Sunday, August 14, 2011

Weld Stops & Starts

My question is… Where can I go to find out about tie-ins being done all in line with each other and not staggered / spaced out as they should be. I am having trouble with my Mig welders making their welds all the same length and not staggering / spacing there stops and starts out. 
I need physical evidence in black in white to show them in a meeting I am having on Tuesday. 
Allen T.

It is understood throughout the industry that most weld defects are attributed to weld stops and starts.  We automate (or change from manual to semi-automatic) for the purpose of improving cycle time and quality.  Most of that quality improvement comes from the elimination of starts and stops.  Starts in welding can lead to overlap, incomplete fusion and slag inclusions.  Stops typically lead to cracks and undercut.  

Some of these discontinuities may be considered acceptable but stacking them on top of one another in a multi-pass weld is just “flurtin’ with disaster”.  

Stops and starts are a necessary evil in welding.  We can minimize their negative effects through techniques during manual welding, and through weld data settings for semi automatic and automated welding.

In GMA (Mig) welding those weld setting that can be added to many wire feeders include:
  •  Adding a burn-back to keep the wire stick-out as short as possible to help with your next arc start.
  •  Adding a post-flow (3.0 sec) to insure the tip of the wire is kept protected and clean.
  •  Adding a pre-flow (0.2 sec).  This insures shielding gas covers the area before an arc is struck.
  • Adding a slow run-in speed (-100 to -200).  This will reduce the “Snap” that often occurs when initially striking an arc.
  •  Increased start voltage (1-2 volts).  This will also reduce the “Snap”.

As for your Welders techniques:
  •  When striking an arc, start out ahead of the desired start point (1-1/2 x the weld size). When the arc is struck, backup quickly to the desired start point then begin your weld.  This will almost eliminate any overlap (cold lap) that is common with weld starts.
  • When extinguishing the arc, weld to the desired stop point then back up (1-1/2 x the weld size).  This will fill the weld crater, slow the cooling rate, fill the undercut and reduce the chances of a crater crack.
You could probably search the internet for an article on weld stops and starts and get plenty of horror stories.  In that same search you will find oodles of “pixie dust” salesmen with a simple wire/shield-gas/power source that can take all those troubles away.  

Stick with common sense.  Set up your feeders with the settings that help improve weld starts and stops, and teach good welding techniques.



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