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Engineering > VMC > Inquiry: 18 Feb 2004 on surface pitting |
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| Inquiry |
An unexpected issue arose
regarding the Inconel plate surface condition which is being used for the
NCSX VV prototype. Below, following this note is a collection of e-mails
regarding this condition. Here's the basic situation: * our spec requires a 32 microinch finish. * Both vessel teams noted that surface "pits" appear after polishing of the plate surfaces. The origin of these pits is not precisely known, but they're believed to be the result of the plate rolling process. * the "density" of the pits vary, but are guesstimated to be in the range of 12 /sq. ft. -Frank, please correct me if I'm wrong on this- * We don't believe there are any stress issues due to the pits; the concern is on the vessel base pressure. * PMW, who makes vacuum vessel, says they often see this type of pitting in plates used in vacuum vessels, but they claim that they don't polish them out and they've had vessels with this pump down to 10-10. We're developing the spec now for the "production" vacuum vessel, and we need to address the surface requirements more rigorously than we have for the prototype. We could specify that they start with thicker plates and then have them ground and polished to final thickness. Our plate thickness is 0.375" - PMW says 0.500" is the next standard plate thickness. Grounding 0.062 off of each side will not be cheap, and therefore my question to the committee is: Can we specify that a 32 micro-inch finish is acceptable with pits up to 0.020" deep as long as the total area of pits do not exceed, say for example, 5% of the wall area (in any sq. ft.) and still meet out base pressure requirements? Alternatively, we could have the pits polished out locally, but it's a matter of time and money. Is it necessary? We'd appreciate the Committee's thoughts on this issue. Would it be possible to get your feedback by the end of the month? Submitted by Phil Heitzenroeder |
| Response |
I don't believe any extra
measures are required that were mentioned in your original e-mail, such as
grinding down thicker walls or polishing out the pits. Henry Kugel and
Steve Raftopoulos from the Vacuum Materials Committee agree that these
additional steps w.r.t. these pits are unnecessary. I've included
comments from people who have weighed in on the matter. Submitted by Bill Blanchard |
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