Five Technical Requirements for Flange Forging Process

The use conditions of flanges refer to the importance, working conditions, assembly and disassembly difficulty, complexity of them, as well as force, working temperature, corrosion degree, and other factors.
From the basic principle that the forging is to serve and meet the use conditions of the parts, the technical requirements of the forging should include two aspects, one is the requirements of the shape, size and surface of the forgings, and the other is the requirements of structure and mechanical performance.
The key to meeting the requirements of forgings is correctly selecting the raw materials, and strictly controlling the production process as well as the quality of the raw materials. Furthermore, through reasonable development of the production process of forgings, effective quality control is implemented.
The selection of forgings is usually determined by the product design, and it is stipulated that in addition to the basic properties of the material, namely the yield strength, tensile strength, plasticity and fracture toughness, in order to reduce the structure weight, besides  two important parameters, strength-to-weight ratio and stiffness-weight ratio, the physical properties, process properties (malleability, hardenability, machinability, weldability, etc.) and economic efficiency must also be taken into account.

1. Requirements for choosing raw materials

Choosing the most reliable raw materials is a prerequisite for the quality of the forgings. The main processes of determining the quality of raw materials are smelting, ingot casting, and semi-product processing. The technical requirements for selecting raw materials can be summarized as follows.

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Chemical composition
The content of alloying elements, harmful impurities, gases and residual elements in the materials should comply with the technical standards and relevant technical conditions or agreements, and be controlled as strictly as possible based on the production conditions. There must be certain requirements for the distribution uniformity of alloying elements.
Smelting technique
Ultra-high-strength steel, titanium alloy and high-temperature alloy are produced by vacuum arc remelting method, among which, titanium alloy and high-temperature alloy require no less than two times; Alloy structural steel, stainless steel and heat-resistant steel are produced by electric arc furnace, or dual process with electric arc furnace and electroslag remelting, or other better smelting methods; Aluminum alloys are usually smelted by flame furnace, electric resistance furnace and induction furnace, and high-quality aluminum alloys require a series of technological measures to strictly control the impurity content and diversify the heat treatment conditions of the materials.
Material specifications, surface quality and dimensional tolerances are based on the production process and quality requirements of forgings. The specifications of the materials include ingots, bars (rolled, forged, extruded), billets, flats, etc. When forgings have strict streamline distribution requirements, care should be taken to select the flow direction of the raw material to coordinate with the streamline distribution specified by the forging. Surface defects of raw materials, such as cracks, folds, crusting, double skin, etc., may cause problems on the surface of the forging, and therefore should be avoided. The dimensional tolerance of raw materials has an important influence on the precise forming of forgings, so the requirements of this should be clearly defined.
Material forging ratio – Materials should be guaranteed to have a sufficiently large degree of deformation, that is, the forging ratio should be specified within a suitable range to ensure sufficient deformation of the material and to reduce or eliminate the cast structure in the material. For large forgings, the forging ratio of raw materials is generally required to be greater than 6-8.
Mechanical property
The mechanical properties of raw materials include properties at room temperature and high temperature, such as strength index, plasticity index, impact toughness, hardness, fracture toughness, permanent strength, fatigue properties, stress corrosion resistance, etc., which should be regulated based on flange forgings and their uses. The differences are specified separately according to the technical requirements for raw materials. Some mechanical properties of large-size raw materials are somewhat low, so be careful when choosing materials.
Microstructure
It is the requirement for the microstructure, grain size (for steel) and purity of the raw materials in the final heat treatment state. The organization of the material has a decisive influence on its properties. Some abnormal structures in the raw material, such as excess ferrite in austenitic and martensitic stainless steels, eutectic compounds in other steels and aluminum and magnesium alloys, the low melting point in the high temperature alloy, the microstructure defects such as carbide segregation and banded structure, and the excessive grain size and low purity will not only seriously affect the performance of forgings, but also will increase the reject rate of forgings. Therefore, there should be clear requirements for microstructure of raw materials and they should be stated in the relevant technical standards.
Macrostructure
It is used to inspect and limit various low-magnification metallurgical defects in raw materials, such as white spots, shrinkage holes and cavities, bubbles, delamination, cracks, slag inclusions, pinholes, segregation, oxide film, etc., which have a serious impact on the performance and processability of forgings, and should be strictly limited and treated according to relevant technical standards.
Supply conditions of raw materials
It refers to the state of raw materials before they are put into production, including whether it is necessary to prepare heat treatment and whether the surface of the raw material needs to be processed to a certain roughness.
Materials manufacturability
It refers to the regulations on the forgeability, hardenability, machinability, weldability, etc. of raw materials. The forgeability of raw materials has an important influence on the forging forming and quality, which is often measured by two indexes – one is plasticity and the other is deformation resistance. The hot forging test is a method of indicating the forgeability. The hardenability, machinability and weldability of the material are the process properties that the forging must have in the process of machining into parts. The technical requirements of the material should be stated in the relevant technical standards.
Regulations for special inspection projects

Raw materials for important aviation forgings should be inspected by ultrasonic flaw detection to prevent or avoid the internal metallurgical defects of materials not found in the destructive inspection. Ultrasonic flaw detection methods and standards, as well as materials to be inspected, should be specified in the relevant technical conditions.

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Repeated tests
If there are inspection results of raw materials that are not in conformity with the regulations, their repeated test should be differentiated and treated with caution. For forgings that are unqualified due to problems in sample processing (including sample heat treatment) and inaccurate test methods, or not caused by material defects, such as mechanical properties, chemical composition, etc., repeated tests are allowed. Low-magnification metallurgical defects of raw materials are not allowed to be re-inspected in principle (except where required and the raw material supplier uses the ultrasonic flaw detection method or other effective methods to screen the metallurgical defects of raw materials. However, in any case, steel with white spots should be discarded once discovered.)

2. Technical requirements for shape, size and surface condition of flange forgings

Based on reliable raw material quality, one of the tasks of flange forging processing is to obtain the required shape, size and surface condition to meet the requirements of parts design, processing and use conditions. The shape and size of the forgings should conform to these of the part and be as similar or close as possible. The main basis of the forging design is the part drawing. At the same time, the characteristics of the forging process, additional machining allowance and process materials, structural elements of the forgings, surface shape tolerances and dimensional tolerances, etc. should be considered.

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Forging surface condition is an important indicator for assessing the quality of forgings. The condition of the non-machined surface of the forging will also directly affect the performance of the part. The starting point of the technical requirements for the surface condition of forgings is to make the surface integrity meet certain specifications.
The basic of ensuring that the parts meet the conditions of use are the structural and mechanical properties of the forgings, which need to correctly formulate and strictly control the forging process.
The structure of forgings includes microstructure and macrostructure. Macrostructure is used to check the flow line distribution of the forging and to judge various metallurgical defects in the forging. Microstructure includes grain size, and purity of the forged part in the final heat treatment state.
The mechanical properties of forgings vary according to the different uses of the forgings and the requirements for their mechanical properties. The room temperature mechanical properties of forgings, such as strength index, plasticity index, impact toughness, hardness, fatigue strength, fracture toughness, etc., will vary depending on the material and forging use. For parts working at high temperatures, there should also be requirements for high temperature instantaneous tensile properties, enduring properties, creep resistance, thermal fatigue properties and thermal stability. The mechanical properties of the forgings should also be consistent with these of the raw materials. For large forgings made from large-size raw materials, the mechanical properties can be appropriately reduced as required. The mechanical performance indicators of various forgings are specified in the relevant technical standards. The test items to be added for the trial production of forgings shall be indicated in the special technical conditions.
Other technical requirements for forgings
Other technical requirements for forgings include forging heat treatment, special project inspection and test methods. According to different materials, the heat treatment of forgings can be divided into preliminary heat treatment and final heat treatment. Steel forgings are supplied in a pre-treated state, and the final heat treatment is carried out during the part processing. High-temperature alloys, titanium alloys and aluminum alloy forgings are mostly supplied in the final heat treatment state.
The processing system for preliminary heat treatment and final heat treatment is noted in the forging drawings or special technical documents. Special project inspection refers to ultrasonic flaw detection and other non-destructive tests of forgings. Due to the complex shape of the forging, ultrasonic flaw detection is only used for important forgings. In addition to the design, the flaw detection part should be considered in the area where the force of the part is large. The characteristics of the process and the weak points of the parts should also be considered. The specific part can be determined by the design, metallurgy and process department, and marked in the product or forging pattern. Other non-destructive tests refer to magnetic particle inspection and fluorescence crack detection. The test methods for checking the technical requirements of forgings must be clearly defined and stated in the technical standards for forgings.

3. Technical requirements for the mechanical and mechanical properties of the flange

The most basic of the parts to ensure that the parts meet the conditions of use are the structural and mechanical properties of the flange. The main means to ensure the structural and mechanical properties of the flange is to correctly formulate and strictly control the forging process.
The structure of the flange includes the low-fold structure and high-magnification of the flange. Low-magnification is used to check the flowline distribution of the flange and to determine various metallurgical defects in the flange. The high-magnification structure includes the microstructure, grain size, and purity of the flange in the final heat treatment state.

The mechanical properties of the flange vary according to the different uses of the flange and the requirements for its mechanical properties. The room temperature mechanical properties of the flange, such as strength index, plasticity index, impact toughness, hardness, fatigue strength, fracture toughness, etc., will vary depending on the material and the purpose of the flange. For parts working at high temperatures, there should also be requirements for high temperature instantaneous tensile properties, long-lasting properties, creep resistance, thermal fatigue properties and thermal stability. The mechanical properties of the flange should also be consistent with and consistent with the mechanical properties of the raw materials. For large flanges made of large-size raw materials, the mechanical performance index can be appropriately reduced as required. The mechanical performance indexes of various types of flanges are specified in the relevant technical standards. The test items to be added for the trial batch flange shall be indicated in the special technical conditions.

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4. Other technical requirements for flanges

Other technical requirements for flanges include heat treatment of flanges, inspection of special items and test methods.
The heat treatment of the flange is based on the material, and the heat treatment of the flange is divided into a preliminary heat treatment and a final heat treatment. The steel flanges are supplied in a ready-to-heat state, and the final heat treatment is carried out during the machining of the parts. Superalloys, titanium alloys and aluminum alloy flanges are mostly supplied in the final heat treatment state.
The process regime for the preliminary heat treatment and final heat treatment is indicated in the flange design or in the special technical documentation.
Special item inspection It refers to the ultrasonic inspection and other non-destructive testing of the flange. Due to the complex shape of the flange, ultrasonic flaw detection is only used for important flanges, such as Class I and II flanges. In addition to the design, the flaw detection part should be considered in the part where the force of the part is large. The characteristics of the process and the weak links of the parts are likely to be considered. The specific part can be determined by the design, metallurgy and process department. On the pattern of the product pattern or flange. Other non-destructive testing refers to magnetic particle inspection and fluorescent inspection. 100% of the parts made of Class I and II flanges should be tested for this type; for parts made of Class III and IV flanges, this can be determined as needed. The proportion and quantity of the tested items. More effective non-destructive testing methods can also be used for certain metallurgical defects. For example, to check the alpha crystallization of titanium alloys, the blue anodizing method can be used and specified in the special technical documents.

Test methods for flanges

The test methods for the inspection of the technical requirements of the flange must be clearly defined and stated in the technical standards of the flange.

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5. Standards for the processing of forged flanges

DIN standard
1) Common pressure levels: PN6, PN10, PN16, PN25, PN40, PN64, PN100, PN160, PN250
2) Flange sealing surface:

  • Raised face DIN2526C protruding flange
  • Grooued acc. DIN2512N

ANSI standard

1) Common pressure levels: CL150, CL300, CL600, CL900, CL1500
2) Flange sealing surface: ANSI B 16.5 RF flanges

JIS standard:

Not commonly used

1) Common pressure level: 10K, 20K

GB standard:

  • GB/T 9112~9124-2010 includes the following 13 standards:
  • GB/T 9112-2010 Steel pipe flange Types and parameters
  • GB/T 9113-2010 integral steel pipe flange
  • GB/T 9114-2010 steel pipe flange with neck thread
  • GB/T 9115-2010 Butt welded steel pipe flange
  • GB/T 9116-2010 Flat welded steel pipe flange with neck
  • GB/T 9117-2010 with neck socket welded steel pipe flange
  • GB/T 9118-2010 Butt welding ring with neck loose steel pipe flange
  • GB/T 9119-2010 Plate type flat welded steel pipe flange
  • GB/T 9120-2010 Butt welding ring plate loose steel pipe flange
  • GB/T 9121-2010 Flat welding ring plate loose steel pipe flange
  • GB/T 9122-2010 Flange ring plate loose steel pipe flange
  • GB/T 9123-2010 Steel pipe flange cover
  • GB/T 9124-2010 Steel pipe flange Technical conditions

The pressure markings specified in this standard are divided into PN marks and Class marks.
PN mark has 12 pressure levels
They are: PN2.5; PN6; PN10; PN16; PN25; PN40; PN63; PN100; PN160; PN250; PN320; PN400
Class mark has 6 pressure levels

They are: Class 150; Class 300; Class 600; Class 900; Class 1500; Class 2500

Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References:

  • https://www.yaang.com

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