What are the facing types and surface finish requirements for steel flanges?

A bolted flange joint only seals as well as the contact between the flange face, the gasket, and the mating flange. Two details decide that contact: the facing type, which sets the geometry of the sealing area, and the surface finish, which sets how the gasket grips that area. Get either one wrong and a joint that looks perfectly bolted can still weep, creep, or blow out under pressure. This article explains the facing types and surface finish requirements for steel flanges, and how to specify them correctly on your next order.

Flange facing types explained

For flanges up to 24 inches, ASME B16.5 defines the facing geometry; for larger sizes, ASME B16.47 applies. Between them, six facing types cover almost every industrial joint. The table below summarizes them at a glance.

Facing type	Typical service	Gasket
Raised face (RF)	General process, most common	Flat ring, spiral wound, metal composite
Flat face (FF)	Low pressure, cast iron equipment	Soft full-face
Ring type joint (RTJ)	High pressure and high temperature	Metal ring (R, RX, BX)
Lap joint	Frequent dismantling	Seals on the stub end
Male and female (M&F)	Heat exchanger shells	Located by the female recess
Tongue and groove (T&G)	Pump and valve covers	Self-aligning matched pair

Raised face (RF)

The raised face is the default and most widely used facing in process plants. The sealing surface sits slightly higher than the bolt circle, which concentrates bolt load onto a smaller area and raises the pressure the joint can hold. The height of the raised strip follows the pressure class: roughly 1.6 mm (1/16 inch) for classes 150 and 300, and about 6.4 mm (1/4 inch) for classes 400 through 2500.

Flat face (FF)

A flat face keeps the sealing surface level with the bolt circle. It is paired with a soft, full-face gasket and is preferred for low-pressure service and for connections to cast iron or non-metallic equipment, where a raised face could over-stress the mating part. Flat face flanges should never be bolted to raised face flanges.

Ring type joint (RTJ)

RTJ flanges carry a machined annular groove that seats a metal ring gasket. As the bolts are tightened, the ring deforms into the groove and forms a metal-to-metal seal. This facing is reserved for high-pressure (class 600 and above) and high-temperature service above roughly 800°F (427°C). Ring gaskets come in R, RX, and BX styles; the R style with an octagonal cross-section is the most common.

Lap joint, male and female, tongue and groove

Lap joint flanges do not seal by themselves; they simply back a stub end, and the actual sealing face sits on the stub end. Male and female facings use a raised male face that fits a matching recess, which locates the gasket and is common on heat exchanger shells. Tongue and groove facings use a raised ring that seats into a matching groove, giving a self-aligning joint often found on pump and valve covers. These three types must always be ordered in matched pairs.

Surface finish requirements for steel flanges

A mirror-smooth flange face is not necessarily a good sealing surface. ASME B16.5 deliberately requires a controlled roughness on RF and FF faces so the gasket can bite into the metal and hold under pressure. The roughness is expressed as AARH (arithmetic average roughness height), measured in micro-inches with a profilometer. A higher AARH value means a rougher face.

Common flange face finishes

Stock finish is the most common and suits the majority of applications. It is cut with a phonographic spiral groove and typically lands in the 125 to 500 micro-inch AARH range. Spiral serrated and concentric serrated finishes use a V-shaped tool to cut 30 to 55 grooves per inch, with a target roughness of 125 to 250 micro-inches (3.2 to 6.3 µm Ra) for standard RF flanges. Smooth finish shows no visible tool marks and is used with metal-jacketed gaskets. Cold water finish is smooth and shiny, in the 85 to 100 micro-inch range, and is used where metal faces seal directly without a gasket.

Why the right finish matters

If the face is too smooth, the gasket has nothing to grip and can blow out under pressure. If it is too rough, the gasket cannot conform to the peaks and the joint leaks. That is why the 125 to 250 micro-inch serrated range is the standard for raised face flanges: it gives a soft gasket enough bite to seal without crushing it. When the finish does not match the gasket, even a correctly torqued joint will not hold.

How to specify facing and finish when ordering

Raised face is the default facing, but a buyer should still state the facing type, the pressure class, and the required finish on every order. For most soft gaskets, specify a serrated finish in the 125 to 250 micro-inch range. For metal or jacketed gaskets, specify smooth. If the finish matters to your process, ask the supplier to confirm the AARH reading and the groove pattern rather than assuming a standard face is right for your gasket.

Getting steel flanges right with EZ Steel Industrial

EZ Steel Industrial Co., Ltd. manufactures and supplies steel flanges and pipe flanges for project and industrial piping systems, covering carbon steel, alloy steel, stainless steel, and copper-nickel materials. The company works to ASTM, ASME, API, EN, ISO, JIS, GOST, and GB/T standards, and its production control includes more than a dozen quality checkpoints plus hydrostatic and ultrasonic testing. That matters for flanges because facing geometry and surface finish are exactly the details that are easy to get wrong and expensive to discover in the field.

Whether you need a raised face, flat face, or ring joint pipe flange for a petrochemical plant, a marine system, or a power project, the EZ Steel team can confirm the facing type, finish, and material against your gasket and service conditions before the order ships. Contact the export team at export@ezsteelpipe.com for a quotation.