Material selection starts with the service conditions, not with a preferred grade. Temperature, chloride content, sour service, cycling and the welding procedure all narrow the choice before the standard and pressure class are considered.
This guide covers the grades we machine and forge most often, with the limits that matter in practice.
Start with four questions
- What is the design temperature? Below −29 °C, carbon steel needs impact-tested grades. Above about 425 °C, carbon steel loses creep strength and alloy grades take over.
- What is the corrosion environment? Fresh water, seawater, chlorides, acids and sour service each point to a different family.
- Is the joint welded? Welding changes the answer — low-carbon “L” grades, stabilised grades or post-weld heat treatment may be required.
- What does the design code require? ASME B31.3, B31.1, EN 13480 and project specifications add rules that override a generic table.
Carbon steel
Carbon steel is the default for oil, gas, water and steam service. It is weldable, available in the full size range and the least expensive option.
| Grade | Standard | Service range | Notes |
|---|---|---|---|
| A105 | ASME B16.5 Group 1.1 | −29 to 425 °C | General piping; not for sour service without hardness control |
| A105N | Normalised A105 | −29 to 425 °C | Better toughness when impact testing is specified |
| A350 LF2 | Low-temperature carbon steel | −46 to 345 °C | Impact tested at low temperature; LNG and refrigeration |
| A694 F52/F60/F65 | High-yield pipeline flanges | −29 to 400 °C | Matches high-strength line pipe |
| Q235B / 20# | GB/T 9112–9124 | −20 to 350 °C | Domestic and general industrial piping |
The most common mistake with carbon steel is using A105 in wet sour service. NACE MR0175 / ISO 15156 limits hardness and requires qualified materials; a standard A105 flange may not comply.
Stainless steel
Stainless covers a wide range of alloys, and “stainless” alone is not a specification. The grade decides whether the flange resists chlorides, high temperature or neither.
| Grade | Standard | Service range | Best for |
|---|---|---|---|
| 304 / 304L | A182 F304/F304L | −196 to 400 °C | Water, food, mild chemicals |
| 316 / 316L | A182 F316/F316L | −196 to 400 °C | Chemical processing, pulp and paper, marine atmosphere |
| 321 | A182 F321 | −196 to 600 °C | Elevated temperature, heat exchangers |
| 347 | A182 F347 | −196 to 600 °C | High-temperature and stabilised service |
| Duplex 2205 | A182 F51 | −50 to 300 °C | Offshore, desalination, chloride-bearing lines |
| Super duplex 2507 | A182 F53 | −50 to 300 °C | Severe marine and sour service |
| 904L | A182 F904L | up to 400 °C | Sulphuric and phosphoric acid service |
Two practical rules:
- Use the L grades for welded assemblies. 304 and 316 can sensitise in the heat-affected zone; 304L and 316L avoid it.
- Do not use duplex above about 300 °C. Sigma-phase formation embrittles the material, and the risk rises with time at temperature.
Alloy steel for high temperature
Chromium-molybdenum grades keep strength where carbon steel creeps. They are standard in refineries and power plants.
| Grade | Composition | Service range | Typical use |
|---|---|---|---|
| F5 | 5Cr–½Mo | up to 650 °C | Refinery and power piping |
| F9 | 9Cr–1Mo | up to 650 °C | High-temperature process lines |
| F11 | 1¼Cr–½Mo | up to 590 °C | Steam and hydroprocessing |
| F22 | 2¼Cr–1Mo | up to 590 °C | Reactors, high-pressure steam |
| F91 | 9Cr–1Mo–V | up to 625 °C | Main steam and reheat lines |
Alloy flanges need controlled welding: preheat, interpass temperature, and post-weld heat treatment. The purchase order should state the PWHT requirement, because it affects both the material condition and the delivery time.
For mechanical parts rather than piping, 4140 and 4340 are the workhorse quenched-and-tempered grades for shafts, gears and couplings.
Selection matrix
| Requirement | Recommended material |
|---|---|
| General industrial, moderate temperature, lowest cost | ASTM A105 |
| Low temperature down to −46 °C | ASTM A350 LF2 |
| High-pressure gas transmission | ASTM A694 F52/F60 |
| General corrosion resistance | 304/304L |
| Chlorides, marine atmosphere, chemicals | 316/316L |
| High strength plus chloride resistance | Duplex 2205 |
| Severe marine or sour service | Super duplex 2507 |
| 550–590 °C steam or process | F11 or F22 |
| Above 600 °C | F91 or a nickel alloy |
| Sulphuric/phosphoric acid | 904L |
Documentation and verification
Whatever the grade, the certificate is part of the product. EN 10204 3.1 certificates report the actual chemical and mechanical results for the heat, and the heat number must match the stamping on the flange. For stainless and alloy heats, positive material identification (PMI) confirms the grade before shipment.
If you are unsure between two grades, send the design temperature, the fluid and the design code — we will narrow it down and quote the matching carbon steel, stainless steel or alloy steel flange.