Best way to remove black oxide skin from hot roll

[rogee07]

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.

[BCRider]

Maybe not perfect but I had decent results from a couple of days soak in common vinegar. After 24hrs it still had a few stubborn patches of scale on the surfaces. Another 15 hrs or so took off all the remaining scale. Surface color was pretty dark and a scrub with some Bar keeper's Friend stainless cleaner brought it up to a nice silvery grey. For what I was doing that was a good and suitable finish.

[JoeLee]

Bead blast works the best. It's fast in there is no mess.
Providing you have a bead blast cabinet.

JL......

[tnotrab]

I use vinegar, or better yet acetic acid, which is basically less diluted vinegar.

I also use a heat lamp if the temperature is below 60 degrees, this increases the speed of the chemical reaction.

Rinse with water and then neutralize with baking soda to stop the chemical reaction.

Bart

[rogee07]

I have read that diluted muriatic acid was a good way, but that the fumes causes things to rust in the shop. Would using vinegar also cause rust.

[Jim Stewart]

Ordinary white vinegar saturated with salt is much faster than vinegar alone.

-js

[psomero]

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"

[rogee07]

{yes mill scale can be kind of tough to machine through, but honestly isn't that big of a deal.}

From that statement can I assume that with a carbide endmill I can clean the material up, before final milling starts, thus saving the hassle of messing with acid or vinegar baths.

[Cenedd]

Only had to do it once but I ended up fly cutting it off. Left hand CCMT lathe tool in the fly cutter and a CBN CCGT/CCMT insert - both cheap imports. Lovely finish. Just don't let the CBN rest on the workpiece as it stick and fracture - eg using it on a lathe at the end of the cut, pull out. If you stop the workpiece with the insert still touching you can wait for another insert....and they only have one cutting point on the cheap ones.

[dian]

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"

i think that makes a difference. sometimes hot rolled has a very rough scale. thats mill scale. the material you get from a diy store has a smooth black coating. is that iron oxide? anyway, a while ago i needed to prep some medium sized such plates for tig. i tried a 15% muriatic acid solution and even heated it to almost boiling but they didnt clean up, even after several days in there. they turned grayish but i had to sand them anyway. no idea if a stronger solution would have stripped them, but that gets expensive fast.

[LKeithR]

If I had to use some of that chemical bath witchcraft every time I needed to machine a piece of hot rolled material
I'd never get anything done. Just throw the damn stuff in the lathe or mill and have at it. If it makes you feel better
start with a used end mill or insert for the first pass to blow the scale off. If you really want to ease your conscience
take a wire wheel--or even a sander--to the piece before you start...

[markx]

Some hotrolled material can have quite a scale on it and it can be quite hard, even to the point where a flap wheel on an angle grinder really can not clean it up. It just skids over and makes the surface shiny, but the layer of scale remains. But as far as machining it off I've never perceived it to be a huge problem in terms of tool wear. Cutting tools wear inevitably and have to be resharpened as they do. Soaking the material in etchants for prolonged periods vs. sharpening the cutting tool a few more times....I personally opt for sharpening. Seems like too much of a hassle to get rid of the scale by chemical means.

[Mcgyver]

To give a sense of how this plays out in industry, the scale is removed by pickling in acid. Its the norm for say a stamping plant to buy picked and oiled coils, have them slit to the width needed and stamp away. For fabrication work, rarely is the scale removed. Its durable and gets painted over (see pic below). Occasionally, more like rarely, a special coating (or special customer lol) will require a clean surface, and would spec'd at say SP6 or 10 sandblast finish. You'd never buy P&O for fabrication. For machined bar stock, afaik its always just machined, don't think I've seen a spec for pickled HR bar stock - everything you ask to have done takes time and has a cost; so to the extent I've seen you don't scale from hot rolled, you machine it as it. I would say mill scale on sheet and structural is a hard, thin durable layer, as is most bar stock. I have seen HR with a thicker flakey layer on bar stock that might change things, but generally, hold the work and machine away.

[RB211]

Last time I flycut it off with carbide.

[LKeithR]

To give a sense of how this plays out in industry, the scale is removed by pickling in acid. Its the norm for say a stamping plant to buy picked and oiled coils, have them slit to the width needed and stamp away. For fabrication work, rarely is the scale removed. Its durable and gets painted over (see pic below). Occasionally, more like rarely, a special coating (or special customer lol) will require a clean surface, and would spec'd at say SP6 or 10 sandblast finish. You'd never buy P&O for fabrication. For machined bar stock, afaik its always just machined, don't think I've seen a spec for pickled HR bar stock - everything you ask to have done takes time and has a cost; so to the extent I've seen you don't scale from hot rolled, you machine it as it. I would say mill scale on sheet and structural is a hard, thin durable layer, as is most bar stock. I have seen HR with a thicker flakey layer on bar stock that might change things, but generally, hold the work and machine away.

Can't say I've ever seen P & O bar stock or structural shapes but it's quite common in thinner plates and sheet.
A lot of laser cutting shops won't use anything but P & O material because it's so much easier to get clean cuts.

We had one project a few years ago where we used about 125 tons of 3/16 P & O plate for laser cut profiles...