Question for metalurgists

[doug]

    I know there are a few professional metalurgists on the list and am looking for an informed opinion.  When annealing, how long does a piece of steel have to remain above the austenitic temperature, for the carbon molecules to migrate to the soft location?  I realize that some have heat treating furnaces and can hold a piece of metal at a given temperature for as long as they wish, but for many of us, annealing is heating the steel red hot with a cutting torch or similar, keeping it red for one or several minutes depending on one's opinion of the day, and dropping it into a bucket of ashes or of lime and leaving it until cool.  About the only thing we can vary is how long the metal is kept red hot before the cooling process starts.

cheers Doug

[LRB]

  Not a metallurgist, but your question is not as simply answered as you might think. Much depends on the steel. Simple, or alloyed. Hypoeutectoid, or hypereutectoid. With hypoeutectoid or eutectoid simple steels, which is to say .85% carbon or less, a couple of minutes should be enough. With simple  steels with more than .85% carbon,  you can do a heat cycleing anneal with a series of heats below the magnetic range, say 1300° or so, letting the steel air cool in between heats, and make it soft enough to drill and file. You can anneal hypereutectoid simple steel as if it were a hypoeutectoid, then heat cycle to make it even softer. Alloyed steel can be a much different animal, and tecniques,  heat ranges, and soak times in annealing can vary greatly.

[D. Taylor Sapergia]

Doug, I had a new tumbler that needed to be drilled and threaded for the hammer screw, and a new drill would not touch it...left a shiny spot but wouldn't cut.  I tried burying it in sand after salmon red heat, with no effect.  Then ash...same result.  I phoned Thomas Curran, who knows everything about steel, and he said to build a little hardwood fire with a nice bed of coals, heat the tumbler orange, and drop it into the fire.  I thought that sounded interesting, so I phoned Cody Tetachuck, who also knows everything about lots of things, and he told me the same story.

So, I did.  I built a little fire of dry birch with some curly maple chunks for longer sustained heat, took my portable oxy/act torch into the back yard, heated the tumbler orange, and dropped it into the coals.  When the fire was just ash, I dug around and found the tumbler, put it into the lathe and "le voila"...cut beautifully.

[KentSmith]

Well, I have a bucket of hardwood ashes from my wood stove.  First I heat a 3/8" X12" piece of wrought iron to orange and put that in the ashes.  Then I bring the piece I work on in the forge and heat to orange.  I remove the wrought and put the steel in where the wrought was.  I leave all day or over night - 8 hours but probably waiting until the ashes are cold would suffice.  I then have no problem.  I am using 1084, 1085 and 1094.

i think the bottom line is heat to orange, put the piece in VERY hot ashes and let it sit until cool.  A couple hours?  Bring the heat down very slowly.

[doug]

     an alternate solution for annealing small pieces is to bury the piece in dry sand in an iron box and heat until the sand is red hot and let that cool slowly.  Essentially you are increasing the thermal mass.  To some extent that is what Taylor was doing.  With sand you do have to be careful not to keep the sand red hot for too long or you get glass with a small part imbedded in it.  :>(
    Back to the original question, when dealing with a larger piece of metal, the question in my mind was does the carbon in a piece of steel move around very quickly relative to the crystaline structure or does it slowly migrate and I think the answer that I got was that it migrates somewhat slowly and requires at least a couple of minutes or 3 to move into its hard location.  I said soft location because my brain is old and foggy.  It moves into the hard location and freezes there when you quench it but slowly returns to soft location if cooled slowly.

cheers Doug

[davec2]

Get a used copy (or new) of the Machinery's Handbook.  Mine is the 21st edition.  All the information you will need about heat treating, annealing, and about 35 million other things.

As taylor noted about Tom's technique, sustained heat and very slow cool down will usually result in a complete anneal.  I don't bother with the torch for gun parts.  I put a dozzen or so Kingsford charcoal briquets in a coffee can with a few holes at the bottom edge for air and get them well started.  Drop the part in and fish it out of the ash tomorrow.   Bigger part?  Same technique, just a bigger fire.  Had to anneal a part the size of a football not long ago.  Just made a bigger charcoal fire in a pit in the yard.  Worked fine.

[J.D.]

Dang, that is what I was going to suggest, but thought better of it, since my knowledge of metallurgy is not up to par with other members of this forum.

IMHO, I prefer to surround the part with wood/charcoal before the fire is lit, so's the temp of the part increases fairly slowly, as compared to putting a cold part in a hot fire.

IMHO, when it comes to heating steel parts, slower is better.

God bless

[Stan]

The longer you keep a piece of carbon steel at critical temp the more carbon you loose.
Heat the item to a clear uniform cherry red & imediately put it into powdered wood ash (not lime).  For small parts I use a coffee can full of ash and plunge the item into the center. Let it cool overnight.

[LRB]

  Again Doug, that depends on the steel. As far as hardening the steel, a soak at the prescribed quench heat is always beneficial, but some require less soak time than others. 1 to 3 minutes soak time is pretty much a minimum for simple steels such as the 10XX types, and you will not lose any appreciable amount of carbon.  Move up to 01 steel, you need about 15/20 minutes of soak at quench temp. Move again up to A2 you need 30/45 minutes, with a lower preheat for the same amount of time. 01, and A2 do require decarb protection, and a controlled heat source.

[Dphariss]

    I know there are a few professional metalurgists on the list and am looking for an informed opinion.  When annealing, how long does a piece of steel have to remain above the austenitic temperature, for the carbon molecules to migrate to the soft location?  I realize that some have heat treating furnaces and can hold a piece of metal at a given temperature for as long as they wish, but for many of us, annealing is heating the steel red hot with a cutting torch or similar, keeping it red for one or several minutes depending on one's opinion of the day, and dropping it into a bucket of ashes or of lime and leaving it until cool.  About the only thing we can vary is how long the metal is kept red hot before the cooling process starts.

cheers Doug

If in doubt I heat it to red then cover with ash to insulate it. I use the ash bucket from the wood stove. Unless a very large piece I don't think the time frame at red is important.
Some steels are AIR HARDENING.
It is also possible to make a piece of steel workable by heating to just short of red, past blue to gray and allowing to cool.

Dan