Thread: Implications of poor prep

I'm a backyard hack. Sometimes, due to circumstance, my prep isn't as good as it could be. E.g. I'm building something now with a mixture of galv, painted and plain milled mild steel. Whatever I have lying around really. I'll prep a joint, tack stuff up, weld a bit, then the day or weekend's over and I won't get back to finishing until the next weekend. By that stage surface rust has formed tight in joints etc that I can't get to with a grinder to effectively remove so I'll go at it with a wire brush and then just blast it with the welder. Other times (prompted by the scale thread), I coarsely go over scale with the grinder with a wire brush but don't spend as much time as I could so it's still not bare, clean metal (as if grinded) before I start welding.

I have no problem laying down a decent weld and these aren't mission-critical joints, but I wonder what the long-term effects are of rust, scale and other contaminants inside a weld?

We talking MIG or stick?

but I wonder what the long-term effects are of rust, scale and other contaminants inside a weld?

In service this will lead to a weld failure its not a matter of if............but rather when!.............

Regards Lou

Lou,
can you explain that a bit please?
I'm like Legion pretty often, but have never had a weld fail in 40 years.
My logic is the heat from arc welding will reduce all iron oxides to iron an burn off the oxygen.
Paint, oil and gease is a little different because it contains hydrocarbons. The carbon will disolve into the iron and changes - potentially measurably - the catbon content in the weld margins. Then as the weld cools - or is quenched with a hose - that layer goes hard and brittle. So that could explain failures.
Very interested in more detailed explanations! Always ready to learn, but not so receptive to unsubstantiated assertions I'm afraid.
Cheers,
Joe

Stick, with 6012 or 6013 rods, 2.0 or 2.6mm. Dog house, BBQ table, etc. One example - I used a piece of 25mm mild angle and a piece of 25mm mild bar to form a z-section as part of a frame. It was tacked and I only got around to running a bead down it the next weekend. By that stage it had been rained on and had surface rust. I ran the angle grinder wire brush down the join (effectively an outside fillet/butt) as best I could but the brush can't get into the root gap and I know there's rust in there. Both pieces were 3mm. The structure is a static frame, no dynamic loading.

Would it be best to grind off the tacks, clean everything, refit and start again in that situation? It would screw up my already poor fitting of the whole frame and alignment etc worries me.

can you get a grinder with a thin cutting disc and run it gently down the seam? to "grind" the rust off. I dont think a bit of surface rust will worry it. I'd be more worried about it twisting outta shape when you weld it.

Not really. Sometimes the root gap is 0mm, sometimes almost 2mm. The edge of the flat bar is often not straight so I tack ends then clamp along it's length to minimise the root gap and tack, tack, tack before welding. So a cutting disk might fit sometimes but often won't. And my frames are often needlessly complex so although I'm burning at least 20 rods/day when I pull out the inverter, sometimes I don't finish things. E.g. last Sunday I fitted up a gate thing about 1000mm x 700mm from SHS with these 25mm flat bar internal flanges with 50mm x 50mm weldmesh welded onto the bar. It's overkill but I'm fully welding all joins to help me learn welding and I didn't quite finish the gate - I needed to grind off one bad start section and probably burn two more rods to finish the job and join up all my welds but it was dark and dinner was getting cold. So I'll have this same problem with the gate this weekend. The missing sections are probably several 50-80mm gaps - no way a disk will fit in there.

Meh, with stick, if the finished weld looks good, it most likely is good. Any paint, rust whatever probably gets cooked up the molten metal and ends up in the slag.

Originally Posted by RustyArc

Meh, with stick, if the finished weld looks good, it most likely is good. Any paint, rust whatever probably gets cooked up the molten metal and ends up in the slag.

Yeah exactly. If you're running your rods hot enough (i.e. correctly) you'll penetrate any light surface rust, get a well mixed weld pool and the oxides and other impurities will get mopped up by the additives in the flux. If you're too cold then there's a chance you'll leave streaks of impurities within the weld which could lead to failure.

One thing I've noticed though is that the slag will peel off nicer if you start with clean metal. Welding on crappy metal tends to end up with slag that takes a fair bit of raking to get off.

Cheers

- Mick

Guys ,
Whatever you weld on needs to have a (self imposed) fitness for purpose standard applied against it .

A domestic gate would have a very different standard applied against it ,than say a set of heavy duty truck axle stands.Other applications of similar nature are pressure vessels and lifting gear which attract legislative standards / controls to their construction.

This is what Lou was getting at.

The outcome of a a weld failure in service of a gate will be much less severe than that of a welded axle stand that collapses under a heavy truck.
Accordingly your welding preparation and procedures will vary to suit.

Where a few damp electrodes or a bit of rust and paint on the gate from won't kill anyone, the same thing on your H/duty axle stand may well have the capacity to cause a severe failure with dreadful effects. It is the load and environment applied to the welded item that must be considered.

The final use of your welded item should dictate the the amount of workmanship put into it.

I hope this clears this up for you.

Grahame