Material Reference
Alloy Steel Flanges
When carbon steel runs out of capability, alloy steel steps in. The chrome-moly grades handle temperatures and creep environments where A105 would soften and yield. The low-temp grades stay ductile and tough at cryogenic conditions where ordinary carbon steel turns brittle. The high-yield pipeline grades match the mechanical properties of modern transmission pipe so the joint does not become the weakest link. We supply the full alloy range in forged and plate-cut configurations across ASME B16.5, B16.47, MSS SP-44, and custom dimensions.
Chrome-Moly Grades for High Temperature
ASTM A182 covers forged or rolled alloy steel flanges for high-temperature service. The chrome-moly family adds chromium and molybdenum to carbon steel to push the creep limit upward and let the flange hold pressure at temperatures that defeat A105. The grade selection follows the operating temperature and the service conditions on either side of the flange.
F11 (1-1/4 Cr - 1/2 Mo)
F11 Class 2 is the lower-strength chrome-moly grade. It serves up to about 1100 F for ASME B31.1 power piping and steam service, with a sweet spot in the 700 F to 950 F range where carbon steel has derated significantly but the heavier alloy is not yet required. F11 takes weld procedures that match the parent piping and tolerates PWHT well. Typical service includes steam piping in fossil power plants, refinery process piping at intermediate temperature, and the upstream side of fired heater coils.
Specification callouts read A182 F11 Class 2. Class 1 is a lower-yield grade rarely used in current piping. Class 2 is the dominant grade for new construction.
F22 (2-1/4 Cr - 1 Mo)
F22 Class 3 is the workhorse chrome-moly for high-temperature refinery and power service. Higher chromium and molybdenum content than F11 pushes the upper temperature ceiling to roughly 1200 F and improves creep resistance and hydrogen attack resistance significantly. Hydroprocessing reactor piping, FCC feed lines, hydrocracker effluent piping, and similar high-temperature high-pressure refinery service is where F22 dominates.
F22 requires careful welding procedure qualification and post-weld heat treatment. The PWHT cycle is non-negotiable on any pressure joint. Specifying F22 without PWHT in the weld procedure is a common bid-stage mistake.
F91 (9 Cr - 1 Mo - V, Grade 91)
F91 is the modern high-temperature alloy, a 9 Cr martensitic grade with niobium and vanadium additions. The mechanical properties at elevated temperature are significantly better than F22, which lets designers use thinner wall pipe and lighter flanges at the same pressure and temperature. F91 shows up in supercritical and ultra-supercritical power stations, in new high-temperature petrochemical service, and anywhere the project budget can absorb the material and welding cost in exchange for weight and thickness savings.
Welding F91 demands strict control of preheat, interpass temperature, and the PWHT thermal cycle. Get any one of them wrong and the heat-affected zone develops brittle phases that crack in service. The flange-side welding procedure must match the project specification exactly.
Low-Temperature Grades A350 LF2 and LF3
On the cold end of the spectrum, ASTM A350 covers carbon and low-alloy forgings qualified for low-temperature service by Charpy V-notch impact testing. The two grades that dominate piping work are LF2 and LF3.
A350 LF2 (Carbon Steel)
LF2 Class 1 is essentially A105 with a documented impact test at minus 50 F. Same chemistry, same tensile properties, same hardness. The difference is the qualification testing: LF2 has been Charpy-tested to confirm it stays ductile in cold service, and the MTR carries the impact result. For any piping that sees temperatures below 0 F, ASME B31.3 practically requires LF2 in place of A105.
Typical service includes ethylene plant cold sections, LNG ancillary piping, hydrogen service in cold climates, and propylene refrigeration. The flange ships from the same forging line as A105 but with an extra QC step and the impact certificate in the documentation package.
A350 LF3 (3.5 Ni Low-Alloy)
LF3 adds 3.5 percent nickel to the chemistry, which pushes the qualified service temperature down to minus 150 F. This is the grade for the cold core of LNG plants, cryogenic ethylene piping, deep refrigeration in petrochemical service, and any process loop where the design minimum temperature drops into deep cryogenic range without going fully to stainless or to a 9 percent nickel grade.
LF3 is a forged grade only. There is no plate-cut LF3 because the qualification depends on the forging-induced grain structure. For applications colder than minus 150 F, the next step is A707 or a fully austenitic stainless grade.
High-Yield Pipeline Grades A694
For transmission pipeline service the flange has to mechanically match the line pipe, not just hold pressure. ASTM A694 covers forged carbon and alloy steel flanges with high yield strength, specifically intended to mate with API 5L line pipe in grades X42 through X70. The grade designation F42, F52, F60, F65, F70 tracks the same minimum yield strength as the matching pipe grade in ksi.
Why High-Yield Matters
A105 has a minimum yield of 36 ksi. X65 line pipe has a minimum yield of 65 ksi. Welding a Class 600 A105 flange to X65 transmission pipe creates a stress discontinuity at the weld: the pipe can carry more stress than the flange. In elastic service the joint works. Under pressure cycling, ground movement, or the kind of bending loads that pipelines experience routinely, the lower-strength flange becomes the limiting component.
A694 F65 has matched yield strength to X65 pipe. The weld no longer creates a strength discontinuity. The joint behaves as an integrated structural element, which is what the API 1104 welding procedure and the project pipeline code both assume.
The economic and regulatory pressure to upgrade gets stronger at higher classes. Class 600 and above transmission service in X52, X60, X65, or X70 pipe rarely accepts A105 anymore. The spec defaults to A694 in the matching grade. MSS SP-44 carries the standardized large-diameter pipeline flange pattern, and most of those flanges are supplied in A694 with the appropriate yield grade.
A694 is a forged grade. Plate-cut work in equivalent chemistry is sometimes available for very large diameters but requires qualification on the project-specific basis. For standard B16.5, B16.47, and MSS SP-44 sizes, forged A694 is the right answer.
Selecting the Right Alloy
The decision tree usually starts with temperature. Above 800 F, chrome-moly takes over from carbon steel. Above 1100 F, F91 becomes a serious candidate. Below 0 F, LF2 is the minimum qualification. Below minus 50 F, LF3 starts to make sense.
After temperature comes the matching pipe grade. Transmission and gathering line work in API 5L pipe needs flanges that match the pipe yield. Any Class 600 or higher service in X-grade pipe should be A694 by default unless the project specification permits otherwise.
The third consideration is the process environment. Hydrogen service, wet sour service, and any application with NACE MR0175 exposure adds chemistry and hardness restrictions that constrain the alloy choice. Confirm the project NACE requirements before the order routes to a specific mill.
Welding Considerations
Alloy steel flanges almost always require preheat before welding and post-weld heat treatment after. The exact thermal cycle depends on the grade and the thickness of the joint. F11 typically requires preheat to 300 F and PWHT at 1250 F to 1300 F. F22 typically requires preheat to 400 F and PWHT at 1300 F to 1375 F. F91 demands tight control of preheat, interpass, cooldown, and a PWHT cycle in the 1400 F to 1450 F range.
On the flange side, the material certification needs to match the welding procedure qualification record (WPQR or PQR) the project is using. For B31.3 process piping, the procedure usually qualifies the base material grade by P-Number from ASME Section IX. F11 is P-Number 4. F22 is P-Number 5A. F91 is P-Number 15E. Mixing P-Numbers across the joint requires additional qualification work.
A350 grades weld as carbon steel for procedure purposes but require maintaining the impact qualification through the weld and heat-affected zone. Most LF2 procedures use low-hydrogen consumables and controlled heat input to preserve toughness. For LF3 the consumable choice usually includes nickel-bearing filler metal to match the parent chemistry.
Documentation and Sourcing
Alloy steel flanges are sourced from domestic forging mills and from qualified import mills depending on project requirements. MTRs in EN 10204 type 3.1 format are standard. For F91 and other high-alloy grades, third-party witness inspection (type 3.2) is sometimes specified, particularly for power plant and high-pressure refinery service.
For A694 pipeline service, additional certifications often apply: API 6D for ball valve trim, NACE MR0175 for sour service, and project-specific testing such as hardness verification on the finished part. Confirm the test requirements before the order routes to the mill so the QA package matches what the project expects.
Lead time on alloy steel varies by grade and size. F11 and F22 in B16.5 sizes are usually available from stock or short production cycle. F91 typically runs longer. A694 in mainline pipeline sizes (24 inch and above) is a project lead time, not a stock pull. Call the sales desk early in the bid cycle for any alloy that is not A105.
Need an Alloy Quote?
Call (281) 484-8325 or email sales@texasflange.com with the grade, size, class, and any project-specific NDE or NACE requirements. Alloy work runs from stock on the common grades and through mill production on the heavier alloys.
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