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  • Annealing hard metal in a fireplace?

    I've seen it mentioned on the forum that hardened steel can be annealed in a wood burning fireplace. After a few years of not using ours, SWMBO & I decided to fire it up this winter (friggin' cold!)

    I have a 2 1/2 ft. piece of auto axle that I drug home a couple years ago, thinking I'd make something with it. The darned thing is hard as Superman's kneecap and I'd like to soften it. If I simply laid it on the bed of coals it seems to me that it would slowly settle down through the coals and end up too close to the fireplace floor to stay hot.

    Do you just lay it across the top of the andirons, pile on the wood and forget about it for a day? Or do you tie it loosely to the bottom side of of the andirons so it'll stay close to the top of the coals? If so, what kind of wire?
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    Let the fire but down to to a good bed of red hot coals higher than your grate. Lay the steel on the bed of coals and build a good fire on top. Let that buren down to a bed of coals and rake the coals up over the steel and leave it to cool down slowly. As long as you got the steel red hot after it cools slowly it should be fully annealed. If you did not get it hot enough you could repeat. Gary P. Hansen
    In memory of Marine Engineer Paul Miller who gave his life for his country 7-19-2010 Helmand Province, Afghanistan. Freedom is not free, it is paid for with blood.

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    • #3
      I welded up an annealing box from 1/8" plate, filled it with ash from the fire place and it works perfectly. Get the material red hot, put it in the box and next day take it out......files etc come out very soft.

      Now, at 2.5 feet you don't want to weld up a box, but the take away is 1) that ash provides a great insulator and isn't bother by the high temps and 2) as Gary said, it won't work if you can't get the metal hot enough....I can't see how the fireplace will get you there, especially for a 2.5' piece, then again i haven't tried so maybe its possible

      can you torch into some manageable sizes ?

      When i need to get a largish piece, like a foot long, red hot, I build a temp structure out of insulated firebrick, bottom, sides, roof and heat the item red hot there and drop it in the annealing box. Last one i did was a big ass file - with the IFB structure i was able to leave the O/A in the corner and got it done with propane, cheap sob that i am.

      anyway, your challenges are getting it to temp and then having it cool slowly.....I'd be thinking the fireplace won't generate enough heat but the ashes you make could be very handy. maybe set up on a couple of pieces of brick in the fireplace (without the grate), lay the piece across the the brick more or less surrounded by ash and get to temp with a a torch then cover it over with ash. just an idea, maybe some domestic issues with wheeling the O/A cart into the house though
      Last edited by Mcgyver; 01-21-2009, 11:45 PM.
      .

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      • #4
        I don't think a fireplace will do the trick either. It has too much airflow to get hot enough. I anneal my pieces in our airtight wood stove which is a great big old Fisher with a box about three feet deep and lined with firebrick on the bottom half. I build up a good fire and let it burn down the toss in the material and keep the fire stoked as usual. Then I let it burn out and cool overnight. Next am it is still pretty hot but below the quenching temperature so it can come out and be washed.
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        • #5
          I don't think you need to get it all that hot, relatively speaking. It's almost certainly just high-carbon steel. To anneal it, you don't need to get it as hot as you would if you were trying to harden it. In fact, just "dull red," then cooled slowly, ought to do it.
          ----------
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          • #6
            Thanks all. I'll try Gary's method since it looks easy. As luck would have it, the temps are climbing and are supposed to be back in the 60's (!!) on Sat. when I was going to try. Never fails; plan something & the weather won't cooperate. I'm not going to complain too loud though.

            McG I was hoping to get the middle soft enough to cut it in half in my bandsaw and then throw 'em back in again. I don't have a torch yet but do have my eye on a set.

            As far as the heat goes, if I open my damper wide open and close the glass doors, leaving the little sliding vent at the bottom open, the coals get REAL hot. I've seen the middle bars on the grate glowing pretty red! I guess I could hit it with a hairdryer to stoke it up even more if need be.
            Milton

            "Accuracy is the sum total of your compensating mistakes."

            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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            • #7
              Sorry, the fireplace trick does work. Fuel and location may change but the fireplace was the standard annealing furnace for Muddle Ingineers in the UK before the "Central Heating" brigade took over.

              The fuel used was coal and the text of many articles in the ME were " Place in the burning fire and leave overnight "

              Regards Ian.

              Annealing temperature is dependent on material being annealed.
              Last edited by Circlip; 01-22-2009, 11:10 AM.
              You might not like what I say,but that doesn't mean I'm wrong.

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              • #8
                I think it all depends on this question --- "can you get it glowing cherry red"

                If so then your fireplace will work just fine, some things to consider

                hardwood creates hotter coals that last much longer -- something like pine will due for a piece thats not as thick but a fiery bed of hardwood coals will get some fairly large material cherry red hot.

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                • #9
                  I've annealed a car axle in my fireplace and there wasn't much to doing it. I laid the axle on the grate (about 4" above the bricks), built a fire using relatively small wood (so it would burn fast and hot) and kept it going for a couple hours. The fire was much hotter than I expected - the axle actually turned dull red.

                  I left it on the grate over night to cool slowly. The axle, which had been really difficult to cut before annealing, turned very nicely afterward and became several MT3's for my mill tooling. The axle spline is visible on the leftmost holder:




                  John

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                  • #10
                    Originally posted by GadgetBuilder
                    The fire was much hotter than I expected - the axle actually turned dull red.
                    Steel's annealing temperature (when the carbides go back into solution) is 1341؛ F, which is dull red

                    Anything below that temperature won't do much.
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      I used a wood burning forge, but I think a fireplace would work too. Like you mentioned, with the damper open, the doors closed and the vent open, you can get things going real well. You'll just have to work a little more at it.

                      Once I got my axle a nice cherry red, I dropped it in a bucket of kitty-litter. I came back 24 horus later and it was still warm! Anyway, it was soft as butter and easy to machine.

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                      • #12
                        Can stay hot a LONG time

                        Originally posted by Fasttrack
                        I used a wood burning forge, but I think a fireplace would work too. Like you mentioned, with the damper open, the doors closed and the vent open, you can get things going real well. You'll just have to work a little more at it.

                        Once I got my axle a nice cherry red, I dropped it in a bucket of kitty-litter. I came back 24 horus later and it was still warm! Anyway, it was soft as butter and easy to machine.
                        I once shoveled ashes out of my woodstove after it had been out for a week. I could put my hand on top of the ashes and not feel any heat. I shoveled them into a plastic bucket, and melted it. The bottom of the stove was still quite warm. I've since switched to a galvanized steel bucket.
                        .
                        Mike

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                        • #13
                          Originally posted by Circlip
                          Sorry, the fireplace trick does work. Fuel and location may change but the fireplace was the standard annealing furnace for Muddle Ingineers in the UK before the "Central Heating" brigade took over.

                          .
                          I can accept the fireplace gets hot enough, like i say, haven't gone the route before, but the point is not will it work for any annealing, but will it work for a 2 1/2' length ......that's a very big fire! It takes a fair bit of heat to get big chucks to red all over...not sure how you'd make a fire to engulf all 2 1/2' at temp as well as have it buried in coals for a slow cool.
                          .

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                          • #14
                            I am glad to see a couple of folks addressed the specific question of whether a presumably forged axle shaft would anneal. I am no metallurgist, but I know some alloys do not return to their annealed state well. It sounds like this may just be a medium to high carbon steel.

                            I think I would be tempted to make a little "tunnel" out of fire brick and sort of pack the thing in charcoal and light it up. This scheme would allow you to heat it evenly and to use forced air (maybe a leaf blower at idle) at one end to generate the needed heat pretty quickly.

                            The only concern I might have (and maybe someone could address) is whether the charcoal might tend to provide enough carbon on the exterior to case harden it if left in there. I think after it reached red heat I would pull it and bury it in dry sand to slow cooling. I have done that with smaller parts and it seemed to work well.

                            Paul
                            Paul Carpenter
                            Mapleton, IL

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                            • #15
                              I once shoveled ashes out of my woodstove after it had been out for a week. I could put my hand on top of the ashes and not feel any heat. I shoveled them into a plastic bucket, and melted it. The bottom of the stove was still quite warm. I've since switched to a galvanized steel bucket.
                              My father caught the house on fire doing that once. Shoveled week old ashes into a metal bucket, then set it outside, next to the house.

                              The fire was contained before significant structural damage occurred. New siding and a few new studs were needed.

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