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Fluid conveyance · Stainless steel flanges

TECHNICAL DATA SHEET

FT-TC-BRI-INOX-001

Issue date: August 5, 2026 · Rev. 001

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Forged stainless steel flanges ASTM A182 F304/F316 — ASME B16.5 Classes 150 and 300: same dimensions as carbon steel, a different rating table

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Forged austenitic stainless steel flanges ASTM A182 grades F304 and F316 (and their low-carbon versions F304L and F316L) in ASME B16.5 Classes 150 and 300, in the four types the TECTUL catalog publishes in stainless: slip-on, welding neck, blind and threaded, plus the two schedule 40 socket welds. The dimensions are identical to those of the carbon steel flange of the same class and size —B16.5 does not dimension by material— which is why this sheet does not repeat the dimensional table: it refers to FT-TC-BRI-AC-001. What does change, and is the backbone of this sheet, is the pressure-temperature table: stainless enters through material groups 2.1, 2.2 and 2.3, and cold it is rated for less pressure than carbon steel (275 psi against 285 psi in Class 150). Where it wins is at the top: at 538 °C a Class 300 in F316 retains 365 psi against the 85 psi of A105.

SKU VT-86920 (stainless steel flange, Class 150, slip-on) · 6 stainless references: 5 in Class 150 and 1 in Class 300 · slip-on, welding neck, blind, threaded and socket weld types · 1/2″–12″ by reference · sold by the unit

1. Technical specifications: same dimensions, different material

First, what does NOT change: the dimensions

ASME B16.5 dimensions a flange by class and nominal size, not by material. A 4″ Class 150 flange in F316 stainless steel has exactly the same outside diameter, minimum thickness, bolt circle and eight holes as its A105 carbon steel twin: they bolt to each other and are dimensionally interchangeable. That is why this sheet does not repeat the dimensional table: the full table for Classes 150, 300 and 600 from 1/2″ to 12″, with outside diameter, thickness, hub, height, bore, bolt circle and drilling in millimetres and inches, is in data sheet FT-TC-BRI-AC-001 and serves this family equally.

What does change with the material is the rating group —and therefore the pressure-temperature table, which is section 2 of this sheet— and the compatible bolting, because of the galvanic couple in section 3.

Bolting for the two classes in the stainless catalog

Quantity, diameter and bolt circle from ASME B16.5, identical to those of the carbon steel flange of the same class. Stud lengths and the tightening torque table are in data sheet FT-TC-BRI-ESP-001; the stud material, however, is not the same as in a carbon steel joint: see section 3.

Material: ASTM A182/A182M, grades F304, F304L, F316 and F316L

The reading that settles the specification is in the last column. Grades F304 and F316 are groups 2.1 and 2.2; the low-carbon grades F304L and F316L are group 2.3, a different group with its own pressure-temperature table. The reason is mechanical and plain to see: dropping carbon from 0.08 % to 0.030 % —which is what the "L" does— costs 35 MPa of yield (from 205 to 170 MPa) and 30 MPa of tensile strength. That loss of strength is the metallurgical trade-off for immunity to sensitization in the weld heat-affected zone. If the supplier delivers a flange with dual F304/F304L marking —very common, because a low-carbon heat can meet both specifications— the flange meets both and the designer may specify it as F304 and use the group 2.1 table, provided the certificate evidences the mechanical properties of the non-L grade.

When 304 and when 316

The chemical difference is molybdenum: F316 carries 2.00–3.00 % and F304 carries none. That molybdenum is what gives resistance to pitting corrosion from chlorides, and it is quantified by the PREN index, which this series develops in the stainless pipe sheet FT-TC-TUB-INOX-001 and does not repeat here. The short selection rule: water, air, clean steam and non-chlorinated chemicals accept F304; seawater, brine, pool water, coastal environments, bleaches and any chloride-bearing fluid demand F316. In pressure and temperature the difference between the two is marginal —group 2.2 yields between 2 and 5 psi more than 2.1 in Class 150— and is not a selection criterion: you choose by corrosion, not by rating.

The 6 stainless steel references this sheet covers

General specifications

2. The stainless pressure-temperature table: three groups, not one

The double error this section corrects

Two false statements circulate about a Class 150 stainless flange at the same time. The first is the usual one: that "Class 150" means 150 psi —it does not: it means 275 psi (19.0 bar) cold—. The second is more specific to stainless and more expensive: assuming that, being stainless, the flange withstands more pressure than the carbon steel one. It does not. In Class 150 and cold, carbon steel is rated 285 psi and stainless 275 psi: 10 psi less. Stainless is not specified for pressure; it is specified for corrosion and for high temperature.

Class 150 — allowable pressure by material group

Class 300 — allowable pressure by material group

ASME B16.5 material groups: 1.1 = A105 and A350 LF2 forgings, A216 WCB castings (carbon steel); 2.1 = A182 F304 and F304H, A351 CF3 and CF8 castings, A240 304 plate (18Cr-8Ni); 2.2 = A182 F316, F316H and F317, A351 CF3M, CF8M and CG8M castings, A240 316 plate (16Cr-12Ni-2Mo); 2.3 = A182 F304L, F316L and F317L and A240 304L and 316L plate (the low-carbon grades). Bar values are those tabulated by the standard where it publishes the metric series (19.6 bar for 285 psi and 51.1 bar for 740 psi) and, in the remaining cells, the exact conversion of the tabulated psi at 0.0689476 bar/psi. The two empty cells in Class 300 are group 1.1 rows this sheet does not publish for want of the tabulated value at hand: for carbon steel the reference sheet is FT-TC-BRI-AC-001, with its full table at −29/38 °C, 204 °C, 427 °C and 538 °C.

The L-grade trap: the group changes and the rating drops

This is the detail no distributor sheet publishes and that does decide a specification. Grades F304L and F316L are not in groups 2.1 and 2.2: they are in group 2.3, with its own table. The difference shows exactly where it matters:

Group 2.3 loses between 15 and 60 psi against 2.1 and 2.2 in the mid-temperature range, and the cause is in the last row: 35 MPa less yield. That is why a flange marked F316L only cannot be specified with the group 2.2 table. The practical and frequent way out is dual F316/F316L marking: a low-carbon heat whose mechanical properties also reach the non-L grade minimum, certified to both specifications. With that certificate you may use the 2.2 table. Without it, you drop to 2.3.

How to read the table, in three steps

1. Identify the flange's real grade from the edge marking and the certificate, not from the commercial name: "304 stainless" may be F304 (group 2.1) or F304L (group 2.3), and they are not the same.
2. Enter the corresponding group's table with the project design temperature, and interpolate linearly if the point falls between two tabulated steps.
3. Choose the class whose cell sits above the design pressure — and then check the gasket, the pipe and the stud, which have their own limits and are usually the ones that govern.

3. Galvanic couple, face finish and why the L grade exists

The carbon steel stud eats itself to protect the flange

This is the most expensive assembly mistake in this family and it stays invisible until disassembly. A stainless steel flange bolted with A193 B7 carbon steel studs in the presence of moisture forms a galvanic cell: the two metals are in electrical contact and immersed in the same electrolyte, and the less noble one corrodes preferentially. In this pair the less noble one is the carbon steel, so the stud is the anode and it is sacrificed.

What makes the case worse is the area ratio: the stainless flange (cathode) has a vastly greater surface than the stud thread (anode), and in galvanic corrosion an unfavourable area ratio concentrates all the current in the small part. The result is not a "slightly rusty" stud: it is localized section loss on the thread flanks, exactly where the load is, with no sign visible from outside. The joint loses tightening and one day the stud fails, not the flange.

The trade-off with B8 and B8M is mechanical: in the usual supply condition they are weaker than B7, so the target preload and the tightening torque differ. The torque table in data sheet FT-TC-BRI-ESP-001 is calculated on B7 yield and does not apply to austenitic bolting. Austenitic material is also prone to thread galling, so assembly lubricant is not optional: it is mandatory.

Sealing face finish: the other figure that decides the gasket

A flange's raised face is not smooth. ASME B16.5 delivers it with a serrated finish, concentric or spiral, with a roughness of 3.2 to 6.3 µm Rₐ. That groove is not a machining defect: it is what gives a nonmetallic gasket something to bite so it does not extrude outwards under pressure. Two practical consequences:

• Nonmetallic gasket (flat gasket, ASME B16.21): the standard finish is the correct one. Polishing the face "so it seals better" makes the seal worse.
• Metallic or spiral-wound gasket (ASME B16.20): some require a different finish —finer, or with another pattern— and in that case it must be ordered: it is not what the standard supplies by default.

In stainless there is one extra detail: the face is softer against impact than people assume, and an austenitic flange whose face has been dented in transport will not seal no matter how much torque is applied. Check the face before fitting the gasket; recovering a dented face requires remachining, which reduces the thickness and may take the flange below the tabulated minimum.

Welding austenitic stainless: why the L grade exists

When welding a normal-carbon austenitic (F304 or F316, with up to 0.08 % C), the heat-affected zone passes through the temperature range in which carbon precipitates as chromium carbide at the grain boundaries. That process —sensitization— robs chromium from the matrix right around the grain boundary and leaves that band without the passive layer that makes the steel stainless: in corrosive service, the part corrodes along the grain boundaries. Dropping carbon to 0.030 % is what the L grades do, and with that the mechanism shuts down because there is not enough carbon to form the carbides.

Hence the decision that orders this family: on a slip-on, welding neck or socket weld flange —the ones that get welded— the L grade is the technical choice when service is corrosive. On a threaded or blind flange —which are not welded— sensitization is not a risk and the non-L grade, with its higher yield and its better rating group, is the reasonable choice. The cost of the L grade is tabulated in section 2: between 15 and 60 psi of rating in the mid-temperature range.

4. Frequently asked questions

Does a Class 150 stainless flange withstand 150 psi?

It withstands 275 psi (19.0 bar) between −29 and 38 °C, almost double what its name suggests. "Class 150" is not a pressure figure: it is the name of a column in the ASME B16.5 pressure-temperature table, and the value comes from crossing that column with the service temperature and with the flange's material group (2.1 for F304, 2.2 for F316, 2.3 for the L grades). That value drops with temperature: 230 psi at 93 °C in F304, 190 psi at 204 °C and 140 psi at 316 °C.

What is the difference between a Class 150 carbon steel flange and a stainless one?

The dimensions are identical and the ratings differ. ASME B16.5 dimensions by class and size, not by material: same outside diameter, same thickness, same bolt circle, same drilling, and they bolt to each other. What changes is the pressure-temperature table, because each material enters through its own group: cold, carbon steel is rated 285 psi and stainless 275 psi. And the compatible bolting changes: on a stainless flange a carbon steel stud forms a galvanic couple and corrodes.

Does stainless withstand more pressure than carbon steel?

No, and this is the family's most expensive confusion. In Class 150 and cold, carbon steel is rated 10 psi more (285 against 275), and at 93 °C carbon's advantage grows to 30 psi against F304 (260 against 230), almost 12 %. The curves cross around 316 °C, where all four groups meet at 140 psi. From there on stainless dominates: at 538 °C a Class 300 in F316 retains 365 psi against the 85 psi of A105. Stainless is specified for corrosion and for high temperature, never for cold pressure.

Can I use the F316 table if my flange is marked F316L?

No, unless it is dual marked and certified. F316L belongs to material group 2.3 and F316 to 2.2: they are different tables and 2.3 is rated for less pressure in the mid-temperature range (560 psi against 620 psi in Class 300 at 93 °C). The cause is yield strength: 170 MPa in the L grade against 205 MPa in the non-L grade. The usual way out is dual F316/F316L marking, a low-carbon heat whose mechanical properties also reach the non-L minimum and which is certified to both specifications: with that certificate you may use the 2.2 table.

When must 316 be specified instead of 304?

When there are chlorides. F316 carries 2.00–3.00 % molybdenum and F304 carries none, and that molybdenum is what gives resistance to pitting corrosion: seawater, brine, pool water, coastal environments, bleaches. In clean water, air, clean steam and non-chlorinated chemicals, F304 is enough. What is not a criterion for choosing between the two is pressure: in Class 150 group 2.2 yields between 2 and 5 psi more than 2.1, a difference of no practical significance. The PREN index, which quantifies that pitting resistance, is developed in data sheet FT-TC-TUB-INOX-001.

Can I bolt a stainless flange with B7 carbon steel studs?

They fit, technically, but it is a mistake as soon as moisture is present. The two metals in electrical contact form a galvanic cell in which carbon steel is the anode: the stud corrodes to protect the flange. And the area ratio makes it worse —the flange is enormous next to the stud thread—, so all the current concentrates in the small part and the section loss appears on the thread flanks, where the load is and where it cannot be seen. The correct bolting is A193 B8 or B8M with A194 grade 8 or 8M nuts. Watch the torque: B8 and B8M are mechanically weaker than B7 and the stud sheet's torque table does not apply.

Should the flange face be polished so it seals better?

No: it makes the seal worse. ASME B16.5 delivers the raised face with a concentric or spiral serrated finish of 3.2 to 6.3 µm Rₐ, and that groove is what gives a nonmetallic gasket something to bite so it does not extrude under pressure. With a B16.21 flat gasket the standard finish is the correct one. Only some B16.20 metallic or spiral-wound gaskets require a different finish, and in that case it must be expressly ordered. What must be checked is that the face has not arrived dented: a dented face will not seal no matter how much torque is applied.

What stainless grade does the catalog supply?

The catalog publishes the references as "stainless steel" without declaring the ASTM grade or the F304/F316 designation. Since the grade decides the rating group —and therefore the allowable pressure— and also decides chloride resistance, the exact grade is confirmed on quotation against the supplier's material certificate, with its heat number. This sheet publishes the tables for all three austenitic groups precisely so the decision can be made as soon as the grade is known.

5. Application notes

6. Technical notice and limitation of liability