Hello all,

This weekend I built an insert for my (formerly) smokey fireplace, using 1/8" steel bought at a local metal shop.

Question: The metal was dark grey on the outside, and shiny bright on the inside. What is that coating? (I believe it to be part of the cold/hot rolling process...?)

Basically, I wanted to prep the metal to make it "shiny and bright" before welding, and realized if I sanded a little deeper I got to shiney metal. The top coating is hard - it took a lot of sanding/grinding to get through. Once through, the metal underneath is softer, and I ended up with some "divots" where I sanded away the soft while leaving the hard.

So I came to the conculsion this is a hard layer on top of the sheet. Welding to this layer seems to work fine - I am an amateur at this, so "fine" is relative - someone like Aweman would do a much better job!!

Thoughts?

Sounds like hot rolled and pickled steel.

paint grip? be careful welding most galvaneal or galvanized is high in zinc content (zero lead) cold rolled and hot rolled that's been pickeled and oiled will be same "steel" gray color. tt

If I am not mistaken, the inside is shiny and bright to reflect the heat and light......not that that helps any.....

Do you know what the AISI # or ASTM # of the steel is? Eg. A36 or something similar? If it is USED Steel It could be almost any alloy and the coating could be almost anything applied by Who knows.
New steel comes direct from the steel mill with a chemically induced coating on it called pickling. {Termed by tradesmen "Mill Scale"} When the steel is new the pickling can easily be ground off Because it is very thin. But in the case of weathered steel the oxidation process actually thickens and hardens this pickling coat. Making it very difficult to remove in some cases. The specific purpose of this pickled surface is to prevent oxidation {Rust} from compromising the steel below the pickled surface. There is no easy way to remove this surface once it had weathered to a hardened state other than grinding it off. Despite what others might tell you about welding the steel without removing the coating, Doing so produces inferior welds due to contamination of the weldment.
When purchasing steel unless you need specific hardened steel properties
Mild steel is ASTM A36 {this number A36 can be any number of specific Alloys in the mild steel range.} Typically 1020 mild steel is what you will want to use for most projects at home.

Thanks all for your replies - AweMan, I'm in awe... . Seriously, I just read your other post in the Triumph forum, where you say the same. Thanks!
http://www.britishcarforum.com/bcforum/ubbthreads.php/topics/523125/all/TIG_or_MIG

Basically, yes, my weld bead "stuck" to the mill scale. However, looking at the other side, I only had a "heat-effected" areas where I ground through the scale to shiney metal. Fortunately, this fireplace surround is a low-stress part. For my car, at 60 mph, my welds better be better!!

Going forward, and again as I am a rookie perhaps this is old news, I will grind my metal to shiney; through all corrosion and scale.

For some more info on removing mill scale:
http://weldingweb.com/showthread.php?p=229498

Mike
66 TR4A