8 lug thug
In my past threads I have discussed some different scenarios, in this one I am going to go a little further into detail on full penetration (full pen) welds.
Open Root (full penetration):
This is the strongest joint that you can make, this is how pipe and other critical structural components are welded together. Simply butting the ends of 2 pieces of steel just won't cut it, look at it as, for lack of a better term, hot gluing metal together. When you weld metal together the point is to create the strongest link possible. Unless its thin material you will need a minimum of 3 passes, 1 root and 2 caps. Here is how to achieve joint greatness.
1) fit the pieces to be welded leaving about a 1/16" (.0625) gap between the pieces.
2) Bevel the ends to be joined to 37.5° (ASME B16.5 spec), less than 45° will work though. You want enough of a bevel to allow for good penetration but not so much of a bevel that the material is paper thin towards the edges, you will blow through and make a mess. There should not be a point at the edges, there will need to be a flat equal to the gap.
3) Clamp the pieces in place keeping the .0625 gap.
NOTE: Here is where a backing plate of brass, bronze, or aluminum comes in handy. If you don't have one, or you want extra strength, cut a piece of steel to bridge the back of the gap. You do not need the backer unless you want a smooth finish on the back of the weld. If you are tig welding and you don't have the ability to back purge, the backing plate is mandatory. If you don't use it the weld we become contaminated or "sugared", this will be weak and need to be ground down on the back and rewelded.
4) Start your weld on the INSIDE of one of the pieces of metal STARTING AT THE BOTTOM if in a vertical position. Rotate the torch from side to side with a slight downward arc, pausing before moving to the other side. DO NOT MOVE YOUR ENTIRE ARM, ONLY THE WRIST. This weld is ONLY a root pass and should fill the gap and a little more. Your heat setting will be lower than needed for the overall thickness of the base metals.
NOTE: If you did it right, there will be a piece of wire sticking out of the back end of the metal if there is no backing plate used at the very beginning of the weld.
5) Grind the weld flat.
6) Turn the machine up to the proper setting for the metal being welded. Run your cap beads only half over the root. Grind each weld before you progress onto the next bead.
DO NOT, I REPEAT, DO NOT try to fill the gap in one pass unless the material is very thin.
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