Grade choices for steel with bainite inclusions
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  1. #1
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    Default Grade choices for steel with bainite inclusions

    So.... Hello. Long time no post. I have been away from the board for a while. As many may remember, I was working on transferring product southward. That work is now almost complete and since last June I have been transitioning into the steel machining side of things. Lots of work to do and a considerable amount of learning also.
    However, that is not the topic of this post.

    We have two forging suppliers, the material specifications are the same for both, but from material properties investigations, there are considerable differences in the two, although both are within the specifications. We have lots of issues with chipping inserts with one supplier and the investigation has found long bainite inclusions and particles >500 microns in size in the material. This was there prior to forging as the bands of bainite flow with the forging shape. Bainite is not found in any significance in the other forging supplier parts.

    Has anyone dealt with this before and have you found a reliable carbide grade to handle it? Our steel is a Cr-Mo alloy and seems to respond better to grades and coatings typically used for stainless steels from my experiments and testing so far.

    Appreciate any information you can share.

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    Glad you're back. Just this morning I was wondering what happened to Tony? As far as the Bainite question goes, I don't have a clue as I have never knowingly dealt with it.

    I did a little reading and my feeble mind sees a relation between cooling time and formation of Bainite from the original pour. So your forgers supplies have some difference in production. If forger that has no Bainite can reliably produce product, i would seriously consider staying with them.

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    Quote Originally Posted by alphonso View Post
    Glad you're back. Just this morning I was wondering what happened to Tony? As far as the Bainite question goes, I don't have a clue as I have never knowingly dealt with it.

    I did a little reading and my feeble mind sees a relation between cooling time and formation of Bainite from the original pour. So your forgers supplies have some difference in production. If forger that has no Bainite can reliably produce product, i would seriously consider staying with them.
    Unfortunately, neither supplier can produce the required volume alone. Additionally, we would not want all of our eggs in one basket either. There are already enough supply chain disruptions as it is.

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    And ....I think more important, finding out how it got there, and how to eliminate it.
    Repeat-ably, and reliably.

    Specifying that on the PO, and testing for it.

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    Is it possible to "forensically" compare both suppliers processes and then have the supplier of the problematic material change their process?

    Could be something as simple as where they get the material to begin with? Or are these guys big enough that they make it "in house"?

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    Tony, I'll have to bone-up on my iron-phases to give you a more scientific understanding of the problem.

    My gut tells me that you're forgings are 4140-ish material, and that they're heat-treated after forging, and before machining. Based on experience, I'm guessing that you're in the 32-38 HRC range, correct?

    You never mentioned weather you're turning, indexable milling, or solid milling. Can you provide some more details? Turning, threading, face-milling, end/profile milling, boring, etc...

    We machine steel forgings - both of US 4140 material, and foreign-sourced 39mn5cr material. We see better repeatability with the US-sourced 4140 material, and may be buying our parts from your same forging supplier. (Given your location, that is...)



    I'm a little puzzled to hear that stainless grades perform better. Usually when steel-cutting, thick CVD aluminum-oxide coatings rule for performance, when turning & indexable-milling. Those trying to go all-out will start treading into cast-iron grades, as they typically have lower cobalt content & thicker coatings.

    Sometimes cermets for finishing & reaming, though they are picky, and cermets might not like the variation in your forging materials.



    I wonder if you're not seeing more success with the stainless grades, as they may do a little better with chip-control.




    Please let us know what type of cutting you're doing - turning, indexable milling, etc... In the meantime, I'll read up more on bainite inclusions.

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    I'm pretty sure this is a common occurrence in a steel mill roll shop.

    I would be in search roll lathe hand to talk to

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    So do you need to specify something like vacuum degassed, remelt refined ?
    or is it showing up the processing downstream ? Like bad forging press technique ?

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    To answer some questions, turning, grooving and boring. Tight tolerance work of course. I will check to be sure, but I believe the material is like a 38CrMo and I am not 100% sure of the hardness specification but it isn't hard. It is actually sort of gummy which is my thoughts on why the stainless grades work better so far. I apologize for not having all of this memorized yet. Still trying to do my former job along with this one and I am not fully up to speed yet.

    From what research I have done, I believe this is happening when the bar is poured at the foundry, they are allowing it to cool too slowly which allows the phase change to bainite.

    Both forging suppliers are non-us (but different countries), which creates a logistics issue in that we have several shipping container loads of these things to machine on the way and even if it was fixed tomorrow, it would be a couple of months before we got the fixed material. In the mean time, we need to see if we can figure out a way to run these without being down half a shift chipping inserts.

    Thus I have turned to the true experts - the folks of PM. I will confirm material and get more details and post back.

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    I don't have any experience with bainite inclusions. Am I correct in assuming that the inclusions are harder than the base material, and hitting them is chipping tools? If that's the case, then I think you need to treat it more like an interrupted cut and go with a tougher grade than you normally would use in that material. Then you would have to lower the speed so that the hard spots didn't wear out the tool too quickly.

    I think this is an application where high pressure coolant directly applied to the cutting edge could give the tougher grade a better chance of not wearing out from the inclusions. With HPC and a tougher grade, you might be able to achieve the same cycle times as on your other forging.

    Another thing to try would be to cut as much material as you can with a low lead angle tool to spread the cut over a longer area. This would be similar to how high feed machining can be used to cut hardened materials.

    Again, I don't have experience with this particular situation, but that's how I would begin to approach it.

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    When you ordered the forgings, was a specific chemistry specified for the material, or just material properties? It's not uncommon for a forging house to source ingots with unique chemistries.

    Is heat treating out of the question? The efficacy would of course be dependent on the material chemistry, but a full quench and temper might/should solve the problem. I'd pay for heat treating over f'ing around with my machining process any day of the week.

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    Bainite formation temperatures are lower than forging temperatures. I'd suspect that the work is being forged at too low a heat for too long or that it isn't cooling fast enough after forging.

    I'm not a metalurgist, nor to I play one on TV...
    Last edited by Mark Rand; 02-23-2021 at 07:06 AM.

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    If available in the shapes you're using, sandvik's 1115 might be worth a try if you haven't tested it already. I mainly use the MM chip breaker, its good for finishing but can also take a pretty good roughing cut.
    I mainly turn various flavors of stainless, some with very interrupted cuts, duplex, inco, and the rare piece of steel/1144, but it does quite well with all of them.

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    Is the bainite inclusions allowed in the finished product ?

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    Quote Originally Posted by Tonytn36 View Post
    To answer some questions, turning, grooving and boring. Tight tolerance work of course. I will check to be sure, but I believe the material is like a 38CrMo and I am not 100% sure of the hardness specification but it isn't hard. It is actually sort of gummy which is my thoughts on why the stainless grades work better so far. I apologize for not having all of this memorized yet. Still trying to do my former job along with this one and I am not fully up to speed yet.

    From what research I have done, I believe this is happening when the bar is poured at the foundry, they are allowing it to cool too slowly which allows the phase change to bainite.

    Both forging suppliers are non-us (but different countries), which creates a logistics issue in that we have several shipping container loads of these things to machine on the way and even if it was fixed tomorrow, it would be a couple of months before we got the fixed material. In the mean time, we need to see if we can figure out a way to run these without being down half a shift chipping inserts.

    Thus I have turned to the true experts - the folks of PM. I will confirm material and get more details and post back.
    Tony,

    I *think* I know what kind of parts you're cutting...

    "Hard" is relative, but my guess is that these parts are heat-treated proper, at the forging supplier, and sent to you in the heat-treated condition. My guess is low 30's HRC. To the toolmakers on this forum, that certainly isn't "HARD" but combined with the high-tensile, doesn't-want-to-break-a-chip-for-anything, yes-it's-harder-than-mild-steel, and, oh-don't-forget-about-the-chemical-and-metaullurgical-variations, it can become a challenge. Especially at volume, when you're trying to keep production machines humping...


    While I still can't understand the bainite inclusions, I would suggest that you reach out to your Walter Tools rep, and begin a conversation with them.

    Walter has some unique grades in their lineup intended for steel & super-alloy cutting, that may yeild benefit here. Their WSM grades use a PVD applied, aluminum-oxide coating that's unique in the industry. It's a thinner coating, which allows for a sharper cutting edge in difficult chip-breaking materials. Yet, being aluminum-oxide, they still allow good heat-resistance. I could see that combination working well in your tricky material. Walter also has a solid lineup for turning & grooving as well.

    If you'd like to provide more details, feel free. But from 350 miles away, that's my recommendation.

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    There is no option for heat treatment after they get here. The specifications are unfortunately pretty wide for the material properties and I think design did that on purpose because of global sourcing. Parts like this are produced by our corporation all around the world. So far, they are within specification for the amount of bainite. It might not even be causing our chipping issue, there is investigation under way, but anecdotally we seem to be able to turn it on and off with the forgings.

    I will reach out to our Walter rep and see what they have.

    Jashly73, You are correct, this stuff will not break a chip. About the only place chips break is during the interrupted cuts. High Tensile chewing gum is what I relate it to.

    I'll also ask about Sandvik's 1115 grade. Maybe they have that in the shape we need for the boring bar.

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    From a metallurgy friend.
    "Bainite is a phase. If both chemistries are the same Cr-Mo chemistry, they should both be bainitic alloys. Bainite is not an inclusion. It is a microstructure like martensite. Look at the supplier cert and look for differences in chemistry and heat treatment."

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    Quote Originally Posted by digger doug View Post
    From a metallurgy friend.
    "Bainite is a phase. If both chemistries are the same Cr-Mo chemistry, they should both be bainitic alloys. Bainite is not an inclusion. It is a microstructure like martensite. Look at the supplier cert and look for differences in chemistry and heat treatment."
    Agreed, it is a phase and from my research can be controlled via cooling rate. From the SEM images of material sections, it looks like inclusions because it is shaped like long strings in some places of the forging and large crystals in others. (I have an in-house Materials Lab available.)

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    Quote Originally Posted by Tonytn36 View Post
    (I have an in-house Materials Lab available.)
    Lucky dog - there aren't many of them left...

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    Quote Originally Posted by Tonytn36 View Post
    Agreed, it is a phase and from my research can be controlled via cooling rate. From the SEM images of material sections, it looks like inclusions because it is shaped like long strings in some places of the forging and large crystals in others. (I have an in-house Materials Lab available.)
    Out of curiosity. I *assume* that you're cutting/sectioning these parts before microscope inspection. And I also *assume* that these inclusions are visible in the material-removal zone. Is that correct? I'm just curios if these bainite inclusions are actually in the zone of material to be removed, or if they're just below the finished part.




    As to the actual shop-floor solution - What have you tried thus far?

    I can't actually "see" your on-the-shop-floor issues, but from what's described, I think I'd investigate roughers with a heavier edge prep, tougher grade, and cutting closer to finished size. Perhaps a finisher in a more wear-resistant, stainless/super grade, that's going to have a better chip breaker for tougher, stringier chips.

    We have good luck with Iscar 807/907 grades, for finishing our pre-heat treated 4140 & 39Cr5Mn pars. In your case, I would investigate Walter's WSM grades too.


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