View Full Version : Weld penetration
Carld
09-20-2009, 01:44 PM
I have noticed this years ago but never asked. When welding with a new rod the penetration is good for the size rod and the amp setting. As the rod gets to about four inches long the penetration gets less. This is with drag welding and if I push back into the puddle I can get better penetration but not like the full length rod does.
I have noticed in many shops they drop the rod when it is about 4" and get a new one. Being somewhat frugal I don't do that.
Is what I have been seeing true and why does it happen?
EDIT: This is with 1/8" 6013 rod AC at about 70 amp. I haven't noticed it with reverse DC.
loose nut
09-20-2009, 05:28 PM
Doubt it's the rod.
When you are welding open root and don't have sufficient tacks to hold the spacing of the root, it will close up on you as you weld there by making it difficult to get the proper penetration. You can also use step welding to help keep the gap from closing.
radkins
09-20-2009, 06:18 PM
When you are welding open root and don't have sufficient tacks to hold the spacing of the root, it will close up on you as you weld
Take two of pieces of 1/4" or so metal strip a couple of inches wide and about a foot long or a couple pieces of angle about the same size. Butt it up edge to edge and start welding it from one end without tacking and see how far you get before the seam spreads so wide you can't weld it!
I have won several bets on this one. :D
Carld
09-20-2009, 07:48 PM
On the stove I tacked every 6" tight. On the inside you could see where the weld penetrated maybe 3/4 the thickness of the 3/16" plate.
I may do some experiments to see what the difference is if there is or if it's an illusion.
I agree, two strips laying butted tight but not tacked will spread apart as the welding heat expands the two welded edges. Been there, had it happen.
loose nut
09-20-2009, 07:51 PM
must be the difference in tacking because it always closes up on me, I haven't tried the trick you are talking about though.
radkins
09-20-2009, 09:15 PM
must be the difference in tacking because it always closes up on me, I haven't tried the trick you are talking about though.
This is an old one and has surprised a lot of people, it just seems to make sense that the seam would tend to close up as the weld shrinks but it turns out just the opposite happens. What I think is happening is the metal expands along the inside edge as it is heated up causing it to "curl" to the outside as the bead progresses then the weld bead solidifies in the opening and it can not contract back to it's original position. I have seen this happen on heavy metal, one case in particular was 2 1/2" thick (large loader bucket lip) and the guy who was welding it tried to weld from one side to the other in one continuous weld causing his tacks, which were quite heavy, to break and allow it to spread open a couple of inches. In any case like I said it is kind of an old trick because it seems backward and we have had a lot of fun with it, along with several more, by taking bets on what would happen.
"As the rod gets to about four inches long the penetration gets less."
The answer to your question is, resistance heating.
As the rod heats in use it's ampacity diminishes. The reason SMAW electrodes have definite diameter/length ratios.
Carld
09-21-2009, 01:22 PM
Carm, that is what I thought was happening but I never really knew for sure. I have suspected the resistance changed causing less amps at the arc giving less penetration.
radkins, that is the reason I will run one rod at one end and then run a rod at the other end of the weld. Then the two parts can't spread open. I too have had that happen to me. It is very troublesome when it does because it's next to impossible to close the gap.
I don't weld a lot but I sure remember the "Oh ****" and try to avoid them.
boslab
09-26-2009, 07:08 PM
you cant beat heat!
mark
Fasttrack
10-02-2009, 05:50 PM
"As the rod gets to about four inches long the penetration gets less."
The answer to your question is, resistance heating.
As the rod heats in use it's ampacity diminishes. The reason SMAW electrodes have definite diameter/length ratios.
Yep, joule (or resistance) heating. As you weld, more and more heat ends up in the electrode, or filler, instead of in the work piece. If it really concerns you, you can swap to a cool electrode and let the short one cool off and then swap it back in. :D