Reduction in thickness in ellipsoidal dish ends

[ukmet]

We were forming (cold forming) ellipsoidal dish ends in-house with our new press and flanging machine. Required thickness after forming should be 15.8mm, we used a plate of 18mm. After forming the thickness reduced up to 12.9mm. We have made many dishes from the same material (SA-516 Gr70) and same company but the last two show extra low thicknesses then required. Following are my questions:

1. Possible reasons for this much reduction and how to mitigate them?
2. Acceptance criteria (if any) after reduction of thickness below minimum?
3. Any repair or patch work on new fabricated dish ends possible in light of ASME or PD?

 

[chicopee]

1. Internal diameters may have increases making the heads deeper.
2. Use ASME code formulae to recalculate new MAWP's
3. I am not sure if cladding would be acceptable.

 

[JStephen]

You might check with the company furnishing the forming equipment for troubleshooting like this.
You could possibly have variations in the material properties that would affect this. Fabrication sequences or procedures might affect it. Maybe it needs to be formed hot.

 

[racookpe1978]

Why did you choose a initial plate Th of only 18 mm?

 

[fegenbush]

1. For many formed head geometries, a 20% thinning allowance is common.
2. The head would be unacceptable as furnished. 18.4% under thickness is more than can be justified by any code with which I am familiar.
3. A butt welded patch plate may be acceptable, but I do not think that you will find the shape after welding to be desirable. The distortion due to welding will be evident.

 

[Duwe6]

And the center 2/3rds of the dished head can be calculated as a spherical section, per ASME. Thus that area will probably meet T-minimum. Your 'impossible' area will be the knuckle area. Consider that spot irreparable. Guess you could press out an annular ring 'donut' to refabricate the outer area of the head, including the knuckle and straight-flange, then weld that donut to the center portion of your thin head.

 

[ukmet]

Thank you very much for the response. Duwe6 can you please further elaborate the point.

 

[dhengr]

Ukmet:
It’s a tough pill to swallow, but you’ll probably spend more trying to fix those heads than they are worth. Maybe you could cut the knuckle off and save the center 2/3rds as a blank for a smaller head. The best advice you’ve gotten so far came from Jstephen, get with the forming equipment and material suppliers, and have them help you figure this out, and possibly share in the cost. Maybe increase the radius at the knuckle a bit. It sounds like you are stretching and drawing the material too much at the knuckle area, in the forming process. Can you slightly reshape the forming dies from the knuckle outward, should you lubricate the dies in this area?

 

[sharmass]

Where exactly the minimum thickness was found? Knuckle or crown.
Was the shape of head after forming close to what you wanted?

Probably answer to these questions may tell you what could have gone wrong