Corrosion in stainless steel after welding

[sampsonr]

Hi Guys,

I am currently doing a research and development project into fibre laser welding of thin ferritic stainless steel (430LNT). 430LNT is a weldable grade of stainless which has been stabilized with Titanium and Niobium. We have had the material tested and is within the 430LNT specs of material composition.

We are struggling to achieve welds that are able to pass a corrosion test in which the sample is exposed to a corrosive environment for 48 hour where no surface corrosion is allowed.

As a raw experiment to imitate conditions produced while welding. We have placed samples in different atmospheres by sealing them in a box and purging it with various gases. The samples were exposed to a defocused laser beam for 5 seconds at 1500W. The test atmospheres include air, Nitrogen UHP 99.999%, Argon zero grade 99.999%, Argon welding grade 99.9%, and Argoshield (Argon 86%, 2% Oxygen, 12% Carbon Dioxide). After this the samples were subjected to an accelerated salt fog corrosion test for 48 hours. All of the samples had heavy corrosion in an isolated portion of the heat effected zone (refer attached image).

I am trying to understand what is the mechanism in this corrosion. I've have heard that oxides can set off corrosion although you would expect a significant difference between samples in air verses the other oxygen free samples. The other suggestion was chromium depletion caused by the nitrogen combining with the chromium although you would expect to see greater corrosion in the nitrogen rich sample. The other interesting point is the samples did not experience corrosion over the entire heated zone and it was isolated to an outer ring. Does that mean that area went through a specific thermal condition?

If anyone has any information or can help in any way I would be greatly appreciated.

Regards,
Roy

 

[EdStainless]

Was there any visible discoloration after the 'weld'?
If so then you have oxidation.
If you can't get good enough shielding with pure Ar to prevent it then the only option is to pickle to remove it (or mechanically clean followed by pickle).
The oxide isn't causing corrosion, it is the layer of metal below the oxide that is now low in Cr (because that ended up in the oxide).

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P.E. Metallurgy, Plymouth Tube

 

[sampsonr]

Hi there,

Thanks for the response.

Yes there was visible discoloration. I did doubt how good our setup was however would you still not expect the argon atmosphere to have less corrosion than the air atmosphere sample? And how does that explain the thin ring of corrosion? would you not expect corrosion over the full face of the laser spot?

Regards,
Roy.

 

[EdStainless]

That is just where it started, your corrosion test is very mild.
In a more aggressive test it would all fail.
Your Ar purge must have been very poor.
I can TIG weld 439 (similar alloy) with only a light straw color forming.

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P.E. Metallurgy, Plymouth Tube