Why Hammer a Weld?

Whats the purpose of hammering a weld while its still hot? Strength?

 

I believe it's to flatten the weld to avoid excessive grinding but if I'm wrong I'm sure someone will set me right.

 

When you weld, the metal gets hot and shrinks a bit. You hammer the weld with a dolly backing it to flatten the weld and stretch the metal back in the desired shape. Stu

 

Thanks Stu! I knew there wa a better reason than the one I gave.

 

That makes sense for sheet metal, but what about things like plate steel, frames, etc? Is there any point to hammering these?

 

Not really any point to it on structural elements. They're stout enough to not need it. Body panels are much more prone to heat induced warpage that hammering these is important to retaining a smooth surface for paint. Frames etc. won't warp that much and if they do you're using way too much heat.

 

Stu-D-Baker nailed it. Thats why when you weld a patch into a door, or any other flat expanse of metal. the friggin thing will always warp. Each section of weld only shrinks a very tiny amount, but when you add up all them "tiny amounts", its enough to warp Hell out of a panel. By heating and hammering the weld against a dolly, it relieves the internal stresses that this shrinkage set up in the metal, and lets the weld bead and the metal close to it (the heat effected zone) stretch back out to the size it was before it shrunk, and in turn straightens out the warped panel. An added benefit is that the weld bead gets flattened considerably during this hammering process, so consequently requires less finish grinding.

 

cool, thanks!

 

To illustrate this, when filling the roof of my '35 sedan I used these little clamps I got from Eastwood that hold ****ed sheet metal together, now, they work really cool but as soon as you do a few tack welds......you can't get these little clampy things out from between the panels until you hammer the welds......the weld actually pulls the panels together where you might have had a .020" gap before welding........all of a sudden you have only .010".

 

I have read that hammering your welds only applies to gas welding, mig welds are too hard and cool too fast. Is this true or is there the same benefit of hammering mig welds?

 

I've hammerwelded tig but not mig. I think the weld is too hard to hammer.

 

Softer wire is available for MIGs that would be easier to grind, but I don't know how well it would hammer out.

 

Usually done to the tack-welds on structural stuff as a form of stress relief, in order to keep the parts from pulling too far out of square during the tacking process.

Once the real welding starts, the bead tends to even things out on its own as you travel around the joint. In certain situations, such as a "T" joint in square or round tubing, the weld is hammered slightly to reduce the warpage ***ociated with that kind of joint.

I did some work with 3x3x 1/4" tubing a couple days ago and instead of hammering the beads for stress relief, I pre-bent the tubing opposite of the way the weld would pull it. Basically, what I did was put a piece of 1/2" plate directly under the joint, then forced both ends down to the table with big C-clamps. The piece was about 6' long, so it wasw pretty easy to flex it that much. This works a lot better than the hammering method when working with heavier steel.

 

Large weldments, as in machine frames, etcetera which are stick or heavy mig welding can not be hammer welded to relieve the stresses from welding. Instead, they are put into a large "oven" and heated to a high temperature for a number of hours to relieve the ac***ulated stresses. If that wasn't done, then as soon as a machinist started to machine these welded frames, the damn things start to "move" and the machinist can not hold accurate tolerances. Sometimes the "moving" happens after the machining is done and the frame has been ***embled as part of a highly toleranced machine.---this is really bad news, as then there is no way to fix it. Some places stress relieve their machine weldments by a ********y process, but I am not a fan of that method myself. On all the machines that I design, I specify 'stress relieve by heat process". www.rupnowdesign.com

 

Hammer welding will shrink the metal and keep the metal thickness uniform. To do it best it takes 2 people, one welding and one hammering. I have always done it myself though.
I hammerwelded all the tub on this car

 

http://http://i61.photobucket.com/albums/h53/tinmann/August5th033.jpg

I chopped this '52 Chev 5 inches and hammer welded the new roof panels using only Mig. So the answer to "should you hammer Mig welds also?" .... is yes, by all means. When I work on sheet metal, I do a series of a million tack welds, each one given a firm smack with a flat faced hammer and firmly held dolly. That way the metal (warpage) behaves itself and does what I want.

 

Opps can't attach to a quick reply.

 

about hammer welding patch panels that use the overlap type of patch do you still hammer weld them, or just cut off the lip and weld as a **** patch?

 

not as effective, but not a bad idea. the overlap will cause alignment issues of its own.

 

Just to add my .02 worth here. It's important to know the only thing that shrinks is just the weld. That draws the metal and causes the warpage. If you keep your hammer/dolly work confined to just the weld the surounding metal will go back to it's origional shape. If you beat it to much or on the surounding sheetmetal you will streach it and cause it to warp again from streaching the metal to much. There is a fine line there and you half to feel it to know when to stop.
The Wizzard

 

When I used those little Harbor Freight clamps to align patch panels for mig welding **** joints the first time, the welds shrunk and wound up pinching the metal up tight against the flat tabs of the clamps, and I had a hell of a time pulling them out. So now I always grind away a little excess from the edge of the panel in just the areas where those clamps go. By doing that you can get the sheet metal ****ed tight -- or tighter than those little clamps will let you pull it. I try to leave a little gap (~.030") for better penetration.

Mig welds are hard, and difficult to hammer once they're cooled down. OxyAcetylene and hammering is the way to go, but I'm better at Mig welding so that's what I do usually. But then I have to do a lot of hammer and dolly work afterwards and usually some heatshrinking to pull down the high spot puckers that show up in other places afterwards.

 

The reason I hammer the welds is revenge,cause the welding slag falls down in my shoes and I just hammer it to get even!!

 

everything I've been told mig welds will crack if hammered. I've helped out doing it with gas welds though. Like willowbilly said. It's easier if one guy welds and another follows up with the hammer and dolly.

 

If you use ESAB EG grade mig wire it can be hammered without cracking. And waaayy easier if one welds and one hammers. With the right touch you need ZERO filler when it's done.

 

FWIW-I've been leadman/welder for 23 yrs. for a company where we build structures for the medical industry (Siemens Med. Div.) that are used for radiation treatments. Using from 1/2" plate up to 4"plate with the wedments weighing 1500-2000 lbs. and all are machined (some to +/- .001) before ***y. We have always used a Formula-62 "shaker" to stress relieve our parts and have never had a problem. In fact this process is called out on the Siemens prints.

 

Just wondering, what if you are welding in a patch panel in a spot where you can't get behind the weld to put the dolly there? Do you just not dolly it??? or cut out enough behind to GET to it??? Also a little bit away from this topic, but when heat shrinking,... like if you didn't hammer the weld in time, when you heat shrink, do you just heat up the raised or sunken area and then cool it immediately with water, like a wet rag or spray bottle???? I am asking because when you fast cool metal doesn't it harden up??? so wouldn't the shrunken area now be prone to a crack from the hardening effect???? I have never done either proccess so i have nothing but questions, with zero answers...Ken

 

Chopperimpala - Yes, if your doing quality work it's not uncommon to cut out innerstructure to get to the back side of a panel to work it. You also plan your cuts to minimize obstructions.

Quenching does harden metal. Cool with compressed air, or not at all.

 

Yeah, I use that ESAB "easy grind" wire too for mig welding sheet metal. It is easier to grind and a little softer than the usual mig wire but it has the same tensile strength as the usual mig wire. I've never had a problem with it cracking.

On a long seam, like welding the bottom half of a door skin on, it seems like it helps a little if you partially weld it with short little welds, then grind down those welds, then weld some more, then grind down those welds, then keep going like that until you're done, letting it cool down in between welds and taking your time. A long weld bead is kind of like a long hot cable, and as it cools, it shrinks and shortens up and puckers and pulls the surrounding sheet metal in with it. The weld bead is stronger and thicker than the surrounding sheet metal, so the sheet metal loses the battle.

Oh, by "grind", I mean sand it down. An actual grinder wheel gets the steel too hot. I use 50 or 60 grit 3" zirconia abrasive "roloc" discs on a rubber backing pad in a right angle grinder. It cuts fast and cool.

 

the wire you are refering to is called EZ-GRIND!,low alloy around 30,000 psi vs 70,000 psi tensile of standard wire. ESAB is the brand name i have seen. less heat-less warp!