Types and specifications of forged flanges

What is a forged flange?

A forged flange is a flange connector with heavy material, tight structure, high pressure resistance and corrosion resistance. It is made by high temperature and high pressure forging process. Forged steel flanges are generally divided into three types: light, medium and heavy. According to the implementation standards, they can be divided into different specifications such as American standard, European standard, Chinese standard, Japanese standard and industry standard.

Forged flange is a product with the best mechanical properties among flange products. Its raw material is generally a tube blank, which is then cut and hammered continuously to eliminate defects such as segregation and looseness in the steel ingot. The price and mechanical properties are one level higher than ordinary cast flanges.

Pipe flanges are parts that connect pipes to pipes and valves, connected to the pipe ends; there are also flanges used on the inlet and outlet of equipment. They are used for the connection between two equipments. They are parts that connect pipes to pipes, connected to the pipe ends. It is an accessory product of pipelines. The main materials of forged flanges are carbon steel, alloy steel and stainless steel.

Classification by international standards:

Forged flanges are classified according to international standards to ensure product consistency and quality. Some of the most famous standards include:

ANSI/ASME B16.5: Applicable to the size and pressure of pipeline flanges.
DIN/EN: European piping system standard.
API: Petroleum industry specification.

In addition, flanges are also classified according to their nominal pressure (called pressure class), which ranges from Class 150 to Class 2500 depending on the application.

Main types of forged flanges:

There are many types of forged flanges, each designed to meet the specific needs of different applications. Forged flanges can be divided into the following according to the connection method:

1. Slip-On Flange (SO)
The pipe is inserted into the inner hole of the flange for welding, which is easy to install and low cost. It is suitable for low pressure and non-violent vibration occasions.

2. Weld Neck Flange (WN)
With a long neck, it is connected to the pipe by butt welding. It has high strength, high pressure and high temperature resistance, and is often used in harsh working conditions.

3. Socket Weld Flange (SW)
The pipe is inserted into the flange socket for welding, which is suitable for small-diameter high-pressure pipes (≤DN50).

4. Threaded Flange (TH)
Internal thread connection, no welding required, easy to disassemble, used in low-pressure, small-diameter or explosion-proof occasions.

5. Lap Joint Flange (LJ)
The flange ring is combined with a short pipe, which can be rotated for easy alignment, but the pressure bearing capacity is low.

Classification by sealing surface:

1. Full plane (Flat Face, FF)
The sealing surface is flush with the flange surface, used for low-pressure soft gasket sealing (such as cast iron flange).

2. Raised Face (RF)
The sealing surface is slightly higher than the flange surface (usually 3-5mm), the most common, suitable for medium and high pressure.

3. Ring Joint Face (RTJ)
With annular groove, matched with metal ring gasket, used for high pressure and high temperature (such as API 6A standard).

4. Male-Female, M/F
The convex and concave surfaces are paired, with good sealing performance, and are used for flammable and explosive media.

Special types:
Blind Flange (BL)
Closes the end of the pipeline for maintenance or system isolation.

Reducing Flange
Connects pipelines of different diameters, with eccentric or concentric center holes.

Material standards for forged flanges:

The materials of forged steel flanges are usually carbon steel, alloy steel and stainless steel.

Stainless steel forged flanges: ASTM A182 F304, F316
Alloy steel forged flanges: ASTM A182 F5, F9, F11, F12, F21, F22
Carbon steel forged flanges: ASTM A105, ASTM A694 F42, F46, F52, F60, F65 and F70, ASTM A350 LF2

ASTM A182/ A182M:
This specification covers forged low alloy steel and stainless steel piping components used in pressure systems. It includes flanges, fittings, valves and similar parts that meet specified sizes or dimensional standards.

ASTM A105:
Forged carbon steel piping components for pressure systems at normal and elevated temperatures, i.e. flanges, fittings, valves and similar parts.

ASTM A350/ A350M:
This specification covers various grades of carbon and low alloy steel forged or ring rolled flanges, forged fittings and valves, primarily for cryogenic applications and for notch toughness testing required for piping components.

ASTM A694/ A694M:
This specification covers carbon steel and alloy steel forgings for pipe flanges, fittings, valves and high-pressure transmission components.

Features and advantages of forged flanges:

1. High strength
Forged flanges have high strength and rigidity and can withstand high pressure, high temperature, high load and other working environments. During the forging process, the metal grains of the pipe flange are pressed together to eliminate defects such as pores and slag inclusions. The material density is high and the pressure is uniform. The tensile strength and impact resistance are significantly better than cast flanges. It is suitable for high pressure (such as Class 1500 and above) and pulsating load conditions.

2. Excellent sealing performance
Using high-precision machining and high-quality sealing materials, forged flanges have excellent sealing performance and can effectively prevent liquid or gas leakage.

3. Good corrosion resistance
During the forging process, the oxide scale is removed from the surface of the forged flange, making the surface of the forged flange have excellent corrosion resistance.

4. Precision size control
Forged flanges have high processing accuracy, high sealing surface finish and bolt hole position accuracy, tight connection and firm structure.

5. Wide material adaptability
Forged flanges can be made of various materials such as carbon steel, stainless steel, alloy steel (such as 16Mn, F51 duplex steel), etc., to meet special requirements such as corrosion and high temperature.

Specifications of forged flanges:

a. Nominal diameter (DN)
Common range: DN15 (½") to DN600 (24"), large-diameter flanges mostly use butt welding or loose sleeve structure.

b. Pressure level (PN/Class)
American standard (ASME B16.5): Class 150, 300, 600, 900, 1500, 2500.
Chinese standard (GB/T): PN6, PN10, PN16, PN25, PN40, etc.
High-pressure flange: up to PN420 or Class 2500 or above.

c. Material
Carbon steel: Q235, A105 (American standard), 20# steel.
Alloy steel: 16Mn, 304/316 stainless steel, CrMo alloy (such as F11, F22).

Special materials: Duplex steel, Hastelloy and other corrosion-resistant materials.

Forging process of forged flange:

‌a. Free forging‌: Suitable for single piece and small batch forgings with simple shapes. The basic processes include upsetting, drawing, punching, bending and cutting. The free forging tools are simple and widely used in small batch production‌.

‌b. Die forging‌: Suitable for mass production, with the advantages of high productivity, simple operation, easy mechanization and automation. The basic processes of die forging include blanking, heating, pre-forging, final forging, punching, trimming, quenching and tempering, and shot peening. The die forgings have high dimensional accuracy, small machining allowance, and more reasonable fiber structure distribution of forgings‌.

Forging process requirements:

1. Use multiple forgings and gradual deformation to avoid excessive cooling and excessive heating.
2. Control the forging temperature and speed to avoid cracks and deformation.
3. Use suitable forging dies and forging equipment to ensure the accuracy and quality of the product.
4. Before subsequent processing, surface treatment and removal of forging residual stress must be carried out.

Application fields of forged flanges:

Forged flanges are widely used in the fields of petroleum, chemical industry, electric power, metallurgy, shipbuilding, medicine, etc., especially in working environments such as high temperature, high pressure, strong corrosion, and strong vibration, where they can give full play to their excellent performance advantages.

Typical application scenarios:

1. High-pressure pipeline system
Such as petroleum cracking unit (Class 900 and above), nuclear power main steam pipeline, rely on the burst resistance of forged flanges.

2. Extreme environment
Low temperature (-196℃ LNG pipeline) or high temperature (power station boiler above 538℃), forged materials have better low-temperature toughness/creep resistance.

3. Corrosive media
In chemical acidic and alkaline environments, forged stainless steel flanges or alloy flanges are used to extend their service life.

4. Vibration conditions
In ships and compressor pipeline systems, the fatigue resistance of forged flanges can reduce the risk of bolt loosening.

Forged flange selection precautions:

a. Working condition matching: select materials and sealing surfaces according to pressure, temperature, and medium (corrosiveness, toxicity).
b. Standard unification: Ensure that the flange is consistent with the pipeline and valve standards (e.g. ASME and GB bolt hole distances may not be compatible).
c. Processing technology: High-pressure flanges require heat treatment (such as normalizing and quenching) after forging to eliminate internal stress.

Read more: Inspection standards for pipe flanges or Standard sizes of pipe flanges