Golden colour in bead

[waqasmalik]

Hi all, i have welded SS 304 with SMAW and using 308L-17 electrode.Resulting bead looks like as i have shown in the attached image. I am confused in understanding why golden and some other colors are appearing in the bead? What is the reason behind this?

Any response will be highly acknowledged.

 

[Guest102023]

The colour is caused by light interacting with a very thin, nearly transparent oxide layer. The colour (aka wavelength) you see after refraction is an indication of the thickness.
On the weld it is not a concern, but on base metal (known as 'heat tint'), it's a different matter and must be removed, mechanically followed by chemically.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[waqasmalik]

The interaction of light with chromium oxide?

 

[waqasmalik]

The interaction of light with chromium oxide? if color indicates thickness then it means chromium oxide layer is there, why this color/thickness should be removed from HAZ?

 

[gtaw]

The color is the result of oxides forming when the hot metal is exposed to air. Welds that are slagged immediately will turn blue. If the weld is allowed to cool somewhat before removing the slag, the bead will appear to be dark straw, or light straw depending on how low the temperature is when the weld surface is exposed to air. If the weld is cooled to a temperature below say 300 degrees F, the weld will appear silver.

If the root surface is not protected, it will be exposed to air while it is still very hot and it will appear to be black and crusty. Again, it is simply oxidized metal, i.e., chromium, nickel, iron, etc. Welders sometimes assume it is "carbon that has been burned out of the metal". Not the case. It is simply oxidized metal.

While the mechanical properties of the base metal are not affected, the corrosion resistance is affected. Thus any oxidized material should be removed if it is used in an aggressive wetted environment.

Best regards - Al

 

[waqasmalik]

Thanx GTAW, What i have learnt from this is that silver color is the most desirable because slag is removed after the weld metal has been cooled to room temperature and blue is most undesirable. Am i right sir?

 

[gtaw]

Sound good to me, but black is worse.

Best regards - Al

 

[EdStainless]

It is there because of water and oxygen present in the weld environment (poor shielding).
The clear or shinny colors are when the oxide is thin enough to be transparent. The color itself is from the thickness of the film (diffraction). When the color gets dull it is thicker.
Remove it by using a clean flap wheel followed by some pickling paste (RedOne).

This is damaging for two reasons. the oxide acts like a surface contaminate and forms a crevice which is much worse from the corrosion standpoint.
The other reason is that the Cr in the oxide came from the alloy, so there is a thin layer of metal under the oxide that is significantly depleted in Cr.
Combine these two and welds (and HAZs) with any discoloration in SS are much more susceptible to corrosion than the clean surface is.

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P.E. Metallurgy, Plymouth Tube