Thank you.
Answering the points you variously raise:
Material:
I was careful to state plate and not bar steel so know it is not leaded, but might hold other elements.
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I had bevelled the plate edges.
That didn't help. I find it very hard to aim the electrode accurately anyway, even if I weld in sufficiently bright sunlight to see the metal before striking the arc. Then almost everything goes black because my auto-darkening mask is if anything too powerful on the MIG setting and won't turn down enough. It might be intended for trade-grade welding with heftier arcs.
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I'd also cleaned the coating back.
I know that zinc will poison the weld… while its fumes try to poison the operator.
What I do not know is what is coating this steel. It is certainly not hot-dip galvanising. It is light grey colour, very thin, easy to scratch off. Some of the metal still in my front porch is beginning to rust. It might even be a lacquer.
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Gas – I thought of that and fitted a new CO2 bottle, after weighing suggested I need do that. No difference.
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Welder settings – they just control the mess I make of it.
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Welder models – SIP 'Weldmate T141P-ARC', I bought new and have hardly used. Clarke 'MIG150TE'. Second-hand. It had a new tube liner a few years ago and little use since. I have fitted a new nozzle and shroud and it feeds properly – whether my settings are correct is another matter.
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Weld appearance –
I have always had such unsteady hands that I could never make anything in the Cherry Hill / Ron Jarvis league! So I cannot guarantee to aim the electrode properly and keep its height and speed steady.
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Here though, I query the result as it differs from normal, whether using open arc or MIG:
– Normal and assuming the steel is of good weldable quality: Shiny, convex splodges of steel, most melting into one side only. Cavities full of slag. Splatter and blobs on surrounding metal. Under quarter of weld length joining both parts. On thicker materials, penetration very unlikely beyond half depth of prepared groove. Full penetration, full length impossible; any thickness. Holes very likely through <3mm thick. Faulty areas left or over-welded where inaccessible for grinding out.
– This Case: Same except – the splodges are very rough and irregular, with very little smooth area, using both welding methods. The MIG set leaves a brown "soot" on the steel.
It is that difference which is the key to my question:
Same conditions, same equipment and electrodes, same proper preparation, same poor welding skill… but noticeably different metallurgical results?