Rolling a 600 mm Shell on a Slip Roll
A workshop must roll a 1200 mm long shell from 3 mm mild steel plate with a 600 mm inside diameter, on a three-roll slip roll rated at 1300 mm × 3.5 mm. The operator cuts the blank using the outside diameter and rolls it with the roll gap set equal to the plate thickness, passing the sheet once in each direction at high pressure. The result has two flat ends, the seam edges overlap by about 8 mm, and the diameter measured across the flats is 592 mm. There is no spare plate in the shop, the next delivery is in three days, and the customer wants the vessel by then. The operator asks whether to simply trim the overlapping edge and weld it.
1. Recalculate the correct blank length and explain, with numbers, why the seam overlaps by about 8 mm.
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Correct length. Rolling bends the plate about the neutral axis, which for a cold-rolled mild steel sheet of this radius-to-thickness ratio is close to mid-thickness. With a 600 mm inside diameter and 3 mm plate:
- Mean diameter = 600 + 3 = 603 mm
- Blank length = π × D_m = π × 603 = 1894.6 mm
If a 25 mm lap seam is used instead of a butt weld, the blank becomes 1894.6 + 25 ≈ 1920 mm (lap length, not circumference, is added).
Why the overlap. The operator used the outside diameter: π × 606 = 1904.0 mm. That is 1904.0 − 1894.6 = 9.4 mm too long. After the seam edges are brought together there is nothing to absorb that excess except an overlap, which is what was measured. So the 8 mm overlap is not a rolling fault; it is a layout error of using outside diameter instead of mean diameter.
Action: do not simply weld the overlap. Cut about 9 mm off one edge (leaving a root gap of about 1.5–2 mm if a butt weld is specified), re-bevel if needed, and close the seam with a proper alignment. Welding an 8 mm lap on a 3 mm shell would double the thickness locally, distort the roundness and leave a crevice that will corrode.
2. Explain the two flat ends and the 592 mm dimension across the flats, and give the correct rolling procedure for the next attempt.
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Cause of the flats. A slip roll only bends the short length of plate that is between the rolls. The leading and trailing edges, roughly the distance between the bottom rolls, are not bent until they have passed right through and been brought back. On a single heavy pass they therefore remain flat. As the flat regions straighten, they pull the whole shell out of round, which is why the dimension measured across the flats is smaller (592 mm) than the true diameter: the flats do not follow the 600 mm curve, so the shell is a polygon rather than a circle. Springback, which opens a rolled diameter outwards when the load is released, adds to the error and is why the shell must be overbent slightly on the machine.
Correct sequence:
- Pre-bend both ends to approximately the final radius using a template or by rolling each end separately with short passes.
- Set the roll gap just under the plate thickness (about 2.8 mm for 3 mm plate), not equal to it, so the plate is nipped without slipping.
- Roll in small increments, passing the sheet back and forth, lowering the top roll a little at a time.
- Check the curvature after every pass with a curved template or by measuring the chord and rise.
- Overbend slightly for springback, then release and check the diameter at several points, 90° apart.
- Weld the seam, then re-roll lightly to correct weld shrinkage of 1–2 mm.
Only then trim the seam edge; trimming before the shell is round produces an ill-fitting seam.
3. The customer needs the vessel in three days and there is no spare plate. What do you tell the operator, and what checks must be recorded before the shell is released?
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Tell the operator to stop and salvage this shell rather than scrap it, because the defects are correctable within the plate already cut:
- Trim about 9 mm from one seam edge to correct the developed length (butt weld), or leave the lap and reduce the circumference by the overlap if the specification allows a lap.
- Re-roll after trimming to remove the flats, using pre-bending on the ends and light, repeated passes.
- If about 8 mm of length is lost, the finished diameter is restored to 600 mm inside because the circumference is now correct; do not try to compensate by hammering, which will work-harden the plate.
Release checks to record: inside diameter measured at 0°, 90°, 180°, 270° (tolerance typically ±2 mm for this class of work); roundness (difference between maximum and minimum diameter); straightness of the shell along its length; seam alignment and root gap (1.5–2 mm); squareness of the ends to the axis; seam weld quality; and that the plate identification and mill certificate match the drawing.
The judgement point is commercial as well as technical: a three-day deadline is not a reason to weld an 8 mm lap into a pressure-containing shell, because the rectification cost after a leak test failure would be far higher than three days of schedule pressure.