How to identify AR-400 mixed with mild plate?

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Haven't been on in a while, but I have a question for you guys.  Kid at work mixed all of our plate steel inventory, stored outside, so that we have AR-400 in the same pile as mild plate. The surface rust has erased some of the marks we made on the 400, and I need a reliable way to tell them apart. Any ideas? We  have 1/4", 1/2", and 3/4" sheets.
Reply:Regular drill bit will not drill AR-400, would my guess
Reply:You can often tell with a punch, it's harder to punch the ar and the mark isn't as deep
Reply:I tried the punch trick, dropped from the same height onto known A-36, and mystery metal.  I think I saw a difference, but would be nice to know for sure. It will be real inconvenient to re-do this job if I use mild steel. Haven't tried a drill. Any one else?
Reply:Hey man. Long time no see.Try a spark test? Dunno exactly what to look for, but maybe there's some info online about it.That's all I got.Expert Garage Hack....https://www.facebook.com/steven.webber.948
Reply:Try grinding it with a flap wheel, the regular steel it'll start taking material off, the ar plate...it will basically just take the rust offAsk me how I figured that outMiller Spoolmate 200 w/t S-52 WirefeederMM 211'09 Miller Trailblazer 302
Reply:There are portable hardness testers such as a Shore Scleroscope and there are portable analyzers such as a Niton gun.  It all depends on how much you value absolute certainty.You can judge a good anvil by the amount of rebound of a large steel bearing ball.  I wonder if there would be enough difference to notice in the rebound between the two steels if you measured carefully?  (This is how the Scleroscope works.)  I think you would have to judge each thickness separately if using the ball bearing.
Reply:Bump for the night shift... Thanks for the answers so far.
Reply:You are supposed to mark it before you shelve it but a spark test or indent test with a punch are probably your only ways of telling if you got the two mixed up (obviously compare them to known alloys).  The other higher alloy material may not rust as quickly if you grind a shiny spot and leave it outside for a few days too.  Take a grinder or a punch to some known alloys and see how they compare to the unknown.
Reply:I think I would hit it on the very edge with something kind of sharp, like the claw on a carpenter's hammer.  The mild steel will take a much deeper dent than the AR.  Or maybe a steel chisel on the pointed edge of the metal?Miller Syncrowave 350Millermatic 252/ 30A spoolgunMiller Bobcat 225g w/ 3545 spoolgunLincoln PowerArc4000Lincoln 175 Mig  Lincoln 135 Mig Everlast 250EX TigCentury ac/dc 230 amp stickVictor O/AHypertherm 1000 plasma
Reply:May sound strange but give it a belt with a hammer, the 400 being harder has a sharp ring to it, ms plate a bit less.....Mike
Reply:The best way to test this that I know of is to lay all the sheets out in a long row.  Strip down and drench yourself in used motor oil.  Start running like you just stole third base and your diving for home plate.  As you're sliding across each sheet, the AR400 will stop you dead in your tracks every time.  Its not pretty but it works....Honest!  I didn't believe it either until I saw it for myself.If you don't want to try that (I highly suggest it though and pictures are mandatory) take a grinder to it.  A36 will spark brighter and more abundantly then AR.  AR will have a duller spark when you grind it.  If that doesn't give you what you need, fall back on plan A.
Reply:Thanks for all the tips guys. Farmfab, I just about spit my coffee on the keyboard. That was great. Good chuckle to start the day.
Reply:I think FarmFab forgot to include the caveat that this test works best with female participants, preferably attractive. Faster results are obtained by laying out several smaller runs of said plate, and using multiple female assistants. Expert Garage Hack....https://www.facebook.com/steven.webber.948
Reply:Never really thought about but yeah mud wrestling is a good application for AR 400 plate...lots of abrasion.
Reply:You might try a file test on a clean edge.  AR400 should be about 42 Rockwell C.  Much harder than regular plate. (400 B vs. < 160 for A36.)"USMCPOP" First-born son: KIA  Iraq 1/26/05Syncrowave 250 w/ Coolmate 3Dialarc 250, Idealarc 250SP-175 +Firepower TIG 160S (gave the TA 161 STL to the son)Lincwelder AC180C (1952)Victor & Smith O/A torchesMiller spot welder
Reply:"   I think FarmFab forgot to include the caveat that this test works best with female participants, preferably attractive.    Faster results are obtained by laying out several smaller runs of said plate, and using multiple female assistants. "I assume bikini's are optional???
Reply:Also if it has good factory edges you will find mild steel will often have shear markings from being cut. AR 400 will have torch cut factory edges.
Reply:Look for the heat number all plates are marked
Reply:Originally Posted by Bob_C"   I think FarmFab forgot to include the caveat that this test works best with female participants, preferably attractive.    Faster results are obtained by laying out several smaller runs of said plate, and using multiple female assistants. "I assume bikini's are optional???
Reply:Could I put the oil on them? Pretty please!
Reply:Sure...on the plate.
Reply:daddyI endorse all of the suggested detection modes - especially the 'lubricity-brake'. Day-one of my first job - decades ago - I was issued one of these.http://www.markal.com/liquid-paint-m...k-tube-marker/The mandate: if you start with a full sheet - mark all of the drop.  If you start with drop - identify all of the 're-drop'. Originally Posted by daddyKid at work mixed all of our plate steel inventory, stored outside, so that we have AR-400 in the same pile as mild plate. The surface rust has erased some of the marks we made on the 400 . . .