Process flow of stainless steel pipe fittings

Generally speaking, the process flow of manufacturing stainless steel pipe fittings includes blanking, forming, heat treatment, surface treatment, cutting, non-destructive testing, surface protection and marking in order of sequence. The following is a detailed description of the steps.


The materials used for pipe fittings are mainly pipes, plates and bars, and the blanking method is selected according to the characteristics of the material and the shape of the blank used in the product. The shape, size and other requirements of the blank are carried out according to the process regulations of different products.
For pipes, commonly used cutting methods include band sawing or bow sawing, gas cutting, and plasma cutting.
For plates, the commonly used cutting methods include gas cutting, plasma cutting, and punch cutting.
For bars, commonly used cutting methods include band sawing or bow sawing, and punching cutting.

Forming (welding)

For all pipe manufacturing processes, forming is an indispensable process. Because the forming process of different products is not the same, the required length is longer, which will be described separately in Section 15.2.4. Here, the heating and welding involved in part of the forming process are briefly introduced.
a. Heating
For pipe fittings manufactured by hot forming, in order to meet the requirements of material deformation in the forming process, the blanks need to be heated during forming. The heating temperature is usually determined according to the material and process requirements.
When the hot push elbow or hot bend pipe is formed, the medium frequency or high frequency induction heating method is usually used, and the flame heating method is also used. This heating method is continuous heating synchronized with the forming process of the elbow or elbow, and the tube blank is heated in motion to complete the forming process.
When hot-pressed elbows, hot-pressed tees or forgings are formed, reverberatory furnace heating methods, flame heating methods, induction heating methods or electric furnace heating methods are usually used. This heating is to first heat the tube blank to the required temperature, and then press or forge it into a mold.
b. Welding
Pipe fittings with welds include two situations. One is pipe fittings made of welded pipes. For pipe fitting manufacturers, the forming process of welded pipes is basically the same as that of seamless pipes. The forming process of pipe fittings does not include welding process; The other is the welding process required by the pipe fitting manufacturer to complete the pipe forming, such as single-piece pressing and then assembled and welded to form the elbow, and the steel plate reel is welded into a tube blank and then pressed.
Commonly used welding methods for pipe fittings are manual arc welding, gas shielded welding and automatic welding.
The manufacturer shall prepare welding procedure specifications to guide the welding work, and shall perform welding procedure qualification according to the requirements of the corresponding specifications to verify the correctness of the welding procedure regulations and qualify the welding ability of the welder.
Welders engaged in pipe fittings welding operations should pass the examination of the quality and technical supervision department and obtain the corresponding qualification certificates before they can engage in the welding of relevant steel types (according to the regulations of some industries, welding pipe fittings used in some industries must obtain the welder examination and Welding procedure qualification, such as welding of marine pipe fittings must obtain welder examination and welding procedure qualification of the corresponding classification society).
The greater the carbon equivalent, the greater the hardening tendency of the welded material, and the weld zone is prone to cold cracks.
The reason why carbon equivalent is related to the tendency of hardening and cold cracking caused by welding heat is because:
When the carbon equivalent is large, it is easy to produce hardened martensite structure in the welding heat affected zone, which is sensitive to cracks and hydrogen quenching. Hardening will form more lattice defects. Under the condition of unbalanced stress and heat in the weld, the lattice defects will be called crack sources, which increase the tendency of cold cracks in the weld.
The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, and the precipitation of carbides may cause stainless steel to produce intergranular corrosion (welding erosion) in certain environments.
Due to the oversaturation of carbon content, the strength and hardness of martensite increase, plasticity decreases, and brittleness increases.
Carbon has duality in stainless steel, because the presence of carbon can significantly expand the austenite structure and increase the strength of steel.
On the other hand, the increase of carbon content in steel will form carbides with chromium, that is, chromium carbide, which will relatively reduce the chromium content in solid solution. The existence of a large number of micro-batteries will reduce the corrosion resistance of steel. In particular, reducing the resistance to intergranular corrosion can easily cause intergranular corrosion of steel, which will increase the hardness and weaker welding performance.
The carbon content in steel increases, the yield point and tensile strength increase, but the plasticity and impact properties decrease. When the carbon content exceeds 0.23%, the welding performance of the steel deteriorates. Therefore, the low alloy structural steel used for welding contains The carbon content generally does not exceed 0.20%. High carbon content will also reduce the atmospheric corrosion resistance of steel, and high-carbon steel in the open stock yard is easy to rust; in addition, carbon can increase the cold brittleness and aging sensitivity of steel.
Carbon (c): The carbon content in the stainless steel tube increases, the yield point and tensile strength increase, but the plasticity and impact resistance decrease. When the carbon content exceeds 0.23%, the welding performance of the stainless steel tube deteriorates, so it is used for welding Low-alloy structural steel generally does not contain more than 0.20% carbon. High carbon content will also reduce the atmospheric corrosion resistance of steel, and high-carbon steel in the open stock yard is easy to rust; in addition, carbon can increase the cold brittleness and aging sensitivity of stainless steel pipes.
Carbon shifts, causing chromium depletion and forming intergranular corrosion.

Heat treatment

The heat treatment process is an important part of pipe fittings manufacturing. Through the heat treatment steps of heating, heat preservation and cooling, the work hardening, residual stress and metal deformation defects generated during the forming process are eliminated, so that the metal structure and properties of the pipe after forming are changed, and the state before the deformation is restored or its performance is obtained. Improve and improve.
Commonly used heat treatment equipment is reverberatory furnace, electric furnace, etc.; the usual control method is that the thermocouple in the furnace is connected to the control device of the temperature-time automatic recorder through the sensor.
Different pipe fitting product standards have different requirements for heat treatment. Not all deformed pipe fittings need to be heat treated. Generally, when the final forming temperature of low-carbon steel pipe fittings is not lower than 723℃ (recrystallization temperature), heat treatment is not necessary, because the final temperature is The state of the organization is basically normalized. Heat treatment should be performed when the temperature is lower than this temperature or higher than 980℃; whether the pipe fittings of alloy steel or stainless steel are cold formed or hot formed, they should be heat treated.
The routine inspection of heat treatment is generally completed by hardness test.

Surface treatment

The surface treatment of pipe fittings is usually carried out by sandblasting, shot blasting, polishing, pickling and other methods to remove rust and scratches on the surface of the product, so that the product can reach a smooth surface and meet the requirements of subsequent processing and inspection.
The surface hardness of pipe fittings that are surface treated by shot blasting will increase slightly.


Machining is the process of completing the welding end, structural size, shape and position tolerance of the pipe fittings. The cutting processing of some pipe products also includes inner and outer diameter processing. Machining processing is mainly done by special machine tools or general machine tools; for oversized pipe fittings, when the existing machine tool capacity cannot meet the processing requirements, other methods can be used to complete the processing, such as the gas cutting and polishing method used for large-diameter elbows.
The appearance and size inspection of pipe fittings are usually carried out after cutting.

Non-destructive testing

Non-destructive testing is an important process for testing possible defects in materials and pipe fittings. Most pipe fitting product standards stipulate the requirements for non-destructive testing, but the requirements are not consistent. In addition to meeting product standards and ordering requirements for non-destructive testing, some manufacturers with stricter quality control also formulate non-destructive testing requirements based on materials, processing techniques and internal quality control regulations to ensure the quality of the products shipped.
In actual work, the determination of the qualified level of non-destructive testing of pipe fittings should be based on the clear regulations of the order requirements or standards. Because the surface of the pipe fitting is basically in the state of the original pipe, plate or forging, for the non-destructive testing (MT, PT) of the surface quality of the pipe fitting, if there is no clear grade requirement, it can be classified as grade II. Defects such as cracks whose depth is not easy to determine shall be regarded as unqualified. For the non-destructive testing (RT, UT,) of the internal quality of pipe fittings, if there is no clear level requirement, radiographic testing should be level II (for example, welding seam testing), and ultrasonic testing should be level I.
In order to prevent possible defects in the product during the heat treatment, the final non-destructive testing of the pipe fittings should be carried out after the heat treatment.
The non-destructive testing of pipe fitting manufacturers in our country usually uses the JB/T 4730 standard. Personnel engaged in non-destructive testing shall obtain corresponding qualifications in accordance with relevant regulations.

Surface protection

The surface protection of carbon steel and alloy steel pipes is usually painted, and the stainless steel is passivated after pickling (passivation is not necessary for all stainless steel pipes that are cut on the surface). The main purpose of the surface protection of pipe fittings is anti-corrosion, and at the same time, it also achieves the beautiful appearance of the product. Usually, the ordering party puts forward specific requirements for surface protection, and the manufacturer completes the surface protection of the pipe fittings according to the ordering party’s requirements.

The mark is an indispensable part of the product and the basis for achieving traceability requirements. Generally, the content and method of marking are specified in product standards. The marking content of the pipe fittings generally includes the manufacturer’s mark or name, material grade, specification, and other content required by the order. Marking methods include permanent marks, such as steel stamping, engraving, and electric erosion; non-permanent marks, such as jet printing, labels, etc.


In addition to the above-mentioned conventional manufacturing process flow, in order to control the quality of raw materials, the manufacturer should also complete the inspection of raw and auxiliary materials to ensure that the materials used are correct; in order to meet the special requirements of the order or material, it should also perform such as metallographic structure and intergranular Corrosion, ferrite and other inspections and tests to ensure that the products provided meet the requirements of customers.

Passivation treatment of stainless steel pipe fittings

Stainless steel pipe fittings can be said to be used a lot in our current life, but you know that the stainless steel pipe fittings you just got can not be used immediately, do you know that advanced passivation treatment is required? All stainless steel parts, such as electroless plating or other coating requirements, generally have to undergo passivation treatment (including pickling to remove black skin, polishing, etc.) before they can be used as finished products or assembled into parts. After passivation, it can improve the thermodynamic stability of stainless steel in environmental media, prevent local corrosion of stainless steel, make the surface of stainless steel have sufficient cleanliness, and can also eliminate hot processing oxides on the surface of stainless steel. The passivation process of stainless steel can be divided into wet method and dry method. It can be divided into many types.
Here is an introduction to the operation method of pickling passivation in the wet process:
Pickling is a method of using chemical reactions to dissolve rust, oxide film and other products on the surface of the workpiece. This method does not affect the base metal. Its purpose is to decontaminate the surface of the workpiece and achieve the purpose of purification. Passivation is the use of chemical reactions to form a dense oxide film on the surface of the workpiece. Its purpose is to build an oxide film or oxygen adsorption layer on the surface of the workpiece, thereby preventing the progress of electrochemical corrosion and improving the corrosion resistance (electrochemical corrosion resistance) of the metal.

Construction procedures


  • (1) Before pickling and passivation, the welding coatings, splashes, burrs and dirt on the pipe fittings must be cleaned up.
  • (2) The oil stains on the surface of the pipe fittings can be wiped clean with organic solvents such as gasoline and acetone. For safety reasons, large areas of oily dirt can be cleaned with steam or 3~5% caustic soda (NaOH) solution, and then rinsed with clean water and dried. Note that the CL-ion concentration of the clean water used should not exceed 25mg/l.
  • (3) Prepare the equipment, tools and labor protection supplies required for pickling and passivation operations (taking three persons working at the same time as an example).

Equipment and tools

  • A. An acid-resistant pickling and passivation tank;
  • B. 3 stainless steel wire brushes or hard plastic nylon brushes;
  • C. One piece of measuring cup, measuring cylinder, table scale, and stirring rod;
  • D. 3 acid-resistant mops.

Labor protection products

  • A. 3 pairs of acid-resistant long-tube rubber shoes;
  • B. 3 pairs of acid-resistant rubber gloves;
  • C. 3 sets of acid-resistant overalls, work caps and masks;
  • D. 3 pairs of glasses.

Precautions for pickling and passivation materials and preparation of pickling and passivation solutions

Preparation of pickling and passivation materials

  • ① Industrial nitric acid (HNO3, γ=1.42);
  • ② Industrial hydrochloric acid (HCL);
  • ③ Industrial sulfuric acid (H2SO4).

Generally, the passivation effect of stainless steel depends on both the passivation process and the stainless steel material itself. The specific influencing factors include the elements contained in the stainless steel, the stainless steel metallographic structure, and the processing state of the stainless steel. Among the constituent elements, chromium and nickel are elements with strong passivation, followed by the passivation of iron. Therefore, the higher the content of chromium and nickel, the stronger the passivation of stainless steel. Austenitic and ferritic stainless steels have a relatively uniform structure and good passivation. Martensitic stainless steels are strengthened by heat treatment, and their metallographic structure is a multi-phase structure, so the passivation is not strong.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (

(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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