Do the physical properties of 4340HT (249-301 BHN) chage when torched?
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    Default Do the physical properties of 4340HT (249-301 BHN) chage when torched?

    I have 12 pcs to quote. The parts are about 300#'s of 4340HT, and need repairs to the bottom surface of the counterbores.
    The Ø2.5" bolts that held the piece down for 10 years, were rusted to the main part.
    The end user used either an Oxyacetylene torch, or an oxy lance to remove the bolt heads.
    They unfortunately gouged (severely) the bottom surface of the counterbore.

    So, the actual question:
    Will the base material be much harder, since the base material was taken above critical, and instantly cooled by the surrounding material?

    Or is there a way to predict the machinability of the part, for a quote's sake?

    My first thought was to price it high enough, that there would be plenty of compensated "trial & error" time.

    Thanks,

    Doug.20200108_131719.jpg

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    Quote Originally Posted by doug925 View Post
    I have 12 pcs to quote. The parts are about 300#'s of 4340HT, and need repairs to the bottom surface of the counterbores.
    The Ø2.5" bolts that held the piece down for 10 years, were rusted to the main part.
    The end user used either an Oxyacetylene torch, or an oxy lance to remove the bolt heads.
    They unfortunately gouged (severely) the bottom surface of the counterbore.

    So, the actual question:
    Will the base material be much harder, since the base material was taken above critical, and instantly cooled by the surrounding material?

    Or is there a way to predict the machinability of the part, for a quote's sake?

    My first thought was to price it high enough, that there would be plenty of compensated "trial & error" time.

    Thanks,

    Doug.20200108_131719.jpg

    Unless they dumped water on it I would expect them to be softer around where the bolts were torched. A lot depends on how long they were in there and how long the heat had to soak into the part. Its anyones best guess. Can you test one with a file or one of those portable hardness tester things that are like an automatic center punch?

    If you are concerned about it being to hard to machine, I have never been able to get 4140 hard enough that I couldn't machine it. I've even played around with quenching it in water and it still didn't get "that hard".

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    This is 4340, not 4140. The torched areas might well be harder. Might be cheaper to just have them all annealed and then re heat treated after you rework them. If someone was hacking on them with a torch, they are probably not precision parts.

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    Yes, it likely will be harder. Won't be softer without a slow cool, especially if the part is substantial as it will cool quickly because at 300 pounds the bulk of the cool steel will suck the heat away from the burned area pretty rapidly. Whether it will be hard enough to cause difficulty with machining you'll just have to find out for yourself. I would just add the cost of getting a carbide endmill/spotface to the quote as insurance and split it across the parts.

    Edit: I see now that these holes are for 2½" bolts. Didn't look before, thought they were for smaller bolts. You should be able to handle touching those CBs up with a boring bar and a cemented carbide tool with no issues.

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    My guess is harder than woodpecker lips. How deep depends on the skill of the guy with the torch. But since he tore up the seat, I'm guessing the skill level wasn't high.

    I'd put a best efforts line in a quote, maybe provide a replacement price.

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    Every piece of flame-cut 4340 that I have seen has been nasty on cutters. Not something I would want to climb cut.

    It can help to spot anneal the holes with a torch, and lay a blanket or something that insulates over it to slow down the cooling.

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    Quote Originally Posted by gbent View Post
    My guess is harder than woodpecker lips. How deep depends on the skill of the guy with the torch. But since he tore up the seat, I'm guessing the skill level wasn't high.

    I'd put a best efforts line in a quote, maybe provide a replacement price.
    This. 300lbs steel chunk to cool the surface and high energy output cutting torch to heat it fast. 60 HRC on a bad day.

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    I would be interested to see what the hardness actually ends up being. 4340 can't even get up to 60C unless you nitride or carburize the surface. It's 4140 with some nickel which does a little but not much to the hardenability. Steel has crap for thermal conductivity and when flame or induction hardening 4340 a liquid quench medium is still required. I would venture to guess that someone who jacked them up with a torch was likely in there for some time and got the piece itself quite warm. And likely left the hot stud and slag sitting there till the whole piece cooled off. Just a couple hundred degrees is enough to slow the cooling from hardening to normalizing.

    I just don't see them being very hard. Would I be shocked if they ended up being a little harder, no, but I don't agree with the majority that they will definitely be harder.

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    What Matti says. 50 HRC if you are lucky, more likely to be touching 55HRC. I've had 4340/EN24 that carbide inserts wouldn't touch. CBN will do a good job if you can get the right inserts for the boring tool and can get a high enough SFM.

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    Quote Originally Posted by Hardplates View Post
    I would be interested to see what the hardness actually ends up being. 4340 can't even get up to 60C unless you nitride or carburize the surface. It's 4140 with some nickel which does a little but not much to the hardenability.
    54 HRC minimum and 60 max on jominy test bar. To about half inch depth.

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    Quote Originally Posted by MattiJ View Post
    54 HRC minimum and 60 max on jominy test bar. To about half inch depth.
    That's with water and no tempering. Perhaps I should have said in a useable condition. There is no way in hell a 300 pound chunk of steel that's had a torch on it will cool as quickly as a 1" bar sprayed with water.

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    I don't know for certain but I expect the mass quench effect of the surrounding metal to be more severe than a water quench. Better include the cost of annealing and reheat treatment in your proposal.

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    Quote Originally Posted by Illinoyance View Post
    I don't know for certain but I expect the mass quench effect of the surrounding metal to be more severe than a water quench. Better include the cost of annealing and reheat treatment in your proposal.
    May not be more severe but I bet it's pretty close. When actual heat treat is performed with an oil or water quench, don't forget that the entire part is heat soaked. The rate of cooling isn't instant there either, by a long shot. The mass of the part plays a big time role in how fast it heats and cools. A torch cut into a 300 pound block of steel just long enough to scarf some bolt heads would probably be just about cool enough to touch as soon as the torching was finished.

    I have actually run up against this exact problem myself, so I know for a fact the steel will be hard where it was torched. Exactly how hard is the only question. Carbide should take care of it since all it should need is a quick touch-up to flatten it back out.

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    Hi Doug:
    I'd make it cost plus and would refuse the job if I couldn't get my way unless there's something really really special about that particular customer.
    After all, you didn't fuck it up in the first place...why should YOU take the risk on the price.
    If they want your expertise, equipment and effort they can bloody well learn to pay for it!

    There you go...that's my pissy rant for the day!
    But seriously...there are real unknowns here that you didn't introduce into the job.
    Make the customer put some skin in the game.
    If they freak out, tell them you'll give them a call if you're getting above a set price and they can then decide if they want to proceed beyond that point or not.
    That gives them a bit of control, but you don't have to take it in the shorts if the job turns into a disaster.
    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Thanks all!
    I quoted the job with a caveat regarding the potential hardness variations, possible extra run time, and extra tooling costs.

    I also asked for 1 piece to test out, if they cannot live with the caveat on my quote.

    Doug.

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    No CBN in the toolbox so if hard it means slow times?

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    I realize that the quote has been submitted, but I wanted to mention an aspect of torch work that has been overlooked. Torch cutting frequently (usually) adds carbon to the steel in the molten zone, which cools rapidly after the heat is removed. Even low carbon steels can hit the high 50 to low 60 Rc range as a result. Those who've tried to mill through a torch cut know what I mean.

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    It’s unusual to have any thermal re-solution under the surface with Oxy/fuel, arc/air or plasma/arc gouging. You DO get an oxide skin that cutting tools don’t like much though… The odd duck would be steels with boron added (like T-1) that do get a significant hardness bump just under the oxide scale.

    With torch boo-boos like these most folks usually take a pencil grinder & stone (slow) or a carbide burr (faster) to clean it. I like the latter & when the orange spark shower stops you’re good to go for normal cutting tools.

    If for some reason somebody IS really worried about re-solution it’s normally found by nital etch, magnaflux or die penetrant. The surface will look like busted safety glass...

    Good luck,
    Matt

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    My concern is less with a hardened heat affected zone (heck you guys already have addressed working thru it)

    The quench could have caused some cracking.

    And then, all the annealing in the world won't fill up the crack.

    It's made from alloy (and not play doh) for a reason, my concern is
    loading in use, and cracks.

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    UPDATE:

    The parts arrived, and were hardness tested in the gouged out surface, and at the top of the counterbore diameter.
    The result was Rc65 or higher!
    I didn't think that 4340 would get anywhere near that high, even with the extra carbon from the lance.

    The parts were picked up, and the PO is on hold until my customer decides on the annealing/re-heat treating.

    Doug.

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