P91 and F91: related grades, different product forms

ASTM A335 P91 applies to seamless ferritic alloy-steel pipe for high-temperature service. ASTM A182 F91 applies to forged or rolled piping components such as flanges. The design intent is a compatible Grade 91 material system, but each product must meet its own specification, heat treatment, testing and marking requirements.

Do not treat an F91 flange as a generic high-alloy replacement. The connected valve body, fittings, branch components, weld metal and fabrication plan should all be reviewed within the same line class.

Dimensional matching at the butt weld

Nominal pipe size alone does not define the weld preparation. State the P91 pipe schedule or actual wall, any minimum-wall purchasing basis, corrosion allowance and required transition. The flange manufacturer can then machine the bore and weld end to the approved detail.

Control the Grade 91 thermal cycle

Grade 91 performance depends on a properly produced and preserved microstructure. The flange should be supplied in the required normalized-and-tempered condition with documented mechanical and hardness results. During welding, control preheat, interpass temperature, consumables, interruption rules and PWHT under an approved procedure.

PMI is important for preventing alloy mix-ups, but it does not confirm microstructure, tempering or internal quality. Hardness mapping and any specified surface or volumetric NDE complete different parts of the verification plan.

Grade 91 joint dossier

• P91 pipe and F91 flange material certificates linked by item and heat number
• Approved flange drawing showing bore and weld-end transition
• Heat-treatment records and mechanical test results
• PMI and hardness reports with locations and acceptance limits
• Qualified WPS/PQR and welder qualification
• Preheat, interpass and PWHT charts for the production weld
• Required MT/PT, UT/RT and final visual or dimensional reports

Retain these records as a connected turnover package. Isolated certificates are less useful when they cannot be linked to the installed joint.