need to cut it but just as a test cut to make sure it was soft). I have a ceramic BBQ so it cools very slowly. I let it sit over night.

The only bummer is that it looks like in spots the coals were hot enough to really soften the steel and the finish isn't so great. I'll be able to polish that out either on the lathe or with sand paper or something like that.

I suppose I probably could re-harden it with the torch but I'm not really sure I have to. It's obviously not as hard as it was but I'm skeptical that I'll wear it out in my lifetime with it in this softened state!