Z-quality and through-thickness properties in offshore steel

Z-quality is a requirement for improved through-thickness ductility. In offshore structures it is commonly specified for plate used in highly restrained welded joints, thick attachments and other details where shrinkage stresses act through the plate thickness.

The common designations are Z15, Z25 and Z35 under EN 10164. In demanding offshore work, Z25 and particularly Z35 are frequently encountered.

What the Z-number means

The Z class is based on the reduction of area in a tensile test taken through the thickness of the product.

EN 10164 defines minimum average reduction-of-area levels corresponding to the class designation. In practical terms, a higher Z class demands better ductility in the short-transverse direction.

This is intended to reduce susceptibility to lamellar tearing, especially where welded attachments impose through-thickness strain.

Z35 does not mean “better UT”

This is one of the most important distinctions.

• EN 10164 Z-quality addresses through-thickness deformation properties.
• Ultrasonic testing addresses internal discontinuities according to the specified UT standard and acceptance class.

They are related in the sense that both matter for internal material quality, but they are not the same test and one does not replace the other.

For plate UT, see EN 10160 ultrasonic testing of steel plate.

Why offshore projects often add Z-quality

Offshore structures can contain:

• thick node plates;
• tubular-to-plate connections;
• diaphragm details;
• heavy brackets;
• cruciform joints;
• highly restrained weld details.

In such geometry, weld shrinkage can create stress through the plate thickness. Conventional tensile properties measured in the rolling direction do not directly describe this behavior.

That is why project specifications may add Z25 or Z35 even when the base steel grade itself is otherwise acceptable.

Ordering matters

Z-quality should be ordered explicitly. A plate being S355, S460 or offshore-grade material does not automatically mean it is Z35.

The RFQ and PO should state, as applicable:

• base steel grade and product standard;
• EN 10164 class, e.g. Z25 or Z35;
• thickness range;
• any required UT class;
• inspection certificate type;
• project-specific sampling or retest requirements.

Can stock plate be tested later for Z-quality?

Sometimes a project needs Z25 or Z35 but the available stock plate was not originally ordered with an EN 10164 Z requirement. In that situation, additional through-thickness testing can be considered, but it should not be treated as an automatic retroactive upgrade of ordinary stock into originally certified Z-quality material.

A practical assessment should separate two questions:

1. Can the actual stock plate be tested?
   If sufficient traceable material is available, through-thickness tensile specimens can be taken and tested to EN 10164. The sampling location, number of tests and represented test unit have to follow the applicable requirements. EN 10164 also links Z-quality to internal soundness requirements, so reduction-of-area results alone may not cover every requirement that would have applied if Z-quality had been specified at the original order.

2. Can the material then be accepted as Z25/Z35 for the project?
   That is a certification and purchaser-acceptance question, not just a laboratory question. A successful additional test demonstrates the result for the tested/represented material, but it does not automatically rewrite the original mill certificate or prove that the whole heat, parent plate or stock lot was originally supplied to EN 10164. The purchaser, designer/project specification and, where relevant, the original manufacturer or third-party inspector should agree how the supplementary results are to be documented and what material they represent.

For stock material, check at least:

• positive traceability to the original heat and plate;
• original ladle analysis, particularly sulphur content, because EN 10164 test-unit rules depend on it;
• whether the required through-thickness specimens can be taken from the correct location;
• the applicable EN 10164 test-unit and sampling rules;
• the internal-soundness/UT requirement applicable to the requested Z class;
• whether cutting the sample from the plate is acceptable for the remaining usable dimensions;
• how the additional results will be incorporated into the inspection documentation; and
• explicit client/designer approval if the project originally required Z-quality material to be supplied as such from the mill.

The conservative procurement position is therefore: post-production Z testing may provide a route to qualify specific traceable stock, but only when the governing specification and purchaser accept that route. It should not be presented as equivalent to having ordered and certified the material as Z25/Z35 from the outset.

Z-quality and offshore grade designations

Some offshore specifications incorporate through-thickness quality into their grade or option structure; others call it up separately. Never infer Z35 only from the offshore grade name.

The correct approach is to read the product standard and project specification together.

Procurement takeaway

If the client asks for Z35, treat it as a distinct material requirement. Do not substitute a plate with good UT results but no demonstrated through-thickness properties. Conversely, a Z35 certificate does not by itself prove compliance with a separately specified UT acceptance class.

References and verification sources

• EN 10164:2018 public text mirror — purchaser information and Z classes
• BSI — EN 10225 family overview