Sub arc Welding gross porosity problem

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Colin Galbraith 21 years ago

I need your help!

I have a problem of gross porosity on our Rear Chassis Beam carriage-mounted submerged-arc welder. It's a square butt, 1-2mm gap, single-sided, full penetration joint with a grooved copper backing strip, in 10mm Domex 350 Swedish steel. Welding is at 700 amps.

The porosity is definitely evolving from the root of the joint - macro and micro studies by Other sources have confirmed this. Typically, the porosity is almost exclusively in the centre-line, but some small pores exist near the top of the reinforcement, although, unlike the main pores, these don't break the surface.

The uniformity of the pores is a point worth remarking upon - I made comment about 'solidification tearing' due to shrinkage stresses across the final liquid zone (although, you are bound to have this due to porosity giving rise to reduced section) I know this sounds pretty straightforward, but we have looked at all the usual causes. One or two more facts to muddy the waters:

We make two variants: large truck - no events observed. Small truck: intermittent and apparently random observations. I am gathering data for each event, by welder, by shift, by position in batch, by defect location, by defect length, etc. I can then use Minitab to see if a relationship exists between occurrences and the different factors. A leading consumable manufacturer have expressed an opinion that there is atmospheric contamination entering the weldment from the underside of the butt, caused by arc blow. I can understand this to a certain extent - we have a DC

+ve power source and there have been problems with the earthing of the mandrel for some time. However, I would expect a much more uniform defect rate, if this was the sole cause - unless it's subject to a build up of magnetism in the beam during production and prior to a mandrelchangeover. The consumable manfacturer has proposed a new AC unit, but the plant they have in mind is a few weeks away from release - I may just go out and buy something from another vendor - if I can't get to the root cause, this should sort it.

Please let me know, your comments and Thanks for taking the time to read this,

Cheers Nevis

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Randy Zimmerman 21 years ago

Some things that comes to mind are simple contamination. I was in a shop where oil mist from air powered grinders was the problem. Once the edges were cleaned with an electric grinder the problem disappeared. Another time I was put in a a postion of authority and of course everyone expected me to have the answer. We had regular porosity on wheel assemblies. Residual oil from the machined bevel preps was one arguement given to me. Another was that the steel was that "cheap chinese" stuff and it was junk. It turned out that the tack welds were big gobs and the sub arc could not burn out the MIG tack welds. Once I enforced tack welds of any length but less thatn 3/16 in section the problem disappeared. Every once and while I had to go back on the war path because it was a lot easier to rest the nozzle on the assembly and just pull the trigger for a second or two. What little experience I have with sub arc is with tight roots and not a backing strip. One suggestion given to me when I asked around with those wheel assemblies was the idea that excessive voltage sometimes will give porosity. It is strange that you get porosity with the smaller but not with the bigger application. That might be a clue. Also the level of porosity is something to address. No weld is normally free of some porosity. What level is acceptable is a valid question. I do know that some wire /flux combinations are more prone to the problem than others. Linclon has some good literature on sub arc. Some of the stuff is available in Adobe for download. There is a set of sheets that is a sub arc trouble shooting giuide. Randy

"Colin Galbraith" wrote in message news:cllsjk$4df$ snipped-for-privacy@sparta.btinternet.com...

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Matt Hoffman 10 years ago

I am experiencing the same issues, the problem has recently come back and my first action is having my guys clean the copper backing strip. We have proven our procedure so, I believe there is a problem somewhere outside the joint and weld settings. Any advice on how you solved your issue?

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Sub arc Welding gross porosity problem

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