Severe Weakness/porosity with plasma welding 1

[Willers]

Hi all,

I am looking for help regarding the welding of seams of generator housings that we produce.

Short question -

How can porosity occuring in a mild steel welded joint be avoided when plasma arc welding (argon, hydrogen/argon)

Long question -

The raw material is rolled into a cylinder, welded along the seam, then the resulting shell is expanded by ~2%. This expanded shell is then swaged at both ends with the diameter at these regions being increased by a further ~12%.

When the production line was installed initially, and our batch of steel was at a tightly controlled 185MPa yield, the parts went through this process with no problems at all.

Maybe 6 months down the line, we moved onto a new style of steel which was pickled and oiled as opposed to scaled. The yield point of this steel was then found to be substantially higher (~215-225MPa). The carbon content as a result was potentially higher. And to top it all off, the standard of maintenance on the tungsten electrode in the welding torch went downhill.

The parts made around this time, and intermittently ever since have cracked at the swaging process and occasionally even when the parts are expanded.

The welds sometimes appear visibly weak (keyholes in the seam), but at the very least feel brittle when the weld joint is broken between two parts, and the ends of the welds appear porous.

I am aware of the problems associated in terms of hydrogen embrittlement etc, and the quality of the tungsten electrode, but I am quite naive in terms of the whole process in general.

Any help with reducing this porosity problem and the associated weakness would be appreciated!

Thanks

 

[EdStainless]

One place to check is cleaning prior to weld. Depending on what the 'oil' is residual can cause poor welds.
Another is your gas lines. Even small leaks will result in a lot of air getting in to your weld gas.


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[metengr]

Because the oiled steel is the problem. The steel must be clean, meaning no oil residue. Oil will cause porosity and possible hydrogen embrittlement in this material.

 

[Duwe6]

Oil will cause porosity and possible hydrogen embrittlement in this material.

Probable hydrogen embrittlement and possible hydrogen cracking.

 

[Willers]

Thanks for the replies.

I will attempt to run through a few batches of material -

Annealed

Fully cleaned

Lasered edges guillotined off

Untreated material

I will hopefully get a definative answer. While the gas lines appear to hold pressure over night, the gas consumption does appear to be higher than it should be.
The production line is in China, and I am not due to go back there for a couple of weeks, so will have to wait until then for further work.

Thanks again!

 

[EdStainless]

If they have used rubber hose for gas lines make them change them out. Metal would be preferred, but lined plastic would work. Rubber get porous quickly and you end up with a lot of moisture and oxygen that way.

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