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Re: [PSUBS-MAILIST] Learning to Weld
Another corrosion issue !!! Intergranular corrosion. More common in non-ferrous (aluminum alloys) than ferrous metals (iron or steel). Most of the time intergranular corrosion is caused by improper heat treatment, or impurities indtroduced at the foundry, or where the material was manufactured.
It can be crud introduced at a weld. It is important (critical maybe a better word) that the surface is clean. How clean is clean? well...get good quality metal and grind down enough that there is no surface corrosion however slight. As when it is induced into a weld it WILL grow.
General for visual inspection we use a 10X magnifying glass, which is a hand held microscope.
Intergranular corrosion is the most dangerous. As it will grow inside the grain boundries of the metal. You probably won't know it untill it is too late.
If you were to grind on an aluminum SCUBA cylinder (even with a non-metalic pad), it causes heat. If it feels hot to touch it's too hot. The extra heat in that one location will probably not cause a problem right away. But the heat treatment in that area has changed, and had developed a different grain in the metal. Along with a grain boundry.
i'm going to try to keep this simple, there are websites if you do a search for "intergranular corrosion".
After a while (how long? who knows !!) corrosion develops inside the wall of the SCUBA cylinder, you can't see, because there is no buldge on the outside. This of course weakens the strength tremendously. So, when the guy at the fill station starts to fill your SCUBA cylinder, the mechanical bomb takes out him (or her) and a portion of the building.
Well...not to scare anyone off !!! Treat the metal properly and you shouldn't have a problem.
I remember the Alaska Pipeline (yes, i'm old !!), they hired lots of welders. I understand that each pin hole (detected by X-ray not by eyes), they were fined $75 each.
Bob
Fly Deep <flydeep4u@yahoo.com> wrote:
A finished weld should look clean and pretty (maybe a relative term). I've seen some welds from professional welders, that were blotchy, and uneven. I wouldn't want them welding on anything of mine. I am an ASME coded high pressure pipe welder and a good weld should look like a stack of dimes.. Most welds are of sufficient strength even if they don?t have a beautiful appearance for what most home built subs require. However the real problem will be come evident down the road. As corrosion creeps in to the welds at spots that have porosity the growing rust in the weld will expand seven times. This creates enormous stress on the surrounding structure and will cause catastrophic failure. DJB
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