Generally, the manufacturing process of metal pipe fittings follows several key steps: cutting, forming (welding), heat treatment, surface treatment, machining, non-destructive testing, surface protection, and final marking. Using our corrosion-resistant alloy as an example:
1. Cutting
The materials used for metal pipe fittings typically include pipes, plates, and bars. The cutting method is chosen based on the material’s characteristics and the shape of the billet required for the product.
- For pipes, common cutting methods include band saw or hacksaw cutting.
- For plates, such as Hastelloy C276, water jet cutting and laser cutting are commonly used.
- For bars, band saw or hacksaw cutting, and shearing are common methods.
2. Forming (Welding)
Forming is an essential step in the manufacturing process of all pipe fittings. Different products require different forming processes. Here, we provide a brief overview of heating and welding involved in some forming processes.
- a. Heating
When manufacturing pipe fittings using hot forming methods, heating is necessary to meet the deformation requirements of the material during the forming process. The heating temperature is usually determined by the material and process requirements.
During the hot push bending or hot bending of pipes, induction heating methods like medium-frequency or high-frequency induction heating are typically used. For corrosion-resistant alloys such as Hastelloy C276, the elements C, P, and S are strictly controlled, and flame heating is rarely used. Preheating and heating to the required temperature is done through induction heating before pressing or forging the material into the desired shape. - b. Welding
There are two cases of welded pipe fittings:- The first is fittings made from welded pipes. For the manufacturer, the forming process for welded pipes is similar to that of seamless pipes, and the forming process does not include welding steps.
- The second involves pipe fittings where welding is required during the forming process, such as elbows assembled and welded after single-piece pressing or tees pre-fabricated by welding steel plates before pressing. For example, large INCONEL 625 alloy elbows are typically formed by single-piece pressing followed by assembly and welding.
Common welding methods for pipe fittings include manual arc welding, gas shielded welding, and automatic welding. For some specialized industries, welders must pass specific industry exams and welding procedure evaluations. For instance, the welding of marine pipe fittings requires certification from the relevant classification society, and in the case of welding in marine desulfurization environmental components, industry-specific certifications are often required.
3. Heat Treatment
Heat treatment is a crucial part of the pipe fittings manufacturing process. Through heating, holding, and cooling steps, heat treatment eliminates work hardening, residual stress, and metal deformation defects that occur during the forming process, restoring or improving the metal’s structure and performance to its pre-deformation state.
Different pipe fitting product standards have varying requirements for heat treatment. Not all deformed pipe fittings require heat treatment; usually, conventional heat treatment methods are similar to those used for pipes.
4. Surface Treatment
Surface treatment for pipe fittings generally involves methods such as sandblasting, shot blasting, grinding, and pickling to remove rust, scratches, and other imperfections, achieving a smooth surface that meets the requirements for subsequent processing and inspection. For example, INCONEL 600 alloy fittings are typically treated with sandblasting.
Pipe fittings treated with shot blasting may have slightly increased surface hardness.
5. Machining
Machining is the process of finishing the welding ends, structural dimensions, and geometric tolerances of pipe fittings. For some products, machining also includes processing the inner and outer diameters. Machining is primarily done using specialized or general-purpose machine tools. For oversized pipe fittings, when existing machine tool capabilities are insufficient, other methods, such as grinding, may be used. For instance, large-diameter elbows may be finished using grinding techniques.
The appearance and dimensional inspection of pipe fittings are usually carried out after machining.
6. Non-Destructive Testing (NDT)
Non-destructive testing is a critical step for inspecting defects that may occur in materials and during the manufacturing of pipe fittings. The criteria for passing NDT should be determined according to the order requirements or specific standards.
For pipe fittings, the surface quality NDT (MT, PT) should adhere to Level II if there are no explicit grade requirements. However, regardless of the specified grade, defects such as delamination and cracks, which are difficult to assess in depth, should always be considered non-compliant.
For internal quality NDT (RT, UT), radiographic testing should adhere to Level II (e.g., for weld seam inspection), and ultrasonic testing should follow Level I if no explicit grade is required.
To prevent potential defects during heat treatment, the final NDT of the pipe fittings should be conducted post-heat treatment. Non-destructive testing in Chinese pipe fitting manufacturing usually follows JB/T 4730 standards. Personnel performing NDT should be qualified according to relevant regulations.
7. Factory marking
Marking is an indispensable part of the product and is the basis for realizing traceability requirements. Usually, the product standards on the content and method of marking provisions. Pipe fittings logo content generally includes trade names, material grade, specifications and other content of the ordering requirements. Marking methods include permanent marking, such as steel stamping, engraving, electro-etching, etc.; non-permanent marking, such as spray printing, labeling, etc..
8. Additional Processes
Besides the standard manufacturing processes mentioned above, manufacturers should also conduct inspections of raw and auxiliary materials to ensure the correct materials are used. To meet special order or material requirements, additional tests and inspections, such as metallographic structure analysis, intergranular corrosion testing, and ferrite content testing, may be necessary to guarantee that the provided products meet customer requirements.
