Plate Product Thickness Variations

With the increased capability of hot mills to rolls heavy coil (up to ½” thick) product in stainless and nickel alloys, has come an increased awareness of the thickness variations that can be present in conventional discrete hot rolled plates.

Until relatively recent times, plate products (3/16” thick and greater) were produced as individual pieces commonly 96” wide, and even wider in special instances. Because of the forces required during rolling, the work rolls can deflect resulting in a crown on the plate where the thickness at the middle is greater than at the edges. The thickness tolerances established for these discrete (Quarto) plates were defined in ASTM A 480, Table A2.17. The minus tolerance of 0.010” applies to all thicknesses at all points. However, a careful reading of this Table reveals that the listed plus tolerances (0.050” on 96” wide ¼” plate, for example) apply only to the long edges of the rolled plate as measured at least ½” from the edge and not more than 3” from the edge. This leaves no plus tolerance requirement on the middle of the plate except for 3” on either edge.

For most users a heavier plate is not a problem, and can be an advantage. Nearly all stainless plate is sold on the basis of a theoretical weight, area or per piece so the pricing is not sensitive to thickness variations. In some cases the extra thickness provided by the crown provides more stock for a cleanup allowance without having to purchase a heavier gauge. Plate produced from coil, which will have less over thickness, can actually be a disadvantage in this regard. However, machine shops performing machining operations on blocks of material cut from the center of a plate can be forced to deal with extra machining. Due to the heavier thickness at the center, a 3/8” thick plate can actually seem like it is half inch plate.

An action is moving through ASTM International to revise standard A 480 to include an upper limit on the permissible thickness. If the change is adopted, the maximum plus side tolerance over the entire plate will be no more than double the permissible over thickness at the edge. In the case of the ¼” plate mentioned above, while the zone along the original long edge must meet to 0.050” maximum, the center of a ¼” plate can measure as much as 0.350” (0.250” + 2*0.050”). As plate production currently satisfies these limits, no change in current production practice is necessary. The revision to the standard will make end users aware of what the upper limits might be. The standard will also note that the edge thickness tolerances in Table A2.17 apply to as-produced mill plates, and not to smaller pieces cut from these plates.