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Best way to remove black oxide skin from hot roll

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  • #31
    I would just ignore it and make sure my first cut is deep enough to get under the rough stuff. If it absolutely needs to be removed for fabrication or finishing, I've had good luck with a rubber backing pad on the angle grinder, and 60-grit sanding discs.
    25 miles north of Buffalo NY, USA

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    • #32
      I've had the flap-type discs basically "bounce off" of thicker scale. They worked at first when new, but a few inches along the area to be welded, they were just rubbing the scale off, no longer cutting.

      Non-flap may work better, even though the flap discs are supposed to "self-sharpen" by wearing off the ends of the flaps. Probably the scale is too smooth to wear them down.
      CNC machines only go through the motions

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      • #33
        The scale is brutal on abrasives and cutting tools. I prefer it gone.

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        • #34
          Originally posted by J Tiers View Post
          I've had the flap-type discs basically "bounce off" of thicker scale. They worked at first when new, but a few inches along the area to be welded, they were just rubbing the scale off, no longer cutting.

          Non-flap may work better, even though the flap discs are supposed to "self-sharpen" by wearing off the ends of the flaps. Probably the scale is too smooth to wear them down.
          This is true -- I've found that the coarser the grit, the better. Have seen sanding discs that go as coarse as 36 grit. They higher quality ones last a while, the cheaper ones dull off easily. The difference is in the type of abrasive (silicon carbide, AlOx vs red garnet) and the resin/bonding chemical -- the more expensive discs can take a higher temperature before the glue melts. SAIT abrasives is what most of the pros use (hopefully on someone else's budget...) Link: https://www.sait-abr.com/en/slider-h...saitdisc_d_9ii
          25 miles north of Buffalo NY, USA

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          • #35
            It's sort of like flame cut.. get under the scale with carbide and conventional cut it clean. All the cleaning is a waste unless you dont have stock to remove.

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            • #36
              Originally posted by Fasturn View Post
              It's sort of like flame cut.. get under the scale with carbide and conventional cut it clean. All the cleaning is a waste unless you dont have stock to remove.
              This is very true -- you will not often see people cleaning the scale off, because it is cheaper to replace a tool tip than it is to pay a guy. Sometimes a job requires it, though. TIG welded parts come to mind, or finely finished and dimensioned machined parts.
              25 miles north of Buffalo NY, USA

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              • #37
                Originally posted by psomero View Post
                mill scale or black oxide?

                one is a byproduct of the hot rolling process, one is a plating process. two different things with different removal methods. vinegar isn't going to do anything to mill scale except make it smell like vinegar. it is kind of like welding slag, in that it's the impurities that accumulate on the surface during the hot work, but it's a lot more tenacious.

                even with black oxide, you'd just be removing the sealing oil with a weak acid, not the actual iron oxide.

                yes mill scale can be kind of tough to machine through, but honestly isn't that big of a deal. if you're really worried about it, and have the stock allowance overhead for it, get an angle grinder with an abrasive-embedded nylon bristle stripping brush or a flapwheel, and cut the scale off. You need someyhing hard like aluminum oxide to chew through it. you'll know when you're down on good bright metal below the "crust"
                Cleaning vinegar alone will remove mill scale, I usenit all the time. there are three different grades of vinegar. cooking vinegar about 5% acetic acid, pickling vinegar about 7% acetic acid and Cleaning vinegar which is 12% acetic acid. Cooking vinegar is not strong enough to remove mill scale in a reasonable time.

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                • #38
                  If you've got the time, put it outside to rust. That lifts the scale off.
                  'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                  • #39
                    Originally posted by J Tiers View Post
                    I've had the flap-type discs basically "bounce off" of thicker scale. They worked at first when new, but a few inches along the area to be welded, they were just rubbing the scale off, no longer cutting.

                    Non-flap may work better, even though the flap discs are supposed to "self-sharpen" by wearing off the ends of the flaps. Probably the scale is too smooth to wear them down.
                    Technically not the best practice and you need to be careful doing it but I will usually use a cutoff wheel on the grinder like a hard grinding wheel for scale removal if it is giving the flap discs a hard time.

                    I find the cutoff wheel cuts through the scale fast without gouging in as much as a typical hard grinding wheel would.

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                    • #40
                      But it really leaves a bad surface. The flap disks are not extra great, but you can use a finer one to follow up. If, of course, it works.

                      Acids seem to be variable. I have phosphoric around, and it does OK, but leaves a very similar coating f iron phosphate in place of the oxide. I won't have hydrochloric around, and sulphuric is surprisingly hard to find, but apparently much nicer as far as rusting everything within 10 m..
                      CNC machines only go through the motions

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                      • #41
                        Originally posted by Jim Stewart View Post
                        Ordinary white vinegar saturated with salt is much faster than vinegar alone.

                        -js
                        That brightens up pennies real nice.

                        JL..............

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                        • #42
                          Originally posted by J Tiers View Post
                          I've had the flap-type discs basically "bounce off" of thicker scale. They worked at first when new, but a few inches along the area to be welded, they were just rubbing the scale off, no longer cutting.

                          Non-flap may work better, even though the flap discs are supposed to "self-sharpen" by wearing off the ends of the flaps. Probably the scale is too smooth to wear them down.
                          Any abrasive type paper even a good quality fabric grinding disc will dull fast and just shine the scale up.
                          Bead or sand blasting washes it right off. I've have yet to see a faster cleaner way to remove it. If you don't like removing it you can always get pickled and oiled sheet and use CF bars and rounds.

                          JL...............

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                          • #43
                            Originally posted by J Tiers View Post
                            But it really leaves a bad surface..
                            The technique with the cutoff wheel I use for weld prep, mostly Mig. If it was a finished part where you would really see it, yes the surface would be bad.


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                            • #44
                              Originally posted by oxford View Post

                              The technique with the cutoff wheel I use for weld prep, mostly Mig. If it was a finished part where you would really see it, yes the surface would be bad.

                              Yeah weld prep, OK.

                              About half of my issue with scale was weld prep for tig, which I was doing on some prototype stuff at work, since I was the only person there who had been trained using tig (local course at the community college, the instructor used to post the class here).

                              ALL issues now are not welding, since I still need to get a welder, so the scale thing is more of an issue.

                              Yes, the scale dulls the angle flap disks in short order. Maybe cubitron ones would last, that stuff eats everything. Have not seen them, but they must exist.
                              CNC machines only go through the motions

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                              • #45
                                Originally posted by J Tiers View Post

                                Yeah weld prep, OK.

                                About half of my issue with scale was weld prep for tig,
                                A lot of times for Tig I will use the Dremel with a carbide rotary file in it. It cuts through it good enough, and you have decent control of what and how much you are taking off and finish is usually a little better than something on the grinder.

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