Duplex and super duplex: selection goes beyond strength

One family, distinct material identities

Duplex stainless steels contain both austenitic and ferritic phases. 2205 / 1.4462 is considered for pumps, valves and process components requiring corrosion resistance together with high mechanical strength. Super duplex 2507 / 1.4410 / UNS S32750 and 1.4501 / UNS S32760 are more highly alloyed options. Super duplex is not a single grade; the purchase document must identify the material number and applicable product standard.

1.4501 identifies S32760, which contains copper and tungsten; it is not 254 SMO. Treating grades mentioned for similar applications as equivalent leaves composition and fabrication requirements uncertain. In particular, changing a grade already specified on a drawing or customer specification requires evaluation by the design authority.

Assess corrosion and fabrication together

Super duplex options generally offer greater resistance to localised corrosion in chloride environments than 2205. Nevertheless, temperature, pH, crevices, deposits, flow and shutdown conditions govern selection. Composition-based indicators such as PREN support preliminary comparisons; they do not independently define a component’s safe operating temperature or lifetime. The term seawater also does not describe the complete service environment.

Welding and subsequent heat treatment affect phase balance. Unsuitable thermal cycles can produce detrimental phases and reduce toughness and corrosion performance. Do not transfer familiar practices for austenitic steels directly to this family. Define the welding procedure, final cleaning and required tests in the fabrication plan; a generic stress-relief treatment should not automatically be considered suitable.

Purchasing checks for bar

High strength also affects the machining and cutting plan, so comparing quotations by price per kilogram alone is incomplete. Specify machining allowance, supplied surface, straightness and heat-treatment condition for the finished component. Sampling location and the required tests can become particularly important as section size increases. Where corrosion or detrimental-phase testing is required, define it in the order rather than assuming it is automatically included.

• Are the grade / UNS, product standard and delivery condition consistent?
• Are fluid data and cleaning conditions sufficiently defined for selection?
• Are fabrication, testing and documentation requirements included in the enquiry?