Oxy/acetylene weld cracking / porus welds

[chap]

Hi, i'm welding a Mini at the moment, chosen to use Oxy/acetylene, but i've come from an AC tig background.

Sometimes i'm getting a sort of 'fizzy' weld pool which is giving a porus weld. What causes this, and how can i prevent it? (btw i've chopped out the rust and am welding to good clean metal)

Onother problem i'm having is that when butt welding a patch in, the weld will hold on one side but then crack on the other side of the patch as it cools. I've tried playing with nature of the flame a little, but i don't really have the experience.. should i use a slightly oxidising flame.. light acet, adjust till no soot, then add oxy till yellow flare goes (neutral flame) then add a little more oxy, till the larger cone nearly meets the smaller cone. Is this right?!

BTW, i'm using a boc 3 tip

I've had to chop out an extra section 'cos the weld was all porus.. (this bit VV) not good enough!



Thanks for any help
Nathan

 

[weldequip]

Too much Oxygen pressure. You want a neutral flame setting & 2psi pressure setting for each gas on a No.3 nozzle. If it isn't taking to the underside it sounds as though you are not distributing the heat evenly on the two sheets. Other possibilities are contaminents or dissimilar metals;
if the two metals are different grades you would be better brazing them (as long as it's not structural).
weldequip

 

[chap]

I presume there are different causes for the different problems..

Too much Oxygen pressure. You want a neutral flame setting & 2psi pressure setting for each gas on a No.3 nozzle.

This would cause the porosity?.. i'll give these settings a go, thanks.

I meant, looking at the example of the gap in the pic (bellow the "VV") say the left hand part of the "U" shape weld would attach be strong, but the right-hand side of the weld (where i finnished) would crack where the join is, as it cooled. It's a butt weld by the way.. think i had good enough penetration. (ended up cutting the patch back out!)

All the steel is mild, the new metal was recycled but clean A panel material.

I want to weld all the non structural joins, so by the time i do the structural ones they are better.

Thanks for the help.

 

[weldequip]

Too much Oxygen pressure would cause the porosity yes; it's a very common problem setting the gas pressures too high on Oxy/Acetylene. Make sure the 2psi is flow ie.set when the torch valves are open; if you just set the 2psi on the guages with the torch valves closed the pressure will drop & be insufficient when you try & light up.
weldequip

 

[chap]

Thanks very much for the info.
I'll have a crack at the weekend, providing it doesn't rain!

 

[peter9763@tiscali.co.uk]

What filler wire are you using?

 

[chap]

Copper plated mild. It's been kicking around with the welding kit ever since i've worked for the Co. (6 yr)

 

[peter9763@tiscali.co.uk]

Make sure the wire is free from grease and dirt, give it a rub over with wire wool. The same as TIG then. Keep the tip of wire inside the flame to protect from atmosphere. Nozzle size? Up to 0.9 mm thick number 1 nozzle. 1.2mm no. 2, 2.0mm no. 3, 2.6mm no. 5. Pressure for DA & Oxy. both 2p.s.i. 0.14 bar. The flame you are describing sounds like a carburising flame used for hardfacing. There is a white excess acetylene flame surrounding the rounded cone. Neutral flame has equal amounts of Oxy. & Acetylene with a rounded cone. Oxidising flame has a very short pointed cone with a smaller flame.

 

[chap]

Thanks Peter.

Looks like I should really be using a No.2 nozzle then.
Would this also require 2psi, or slightly less?

I had a crack with the different settings today.. the adjustment on the regulators is a bit corse, and i have a feeling that they are almost antique, but i tried to set the 2psi as best i could for both bottles.
Had much better results with new welds, even managed to weld over the original porus ones that i did. Will post some pics later, if i got a decent one.
Also didn't have the problem with the welds cracking.

 

[peter9763@tiscali.co.uk]

Pressure will be the same for all those nozzle sizes.