Technical reference
Plate tolerances: EN 10029 & ASTM A6
Incoming inspection at the yard usually checks measured thickness against a tolerance standard, not just the nominal value ordered — here is how the two standards buyers ask about most often are structured.
Below is how each tolerance class works, not the exact mm figures for every thickness — those vary by both thickness and width and are set out in the standard’s own tables. The applicable class should be agreed at enquiry, and the actual measured thickness for a shipment is reported on the mill test certificate for that heat.
EN 10029 thickness tolerance classes
EN 10029 covers hot-rolled steel plate 3 mm and thicker. It defines four tolerance classes that differ in how the allowed deviation is split between under and over nominal thickness; Class A applies by default if no class is specified.
- Class A
- A minus tolerance only — the plate is guaranteed not to be thinner than a set amount below nominal, with a larger allowance on the plus side. The most commonly specified class for general structural use.
- Class B
- A fixed, tighter minus tolerance regardless of thickness — used when the minimum thickness matters more than staying close to nominal.
- Class C
- All tolerance on the plus side — the plate is guaranteed to be at least nominal thickness, useful when a guaranteed minimum section is the priority.
- Class D
- A symmetrical tolerance split evenly above and below nominal thickness.
ASTM A6 / A6M
ASTM A6 is the general dimensional-tolerance standard referenced by ASTM A131 for shipbuilding plate in the U.S. market — it sets allowable variation in thickness, width, length, camber and flatness, with the permitted thickness tolerance depending on nominal thickness, width and the specified minimum yield strength range of the grade.
Flatness
Flatness tolerance is specified as a maximum deviation over a given length, commonly expressed in mm per metre, and is harder to hold on thinner, higher-strength plate where rolling residual stress has more effect. This is the exact problem the low-residual-stress hot-rolled option was developed to reduce.
About low-residual-stress plateMeasured thickness vs. the standard
Tolerance standards set the allowable range; the mill test certificate for each heat reports what was actually produced. Reading the two together — the standard for the allowed range, the MTC for the real number — is how incoming inspection is normally done.
How to read a mill test certificateTell us the grade, thickness and tonnage.
Send grade, thickness, dimensions and tonnage. You get availability, price and a lead time, normally within 24 hours.