Forged Flanges

A flange is a ring-shaped component that joins sections of pipe, or joins pipe to a valve, pump, vessel or instrument, using bolts and a gasket. Unlike a welded joint, a flanged joint can be opened for inspection, cleaning or replacement — which is why flanges appear at nearly every piece of equipment in a process plant.

Flanges are part of the pressure boundary. A defect in one does not cause inconvenience — it causes a leak of whatever the line carries, which may be hot, pressurised, toxic or flammable. That is why flanges for serious service are forged. Forging produces a fully dense structure with no interconnected porosity through which fluid could weep, and it gives consistent, verifiable mechanical properties.

It is also why ASME B16.5 does not permit most flange types to be simply cut from plate: the standard requires forgings (or, for some materials, castings), with plate use limited essentially to blind flanges. Rolled plate has directional properties and possible laminations that a forged flange avoids.

How forged flanges are made

Smaller flanges are produced by closed die forging from a billet. Larger flanges are typically made by upsetting and piercing a billet, then either forging or ring rolling it to a ring with the grain running around the circumference. The forging is heat treated, then machined: bore, raised face, bolt holes and, for weld neck flanges, the hub and weld bevel.

Each flange type suits a different combination of pressure, service conditions and installation practice.

Weld neck flanges

The long tapered hub of a weld neck flange spreads stress smoothly from the flange ring into the pipe wall, and the butt weld can be fully radiographed. That makes it the standard choice for high pressure, temperature cycling and critical service. The flange bore is machined to match the pipe's schedule so there is no step at the joint.

Slip-on flanges

Slip-on flanges are cheaper and easier to align during installation, since the pipe can be positioned before welding. Their fatigue strength is lower than that of weld neck flanges, so they are used in lower-pressure, less demanding service.

Blind flanges

A blind flange closes an opening. Because nothing supports it from behind, it resists pressure across its whole face, and at higher classes it is correspondingly thick.

The class number on a flange — 150, 300, 600, 900, 1500 or 2500 — is a pressure rating designation, not a pressure in psi or bar. The allowable working pressure depends on two further things: the material group the flange is made from, and the operating temperature. As temperature rises, steel loses strength, so the allowable pressure falls.

This is the single most misunderstood point about flanges. A Class 150 flange that is comfortably rated at ambient temperature may be well under-rated for the same line running hot.

Higher classes rise in proportion: at −29 to 38°C, group 1.1 is rated at about 153.2 bar for Class 900, 255.3 bar for Class 1500 and 425.5 bar for Class 2500. Other material groups — stainless steels and chromium-molybdenum alloys — have their own tables.

ASME B16.5 covers flanges from 1/2 inch to 24 inch. Larger diameters are covered by ASME B16.47. Our page on ASME B16.5 explains the standard in more detail.

The face of the flange is where the gasket seals, and its geometry and roughness decide whether the joint stays tight.

• Raised face (RF) — the most common facing. The gasket contact area is raised above the bolt circle, concentrating bolt load on the gasket. Under ASME B16.5 the raised face height is 1.6 mm for Class 150 and 300, and 6.4 mm for Class 600 and above.
• Flat face (FF) — used mainly when mating to cast iron or other brittle equipment flanges, where a raised face could crack the mating part when bolted.
• Ring type joint (RTJ) — a machined groove holds a solid metal ring gasket, used for high pressure and high temperature service.

Why the serrations matter

Raised faces are normally machined with a concentric or spiral serrated finish, commonly specified in the range of about 3.2 to 6.3 µm Ra (125 to 250 µin). The serrations bite into soft gaskets and stop them extruding under pressure. A face that is too smooth lets the gasket slip; one that is too rough leaves leak paths. Surface finish is therefore a functional requirement, not a cosmetic one.

Gaskets for these flanges are specified in companion standards — spiral wound gaskets in ASME B16.20 and non-metallic sheet gaskets in ASME B16.21 — and must suit the fluid, temperature and facing.

Flange material is chosen for strength at temperature, toughness at low temperature, and resistance to whatever the line carries.

ASTM A105 in detail

A105 is the workhorse specification for carbon steel flanges. It requires a minimum tensile strength of 485 MPa and yield strength of 250 MPa, caps carbon at 0.35%, and limits hardness to 187 HB maximum so the flange remains weldable and machinable. It also limits carbon equivalent, because a heat can pass every individual element limit and still weld poorly.

Heat treatment is required under A105 in defined cases — including flanges above Class 300 — and many buyers specify A105N, the normalised condition, as standard for more uniform properties and toughness.

Stainless and alloy flanges

Stainless flanges under A182 must be solution annealed, and where intergranular corrosion is a concern, tested to ASTM A262. For welded assemblies, low carbon L grades such as F316L prevent sensitisation in the heat-affected zone. Stainless flanges must also be handled on tooling kept separate from carbon steel, because embedded iron particles rust on the surface. See 304 versus 316 for grade selection.

1. 01Material verification. Each heat lot is spectro tested against its mill certificate and segregated by heat number.
2. 02Billet cutting. Billets are cut to a weight calculated for the flange size and forging route.
3. 03Heating and forging. Billets are induction heated, then forged in closed dies or upset, pierced and forged or rolled to a ring for larger sizes.
4. 04Heat treatment. Carbon steel flanges are normalised where required; stainless flanges are solution annealed; alloy flanges receive the treatment their specification demands.
5. 05Machining. Bore, facing, hub and weld bevel are CNC turned; bolt holes are drilled to the standard pitch circle.
6. 06Facing finish. Raised face serrations are machined to the specified roughness.
7. 07Inspection. Dimensions are checked against the standard, hardness is verified, and NDT is carried out where specified.
8. 08Marking. Flanges are marked with manufacturer, material, class, size, schedule where applicable and heat number, following ASME B16.5 and MSS SP-25 practice.
9. 09Protection and packing. Faces are protected against damage and corrosion before dispatch.

Every flange can be traced by its heat number back through machining, heat treatment and forging to the steel heat it came from.

Flanges are supplied with EN 10204 3.1 material test certificates showing actual results for the delivered heat. Third-party inspection agencies and customer inspectors are welcome to witness testing at our works by appointment. Pressure testing is normally carried out on the assembled piping system rather than on individual flanges.

A perfect flange still leaks if the joint is assembled badly. Most flange leaks trace to installation rather than to the flange.

• Use matched bolting. For carbon and low alloy flanges, ASTM A193 B7 stud bolts with A194 2H heavy hex nuts are the common pairing; low-temperature and stainless service use other grades.
• Check faces before assembly. Radial scratches across a raised face create leak paths; the face should be clean and undamaged.
• Align the flanges. Forcing misaligned flanges together with the bolts distorts the joint and overloads the gasket unevenly.
• Tighten in a cross pattern. Bring bolts up in stages following a star sequence so gasket compression is even around the circumference.
• Use a new gasket. Compressed gaskets do not reseal reliably when reused.

Flange enquiries are frequently incomplete, and each missing item forces a question before a quotation can be issued. A complete flange description follows a consistent order: type, facing, size, class, bore or schedule, material, standard, then any finish, testing and certification requirements.

Example descriptions

• Weld neck, raised face, 4" NPS, Class 300, bore to Schedule 40, ASTM A105N, ASME B16.5, serrated face 125–250 µin, EN 10204 3.1
• Slip-on, raised face, 2" NPS, Class 150, ASTM A105, ASME B16.5
• Blind, raised face, 6" NPS, Class 600, ASTM A182 F316L, ASME B16.5, EN 10204 3.1
• Weld neck, ring type joint, 3" NPS, Class 1500, bore to Schedule 160, ASTM A350 LF2 Class 1, ASME B16.5, Charpy tested

The complete checklist

1. 01Flange type — weld neck, slip-on, socket weld, threaded, lap joint or blind
2. 02Facing — raised face, flat face or ring type joint
3. 03Nominal pipe size
4. 04Pressure class
5. 05Bore or pipe schedule, for weld neck and socket weld flanges
6. 06Material specification and grade, including any heat treatment suffix such as A105N
7. 07Dimensional standard — ASME B16.5, B16.47, DIN, IS or customer drawing
8. 08Facing finish, if different from the standard serrated finish
9. 09Testing — impact, ultrasonic, intergranular corrosion — where the service requires it
10. 10Certification type and any third-party inspection
11. 11Quantity and delivery requirement

Low-temperature service

Carbon steel loses toughness as temperature falls and, below its transition temperature, can fail by brittle fracture without yielding first. Standard A105 carries no impact test requirement, so it is not intended for low-temperature duty. ASTM A350 LF2 is the usual alternative: LF2 Class 1 is impact tested at −46°C (−50°F), giving verified toughness for cold climates, refrigerated and cryogenic-adjacent systems, and depressurisation events where gas expansion chills the line.

High-temperature service

At elevated temperature the governing concern shifts to strength loss and, eventually, creep — slow permanent deformation under sustained load. Carbon steel flange ratings fall steeply with temperature, as the ASME B16.5 table above shows. Chromium-molybdenum grades such as A182 F11 and F22 retain strength far better at temperature and are standard in steam, power and refinery heater service. Their pressure-temperature ratings are published in their own material group tables.

Sour service

Where a line carries hydrogen sulphide, steels can suffer sulphide stress cracking at stresses below yield. Materials for sour service are selected under NACE MR0175 / ISO 15156, which limits hardness — for carbon and low alloy steels, generally to about 22 HRC — along with other material requirements. A105's own 187 HB maximum sits comfortably below that ceiling, but welds and heat-affected zones must also comply. See NACE MR0175.

When a flanged joint gives trouble, the cause is usually identifiable. The table lists the problems we are asked about most often.

For related products see forged valve bodies, forged pipe fittings and our work for the oil and gas industry. To enquire, send the flange type, size, class, facing, material and quantity, and request a quote.