Root pass and Multi pass

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OK, I keep seeing you tiggers referring to a "root pass." What exactly is the purpose of it?Also, with MIG when multiple passes need to be made do to the thickness of the material, what is the correct process for making multiple passes?If I lay my logic down on doing multiple passes, its direct your heat and wire to the piece that will heat the quickest while just touching the other. Then run your second pass aimed more on the other piece taking advantage of the fact its been "pre-heated" by the first pass. Am I close?
Reply:Root pass is the first one..preheated or not..When you do just one pass and then do multiple or "stitch" over it the first one is a root..If you do one pass and call it a day then its "welded"..The cover passes are after the root..You cant grow a flower without a "root"...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:The root of a weld bead is that bottom Vee corner.  Root pass is the first pass.  The objective is to fuse the corner of the vee.  In the case of plates that are bevelled and you are welding at the bottom of the joint with only an airspace behind you must fuse both edges without getting too much build up or grapes on the back side.  It is not so much of a concern if you can access the other side.  There are several things to consider if you are doing multiple passes.  If it is downhand on the flat into a butt joint you want your edges of each bead to fuse to the adjacent edges.  At the same time you want a relatively flat bead.  If it is highly crowned you will have trouble fusing the edges and there will be what is call discontinuities along the edge of the earlier bead that you will bury.  They show up on an X-ray as wagon tracks.  It looks like the track made from a wagon in the mud, one on each side of the under-bead.  With fillet welds in the horizontal you must plan your weld beads so that they start in the corner and build from bottom up against the vertical surface.  Most welding texts give suggested sequences.   Your last question about where to direct the heat is critical.  You want both sides of a joint to heat up equally, coming up to melting temp at the same time.  Usually that means you direct the heat against the thicker section.  The thicker section is the larger heat sink.
Reply:Originally Posted by OpieOK, I keep seeing you tiggers referring to a "root pass." What exactly is the purpose of it?
Reply:(Edit: Blast, zap and lotechman beat me to the post.... I gotta learn to write faster....)
Reply:Thanks guys.Im good with hands on type stuff, but when it comes to proper terminology for what Im doing I usualy end up lost. Go Figure.I figured thats what a root pass was. Does it server any purpose other than filler? I assumed it was also to "preheat" the material as well.As for when one piece will warm quicker than another...In a T configuration while welding the piece with the edge will warm faster than the piece where you are welding in the center, right? In this scenario if you need to do a multi pass you do your root, and then which piece do you lay your second bead onto? There has to be some sense to it in where you want your heat to end up first.
Reply:Another tip that might be useful for fillet welds in a Tee joint:  Aim your electrode or your wire at 45 degrees but one diameter lower so that it is not quite aimed precisely at the corner.  This gives the bottom plate that small amount more heat.  If you decrease your 45 degrees you direct too much heat against the vertical face risking undercut.  Tilt it higher than 45 and you spread the weld out onto the lower plate and risk undercutting the vertical face as you try moving your electrode closer in to counteract.
Reply:At work I have to do two types of multi passes  (mig) one for corners and one flat. The purpose is to make it water tight. Below is the best way to explain in lame terms.Corner: - First, start at bottom corner and weld vertical 6". Then chip your weld.- Second, start from right side of corner and push weld to the left side, this will be one complete pass over lapping the corner weld. Chip your welds again. Flat (Butt Weld):- Make one complete pass @ 8". Chip weld- Make a second pass (push) 4" from center under your first weld. Chip weld.- Make a third pass again @ 4" from center above your first weld. Chip- Done!This makes since for all welds penetrate and makes contact from both pieces of material. If I can take a snap shot off my cell phone tonight of this process I can post up a picture if you like.- MattBMP Barge Marine Products "Welder"Mig, Stick, Air Arc, Submerge, & Torch
Reply:In general, it is better to use several narrow "stringer" beads to fill a joint, versus fewer large "wash" beads.  Somewhere there is a rule of thumb on this, some codes limit the weld bead width.  The logic has to do with the higher heat input of the large bead producing a larger heat affected zone, and the possibly the larger bead being more prone to centerline cracking.  With a series of smaller beads, the heat into the previous bead is less, and in some steels/applications is beneficial as a tempering treatment that increases the impact toughness of the weldment.The previous bead preheating for the next certainly could be a factor, but have heard it discussed as a reason for choosing multi-pass over single pass.Last edited by pulser; 03-15-2007 at 02:29 PM.