Material selection for a valve looks like a temperature lookup and rarely is. Temperature narrows the field; four other constraints usually decide.
Start with the piping class
On most projects the material is already decided, because the valve should match the pipe it welds into. A dissimilar-metal weld at the site interface brings its own procedure qualification, its own PWHT requirements and its own long-term behaviour, none of which anybody wants to introduce voluntarily.
So the first question is not “what material suits this temperature” but “what is the line made of”. If you have the piping class extract, send it. It answers the question faster than any selection table.
Where the valve genuinely has freedom - auxiliary systems, some retrofit work - the rest of this applies.
Constraint 1: rating at design temperature
The ASME B16.34 rating table gives allowable pressure by material group and temperature. Work from the design temperature, find the grades whose rating at that temperature covers your design pressure at the class you want, and you have the candidate list.
This eliminates grades quickly. It rarely leaves only one.
Constraint 2: availability in the required section
A grade that is metallurgically ideal and unobtainable as a forging in your section size on your schedule is not a candidate.
Large F92 forgings are the common example. The material is right for the duty; the supply base is narrow, and lead times reflect that. This constraint is real and it is best surfaced at the enquiry stage rather than after the order.
Constraint 3: weldability and heat treatment
Every pressure boundary weld - seat rings, overlays, end preparations, and the site weld into the line - has to be qualified for the grade and heat treated correctly afterwards.
F91 and F92 have narrow PWHT windows, and getting them wrong permanently damages creep strength. F22 is more forgiving. A105 more still. Where two grades both meet the rating requirement, the more forgiving one is often the better engineering choice - particularly if site welding will be done under time pressure at height.
Constraint 4: what the specification permits
Client specifications frequently restrict the acceptable grades, sometimes to one per system. This governs regardless of what a selection table says. Where we think a specification names the wrong grade, we say so as a technical query - but the specification, not our opinion, is what we build to.
Trim is a separate decision
Body material and trim material are chosen against different criteria. The body is chosen for pressure containment at temperature. The trim is chosen for galling resistance, seating performance and erosion.
A standard steam trim is:
- Seat and closure member facing: Stellite 6 overlay
- Seat ring base: 13Cr, or the body grade where the seat is integral
- Stem: F6a for moderate temperature, F6NM or 422SS where strength at temperature is needed
- Back seat: hardfaced
Note that trim is a combination and has to be stated as one. “13Cr trim” does not say what the seat facing is.
The special cases worth knowing
WB36. Common in European feedwater system designs. It is not an F22 substitute - different alloy, different properties, different welding requirements. If the specification names it, treat it as a hard requirement.
Austenitic bodies (F304, F316, F347H). Selected for corrosion resistance or where the connecting pipe is austenitic. They have higher thermal expansion than ferritic grades, which matters at the interface, and they are not selected for creep performance in preference to F91.
Hydrogen service. In refinery applications at temperature, high-temperature hydrogen attack governs material selection through the Nelson curves, not through the ASME rating table. This can rule out grades that would otherwise be fine.
Sour service. NACE MR0175 / ISO 15156 imposes hardness limits that affect the trim as much as the body. It cannot be added to an existing design by changing the paperwork.
What we need to give a real answer
- Design pressure and design temperature
- Medium and, where relevant, its composition
- Piping class extract or the connecting pipe specification
- Any client specification restricting materials
- Whether the plant cycles
- Section size and quantity, so availability can be checked
With those, we will state the grade we propose, the rating basis, and any point where we would recommend something different from what is specified - with the reason. The recommendation is advice; the specification decides.