Sub-Arc Waterwall Panels

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I work in a shop that builds boiler replacement parts. The last few years I have been running a straight line Sub-Arc that welds membrane together with tubes to make waterwall panels for power house boilers. The problem we are running into is when we weld the seams on one side of the water wall, then flip it over to weld the other side we are getting alot of gas pocket holes in the weld.  The material is SA210 A1. We are using L-50 er70 Lincoln Brand 5/64 wire and the lincoln 761 flux. All of the materials have been cleaned in a shot blaster before tacking.Does anyone have any ideas on what to try to get rid of the gas pockets?
Reply:a shot blaster may be pounding impurities into the metal. to find out,try grinding the surface on a test piece before welding.
Reply:Originally Posted by TigwldrI work in a shop that builds boiler replacement parts. The last few years I have been running a straight line Sub-Arc that welds membrane together with tubes to make waterwall panels for power house boilers. The problem we are running into is when we weld the seams on one side of the water wall, then flip it over to weld the other side we are getting alot of gas pocket holes in the weld.  The material is SA210 A1. We are using L-50 er70 Lincoln Brand 5/64 wire and the lincoln 761 flux. All of the materials have been cleaned in a shot blaster before tacking.Does anyone have any ideas on what to try to get rid of the gas pockets?
Reply:If the weld on the front side is having no problems, I would suspect the heat from the sub arc weld is pushing metal impurities to the surface on the back side. Try grinding or power brushing the back side side before welding.6"XX P5P8 6G
Reply:Whats the material thickness/joint prep?Have we all gone mad?
Reply:It is a problem that has been around for a while but seems to be getting worse and worse as time goes on. We have tried brushing and cleaning the second side but it doesn't seem to help. All the flux is new. We receive a pallet on a monthly basis direct from Lincoln. The Membrane thickness is usually.250 with the tube wall thickness varying from .150-.325.
Reply:Originally Posted by TigwldrIt is a problem that has been around for a while but seems to be getting worse and worse as time goes on. We have tried brushing and cleaning the second side but it doesn't seem to help. All the flux is new. We receive a pallet on a monthly basis direct from Lincoln. The Membrane thickness is usually.250 with the tube wall thickness varying from .150-.325.
Reply:what's the max interpass temperature? It might be possible that you're already baking the flux on the backside due to the base metal temperature after the first pass. I've seen things like this happen over the course of a day when copper backing fixtures heat up and no longer sufficiently cool the metal.Welding EngineerCertified Scrap Producer
Reply:Originally Posted by boatbuoya shot blaster may be pounding impurities into the metal. to find out,try grinding the surface on a test piece before welding.
Reply:We will be testing that next week. We also have a different tube polisher but we only use it for the tubes we overlay.  Lincoln is also suppose to send someone in the next couple of weeks to see what were doing and see if they have a better flux for our application.
Reply:Something new has been added to the job, and it is causing the problem.That could be anything from a different supplier to a different operator, but it is there showing itself in the gas pocket.Time to sit down and compare today -v- last month and find the problem.
Reply:Sounds like time to design some experiments, whether that is different flux, adjusting the welding parameters, whatever.  I have no knowledge about the sub-arc process, but I know how to design experiments to answer the question about which factor is the important one, and what value that factor should be.Most likely, the process has been semi-marginal all along, just good enough to pass, but not what would be considered a robust process.  Something changed, whether the machine, the settings, the operator, the flux, ......  It changed just enough to cause the process to produce results that are not quite good enough.  A really robust process still produces good results even if something changes.Usually, for design of experiments, the experts and/or the operators suggest which factors they think are likely at fault.  Every process has several factors, some of which are critical, and some of which are less critical, but you may not know which are which.  The flux example is simple, you just try another flux, and run enough to know if the results are the same, better, or worse.  For all you know, one production run of flux runs differently than the previous ones, or the flux manufacturer has changed the formula without telling you.  If it was a machine setting, you would turn it up and check the results, and then turn it down to check the results.  It is time consuming to do these experiments, but if you don't do the experiments, you and the experts are just guessing.  You might even find that there is more than one thing that is off, and by fixing both you could get a very robust process.Yeah, I used to be a manufacturing engineer.RichardSculptures in copper and other metalshttp://www.fergusonsculpture.comSyncrowave 200 Millermatic 211Readywelder spoolgunHypertherm 600 plasma cutterThermal Arc GMS300 Victor OA torchHomemade Blacksmith propane forge