Freeze Quenching A-2 Tool Steel?
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    Angry Freeze Quenching A-2 Tool Steel?

    I have a bend mandrel that I'm working on, and it's machined from annealed A-2 (purchased from Bohler Uddelholm with test reports). All of the machining is complete, and it has been sent out to be heat treated to 62 HRC. Fast forward to now, the part came back with a rusty layer on it, and it's definitely not 62 HRC. The surface rust made me raise my eyebrows, so I hit it with a file and the file dug right in with relatively light pressure. I also have parts (heat treated by the same company) that are truly 58-60, and the file just skates right off the surface; there's a night and day difference.

    So I have contacted the heat treater that I used, and they told me it had to be heat treated three times, with a final freeze quench as they struggled to get it to 58 the first few times. At this point, I'm assuming this part hasn't even been tempered. It's "as quenched" right now, but there's still no way it's 58. From what I know, A-2 should be able to get up to 64 HRC, so I don't see how they struggled to get 58 (if the part even is that hard).

    Has anyone else had this experience with A-2? I'm just wondering what to do next since I need to use this part, but I need it hard. The paperwork that came back with this part claimed that it was 58 HRC, but I'm thinking of having someone else test it for a second opinion. My final questions are:

    1. If it is hard, just not 58, what should I do with it? Full anneal? Sub anneal? I need it to be at least 60 HRC. I'm also going to go to a different heat treater with a better reputation.

    2. Is there any risk in bringing it to full soft and then hardening it again? Greater risk of cracking?

    Any advice is greatly appreciated. The part dimensions are roughly 2"x2"x16" and care was taken to machine the part leaving no sharp corners/edges.

    img_2303.jpgimg_2302.jpgimg_2301.jpg

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    Quote Originally Posted by controlled_burn View Post
    I have a bend mandrel that I'm working on, and it's machined from annealed A-2 (purchased from Bohler Uddelholm with test reports). All of the machining is complete, and it has been sent out to be heat treated to 62 HRC. Fast forward to now, the part came back with a rusty layer on it, and it's definitely not 62 HRC. The surface rust made me raise my eyebrows, so I hit it with a file and the file dug right in with relatively light pressure. I also have parts (heat treated by the same company) that are truly 58-60, and the file just skates right off the surface; there's a night and day difference.

    So I have contacted the heat treater that I used, and they told me it had to be heat treated three times, with a final freeze quench as they struggled to get it to 58 the first few times. At this point, I'm assuming this part hasn't even been tempered. It's "as quenched" right now, but there's still no way it's 58. From what I know, A-2 should be able to get up to 64 HRC, so I don't see how they struggled to get 58 (if the part even is that hard).

    Has anyone else had this experience with A-2? I'm just wondering what to do next since I need to use this part, but I need it hard. The paperwork that came back with this part claimed that it was 58 HRC, but I'm thinking of having someone else test it for a second opinion. My final questions are:

    1. If it is hard, just not 58, what should I do with it? Full anneal? Sub anneal? I need it to be at least 60 HRC. I'm also going to go to a different heat treater with a better reputation.

    2. Is there any risk in bringing it to full soft and then hardening it again? Greater risk of cracking?

    Any advice is greatly appreciated. The part dimensions are roughly 2"x2"x16" and care was taken to machine the part leaving no sharp corners/edges.

    img_2303.jpgimg_2302.jpgimg_2301.jpg
    Just curious, where do you get the magic "needs to be at least 60Rc"? How do you know 56-58 won't work? If it is a wear issue, I would use D2 or a cpm steel, if it is something else, might want to try S7 (shock resistance), or H13 for work hardening properties...

    Can't say if this the final word, but this shows max hardness at 62, which could be why your heat treat is struggling to get there..
    Speedy Metals Information for A2 Tool Steel Plate

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    I am no heat-treater, but IIRC A-2 requires no quench, but is susceptible to surface decarburization. To me it seems either what you got is not A-2, or the heat-treater allowed surface to decarb. Can you file off 1/16" somewhere to see if it is harder under the surface? Can you get the piece shot with alloy gun to see if it really is A-2?

    If you have lots of time in it, and it is not A-2, perhaps you can have it case-hardened

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    I would wonder if it is really A2? I would look into getting the chemistry checked.

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    The reason I chose A-2 is that we have the original mandrel made by Tube Bending Concepts, and the mandrel itself has the type of tool steel and hardness engraved on it. We decided to remake this one in-house because their lead time was astronomical and we need this relatively soon.

    Another thing is that we send them A-2 and D-2 all the time, and the parts usually come back a charcoal color and hard as a rock. They should be able to hit 60 or at least 58. It just doesn't make sense to me because my file bites into it like it's still machinable. I think they are claiming that it is 58 when it actually isn't.

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    My vote is also surface decarb. If you need it to size you are hosed. If you can stand to polish off the decarb material you will find it hard underneath.

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    Hi controlled_burn:
    You are correct; there's no reason they should not be able to get it to 60-62 RC if it's A-2.
    If they accidentally decarburized it in HT the decarb layer should not be very deep...a few thou unless they cooked the shit out of it.

    So you are left with a deficient piece.
    You could try to have it rehardened.
    You could try to nibble a bit away on the underside to see if it has in fact been decarburized.
    You could get it nitrided if it's to final dimension.
    If you nitride it it's gonna be hard, but only the surface few thou will get the full benefit.


    Cheers

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

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    Quote Originally Posted by controlled_burn View Post
    I think they are claiming that it is 58 when it actually isn't.
    Why dither over something you can measure? Find somebody with a Brinell tester. Get the number.
    Call the heat-treat shop back and say they fucked up, they're not getting paid and they need to do it again.

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    I am with the guys above. It's been decarburized, or improperly treated, or both, and the HT guys absolutely should have controlled that. The "have to HT three times" thing is BS. I get perfectly good results doing A2 in an old electric furnace in my own shop, sealed in stainless foil, air cooling/quenching just sitting on a rack on the shop table, and temper in a very used kitchen oven.

    There is such a thing as cryogenic quenching, but the lack of it is not why A2 isn't hitting RC58 or better.

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    Quote Originally Posted by sfriedberg View Post
    There is such a thing as cryogenic quenching, but the lack of it is not why A2 isn't hitting RC58 or better.
    Not relevant to A2 but Edwards always did cryo on 9310, which might be why I never had any problems with it. I've heard other people complain about how "tricky" 9310 is for ht but with Edwards it was a piece of cake.

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    I would recommend choosing an area of the part that is relatively unimportant, doing 3 hardness tests in that area with the part 'as-is', then giving that same area a really good scrub (like at least a few minutes) with some 220 or 320 grit sandpaper and doing another 3 hardness tests in the same area (at least a 1/16" away from any previous test locations). If the test results vary a lot before/after cleaning the surface this indicates the presence of a decarburized surface layer (which you removed by sanding) and means you'll have to resort to carburization/nitriding to get surface hardness back. If the results are nearly the same hardness before/after sanding then it means the heat-treat itself was the culprit and heat-treating it again (properly) should fix it up...

    Further notes on heat-treating A2:
    A2 coming back from your heat-treater 'a charcoal color' is not a good sign in my opinion. The charcoal color means it's getting exposed to oxygen at some point, and any amount of oxygen exposure during the high heat process (austenitization) means carbon is being removed from the steel and the surface hardness is going down.

    A2 is very easy to heat-treat in an integral quench vacuum furnace as it does not need a particularly fast quench to harden fully. Heat-treat in a vacuum furnace is the absolute best way to deal with pretty much any tool steel, except those requiring an oil quench (or nitrogen steels but they are a different story!).

    All my A2 gets heat-treated in a vacuum furnace with a 1 bar pressure quench in nitrogen, then cryo treatment in between 2x 400ºF tempers and I reliably get 62-63HRC. After heat-treat in a vacuum furnace the only color on the steel will be from the temper so it will usually come back a really nice golden color.

    Hardnesses as high as 65HRC (with 400ºF temper) are easy with A2 if you do a sub-zero or cryo treatment before the first temper, but this will reduce the toughness of the steel.

    Some heat-treat places may not be able to achieve really high hardness in their vacuum furnaces and may not quite understand why... The main reason I have seen is they are heavily loading the furnace and therefore won't be able to quench fast enough with all that thermal mass in there. I pay extra to have my parts run in a furnace on their own to avoid this issue, though I also heat-treat largish batches of parts at a time which makes it economical.

    Happy to answer any questions you might have about heat-treating A2. Heat-treat is a very critical part of my process/product and I control it very tightly, A2 is the main material I work with so this thread might as well have been made for me

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    Unless your heat treat shop has never done large pieces of A2, you have said they know how to do it and you have been happy with their previous work.
    Maybe your supplier pulled a piece of O1 off the shelf by mistake and sent you the correct A2 paper work. No matter how any times you try to air harden it is never going to be right.
    Lots of comments about verifying the material and checking for decarb. The problem is one or the other.
    The shop where I did a little A2 work we put a piece of paper next to the part before wrapping it with the idea that in burning it would remove the oxygen from the pack. It did make the surface of the part a little dark. Seems like a more professional heat treater would use an atmosphere where you would not see that.

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    Quote Originally Posted by magneticanomaly View Post
    I am no heat-treater, but IIRC A-2 requires no quench, but is susceptible to surface decarburization. To me it seems either what you got is not A-2, or the heat-treater allowed surface to decarb. Can you file off 1/16" somewhere to see if it is harder under the surface? Can you get the piece shot with alloy gun to see if it really is A-2?

    If you have lots of time in it, and it is not A-2, perhaps you can have it case-hardened
    So I ended up doing this, and you, along with all of the others here that said surface decarburization, are spot on. I didn't have to file much until my file wouldn't bite anymore. I appreciate your help. My only issue now it that I need to have it hard-chrome plated, and I don't know if I can do that with a layer of decarb.

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    I once worked on a chemical process that involved HF. A LOT of HF. This was truly a potentially lethal service, and so we insisted upon certified drawings, materials etc. Was inspecting a part with a plastic liner. The certified drawing (had the right part number, order number, our customer number and name) showed a part with more pipe flanges than the actual part. And this was a certified drawing. So this might end up being not A-2.

    That said, given that it came back with surface oxidation tells me that it was heated in air, regardless of the tool steel type. Others have pointed out the same thing.

    So, the question depends upon why you used A-2. Especially if A-2 was used for its through-hardening properties in deep sections, you must ensure that its really A-2. Would it be possible to unobtrusively grind off some oxidized surface and check alloy with an XRF gun? If it IS A-2, then it's clearly be decarburised and you have a choice about trying to case harden the surface that was carbon depleted or (if possible) grinding off the decarburized iron on the surface. Neither of these sound like a great option to me but...

    If A-2 was used for its dimensional stability in heat treat, and its not A-2, you could try to HT per the actual material spec. Pack in a pipe with packing compound or cast iron borings and HT, quench in oil. Test hardness. It might retain geometry ok.

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    Quote Originally Posted by aarongough View Post
    I would recommend choosing an area of the part that is relatively unimportant, doing 3 hardness tests in that area with the part 'as-is', then giving that same area a really good scrub (like at least a few minutes) with some 220 or 320 grit sandpaper and doing another 3 hardness tests in the same area (at least a 1/16" away from any previous test locations). If the test results vary a lot before/after cleaning the surface this indicates the presence of a decarburized surface layer (which you removed by sanding) and means you'll have to resort to carburization/nitriding to get surface hardness back. If the results are nearly the same hardness before/after sanding then it means the heat-treat itself was the culprit and heat-treating it again (properly) should fix it up...

    Further notes on heat-treating A2:
    A2 coming back from your heat-treater 'a charcoal color' is not a good sign in my opinion. The charcoal color means it's getting exposed to oxygen at some point, and any amount of oxygen exposure during the high heat process (austenitization) means carbon is being removed from the steel and the surface hardness is going down.

    A2 is very easy to heat-treat in an integral quench vacuum furnace as it does not need a particularly fast quench to harden fully. Heat-treat in a vacuum furnace is the absolute best way to deal with pretty much any tool steel, except those requiring an oil quench (or nitrogen steels but they are a different story!).

    All my A2 gets heat-treated in a vacuum furnace with a 1 bar pressure quench in nitrogen, then cryo treatment in between 2x 400ºF tempers and I reliably get 62-63HRC. After heat-treat in a vacuum furnace the only color on the steel will be from the temper so it will usually come back a really nice golden color.

    Hardnesses as high as 65HRC (with 400ºF temper) are easy with A2 if you do a sub-zero or cryo treatment before the first temper, but this will reduce the toughness of the steel.

    Some heat-treat places may not be able to achieve really high hardness in their vacuum furnaces and may not quite understand why... The main reason I have seen is they are heavily loading the furnace and therefore won't be able to quench fast enough with all that thermal mass in there. I pay extra to have my parts run in a furnace on their own to avoid this issue, though I also heat-treat largish batches of parts at a time which makes it economical.

    Happy to answer any questions you might have about heat-treating A2. Heat-treat is a very critical part of my process/product and I control it very tightly, A2 is the main material I work with so this thread might as well have been made for me
    Thanks a ton for the information. I will try what you mentioned regarding the sandpaper. I do have a feeling it is decarb and not a bombed heat-treat. Another guy mentioned filing away some material to see if it would eventually not bite, and that's exactly what happened when I tried it yesterday. I sent a few other A-2 parts in the same lot to get treated as the part mentioned in my post, and those all came out the same way. The material was even from a different supplier (McMaster), so that leads me to believe I have all good material (legit A-2).

    Anyway, I'll do what you mentioned and report back. I am curious about plating over a layer of decarb though, or what my options are. We originally intended on having this mandrel hard-chrome plated (as was the old one we are copying). Would it be okay to hard-chrome over a layer of decarb? Or would some other process/coating give better results? My gut says it would be a bad idea coating a soft layer with a hard plating. You mentioned carburization and nitriding. Would that be comparable to hard-chrome if I went that route?

    Thanks again for the advice!

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    Hard crome on soft core probably fails if the use is any heavier.

    Spec for long nitriding to get enough deep case
    Longwear Surface Treatments nitriding technical data and cycle times

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    Quote Originally Posted by MattiJ View Post
    Hard crome on soft core probably fails if the use is any heavier.

    Spec for long nitriding to get enough deep case
    Longwear Surface Treatments nitriding technical data and cycle times
    I agree with MattiJ, do not hard chrome over the soft surface. Either try a nitriding method (but make sure it goes deep enough to fully harden any damaged metal), or look into a more traditional case hardening if it won't hurt the properties of the "good" A2.

    Once you have a sound, hard surface, then you can go with hard chroming. One thing you should check - make sure whatever method of surface hardening you use doesn't compromise the adhesion of hard chrome. I'm not sure it's an issue, but I'd look into it to be safe. As well, make sure the parts are baked right after plating to limit the risk of hydrogen embrittlement.

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    Quote Originally Posted by Milland View Post
    I agree with MattiJ, do not hard chrome over the soft surface. Either try a nitriding method (but make sure it goes deep enough to fully harden any damaged metal), or look into a more traditional case hardening if it won't hurt the properties of the "good" A2.

    Once you have a sound, hard surface, then you can go with hard chroming. One thing you should check - make sure whatever method of surface hardening you use doesn't compromise the adhesion of hard chrome. I'm not sure it's an issue, but I'd look into it to be safe. As well, make sure the parts are baked right after plating to limit the risk of hydrogen embrittlement.
    Thanks for the tip. I'm sending the part back this week, so I'll report back when we make some progress.


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