Severe Weakness/porosity with plasma welding

[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

 

[TVP]

Porosity is usually caused by contamination, either on the joint surface(s), on the filler metal surface, or in the shielding gas. If the new steel is pickled and oiled, how is the oil removed prior to welding? Is this an autogenous process, or is there some type of filler wire? If the latter, is it clean? Contamination of the shielding gas can come from the bottle or due to mixing with the atmosphere during welding (too low of a flow rate, etc.). Here are some previous threads that discuss porosity:

http://www.eng-tips.com/gsearch.cfm...&ie=ISO-8859-1&q=weld+porosity&sa=Go&siteurl=

http://www.eng-tips.com/index.cfm#1249

 

[Willers]

Thanks for the reply TVP,

There is no filler wire in the process, it is just a hydrogen/argon ionized gas with an argon shield.

Luckily the welding gas and flow rates seems to be one of the few constants in this process between good and bad batches of parts.

The material is strip mill pickled and oiled, so the oil is only a fine film, and I have tried cleaning parts with thinners before they are welded, but have not seen a noticable difference. When parts are passing under the welding torch however, only the direct welding area has been cleaned, and there is a lot of smoke from the oil in the atmosphere around the weld, but as I say I have never been sure if this is significant enough to cause problems with the weld.

The problem I have is the shear randomness in part quality, and the ammount of variables there is to cure in finding the source/s of the problem.

We have had continuous runs of parts where the surface has remained oiled and the quality is good, and some where the quality is bad.

I will have a look at the link you posted as well

Thanks

 

[StrykerTECH]

Sounds like you should run of a "design of experiments" and help quantify what variables will make the material crack. It sounds like you have some good parts and some bad parts.
A DOE should help you converge on what variables drive poor quality.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[Willers]

I have been trying to push this on my MD for a while now, however it is looking unlikely due to various customer pressures. I will continue trying to get this done though, thanks!

 

[ornerynorsk]

Sheet edge is notoriously contaminated as supplied. We have had this issue in the past where 2 edges are being essentially fusion welded with no filler.

As stryker mentioned, a design of experiments would do well to isolate the actual problem. I would personally start with wire brushing the sheet edge and see how the weld turns out.

Good luck.