Not all steel is equal, and not all material certificates tell the same story. For marine structural applications from Chinese mills, IACS UR W7 closes a gap that a standard EN 10204 Type 3.1 review leaves open.

IACS Unified Requirement W7 governs the testing and certification of steel plates, sections, and bars intended for ship construction. When a marine contractor or shipyard sources structural steel from a Chinese mill, the applicable standard is not the mill's internal quality standard or even a generic EN 10204 document. It is the class society's approved test requirements under W7, administered through a class surveyor present at the mill.

Here is what W7 compliance requires that a standard material certificate review does not automatically verify:

Charpy impact testing at specified temperatures. For Grade A to AH40 and higher steels, W7 specifies impact test temperatures that vary by grade and thickness. A standard MTC from a Chinese mill may show impact values tested at a temperature that satisfies the mill's internal specification but does not match the W7 requirement for the specific grade being supplied. The numbers look plausible. The test temperature is wrong.

Survey by a class-approved surveyor, not factory QC. W7 requires that the class society surveyor witnesses the mechanical test sampling, not just countersigns a mill certificate produced by factory QC. In practice, Chinese mills operating under IACS approval have a class surveyor present during test coupon cutting and mechanical testing. Mills that submit documentation without surveyor presence records are not W7-compliant regardless of what the certificate header says.

Heat-by-heat chemistry control, traceable to ladle analysis. W7 requires ladle analysis records for each heat, not just product analysis from finished plate samples. The two can diverge in carbon and sulphur content, particularly on heats with ladle treatment. If a Chinese mill's certificate does not distinguish between ladle and product analysis, the traceability is incomplete.

Z-grade qualification for thickness direction loading. For hull plate applications under transverse loading, Z25 or Z35 through-thickness ductility testing is required under W7 for applicable grades. A plate that meets all in-plane mechanical properties can fail in laminar tearing under through-thickness load if Z-grade qualification is absent.

When we qualify Chinese steel mills for marine structural supply, we require class society certificates with surveyor attendance records, not domestic inspection certificates referencing W7 by name without compliance evidence.

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