You ask whether seam welding or MIG would be better.To weld a seam, you need a welding process,MIG is one of the welding processes,like MMA,GTAW,SubArc,FCAW,etc you have to decide what welding process is the most suitable for your line of work.
You dont say what process is being used at present to weld ducts.Is it MMA,GTAW?
All welding processes require a skilled person,not MIG alone, provided of course you want a quality job to an acceptable standard.
Are you trying to improve from a quality perspective or from a production perspective? or from another?
If it is from a production perspective,ie you want to increase diameter inch per day production, then MIG as a continous process should generate more as opposed to MMA/GTAW,as it is fed from a roll allowing you to weld more on a continous basis whereas MMA/GTAW you are more restrained by length of individual MMA electrode/GTAW wire.
But at these thicknesses i personally dont think MIG is a practical option, maybe other members could advise of thier experience of using MIG at these thicknesses.I myself have not seen MIG used at those thicknesses, it has been GTAW.
If it is from a quality perspective,i from a personal point of view,would advise the use of GTAW on the Stainless and aluminum and on the galvanised although using GTAW on galvanised tends to wear out tungsten very quickly.I believe GTAW at these thicknesses would give better control of welding input and a better quality of weld.
I dont think length of welding is such an issue, more as to whether welder is capable of welding and controlling welding at these thicknesses and that welding, heat input etc is monitored/controlled.
For me,the problems we encountered during construction of our process plant control buildings hvac, were more due to insufficient monitoring/controlling of welding,inexperienced welders, rather than length of welding,these factors caused contributed to distortion, poor quality of welding, blow throughs from welding etc.
Again,in my opinion,if you want to improve the welding, you could use GTAW and if distortion is a problem,you could sequence weld the seam to be welded.
To sequence weld, you could, for instance mark 100mm segements inside and outside, and then number them,for example 1 through 11.Weld outside number 1, 6, 11,then go inside and weld numbers 2,5,7,10, then outside 3,9 and then inside 4,8,then repeat process for outstanding areas to be welded inside and outside.
If you dont want to use numbers you could simply start on the outside, at one end, then weld 100mm then miss 100mm, weld 100mm etc and then reverse that inside, where you miss 100mm then weld 100mm, miss 100mm etc.After this is completed then areas not welded on the inside and outside follow the same sequence.
While either of these are being done, welding is monitored to ensure that welding is not causing distortion of ducts.
Sequence welding simply allows welding to be done in a controlled and organised manner.
Have you got an ITP in place to reflect what you want to achieve, what you want to do and how you want to do it?
These are only some of many ways to control quality welding and welding input and improve welding and i am sure many of the other members have various ways as well.For me there is no right way, only a better way!
Hope this helps
...Mike
