Strain-ageing testing in offshore structural steel

Strain-ageing testing is used to check whether steel retains adequate toughness after plastic deformation and subsequent ageing. It is particularly relevant when plate will be cold formed into heavily strained structural components.

In offshore construction, this can matter when plate is formed into items such as SAWL tubulars or other critical cold-formed components.

What problem is the test trying to simulate?

Cold forming plastically strains the steel. In susceptible material, subsequent ageing can reduce toughness.

A strain-ageing test therefore introduces a prescribed amount of deformation, applies an ageing treatment, and then evaluates toughness — commonly by Charpy testing — after that simulated history.

The important idea is:

The test is not measuring ordinary as-delivered toughness. It is checking toughness after a controlled simulation of forming plus ageing.

How the test is usually performed

The exact procedure comes from the governing standard or project specification, but the usual sequence is:

1. Take a representative sample from the plate/material and keep full traceability to the heat, product and sampling orientation.
2. Apply a controlled amount of plastic strain to simulate the cold-forming operation. This may be produced by tensile/compression deformation or by an agreed bending/forming simulation.
3. Artificially age the strained material. A widely used offshore qualification condition is ageing for 1 hour at 250 °C after the prescribed pre-strain.
4. Machine the required mechanical-test specimens from the strained-and-aged material.
5. Perform Charpy V-notch testing at the specified temperature and orientation; some qualification programmes also include tensile testing or a transition curve over several temperatures.
6. Compare the results with the specified acceptance criteria for the grade/project and document the pre-strain, ageing temperature/time, specimen orientation, test temperature and results.

For EN 10225-type offshore qualification work, 5 % pre-strain followed by 1 hour at 250 °C is a common reference condition. Broader cold-forming qualification programmes can also examine higher imposed strains, for example 7.5 % or 10 %, depending on the forming severity and the applicable option/specification. These values should therefore never be copied into an RFQ without first checking the governing edition and project requirement.

The aim is to reproduce, in a controlled and conservative way, the loss of toughness that could occur after forming and subsequent ageing; it is not simply an additional Charpy test on untouched material.

EN 10225 context

In EN 10225-1:2019, strain-age testing is an option rather than an automatic test on every plate. Publicly available text for the standard notes that the option is recommended when plate will be used for SAWL hollow sections under the EN 10225 tubular framework or other critical offshore components.

That makes it a good example of why the base grade alone is not enough in offshore procurement.

What must be specified?

Where strain-age testing is required, the order/specification needs to establish the applicable details, such as:

• governing standard and edition;
• required option/project clause;
• imposed strain level;
• ageing treatment;
• Charpy test temperature and orientation;
• acceptance energy;
• test frequency and reporting.

These details are edition- and project-sensitive and should be verified directly against the licensed standard/specification.

Do not confuse strain ageing with normalizing or PWHT

Strain ageing is a test simulation of material behavior after deformation and ageing. It is not a mill delivery condition such as +N or +M, and it is not the same as post-weld heat treatment.

Procurement takeaway

If a fabrication route involves significant cold forming, check whether the project specification requires strain-age testing before accepting stock plate. A plate can comply with ordinary tensile and Charpy requirements in the as-delivered condition yet still lack the specific strain-age qualification required for the intended component.

References and verification sources

• Public text of BS EN 10225-1:2019 — strain-age option context
• BSI — EN 10225 family overview