----- Original Message -----
Sent: Friday, April 17, 2009 1:01
AM
Subject: [PSUBS-MAILIST] Re: Pressure
vessel welding
Glen,
A 200 amp MiG welder is not adequate for the purpose of welding together
the pressure hull of your submarine. The GMAW process commonly known as
"MIG" produces very brittle welds when applied to metal thicknesses past
the "sheet metal " range ( 10 gauge being the maximum ). A MIG weld might have
an excellent appearance but if you cut out some sample "coupons" and subjected
them to a "side bend test" you would have an unacceptable failure rate.
Yes, you could TIG (GTAW) weld your whole
sub. Tig done properly produces some of the highest quality welds possible,
but , it is really slow. For the average person who wants to build his own sub
( and is on a budget ) TIG welding the "roots" and "stick" (SMAW) welding the
remaining passes is the way to go. Combination SMAW/GTAW machines are a real
bargain and very versatile. The "inverter" types have excellent arc
characteristics , are energy efficient and can be plugged into your
clothes dryer electrical outlet. Alec has a terrific unit he bought off
the internet and to my knowledge is still going strong. He might be willing to
share some info on it. I have welded with it and I like it alot !
Use 7018 Low Hydrogen welding rods and 70,000psi tensile strength
TIG wire . 3/32" and 1/8" diameter are the recommended sizes.
While welding, multiple small passes are much more desireable than one large
single pass. Large single passes over heat the metal and sets up
stresses , conversely each successive smaller pass tends to
anneal the one prior to it. ( Less heat , less stress ) .
If you have not done much welding then by all means take a
welding course and practice on some scrap first.
When you do start welding on your hull ; bevel, fit , and do the
majority of the welding from the inside of the sub so when it comes time to
back grind the roots you can be on the outside. Weld the
joint completely ( root, hot pass, filler passes and cap passes )before you do
any back grinding.
Daniel Lance
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