help sought with tempering spring steel rod

G'Day,
We've got a piece of 1/4" spring steel rod (as hard as piano wire) - typical grade spring steel (1080) which we need to make a tightly wound shape with.
The local spring guy told us to form the tight curves (around a 3/8" bolt) by annealing the wire to dull red and winding around the form, but obviously this then leaves the rod in an annealed state.
How to we then harden and temper the rod back to a spring?
The shape is best visualised as the metal frame of a slingshot (it's actually part of a motorcycle seat) so it won't make full contact on a flat surface.
If someone with some experience in this type of work can tell us the proper way to harden and temper this we'd appreciate it. We have oxy-acetylene, and propane torches available, and our previous experience with this spring steel grade has been to quench in oil. If you can tell us the colours to heat to, or temperatures it'd be great - similarly if you have any tempering methods to acomodate the irregualr shape (is a domestic overn OK to use?)
Thanks, Des
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On Mon, 01 Feb 2010 10:06:40 +0000, des bromilow wrote:

I don't see how you're going to get it up to uniform temperature for quenching without some sort of furnace. You may be able to get away with something primitive built up from a pile of firebricks, but I think you'll need something to contain the heat. Just torches probably won't give you even enough heat.
--
www.wescottdesign.com

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You need to heat it to about 1500F to 1600F, oil quench, temper at 900F to 1100F depending on exactly where you are trying to get to. Neither one of those temp ranges is easy to get to with just a torch or a kitchen oven.
des bromilow wrote:

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So what happens when this spring breaks?
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Here's one more opinion:
Why not take it to the local spring guy after you have shaped it?
If you do decide to do it yourselves with a torch, here's my 2 cents: Depending on how you heat it to bend it, you may well develop stresses that would call for normalizing before hardening. I agree with an earlier post that you need at least an "oven" made with a half dozen fire bricks if you want to have any chance at consistent finished properties. As a blacksmith, here's what I'd do (if you don't take it to the spring guy: Make an oven by just making a 3 sided box with bricks, with a couple of bricks for a top, and an open front. This "oven" would need to be sitting on flat surface made of more firebricks. Use a rose bud on the OA or Oxy propane torch. Heat the insides of the oven until the walls start to have some color. Place the part inside and continue heating until the part starts to glow a dull red. Either keep the torch moving or don't directly let the torch hit the part at all because you don't want to overheat any local area of the part. Get a magnet on a steel rod. Use the magnet to determine when the part reaches its critical temp, a medium to high red color, or therabouts. When the part isn't attracted to the magnet, you have reached or exceeded the critical temperature for hardening plain carbon steel. Purists will grab me by the throat, but in general, it works. Bright orange is too hot for this process. Hold that temp at low to medium red for at least 10 minutes to normalize it some. In industry, they'd probably hold for a couple of hours. Let the part cool until no color shows, in the oven. Reheat the part just to the point where the magnet is not attracted to the part. This is roughly the critical temp for hardening. Grasp the part in a location that isn't critical, (not the coil part, for instance) and QUICKLY dunk it into some used motor oil that has been previously heated to about 200F to drive out any water and then allowed to cool to about room temp. Swich the part around vigorously while keeping it full submerged. If you don't have the part fully submerged during the first few seconds of the quench, I can almost quaranteee unwanted results. (cracking, for instance.) I would normally consider this 1080 plain carbon steel to be water quenching, but being the part is this thin, you may get a full harden. To test for hardness, attempt to file on the part in a nonessential location. The file should totally skate on the surface. You should not be able to cut the part with the file at all. If this is not the case, you will need to repeat the process, using water as a quenching medium. As I look back over these directions so far, it is clear the you must try this process first with a test piece, not your finished product. If you still haven't decided to take the part to your "spring guy" and if you DID get full hardness in the quench, then shine up the part on buffing wheel in preparation for the tempering process. And when I say "Shine up" I mean that you need to eliminate any nicks or gouges that might have occurred during the forming and handling of the part. These nicks will cause stress points where cracks WILL occur. I guess I should have had you do this "nick removal" before you ever heated the part. Okay, so now you have a bright, shiny part, ready to temper (ie: remove some of the brittleness while still retaining the springiness characteristics: Place the part back in the oven and begin GENTLY heating the oven walls. Gently! The goal is to reheat the part until "temper colors" begin to appear. These are the rainbow type colors that you see part way up a shaft that you have stuck in front of a torch. Each color represents a certain temperature range. The higher the temper temperature, the softer and less srping-like the part becomes. Once you have exceeded the temper color you really wanted, it is too late. You have to anneal the part, reharden, and retemper. So SLOW and EVEN is the order of the day. As you SLOWLY heat the part, it will first turn from shiny to a light straw color, dark straw, brown, reddish, purple,and then some shades of blue. When you get the part to a medium blue, grab it out of the oven and qunceh it in water. If the colors are changing rapidly, you have to move REAL fast to keep from going to a higher temperature, which means a softer part. Another reason to do a test piece first. That's all there is to it.
Pete Stanaitis ----------------------------

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We'd love to have the spring guy deal with it - he's the pro and quite good at it, but he's on the way to closing down. - typical story - urban encroachment surrounding his workshop, new houses full of whinging greenies who complain about the smell and noise of the machinery/forges - and with the various govt "suggestions" about taking the older vehicles (mostly trucks) off the road, the newer ones having air springs, he's pretty much out of work, and out of the nieghbourhood. He's lucky to work 8 hours per week spread over two or three days so it's pretty much a given we can't use him for much longer.
The piece is to replace a seat part in a veteran motorcycle, so appearance and function is what we're aiming for. We'll go with the firebrick forge/hearth and try and make it work.
Thanks All, Des
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The classic method for repeatable spring tempering is heating in a lead bath. For higher temperatures, there are salt baths. Obviously, the bath has to be large enough to handle the part, if it's too big, I'd suggest hunting up a heat treat outfit with a suitable furnace. You can get thermometers made for lead baths, salt baths run hot enough to require a pyrometer.
If you're stuck with torches and it's a small part, make up an enclosure with fire bricks first, then heat up beyond the non-magnetic point and quench in oil. Temper in the lead bath afterwards.
This assumes that your wound kinks aren't done too tightly and there's no nicks or abrasions in the surface. Otherwise your rider is likely to get the point when it breaks! It's going to take some testing before you can strap in onto the bike.
If it's true music wire, you might end up with a brittle mess. Music wire depends on the drawing process for a lot of its spring. Annealing will remove that stress, leaving you with a not so-springy result. Hardening and tempering afterwards is kind of iffy, in my experience. It usually breaks at a bend.
Stan
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wrote:

The classic method for repeatable spring tempering is heating in a lead bath. For higher temperatures, there are salt baths. Obviously, the bath has to be large enough to handle the part, if it's too big, I'd suggest hunting up a heat treat outfit with a suitable furnace. You can get thermometers made for lead baths, salt baths run hot enough to require a pyrometer.
If you're stuck with torches and it's a small part, make up an enclosure with fire bricks first, then heat up beyond the non-magnetic point and quench in oil. Temper in the lead bath afterwards.
This assumes that your wound kinks aren't done too tightly and there's no nicks or abrasions in the surface. Otherwise your rider is likely to get the point when it breaks! It's going to take some testing before you can strap in onto the bike.
If it's true music wire, you might end up with a brittle mess. Music wire depends on the drawing process for a lot of its spring. Annealing will remove that stress, leaving you with a not so-springy result. Hardening and tempering afterwards is kind of iffy, in my experience. It usually breaks at a bend.
Stan
====================================If it's 1080, it's not music wire, but its hardenability is close. Music wire runs between 0.9% and 1.2% carbon. That doesn't make it any more quench-hardenable, but it does increase the chance you'll get maximum hardness from *drawing*, which is how music wire is hardened.
However, if this job is the least bit safety-related, I agree with the other posters that this job should be done in a good furnace. The configuration doesn't lend itself to heating with a torch, even with a makeshift firebrick furnace. The "local spring guy" probably can toss it in with a batch and do it cheap.
Lead baths can be effective for tempering springs (nitrate salts are more common in commercial use, for up to around 1150 deg. F), but, again, the spring guy probably has an electric or muffle furnace for doing that work with some precision. And you have to temper the part immediately after the quench-hardening step, so you can't have someone else harden it and then temper it yourself.
All in all, it sounds like it's worth having it done by a specialist.
--
Ed Huntress








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Ed Huntress wrote:

Or time to redesign the part to use an off-the-shelf spring.
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