Two flange quotes land in your inbox. One is 30% cheaper. The cheaper lot arrives with a mill certificate that does not match the heat number stamped on the body, and your third-party inspector rejects the whole consignment at site. That sequence plays out often enough in Indian procurement to be predictable. This guide gives you a working method for shortlisting a forged flange supplier: how to read the material grade, what ASME B16.5 actually fixes, which tests are worth paying for, and how to verify an MTC in under ten minutes. Krishna Forge Fitting manufactures forged flanges at Shapar, Rajkot, so what follows is written from the production side of the transaction.
Why Forged, Not Cast or Plate-Cut
A flange can reach the same shape by three routes, and they are not equivalent.
- Forged — heated bar or billet worked under a press or hammer. Grain flow follows the part contour. No porosity. Highest fatigue and impact performance.
- Cast — molten metal poured into a mould. Cheaper for complex shapes, prone to internal voids, unsuitable for cyclic or high-integrity service.
- Plate-cut — profiled from rolled plate, then bored and faced. Acceptable for low-pressure water and large diameters, but grain runs in one direction only, which matters at the hub.
Most tender specifications for process piping call out closed-die forging specifically. If a supplier quotes “forged” but the part was cut from plate, the paperwork will usually say so in the material standard — A516 Gr.70 instead of A105.
Step 1: Fix the Material Grade Before Anything Else
Price comparisons mean nothing across grades. Settle the metallurgy first.
Carbon Steel
ASTM A105 is the default forging grade for ambient and moderate temperature service, usable from roughly -29°C to 538°C depending on class. It covers most water, air, steam and general process lines.
ASTM A350 LF2 is the low-temperature counterpart. It carries mandatory Charpy V-notch impact testing, which is the whole point of specifying it. If your line runs below -29°C, A105 is not a substitute regardless of what a supplier claims.
Stainless Steel
ASTM A182 F304 / F304L for general corrosion resistance, demineralised water, food contact and mild chemical duty.
ASTM A182 F316 / F316L where chlorides are present — seawater, coastal plants, bleach dosing, many acids. The molybdenum addition is the difference, and it is worth the premium when chlorides are in play.
The low-carbon “L” grades resist sensitisation in the weld heat-affected zone. Specify them whenever the flange will be welded and then exposed to a corrosive fluid.
Alloy Steel
F11, F22 and F91 chrome-moly grades for elevated-temperature and creep service in boiler and power piping. These carry mandatory heat treatment conditions that must appear on the certificate.
Step 2: Read ASME B16.5 Correctly
ASME B16.5 governs flanges and flanged fittings from 1/2″ to 24″, in Classes 150, 300, 400, 600, 900, 1500 and 2500. What it controls:
| What B16.5 fixes | What it does not fix |
| Outside diameter, thickness, hub dimensions | Material chemistry (that is the ASTM spec) |
| Bolt circle diameter, number and size of holes | Gasket selection |
| Raised face height and facing finish | Bolting grade |
| Pressure–temperature ratings by material group | Surface coating |
| Marking requirements | Delivery condition beyond heat treatment |
Two points buyers get wrong. First, a Class 150 flange is not rated for 150 psi — it is rated per a table that falls as temperature rises, and at ambient an A105 Class 150 flange rates considerably higher than 150 psi. Second, bolt circle and hole size come from a single table, so a flange that conforms will mate with any other conforming flange of the same size, class and facing. Re-drilling at site always means something did not conform.
Flange Types and Facings
Types: weld neck, slip-on, blind, socket weld, threaded, lap joint. Facings: flat face (FF), raised face (RF), ring type joint (RTJ).
The common mistakes are mating an RF flange to an FF cast iron body, which cracks the casting, and ordering a bore that does not match the pipe schedule on a weld neck flange. A weld neck is bored to a specific schedule. State it.
Above 24″, ASME B16.47 Series A or B applies, and the two series are not interchangeable.
Step 3: Decide Which Tests You Actually Need
Testing is where cost and risk meet. Match the test to the consequence of failure, not to a generic boilerplate list.
- Chemical analysis and mechanical testing — baseline. Tensile, yield, elongation, hardness. Non-negotiable for any code work.
- Charpy impact testing — mandatory for low-temperature grades, and sometimes called out for sour service.
- Hydrostatic testing — proves leak tightness of the assembly. Note that B16.5 does not require hydrotest of individual flanges; it is usually applied to the assembled joint.
- Positive Material Identification (PMI) — a handheld XRF reading that confirms the alloy on the delivered part. The single most useful check against grade substitution, and it takes seconds.
- Ultrasonic (UT) and magnetic particle (MPI) or dye penetrant (PT) — for internal and surface defects in critical service.
- Hardness testing to NACE MR0175 — required where H2S is present.
- Dimensional inspection report — flange face, bolt circle, thickness, bore, verified against the B16.5 table.
The Test Most Specifications Leave Out
Ask for the forged blanks to be witnessed before machining. Once a flange is machined and painted, surface defects that were present in the forging are much harder to find. Tenders from established buyers routinely require inspection at the forging stage for exactly this reason, and it costs nothing extra if the supplier forges in house.
Step 4: Verify the Mill Test Certificate
An MTC under EN 10204 Type 3.1 is issued by the manufacturer’s own authorised inspector; Type 3.2 adds an independent third party. Most Indian supply runs on 3.1.
Run these five checks on every certificate:
- Heat number on the certificate matches the heat number hard-stamped on the flange body. If the part is not stamped, that is the finding.
- Chemistry falls inside the ASTM range for the grade claimed — carbon, manganese, chromium, nickel, molybdenum.
- Mechanical results meet the minimums for tensile, yield and elongation.
- Heat treatment condition is stated — normalised, quenched and tempered, solution annealed, as applicable.
- The certificate names the actual forging works, not just a trading firm.
A traded flange often carries a certificate belonging to a different heat of the same grade. Heat-number correlation is the check that catches it, and it is why buyers increasingly prefer dealing directly with the works rather than through a stockist chain.
Step 5: Assess the Supplier, Not Just the Quote
Shortlist on these questions and ask for written answers.
- Do you forge in house, or buy forgings and machine them?
- Can you supply heat-number-stamped parts with correlated MTCs as standard?
- Which classes and sizes are in your regular production range?
- What is your lead time for standard carbon steel versus special grades?
- Will you accept third-party inspection at your works?
- Can you supply dimensional inspection reports against B16.5 tables?
Lead time is where special grades surprise people. Standard A105 moves quickly from stock bar. Duplex, Inconel and other specials run longer because the raw material itself has to be sourced. Build that into your schedule rather than discovering it three weeks before commissioning.
FAQs
What is the difference between EN 10204 3.1 and 3.2 certificates? A 3.1 certificate is validated by the manufacturer’s own independent inspection department. A 3.2 certificate is countersigned by an external inspector or the purchaser’s representative. Projects with third-party inspection clauses usually require 3.2.
Does ASME B16.5 cover flanges above 24 inches? No. B16.5 runs from 1/2″ to 24″. Larger sizes fall under ASME B16.47, which has two non-interchangeable series, A and B. Confirm which series your plant standard uses.
Can I use an A105 flange on a cryogenic line? No. A105 has no impact-test requirement and can fracture in a brittle manner at low temperature. Specify A350 LF2 with the Charpy testing done at the service temperature.
How do I check a flange grade on delivered material without a lab? Use a handheld PMI gun. It gives alloy composition in seconds and reliably separates 304 from 316 and carbon steel from stainless. Most inspection agencies in India offer it as a site service.
Why does my flange class not match the pressure I expected? Pressure class is a reference figure. Actual allowable working pressure comes from the pressure–temperature rating table for the material group at the operating temperature, and it drops as temperature climbs.
About Krishna Forge
Flange procurement goes wrong in the paperwork more often than in the metal. Krishna Forge Fitting manufactures and exports forged flanges along with industrial fittings, elbows, tees, couplings, end caps, nipples and pipe sockets in mild steel and stainless steel, from our works at Shapar, Veraval, Rajkot, Gujarat. We supply water treatment plants, sprinkler systems, steel plants, fertiliser units, boilers and heat exchangers, and we welcome inspection at our premises.
