Welds Cracking on rust patches

[at-jefft]

Ive been doing some rust repair on my spare tire area. I have been cutting patches and butt welding them. Most of the process is going well but VERY slowly (couple hrs for a 1 inch patch, sound right?) My issue is that I'm 90% sure I'm getting good penetration (if not too much penetration) and I've had spots on both patches where the welds crack.

 

The welds look good from the top and bottom, then I grind it flush and when I finish grinding I find a spot that has cracked?

 

My guesses are:

1) That the patch is initially too tight(they hold them selves in place even before welding) and either the welding or grinding causes expansion and then cracks.

2) I'm grinding it down too quickly and again causing heat to weaken/expand the metal.

3) The patch and the body may not be flush in the spots that crack

 

 

Anybody have any ideas on why some seemingly good welds are cracking?

 

 

PIC(ask for more if needed):

Edited May 11, 2010 by at-jefft

[Scott_M67]

Look how hot you got the metal when grinding, its blue. Instead of a grinder use a cut-off wheel and nip away at your excess material to get the majority taken off, then very lightly use a 40-60 grit flpper wheel on smooth it out.

 

Ive been doing some rust repair on my spare tire area. I have been cutting patches and butt welding them. Most of the process is going well but VERY slowly (couple hrs for a 1 inch patch, sound right?) My issue is that I'm 90% sure I'm getting good penetration (if not too much penetration) and I've had spots on both patches where the welds crack.

 

The welds look good from the top and bottom, then I grind it flush and when I finish grinding I find a spot that has cracked?

 

My guesses are:

1) That the patch is initially too tight(they hold them selves in place even before welding) and either the welding or grinding causes expansion and then cracks.

2) I'm grinding it down too quickly and again causing heat to weaken/expand the metal.

3) The patch and the body may not be flush in the spots that crack

 

 

Anybody have any ideas on why some seemingly good welds are cracking?

 

 

PIC(ask for more if needed):

Edited May 11, 2010 by Scott_M67

[Zinpieces]

What size wire are you using? I find for lightweight sheet metal #23 wire is about right. Finding the right balance of wire speed and heat are a matter of practice and trial and error.Your patch panels can stand a slight gap that you can back with a piece of copper to contain the weld pool and act as a heat sink. With very light metal you can weld fairly quickly. Not for a long time, just quickly. with plate steel you can actually allow the weld puddle to form and drag it along the workpiece. with tin it's a matter of practice. If you wait to see the result you will blow through. TIG welding is another option, then the panels can be tight as you are melting and fusing the metal without or with very little rod. Try to keep your weld as flat as possible, it reduces heat and grinding. Practice makes perfect. If there is any rust on the metal you are welding cut back to solid metal. Your patch may be larger,but chasing rust is a bear. Hope this helps. Some guys are more comfortable dragging the gun, some prefer to push. Keep the gun at a 45 degree angle from the workpiece This will help with the flow of gas over the weld.

[Brad-ManQ45]

I'm not a professional welder, but have taken classes and just got done welding in 1/2 of a firewall and 1/2 an inner fender panel and fabricating part of a battery tray and welding it in. I could weld about 2 feet in 10 minutes skipping around to keep the heat down.

 

I haven't had your problem so I will tell you how I did my welding and smoothing in the hope that your process will go faster and with better results.

 

I use Harris Twenty Gauge wire. This has to be used with gas, but has a powdered metal core. This is the easiest wire I have used on sheet metal, but I try to always have a piece of flattened copper behind what I am welding held by a magnet to act as a heat sink - once again to minimize heat buildup and in some cases to actually keep the patch in position - nothing like killing two birds w/one stone!

 

If I didn't have to reposition this piece of copper I could weld faster maybe - but then I also might be more afraid of burn through and maybe not get penetration, but I really don't think lack of penetration is a problem with sheet metal...

 

I really don't like using a grinder on sheet metal - used it plenty when building my rotisserie, so I use cut off wheels and flapper disks as suggested already.

 

No big secrets except for maybe the Harris wire.

 

Hope this helps.

[at-jefft]

Scott_M67- I hadn't noticed the blue(haha), I'll try the cut-off wheel/flapper. Thanks.

 

What size wire are you using? I find for lightweight sheet metal #23 wire is about right. Finding the right balance of wire speed and heat are a matter of practice and trial and error.Your patch panels can stand a slight gap that you can back with a piece of copper to contain the weld pool and act as a heat sink. With very light metal you can weld fairly quickly. Not for a long time, just quickly. with plate steel you can actually allow the weld puddle to form and drag it along the workpiece. with tin it's a matter of practice. If you wait to see the result you will blow through. TIG welding is another option, then the panels can be tight as you are melting and fusing the metal without or with very little rod. Try to keep your weld as flat as possible, it reduces heat and grinding. Practice makes perfect. If there is any rust on the metal you are welding cut back to solid metal. Your patch may be larger,but chasing rust is a bear. Hope this helps. Some guys are more comfortable dragging the gun, some prefer to push. Keep the gun at a 45 degree angle from the workpiece This will help with the flow of gas over the weld.

I'm using a .024 wire and I have a piece of bronze that I secure with a magnet on the back of my welds.

 

ya, I took auto body in High school and learned a lot of what you said(through a lot of REALLY boring videos) about welding technique(always good to refresh though seeing that was over a yr ago). We mostly worked with 16ga metal though so the 20ga is still kinda new to me, Thanks for the info on light metal. I think Scott_M67 got it right, I'm probably overheating the metal with all my grinding.

[at-jefft]

Brad-ManQ45 - Thanks for the info, sounds like I'll be getting some flapper discs. Hmmm... I wonder if harbor freight sells those haha(I'm a poor college student, can ya tell?)

[Apollyon12]

I don't think that the heat from grinding is doing this. What it looks like to me is that your weld is not melting the base metal as you go and is just ends up sitting on top rather than forming one continuous piece of steel. I am not a professional welder, but I do have some experience, so I can't really tell you how to fix the problem, but I think that you need to go a little hotter. Also, when you butt weld are the two pieces of metal chamfered or square? \/ or || ? If you don't chamfer them, you won't get as good a weld. Especially if you are grinding back, which is removing a majority of the metal that you have layed down to form the joint (if the edges are straight).

 

Thinking about it, it may also be the area prep too. Make sure all the metal is bare and there is no rust.

 

The heat that you are generating by grinding the weld isn't really a problem unless it was a body panel that you are trying to get flat. Then the metal is going to warp and, in some cases, create a weird contour in the metal that is worse than the original dent. In this case, you said that you are working on the spair tire well and that doesn't exactly need to be perfect.

Edited May 12, 2010 by Apollyon12

[ozzzzz]

What size wire are you using? I find for lightweight sheet metal #23 wire is about right. Finding the right balance of wire speed and heat are a matter of practice and trial and error.Your patch panels can stand a slight gap that you can back with a piece of copper to contain the weld pool and act as a heat sink. With very light metal you can weld fairly quickly. Not for a long time, just quickly. with plate steel you can actually allow the weld puddle to form and drag it along the workpiece. with tin it's a matter of practice. If you wait to see the result you will blow through. TIG welding is another option, then the panels can be tight as you are melting and fusing the metal without or with very little rod. Try to keep your weld as flat as possible, it reduces heat and grinding. Practice makes perfect. If there is any rust on the metal you are welding cut back to solid metal. Your patch may be larger,but chasing rust is a bear. Hope this helps. Some guys are more comfortable dragging the gun, some prefer to push. Keep the gun at a 45 degree angle from the workpiece This will help with the flow of gas over the weld.

There is a reason for pushing or pulling the puddle. Providing the settings are the same, pushing makes a bead wider with less penetration. Pulling makes a taller bead with more penetration. This is in reference to stringer beads. It does look like the welds are lacking penetration. Turn up your machine and try doing a series of overlapping spot welds. The object is to move over slightly and use the previous spot weld to help back up the next.There will be a slight pause between welds. If you are using flux core try not to let it cool too much as you will trap slag from the flux. The amount of stick out also effects the penetraction. You can always make a larger patch and flange the edges giving you a little overlap.

[johnc]

No base weld melt into the bead. As mentioned above it could be dirt, not enough heat, or moving too fast.

[at-jefft]

Ok, from what I gathered from the last 3 posts, I'm going to start chamfering the joints, turn up the voltage, and clean the workpieces.

 

I'll post my results after I do some work tomorrow. Thanks everyone.

[at-jefft]

Alright, so today I decided to scrap fixing the spare tire well. I would have to replace at least half of it with new metal so I'll be looking for a better one to weld in.

 

I cut two pieces of 20ga to practice with. I butted them up against each other and welded them up. They were clean and as some points I turned the voltage up a notch.

 

Pics:

 

(Used a 60grit flapper disc after cutting the excess off with a cutoff disc.

 

(Penetration and when I changed the settings)

 

(Penetration where I used my normal settings)

[rturbo 930]

Some of your welds on that scrap piece look like they're only on one of the pieces. Take a look at the weld right before the one you circled.

[at-jefft]

Some of your welds on that scrap piece look like they're only on one of the pieces. Take a look at the weld right before the one you circled.

Its not completely on on side but I probably could have done that one better.

[zclubhouse]

Its not completely on on side but I probably could have done that one better.

 

You may want to try a series of spot welds instead of trying to lay a bead on 20ga. Start an inch apart and then alternate filling in the middle (ie 1" - 0.5"- .25" etc..) till it appears you have a continuous weld. Through my personal experience I have learned that the speed required to not blow throw thin metal leads me to missing the desired weld area. Taking your time with lots of spot welds gives you a chance to aim at the joint a bit better, and reduces the risk of warping the piece or blowing through. It does however require a very tight fit and as always, a clean piece to weld to.

Good luck!

[Derek]

Be careful not to cool the weld down. Also dollying the weld before grinding will help reduce the tension created by the weld shrinkage. 

 

 

 

 

Just some thoughts

 

 

 

 

Derek

 

 

[Nismo280zEd]

I read through you post but I can't find where you mention what you are using?

 

I use a Lincoln MIG it has 4 amperage settings. Often I find working on the Z, I have to jump up to the next amperage setting and change my wire speed to accomodate because of the Metal on the Z. I guess the older metal absorbs oils and other media from the air and paint, etc. On brand new clean metal I don't need as much amperage but the old metal on the Z, even cleaned properly still can prove tricky.

 

Also check your wire. The wrong wire will make your life hell.

 

I wasted 2 days trying to weld up an exhaust on my girls 350z till I realized I had the polarity reversed on my welder for flux core. People make dumb mistakes and we get so focused we sometimes forget to check the obvious.

 

Good luck, I'm still learning tricks myself.

-Ed