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Up Topic Welding Industry / General Welding Discussion / TIG WELDING-DISCOLORATION
- - By davidlnrd Date 10-13-2010 10:04
I have a problem when tig welding 4140 casted components. I am seeing discoloration after welding to fill a hole after welding next is buffing, sand blasting and then buing and the end result is discoloration in spots from dark to light more on the dark side. Any assistance or feedback would be greatly appreciated.
Parent - - By TimGary (****) Date 10-13-2010 12:50
The discoloration is a direct result how how much oxygen was absorbed into the surface of the base metal while at elevated temperatures from the welding process, or directly after the shielding gas passed over the heated area as it was cooling down below the point at which the material will no longer absorb oxygen.
This is critical in stainless steel applications because it will affect corrosion resistance properties and for Titanium as it will affect the hardness of the weld / base metal. It may affect other materials as well to a certain extent.
As for carbon steel, such as 4140, this discoloration is not considered an issue, especially considering that your material is to be buffed / blasted, as this discolation only goes a few microns thick.

Tim
Parent - - By davidlnrd Date 10-14-2010 11:52
Thank you very much Tim for your feedback. Based on your suggestion on the 4140, I buffed and sand blasted the component and after that I blued it. Unfortunately the discoloration or scar brought about by the welding is still visible. I do not want this visible in the finished product. My process is TIG-weld a “blue hole” or tiny pin hole in the 4140 investment casting material, then file and buff, then anneal the welded portion by means of oxy/acetylene. Then buff and blasting with emery grit # 80, then gun blue for max of 1 hour. After bluing, that's where the discoloration will appear. Sometimes the welded portion is darker than the rest of the work piece, at times the same color with the material but the sides of the welded portion or circling is darker. Any suggestions? Anybody??
Parent - - By MMyers (**) Date 10-14-2010 13:28
Mind you, I know nothing about bluing, but it sounds like the bluing operation is revealing the difference in microstructure between the weld metal, HAZ and base metal.  If this is the case, your annealing operation isn't doing what it's supposed to do which is homogenize the materials so they are roughly the same microstructure.
Parent - By TimGary (****) Date 10-14-2010 18:16
I agree with Mike.
Parent - By DaveBoyer (*****) Date 10-15-2010 01:32
Mike is on the right track, but it might not have to do with the annealing. Minor differences in alloy or microstructure can show up after bluing, it is similar to an acid etch.
Up Topic Welding Industry / General Welding Discussion / TIG WELDING-DISCOLORATION

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