What is the dark surface on DOM tubing?

[BrianE22]

Is the dark surface an oxide? We just got some DOM tubing in from our steel supplier and about half the pieces look like the dark surface (whatever it is) was cleaned off or removed. I want to make sure I know what was done.

 

[stanweld]

Please define DOM tubing?

 

[BrianE22]

Drawn Over Mandrel (DOM)

 

[stanweld]

Assuming this is carbon steel tube, it is most likely mill scale or mill scale with mill lacquer.

 

[swall]

BrianE22--In order to draw tubing, it has to be lubricated first. What you are seeing is probably residual drawing lube. I am assuming for your appication, it needs to be cleaned off?

 

[BrianE22]

After machining, the parts are sent off to the plater for zinc phosphate prior to power-coating.

Looks like someone then has cleaned the OD of these tubes. That's not a problem. I just get alarmed when the material looks different than what we've been getting.

 

[TVP]

As already mentioned, steel tubes are lubricated prior to drawing. A mixed oxide scale/burned lubricant layer can be produced if the tubes are stress relieved or otherwise heated without properly cleaning the tubes first. This layer definitely needs to be removed in order to develop proper phosphate crystal size and coverage. I suggest sectioning, mounting, and polishing a metallographic specimen to evaluate the near surfaca layer.