Grain-size testing for offshore structural steel

Grain-size testing is a metallographic examination used to determine the apparent grain size of steel. It is not a tensile test, Charpy test or ultrasonic examination.

For offshore structural steel, a project specification may require grain-size verification because grain refinement is closely associated with toughness and metallurgical control. But the presence of a grain-size requirement, its acceptance criterion and its testing frequency must come from the governing standard or project specification.

What standard is normally used?

ISO 643 is the principal international standard for the micrographic determination of apparent grain size in steels. The current edition is ISO 643:2024.

It defines methods for revealing grain boundaries and estimating apparent ferritic or austenitic grain size.

What the test actually tells you

A polished and prepared metallographic specimen is examined under a microscope. The observed grain structure is assessed using an accepted method defined by the applicable standard.

The result describes apparent grain size. It does not directly prove:

• Charpy energy;
• yield strength;
• weldability;
• UT soundness;
• through-thickness ductility.

Those properties require their own tests.

Why can offshore specifications ask for it?

Offshore material specifications frequently aim to control not just minimum strength but the entire metallurgical route. Fine and controlled grain structure can be relevant to toughness and consistent performance, particularly in demanding low-temperature or welded applications.

A project may therefore request grain-size reporting as an additional verification item even where the base product standard does not require routine reporting for every order.

Do not invent a universal acceptance number

There is no safe rule such as:

“All offshore S355 must have grain size X or finer.”

The required criterion depends on the exact grade, standard edition, option and project specification.

The procurement team should therefore ask:

• Is grain-size testing mandatory for this grade or only an additional project requirement?
• Which test method/standard applies?
• What acceptance criterion applies?
• At what frequency is it tested — per heat, plate, lot or qualification batch?
• Must the result appear on the MTC?

Procurement takeaway

Treat grain-size testing as a separate metallurgical verification item. Do not infer compliance from normal chemistry, Charpy or tensile results. If the requirement is project-specific, put the method, acceptance criterion, frequency and reporting requirement explicitly into the RFQ and PO.

References and verification sources

• ISO 643:2024 — official ISO page
• BSI — EN 10225 family overview