Duplex Stainless Steel
Duplex Stainless Steel Tube
Duplex tube is reached through material selection, not by moving up a grade table. The service duty has to be assessed first, the choice released by design, and the fabrication controls that come with the family agreed with the fabricator — before a mill route, a lead time or a price mean anything.
No composition values, mechanical properties, corrosion rates, temperature limits or welding parameters are published here. Those come from the governing standard edition, the fabricator’s qualified procedure and the project material selection.
Families
Four duplex groups and what separates them
“Duplex” covers more than one alloying level and more than one designation system. Which group applies is a material-selection decision, and the grade name alone will not tell you which one you have been offered.
| Family | What it is selected for | Before you enquire |
|---|---|---|
| Duplex grades | A two-phase stainless family, selected where the duty asks for a combination of strength and corrosion behaviour the austenitic grades are not chosen for. | The grade and its fabrication requirements come from the project material selection. |
| Super-duplex grades | A more highly alloyed group within the family, used on more demanding duties again. | Confirm the approved grade rather than selecting a tier from a catalogue. |
| Lean duplex grades | A group developed around specific duties where the full alloying level is not required. | Availability and suitability are order-specific and confirmed at quotation. |
| Grade naming across systems | Duplex designations appear in more than one designation system, and similar names are not automatically equivalent. | Name the designation and the standard edition the contract invokes. |
This page maps the family and the decisions around it. Selection against a specific environment is an engineering decision, and the values come from the governing standard or the project specification — not from this page.
Selection basis
Why duplex is a different decision, not a better one
The common stainless tube grades and the duplex families are reached by different routes. Treating the second as an improvement on the first is how an unapproved substitution ends up inside a purchase order.
| Selection point | Why it matters | Before you enquire |
|---|---|---|
| Start from the service, not the grade | Duplex is a material-selection outcome. The environment, temperature and duty drive the choice, and the choice is released by design. | Attach the service envelope and the material selection that approves the grade. |
| It is not an upgrade to an austenitic item | A higher-strength or more highly alloyed family is not an approved substitute for an austenitic grade on an existing drawing. | Get the substitution released in writing before it enters a quotation. |
| Fabrication moves with the material | The welding, heat-input and cleanliness controls are part of the selection, not a downstream detail. | Confirm the fabricator's qualified procedures before the material is ordered. |
| Availability is order-specific | Duplex tube is a smaller part of the market than the common austenitic grades, and mill route and lead time vary. | Ask for the offered mill, origin and route, and treat dates as unconfirmed until in writing. |
| It is not a coating decision | There is no applied layer to repair at a cut or weld, and the acceptance basis is the alloy and its condition. | Define the delivered condition of the alloy, not a surface treatment. |
Duplex is not offered here as a substitute for an austenitic grade on an existing drawing. Where a change of material is being considered, the design authority releases it first — this page supports the enquiry, it does not approve the change.
Fabrication controls
The controls that follow the material
Duplex tube that is correct at the mill can still be wrong by the time it is installed. These controls are part of the material selection rather than an afterthought at the bench.
| Control | Why it matters | Before you enquire |
|---|---|---|
| Welding procedure | The joining procedure for duplex is a qualified, material-specific decision rather than a general stainless practice. | State that the fabricator holds procedures qualified for the grade. |
| Heat input and interpass control | Fabrication controls affect the phase balance the material was selected for. | Agree the controlling parameters with the fabricator before release. |
| Cleanliness and contamination | Duplex surfaces and welds need contamination control through handling, cutting and installation. | Define the cleaning, protection and segregation requirements for delivery and site. |
| Post-fabrication verification | What is verified after fabrication, and by whom, is agreed in advance. | Name the inspection scope and the acceptance basis behind it. |
Order fields
Eight fields that make a duplex tube order complete
An enquiry that names a duplex family leaves the grade, the standard, the route and the fabrication basis unresolved — and none of those can be inferred from the other.
Standard and edition
The document the contract invokes for duplex tube, named with its edition.
Grade designation
The exact designation from the project material selection, not the family name.
Material selection reference
The document that approved duplex for this duty, so the basis can be checked.
Manufacturing route
The permitted seamless or welded route, with any difference in test or condition it carries.
Dimensions and tolerances
OD, wall and length in one unit system, with the controlling dimension identified.
Delivered condition and finish
Heat treatment, surface condition and cleanliness as delivered, not as installed.
Fabrication and welding requirement
Whether welded fabrication is planned, and the qualified procedure basis that applies.
Inspection and documentation
Certificates, tests and traceability required, and how they link to the supplied items.
Price on request. Availability, mill route, origin and delivery dates are order-specific and require written confirmation.
Next decisions
Where duplex sits against the grade pages
The austenitic grade pages carry the decisions for the grades most orders are actually placed on. Duplex is reached from the material selection that follows them.
- Confirm the selection route. Check whether the duty is served by an approved austenitic grade before moving the material family at all.
- Release the change in writing. A material change on an existing drawing needs the design authority’s release, not a supplier recommendation.
- Agree the fabrication basis early. Qualified procedures and contamination controls are settled before material is ordered, not after it arrives.
- Name the grade and the edition. State the exact designation and the standard edition the contract invokes, so offers are compared on one basis.
- Treat dates as unconfirmed. Mill route, origin and lead time are order-specific and are confirmed in writing at quotation.
Compare against the austenitic grades
A269 TP316L is a separate austenitic grade with its own composition requirement and its own service and connection decisions. Where duplex is being considered, that page sets out what the existing grade actually settles — and what it does not.
The wider stainless family, across all its grades and finishes, is set out on the stainless steel tube page.
Enquiry
Turn a material selection into a reviewable enquiry
Send the standard edition, grade, the material selection that approved it, the permitted route, dimensions, delivered condition and the fabrication basis. Terms are confirmed in writing.
Price on request. Every figure, availability and delivery date requires written confirmation against a specific order.