Open almost any flange or forged fitting enquiry for stainless, duplex or alloy steel and you will see the same specification: ASTM A182. Behind that one standard sit dozens of grades — F304L, F316L, F51, F53, F11, F22, F91 — each for very different service. Knowing what each grade means helps you quote faster and spot errors in enquiries.
What ASTM A182 covers
ASTM A182 is the specification for forged or rolled alloy and stainless steel pipe flanges, forged fittings, valves and parts for high-temperature service. It sets chemistry, heat treatment and mechanical property requirements. Dimensions come from separate standards such as ASME B16.5 (flanges), ASME B16.47 (large flanges) and ASME B16.11 (forged fittings).
For carbon steel forgings, the companion specification is ASTM A105; for low-temperature carbon steel, ASTM A350 LF2; and for high-yield pipeline flanges, ASTM A694 (F52, F60, F65). Our flange types guide covers the shapes these grades are made in.
Austenitic stainless grades
| Grade | UNS | Notes |
|---|---|---|
| F304 / F304L | S30400 / S30403 | General corrosion service; L for welded joints |
| F316 / F316L | S31600 / S31603 | Molybdenum for chlorides; the most ordered grade |
| F316H / F304H | S31609 / S30409 | Higher carbon for high-temperature strength |
| F321 | S32100 | Titanium-stabilised; high temperature |
| F347 | S34700 | Niobium-stabilised; high temperature |
| F44 | S31254 | 6Mo super austenitic; seawater |
Dual-certified F304/F304L and F316/F316L are standard in the market. See SS 316 vs 316L for why.
Duplex and super duplex grades
| Grade | UNS | Common name |
|---|---|---|
| F51 | S31803 | Duplex 2205 |
| F60 | S32205 | Duplex 2205 (higher N/Cr/Mo) |
| F53 | S32750 | Super duplex 2507 |
| F55 | S32760 | Super duplex (Zeron 100 type) |
Duplex grades combine high strength with excellent chloride resistance. Buyers often require ferrite content checks, ASTM A923 testing for harmful phases, impact tests and a minimum PREN. See our PREN guide.
Chrome-moly (alloy) grades
| Grade | Nominal chemistry | Typical service |
|---|---|---|
| F5 / F5a | 5Cr–0.5Mo | Refinery heaters, hydrogen service |
| F9 | 9Cr–1Mo | Refinery heater and furnace lines |
| F11 (Class 1, 2, 3) | 1.25Cr–0.5Mo | Steam, power plant, high-temperature process |
| F12 | 1Cr–0.5Mo | High-temperature service |
| F22 (Class 1, 3) | 2.25Cr–1Mo | Power plants, hydrocrackers |
| F91 | 9Cr–1Mo–V | Advanced power plant steam lines |
Chrome-moly grades are chosen for creep strength and resistance to hydrogen attack at high temperature. Classes (for example F11 Class 2) define different strength levels and heat treatments. These grades match pipe grades such as ASTM A335 P11, P22 and P91.
Heat treatment requirements
ASTM A182 requires specific heat treatment for each grade:
- Austenitic stainless: solution annealed and quenched.
- Duplex: solution annealed and rapidly cooled to avoid sigma phase.
- Chrome-moly: normalised and tempered, or quenched and tempered.
The certificate must state the heat treatment. Missing or vague heat treatment details are a common reason for rejection. MTCMitra helps you include them on every certificate.
Marking a forged flange
A typical A182 flange marking reads:
ABC 4" 300# WNRF SCH40 ASTM A182 F316L B16.5 HT No. 12345
That covers manufacturer, size, class, type and facing, bore, specification and grade, dimensional standard and heat number. Our heat number traceability guide explains why that last item matters.
Supplementary requirements buyers add
ASTM A182 includes optional supplementary requirements that buyers can call up in their purchase orders. Watch for these in enquiries, because each one adds testing time and cost:
- Impact testing at a specified low temperature, common for duplex and for chrome-moly grades in some services.
- Ultrasonic examination of the forging for internal soundness.
- Intergranular corrosion testing (for example ASTM A262 Practice E) on austenitic grades.
- Hardness limits for sour service, often referencing NACE MR0175/ISO 15156.
- Ferrite and phase testing for duplex grades, typically ASTM A923 and ferrite count checks.
When an enquiry mentions NACE, sour service or hydrogen, read the specification carefully. Hardness limits and heat treatment controls may rule out stock material and require fresh production. Mention any such requirement as a separate line in your quotation so the buyer can see why the price is higher than a plain stock offer.
Common enquiry mistakes
- "SS 316 flange A105" — A105 is carbon steel; the buyer probably means A182 F316.
- "F51 super duplex" — F51 is standard duplex; super duplex is F53 or F55.
- "F11" without class — ask which class is needed.
- Grade without dimensional standard — confirm ASME B16.5, B16.47 Series A/B or EN 1092-1.
Clarifying these points before quoting shows expertise and prevents wrong production.
Frequently asked questions
What is the difference between ASTM A182 and A105?
A105 covers carbon steel forgings. A182 covers alloy and stainless steel forgings such as F11, F22, F304 and F316.
Is F51 the same as 2205?
F51 corresponds to UNS S31803, the original duplex 2205 chemistry. F60 (S32205) is the modern, slightly richer version. Many mills dual-certify F51/F60.
What does F91 mean?
F91 is a 9Cr–1Mo–V modified alloy used in high-temperature power plant steam lines, matching ASTM A335 P91 pipe.
Can A182 F316L flanges be used with A312 TP316L pipe?
Yes. They are the matching forged and pipe specifications for the same grade.
Key takeaways
- ASTM A182 covers stainless, duplex and chrome-moly forgings; A105 covers carbon steel.
- F304L/F316L for general corrosion, F51/F53 for chlorides and strength, F11/F22/F91 for high temperature.
- Heat treatment and class must be on the certificate.
- Clarify confusing enquiries before quoting.
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