Class 600 vs Class 900 Alloy Steel Flanges

Technical NewsReviewed: 3 August 2026By SASA FLANGE Technical Content Team

Direct answer

Class 900 flanges generally have heavier dimensions and a higher pressure-temperature rating than Class 600 flanges of the same material and size, but the class number is not a pressure in psi. Select the class from the applicable ASME rating table for the exact material group and design temperature, then verify dimensions, bolting, facing, connected equipment and system loads.

Key takeaways

Class 600 is not limited to 600 psi, and Class 900 is not limited to 900 psi.
Material group and temperature change the available pressure rating.
Changing class affects the entire joint and may affect connected equipment.

What the pressure class designation means

ASME pressure class is a rating designation, not a direct pressure value. The allowable working pressure at a given temperature is found in the adopted standard’s pressure-temperature tables for the material group. As temperature rises, available pressure commonly falls because material strength changes.

An ASTM A182 F22 flange and an F91 flange can have the same Class 600 designation yet require separate rating-table review. The governing piping code, design margin and any owner limits also apply.

Physical and commercial differences

Factor	Class 600	Class 900
Rating	Lower than Class 900 for the same material and temperature	Higher than Class 600 for the same material and temperature
Geometry	Class-specific thickness, hub and bolting	Generally heavier class-specific geometry
Weight and cost	Generally lower	Generally higher due to material, machining and bolting
Interchangeability	Not assumed; mating class, dimensions, facing and equipment rating must match

The exact dimensional difference varies by NPS, flange type and standard. Use controlled tables or manufacturer drawings rather than a generic weight factor.

A five-step class selection workflow

Confirm design pressure, design temperature and upset cases.
Identify the exact material specification, grade and ASME material group.
Use the adopted ASME B16.5 or B16.47 rating table.
Check flange type, facing, gasket, bolting and the mating equipment class.
Verify external loads, cyclic duty and any project margin or derating.

Do not upgrade one flange to Class 900 without checking the mating flange, valve, nozzle, gasket, bolting and available installation space. The weakest approved component or joint condition can control the system.

What buyers should state in the RFQ

ASME standard and edition
NPS, Class 600 or 900, and flange type
ASTM A182 grade and required condition
RF or RTJ facing, finish and gasket reference
Pipe schedule, bore and weld-end detail
Bolting specification when supplied as a joint package
PMI, hardness, NDE, dimensional and MTC requirements

If the supplier is expected to review suitability, provide the design conditions and line-class reference. A manufacturer cannot derive the complete system design from size and class alone.

Frequently asked questions

Does Class 600 mean 600 psi?

No. It is a class designation. The permitted pressure depends on material group, temperature and the adopted ASME standard edition.

Can a Class 900 flange bolt to a Class 600 flange?

Do not assume compatibility. Dimensions, bolting, facing and ratings differ, and the joint requires engineering review.

Is Class 900 always the safer choice?

No. It must be compatible with the complete system and does not correct an unsuitable material, gasket, bolting or weld design.

Primary standards referenced

Standards were checked against publisher pages on 3 August 2026. Always purchase and apply the edition specified by the project.

Request an alloy steel flange quotation

Send the material grade, standard, NPS, pressure class, flange type, facing, pipe schedule, quantity and inspection requirements. Our team will review the technical description before quotation.

Send your RFQ View flange products