Re: [PSUBS-MAILIST] Hatch fabrication dilemma

Hi all,

I've just run into a good example of the practical differences between design and construction. I know there's a bunch of you out there with more practical experience than myself, so really look forward to suggestions.

I'm building a hatch which is comprised of a "dished-only" head in 516 grade 70 and a stainless ring half an inch thick. "Dished-only" means the head has no flange, so it meets the SS ring at an acute angle. This is similar to a K-350 hatch, though my dimensions are a little different.

The head was beveled from the outside, which allowed me to make a full penetration outside TIG weld. I should probably have left it at that. But I had an opened box of SS electrodes going to waste, so I decided to weld it on the inside as a little extra. Initially I thought it was virtually impossible to do that inside weld, because one had to make the weld blind. But with the help of a little mirror and by bending the electrodes into the right shape, I was actually able to do it. I took the inside weld the whole way to the inside edge of the SS ring, which is now nicely faired into the head. This inside weld is, due to the difficulties of the position, surely full of flaws. But again, it's just supposed to be bonus safety margin on top of the high-quality outside weld.

Now for the problem?

I expected the inside weld might pull the ring inwards a little, but I never expected HOW MUCH it would do so. The half inch SS ring is deflecting inwards a quarter inch. My next step was going to be putting the hatch on a lathe to true the SS ring surface and cut the O-ring groove. There's still enough material to do that. But what worries me is that the pull of the inside weld must be putting enormous tension on the critical outside weld. This tension is in the same direction as the tension that will be produced by water pressure at depth. So instead of bracing the outside weld, I have the impression I'm actually adding to the tension that will be stressing it when submerged.

So what do people think should be done about this?

1) Nothing, machine it as planned.

2) Is possible to relieve the tension with a heat treatment?

3) Grind away the whole inside weld? (oh no, please no!!!)

4) Use a cutoff wheel between the inside weld and the SS ring to let the ring pop flat again, then just fill the gap with an additional weld pass?

I like the idea of #4 because it would relieve the tension while still providing the bracing I was after originally. I'd need to be careful about the depth of the cut, I don't want to go anywhere near the outside TIG weld. But again, I'm just wondering what more experienced builders might suggest!

Thanks,

Alec

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