What Are Carbon Steel Fittings and Flanges?

Carbon steel fittings and flanges are the connecting components of a piping system, made from steel whose main alloying element is carbon (plus manganese and small amounts of silicon) rather than chromium or nickel. They split into two jobs:

• Fittings change a line's direction, size or branching: elbows, tees, reducers, caps, crosses, couplings, unions and olets.
• Flanges make a joint that can be bolted together and taken apart again, for connecting pipe to valves, pumps, vessels and other pipe.

They are made two ways. Flanges and small-bore forged fittings are hot-forged from a billet or bar. Larger buttweld fittings are usually formed from seamless or welded pipe or plate by pushing, hot forming or press forming. The manufacturing route matters because each has its own ASTM material specification, which we cover below.

Why Carbon Steel Is the Default Choice

Carbon steel isn't the best material for any single property. It wins because it is good enough across the board and cheap enough to use in volume.

• Strength for the money. Standard grades give around 250 MPa yield and 485 MPa tensile strength, enough for most pressure-piping duties.
• Weldability. Low carbon content makes field welding straightforward with ordinary procedures and qualified welders.
• Availability. Billets, pipe, plate and filler metals are stocked worldwide, which keeps lead times sensible.
• Wide service range. With the right grade it handles everything from cryogenic-adjacent cold service to high-temperature steam.

The trade-off is corrosion. Carbon steel rusts, so it is used where the fluid is benign, where a corrosion allowance and coating are acceptable, or where inhibitors control the environment. We come back to this in the limits section.

Carbon Steel Flange Grades: A105, A350 LF2 and A694

Three ASTM forging specifications cover nearly every carbon steel flange you'll buy. They overlap in appearance but are not interchangeable.

ASTM A105: the everyday workhorse

A105 is the standard forged carbon steel for piping components at ambient and higher temperatures. If a drawing just says "carbon steel flange" without a grade, this is almost always what's meant. It carries a minimum yield of 250 MPa (36 ksi), minimum tensile strength of 485 MPa (70 ksi), and a hardness ceiling of 187 HB.

ASTM A350 LF2: when it gets cold

A350 LF2 is the low-temperature carbon steel flange grade, impact tested at -46°C (-50°F). Its tensile range runs 485 to 655 MPa. The chemistry looks a lot like A105, but LF2 comes with controlled processing and mandatory Charpy testing. That testing is what you are paying for. For pipelines in northern climates, LNG-adjacent systems and offshore topsides, plain A105 is not an acceptable substitute.

ASTM A694: high-yield pipeline flanges

A694 covers high-yield forgings, graded F42 up to F70 by minimum yield strength in ksi, for pipeline service where flanges are welded to API 5L line pipe. Choosing the F-grade is about matching the line pipe: F52 to F65 are common, and F65 gives 450 MPa minimum yield. Using a grade much weaker than the pipe creates a weak point; much stronger can complicate welding.

Values are the standard minimums for the common grades. Always confirm against the current edition of the ASTM specification, as the grade tables carry notes and size limits that a summary can't.

Carbon Steel Fitting Grades: A234 WPB, WPC, A420 WPL6 and WPHY

Buttweld fittings follow different material specs than flanges, and the naming trips people up. The "WP" in each grade stands for wrought piping.

• ASTM A234 WPB is the most widely ordered carbon steel fitting grade, for moderate and elevated temperatures. It is made to match A106 Grade B or API 5L Grade B pipe, which is why the two so often appear together on a bill of materials.
• ASTM A234 WPC is the higher-strength sibling, matching A106 Grade C. You'll see it on heavier duty lines.
• ASTM A420 WPL6 covers low-temperature fittings tested at -46°C, the fitting counterpart to LF2 flanges. For colder duties, WPL3 (nickel steel) goes lower.
• ASTM A860 WPHY is the high-yield grade, graded by yield strength (WPHY 42 to 70), made to pair with high-yield pipeline pipe. These are often specified together with MSS SP-75 for high-test wrought buttweld fittings.

Carbon Steel Flange Types, Pressure Classes and Facings

Choosing a flange involves three separate decisions that are easy to mix up: the type (how it attaches), the class (how much pressure it handles) and the facing (how it seals). Our guide to flange facing types and finishes covers the third in depth, so here's the quick version.

Flange types

Weld neck flanges suit high-pressure, high-temperature and cyclic service because the long tapered hub moves stress away from the weld. Slip-on flanges are cheaper and easier to align, but have lower fatigue strength. Socket weld flanges fit small-bore, high-pressure lines. Threaded flanges avoid welding in areas where hot work is restricted. Blind flanges close line ends and nozzles, and lap joint flanges pair with stub ends where frequent dismantling or bolt-hole alignment is needed. Specialty types such as orifice flanges and spectacle blinds are made to the same materials.

Pressure classes

ASME B16.5 covers NPS 1/2 to 24 in Classes 150, 300, 400, 600, 900, 1500 and 2500. ASME B16.47 extends the range from NPS 26 to 60 (Series A and Series B, which are not interchangeable). Class doesn't equal a fixed pressure: allowable pressure falls as temperature rises. For carbon steel in material group 1.1 (which includes A105 and A350 LF2), the ratings at ambient temperature are:

Those figures are the cold ratings. At 400°C the same Class 300 flange handles noticeably less, so always read the pressure-temperature table in ASME B16.5 for your actual design temperature rather than the headline number.

Facings and gaskets

Raised face (RF) is the default on most industrial lines. Flat face is used against cast iron equipment, and ring type joint (RTJ) is used at higher classes where a metal-to-metal seal is required. Pairing matters: bolting a raised face flange to a flat face cast iron flange can crack the casting. The gasket type, spiral wound or ring, then determines the finish to ask for.

Bolting

Carbon steel flanges are normally bolted with ASTM A193 B7 studs and A194 2H nuts. For low-temperature lines using LF2, A320 L7 studs with A194 Grade 4 or 7 nuts keep the whole joint notch tough. It's an easy item to overlook when the flange itself is correctly specified.

Carbon Steel Buttweld Fittings (ASME B16.9)

Buttweld fittings are welded directly into the line, giving a continuous, leak-resistant joint with smooth flow. ASME B16.9 covers factory-made wrought buttwelding fittings from NPS 1/2 to 48: elbows, tees, reducers, caps and stub ends. ASME B16.28 covers short radius elbows and returns, and B16.25 defines the weld end preparation.

Two details save headaches on site. First, order the wall thickness to match the pipe you're welding to. A Sch 40 elbow on Sch 80 pipe leaves a mismatch the welder has to deal with. Second, specify long radius elbows unless there's a hard space limit. They give lower pressure drop and gentler flow, and short radius elbows should be a deliberate choice. Our ASME B16.9 dimension tables list the figures by size.

Forged Socket Weld and Threaded Fittings (ASME B16.11)

For small-bore piping, NPS 1/8 to 4, forged fittings made from A105 (or LF2 for cold service) are the norm. ASME B16.11 sets their dimensions and pressure ratings, and the "class" naming is different from flanges.

Threaded joints are convenient but they are also the likeliest leak paths in a plant, especially under vibration or thermal cycling. Where the fluid is hazardous or the line is high pressure, a socket weld connection is the safer call. On socket welds, leave the standard small gap (about 1.6 mm) between pipe end and socket bottom before welding. It keeps thermal stress from cracking the weld as it cools. See the forged fittings dimension tables for sizes.

Chemistry, Heat Treatment and Weldability

The properties come from the composition and the heat treatment. A105 limits carbon to 0.35% maximum and manganese to 0.60–1.05%, with controlled residual elements. A350 LF2 caps carbon at 0.30% and allows a higher manganese range to support toughness. Lower carbon generally means easier welding and better impact behaviour.

Carbon equivalent

Weldability is usually judged by carbon equivalent (CE), a formula that converts the effect of manganese, chromium, molybdenum, vanadium, nickel and copper into a carbon-equivalent figure. The higher the CE, the more likely the heat-affected zone is to harden and crack, particularly in thick sections or in cold weather. Thick forgings and high-class flanges often need preheat and controlled interpass temperatures even though the steel is "plain carbon".

Heat treatment

Forgings are supplied normalized, normalized and tempered, or quenched and tempered depending on the grade, size and class. Normalizing refines the grain structure and evens out strength and toughness, and low-temperature grades such as LF2 depend on it. A105 allows as-forged supply for smaller sizes and lighter classes, but larger and heavier-class forgings need heat treatment under the standard. Ask for the heat-treatment condition on the certificate rather than assuming.

Testing and Certification: What to Ask For

A fitting that looks right isn't necessarily right. The paperwork and tests are how you know. A sensible carbon steel order usually covers:

• Mill test certificate (EN 10204 Type 3.1) showing heat number, chemical analysis, mechanical properties and heat treatment, traceable to the product's marking.
• Mechanical tests: tensile, yield, elongation and hardness; Charpy impact at the specified temperature for LF2 and WPL6.
• Dimensional and visual inspection against the ASME standard and the order drawing.
• Non-destructive testing such as magnetic particle or ultrasonic examination where the specification or project requires it.
• Positive material identification (PMI) to confirm the material on the finished piece, useful on mixed-material orders.
• Third-party inspection by an agency of your choice for critical projects.

For sour (H2S) service, add NACE MR0175 / ISO 15156 (or MR0103 for refining) to the order. The main control for carbon steel is a hardness limit, normally 22 HRC maximum, along with limits on weld hardness. It has to be asked for up front, since it affects heat treatment and can't be fixed afterwards.

Every product should also carry permanent markings: grade, size, class or schedule, heat number and the manufacturer's identification. Markings are your first line of defence against mix-ups at the warehouse.

Temperature, Corrosion and Coatings

Temperature limits

Carbon steel has a working window, and both ends have rules. At the cold end, A105 is generally acceptable down to about -29°C (-20°F) without impact testing under ASME B31.3's rules, though exemptions depend on thickness and stress; below that, move to LF2 or WPL6. At the hot end, ASME B16.5 permits carbon steel flanges up to 425°C (800°F), but long exposure above that range can turn the carbide phase into graphite, which weakens the steel. Beyond it, designers move to chrome-moly alloy steel.

Corrosion

Plain carbon steel suffers general corrosion in wet, oxygenated or acidic conditions. Design practice is to add a corrosion allowance (commonly 1.5 to 3 mm) to the wall thickness, use inhibitors or linings, or step up to stainless or alloy for aggressive fluids. Our pages on alloy steel and stainless steel fittings cover those upgrades.

Surface protection

For shipping and storage, carbon steel gets a black varnish or rust-preventive oil. For service, options include epoxy or fusion-bonded epoxy (FBE) coating, hot-dip galvanizing to ASTM A153 for outdoor structures and fire-water lines, and rubber or cement lining for slurry and water duties. Choose the coating with the end-use in mind, since a coating that suits transport may not suit buried service.

How to Choose the Right Carbon Steel Fitting or Flange

Work through it in this order, and most of the decisions fall into place:

• Start with the lowest design metal temperature. If it is below about -29°C, the grade has to be LF2, WPL6 or equivalent, with impact testing.
• Match the pipe. Pick the fitting and flange grade from the pipe grade (A106-B to WPB, API 5L X-grades to A694 and WPHY).
• Set the class from pressure and temperature together, using the actual B16.5 table, not just the ambient figure.
• Pick the flange type for the duty. Weld neck for severe or cyclic service, slip-on for economy on moderate duty, socket weld or threaded for small bore.
• Check the facing and finish against the mating equipment and gasket.
• Match the schedule of fittings to the pipe wall at the weld.
• Add the special requirements early: sour service, NDT, third-party inspection, coating, marking, packing.

If you aren't sure whether a duty calls for LF2 or A105, ask. A short conversation before the order is far cheaper than a rejected shipment.

Mistakes We See on Enquiries

After years of reviewing requests from buyers, the same slips come up again and again:

• "Carbon steel flange" with no grade. A105 and LF2 look identical and cost differently. State the grade.
• Ignoring the minimum design temperature. A line that is fine in summer can see well below zero in winter.
• Mismatched schedules. The fitting is right, but the wall doesn't match the pipe.
• Mixing Series A and Series B large-diameter flanges. B16.47 Series A and B have different bolt circles and can't be paired.
• Forgetting the bolting. Cold-service flanges with ordinary studs undo the benefit of the LF2 grade.
• Leaving out the certificate type. Ask for EN 10204 3.1 up front; adding it later delays dispatch.

Frequently Asked Questions