grinding down old welds to re do them?

[pondy]

made a frame up out of 3mm 50x50 box section, halfway through my old mig started to loose power and welds weak. i welded 2 bits of box section together and put them in a press, at 7 ton the metal bends but weld held fine, mid way i did the same thing and gauge didnt move before before weld broke.

so as i dont know how stong these welds are and as i have a new mig planned, it is ok to grind as much weld material off the joints and re weld them with a better machine to create a strong joint

 

[Dev]

If it were me I would cut them apart and reweld but don't take my word for it.

Someone who knows what they're talking about should be along shortly

 

[Wozzaaah]

Running over the top of an existing weld will add little, if any strength to the joint as you're having to penetrate the new weld material, not the parent metal.
If they need rewelding then I'd suggest grind them off as much as possible so you're near as dammit back to the parent metal.

 

[pondy]

cutting each joint isnt an option as there is alot and some are located were you wouldnt be able to cut. i was thinking of just grinding as much of the weld material away as i can then re welding with slightly more current than would be needed to compensate for any material that was added before

 

[Wozzaaah]

As long as you remove most of the bad weld it should work fine.
My comment was more to emphasise the point that some people think going over the top of a bad weld will make it a good weld when in most cases, it won't.

 

[Hitch]

cutting each joint isnt an option as there is alot and some are located were you wouldnt be able to cut. i was thinking of just grinding as much of the weld material away as i can then re welding with slightly more current than would be needed to compensate for any material that was added before

Far from ideal, you can normally tell if someone has just laid another weld over the top of a rubbish one without grinding any off, normally look like a big fat ugly slug


Grind as much out as possible whereever possible, if you can't do it properly, try and get the edges down a bit at least, that way you may end up with a high weld, but the edges melted in properly.

Its the same old thing isn't it... in the perfect world, you would grind away all the old weld and redo, even if it meant cutting the job apart to allow you to do this..... But if its something like a pond cover, is it really necessary to go this far.

Whats the job in hand out of interest?

 

[pondy]

its this

http://www.mig-welding.co.uk/forum/showthread.php?t=34871&page=6

its the last few welds i did that didnt feel the same, braces going from the wall frame to the centre bar at the bulk head (not pictured yet) they seemed to sit on the surface of the metal were as the others were alot flatter like they had melted into both materials, so alot of them should be fine but due to the application i was to make sure everything is sound.

 

[pondy]

thinking this is a duty cycle thing. i only noticed this problem after alot of welding on the max setting, when the welder gets hot does the power drop or just turn off to protect itself till it cools down? just tried it and it will just penetrate 3mm?

 

[the snooper]

as the coils heat up the resistance changes so it will affect the power

 

[pondy]

which explains why i only noticed this drop after welding for along time on the max power. asking to much from a small machine

 

[the snooper]

thats why the duty cycle is stated, maximum times before the output will suffer, although find me someone that hasnt gone over at some point

 

[@Turbo]

is it at a right angle?

could cut some angle iron and weld them in.

 

[gandalff the grey]

made a frame up out of 3mm 50x50 box section, halfway through my old mig started to loose power and welds weak. i welded 2 bits of box section together and put them in a press, at 7 ton the metal bends but weld held fine, mid way i did the same thing and gauge didnt move before before weld broke.

so as i dont know how stong these welds are and as i have a new mig planned, it is ok to grind as much weld material off the joints and re weld them with a better machine to create a strong joint

hello ,
this could be because you are losing gas pressure or running out of gas.
before welding any steel it should always be cleaned grinding away all the mill scale and rust and grease and contaminates if you don't thease then become part of your weld and make it weak
.another reson for cleaning the steel properly is to obtain a good earth and arch if you have a burn back set it too 6.5 gas pressure too 22 liters/min and try again.alot of welders think you can run on 15 liters per min but I think this causes cold metal transfer and a lot of spatter and that's why I like too use more gas to get a more effient burn less spatter more penetration and a lot stronger welds I found the optimum pressure too be 22 liters per min but the more amps you use the more gas you need and in special sercomstances use pure helium for hevey welding jobs