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MIG Welding a 3g Plate Test open butt 

This weeks Video shows tips for Mig welding a 3g vertical open butt plate test using short circuit mig process (gmaw-s).

Its a common test joint used a lot in schools to prepare a welder for pipe welding.

Why? Because plate is cheaper and a bit easier to weld than pipe and because the same exact settings that work on plate will transfer to pipe as long as wall thickness is fairly close and the same bevel and fitup specs are used.

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so here are the details for this 3g plate test

  • 3/8" (9.5mm) A36 carbon steel plate
  • 37.5 degree bevels
  • 3/32" (2.4mm) gap
  • slight land ( around .030" or .8mm)
  • .035" er70s6 wire (.9mm)
  • 19 volts and approximately 220 ipm wfs
  • downhill root and uphill fill and cap

Depending on the acceptance criteria, the mood of the inspector, the welding code, etc.

You may be required to weld the root pass in a way that is is flush, or you may be required to push it thru a bit for more reenforcement.

The voltage and wire speed along with the gap , bevel, and land all come into play.

But with a given fit up scheme, you can control the penetration to an extent by adjusting the voltage and wire speed.

For example, I welded the root pass in the video at 19 volts and about 220 inches per minute of wire speed and it was pretty much a flush root pass on the back side.

In order to make it push thru more, I could have decreased the voltage to 17-18 and increased wire speed to around 250 ipm.

In an open root situation, wire speed can be used to control root profile.

There are even special MIG welding power sources specifically designed to weld open roots.

  • Miller has RMD ( regulated metal deposition) for pipe root passes
  • Lincoln has a process called STT ( Surface tension transfer)
  • Fronius has CMT ( cold metal transfer)

These are all variations of short circuit mig where the welding current is manipulated and tweaked to improve results on open root welds.

The point is that even slight settings can have an effect on the root pass profile and The Big welding machine manufacturers have gone to a lot of effort to tweak the arc in order to make for better root passes with less operator training.

But a standard Power source without all those bells and whistles will work pretty well in the right hands.

Watch more mig welding videos and learn more

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