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Best way to remove black oxide skin from hot roll

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  • Cenedd
    replied
    Originally posted by darryl View Post
    I don't know- for my part I'm often looking for say a 3/4 diameter part, and the material I find is hot rolled. If I remove the mill scale by machining, I no longer have a 3/4 diameter part. I don't really want to turn down a 1 inch diameter so I can reach an exact 3/4. I don't buy much cold rolled- and it has too much stress in it anyway to produce a good machined part from in many cases.
    Can you get metric dimensioned stock? I find that sometimes imperial stock is slightly larger than the nearest metric and gives you the space to skim it. For example 1 inch stock is slightly larger than 25mm and not really any more expensive. You might be able to get 20mm stock that just a smidge larger than 3/4".

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  • darryl
    replied
    I suppose a good question to ask is whether you need to use the material as is, or will it be machined. And if you use it as is, will it be accurate enough for the application. I ask this because it seems that during the forming process, whatever crust is forming could be pressed into the material, and it may not be perfectly flat or round- or even uniform in composition. Anything with mill scale on it is probably never considered as having close dimensional accuracy.

    I don't know- for my part I'm often looking for say a 3/4 diameter part, and the material I find is hot rolled. If I remove the mill scale by machining, I no longer have a 3/4 diameter part. I don't really want to turn down a 1 inch diameter so I can reach an exact 3/4. I don't buy much cold rolled- and it has too much stress in it anyway to produce a good machined part from in many cases.

    There are times when I paint over the scale because it doesn't matter to me- other times I go at it with a chisel to get most of it off, then sandpaper to clean up the scratched mess left behind. I'm an impatient sort- not readily willing to leave something in a bath for days before I can use it.

    I have recently acquired an ultrasonic cleaner- I wonder if that would be useful to remove scale? Small parts only of course-

    And then there is laser removal- seems to work like magic to remove rust. Perhaps it works for scale too?
    Last edited by darryl; 03-17-2021, 04:09 AM.

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  • nickel-city-fab
    replied
    Originally posted by psomero View Post

    Soooo... I went and tried it and stand corrected.

    It'd be good to note that my ultrasonic has a heater and once it's turned on, it'll sit idle and keep the bath at ~120°F or so. Total soak time was about 15 hours. It is a slow process indeed, however if you were to use a higher concentration of acetic acid or "industrial vinegar" (the stuff from Home Depot for killing weeds/cleaning), I could see this being reasonably effective.
    Straight vinegar works if one is patient. I've gotten lots of "free" tools that way. You find them in the road after they spend all winter under the snow, ice, and salt. I never tried the HD vinegar, just didn't have the money. So I used the regular stuff and let them soak for 2 weeks. The result was a lot like Evaporust. Scrubbed them off in the sink with a kitchen scouring pad, and oiled them. Filled up a drawer with pliers and screwdrivers for free that way. Adding salt makes it more aggressive but I don't think its a good idea. Because adding salt turns it into a mild hydrochloric acid, and that does not stop eating the metal.

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  • psomero
    replied
    Originally posted by MattiJ View Post

    I assume you haven't actually tried the vinegar pickle?

    It works but it's not the fastest process, might take day or two especially if the solution is on cold side. And as mentioned adding salt seems to make it faster.
    Soooo... I went and tried it and stand corrected.

    I had a piece of hot rolled rod that went through a coal fired forge and was blacksmithed on, then quenched in motor oil. There was hot roll mill scale, the slaggy stuff, slaggy stuff from the forging process (hot oxides off the orange steel) and just dirt/grime from the oil quench. Needless to say, the part was filthy as hell. I put it in a ziplock baggie, filled it up with heinz pickling vinegar (might have come from the hardware store, wouldn't ingest from this old, dingy jug under my garage counter) and left it overnight in my little ultrasonic cleaner. The ultrasonic will only run for a max 30 min, but I hit it a few times over the evening and ran it once more in the morning. The black from the oil quench sloughed right off. The loose oxides/scale from forging (where the orange/yellow hot steel was bending and the slag stayed in place) fell right off. Some of the dull dark gray mill scale / hot roll finish came off as well. The part isn't bright or a clean, matte surface like I'd hope for from an etching process, but it certainly etched some schmoo off the thing.

    It'd be good to note that my ultrasonic has a heater and once it's turned on, it'll sit idle and keep the bath at ~120°F or so. Total soak time was about 15 hours. It is a slow process indeed, however if you were to use a higher concentration of acetic acid or "industrial vinegar" (the stuff from Home Depot for killing weeds/cleaning), I could see this being reasonably effective.

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  • Illinoyance
    replied
    Originally posted by Cenedd View Post

    ...and you should take extra care not to get it twisted; I hate it when I get my Swartz twisted!
    That is Schwanz you don't want twicted.......or stepped on

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  • MetalByter
    replied
    This was given to me by a friend, it is fantastic to remove the scale and very fast. Leaves a finnish like sand blasting.
    You may only view thumbnails in this gallery. This gallery has 3 photos.

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  • Ironbearmarine
    replied
    Unless you are under industrial or commercial requirements to descale, it seems to come down to how much fun you want to have preprocessing your A36 stock. Personally, for machining projects i will just cut away the scale rather than abrade it, for fabricated small projects, abrading is easy. For larger projects that will get painted or powder coated i prefer having it sandblasted. If i have lots of small components i pickle them in a vinegar and salt bath aided with a 12 volt power supply. My 2 cents.

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  • Fasttrack
    replied
    Originally posted by tom_d View Post
    I treat mill scale the same as the surface of a sand casting. Whenever possible make the first cut deep enough to get completely under the scale. I've found the greatest tool wear comes from a light pass with the tool coming into and out of contact with an irregular surface. As long as the tip of the tool is under that scale just plow away. Normally I'm using HSS too, as I'm just taking the top layer off, even if there is some tool wear it's just a quick touch up on the grinder and then back to cutting. Quick and easy to touch up HSS.
    That's been my experience as well. I think the small amount of deformation as the chip is formed is enough to break and flake the scale off ahead of the cutting edge, anyway.

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  • dian
    replied
    after several hours simering in 15% muriatic. then 4 hours simering in 5% vigegar/salt solution. no tig on that. now, afrer two days there is plenty of rust.


    Attached Files

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  • J Tiers
    replied
    Originally posted by nickel-city-fab View Post

    The SAIT link had these new ceramic ones, would that be cubitron? I have heard they exist but have not used them.
    Probably similar, I looked up what the cubitron is, but do not recall the details. I know they eat steel, even with a wimpy grinder

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  • nickel-city-fab
    replied
    When I have slag and scale, I just use the 1/4" thick type 27 grinding wheels on whatever size angle grinder is available. If I'm worried about the finish after that, I follow up with a flap wheel. 98% of the time it isn't necessary for what I'm doing nowadays, though

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  • JRouche
    replied
    Originally posted by rogee07 View Post
    Ive got some pieces of hot roll A36 that I will be using on a project, what would be the best way to remove the black oxide skin prior to machining? Or is it even worth worrying about. I have never worked with hot roll before, but have heard that the outer skin is tough on HSS end mills, however I don't know this from personal experience.
    I use these three abrasives for mill scale. The PFERD is great if you have a powerful grinder. I use them in my 9" mil, they should have them in 4-1/2".

    The CGW 4-1/2" disc in 40 grit will eat everything, including skin and clothes.

    The lil cutoff wheel is good for tight spaces (on edge only).

    That Hilti flaring cup is what I used on my thick welding table. Again, needs the power but will knock down scale on a flat surface easily. JR

    Click image for larger version

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  • nickel-city-fab
    replied
    Originally posted by J Tiers View Post

    Yeah weld prep, OK.

    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.
    The SAIT link had these new ceramic ones, would that be cubitron? I have heard they exist but have not used them.

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  • oxford
    replied
    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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  • J Tiers
    replied
    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.

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