What are the factors affecting the performance and service life of stainless steel pipe fittings?
In the future, China’s stainless steel pipe fittings markets will have a rapid development, how to deal with the development? This paper analyzes the factors affecting the performance and service life of stainless steel pipe fittings through stainless steel materials, forming process, wall thickness change and solution (heat treatment) process, so that enterprises can learn better and produce high-quality pipe fittings; users can learn more about and select products with good quality and performance, and accelerate the application of stainless steel pipe fittings in many fields.
- 1. Factors affecting the performance and service life of stainless steel pipe fittings
- 1.1 Effect of material on service life of pipe fittings
- 1.1.1 Influence of forming process on the performance and service life of pipe fittings
- 1.1.2 Influence of wall thickness on pipe connection and service life
- 1.1.3 Influence of wall thickness change on quality during production
- 126.96.36.199 Influence of wall thickness on pipe connection
- 188.8.131.52 Confirmation and selection of wall thickness in use
- 184.108.40.206 Anti corrosion function of solid solution process and influence of service life
- 1.1.4 Production control and improvement of stainless steel pipe fittings
- 2. Control the quality of stainless steel materials in procurement
- 2.1 Reasonable process and technical measures to control pipe wall thickness
- 2.2 Solid solution is an indispensable process to improve the quality of pipe fittings
- 3. Conclusion
Stainless steel pipe fitting has a long service life. From the use of stainless steel pipe fittings abroad, the service life of stainless steel pipe fitting can reach 100 years, at least 70 years, the same as that of buildings. The most outstanding advantage of stainless steel pipe fittings is corrosion resistance. If the stainless steel pipe fittings can be used and maintained correctly in the use process, the service life can be effectively extended.
(I) Effect of material on service life of pipe fittings
The main difference between 304 and 304L stainless steel material is that the C content of 304 is ≤ 0.08%, while that of 304L is ≤ 0.03%; the Ni content of 316 material is 1% – 2% higher than that of 304L, which is mainly due to the decrease of Cr content and the increase of Mo (MO) by 2% – 3%, so their corrosion resistance and corrosion resistance environment are different.
L represents low carbon, for example, 304L contains less carbon than 304, h and s represent high temperature resistance, for example, 310S is better than 310 in high temperature resistance, 304H is better than 304 in heat resistance, and 316 and 316L are marine materials with strong corrosion resistance.
There are two kinds of 304 stainless steel materials: first grade and second grade. The second grade material is the inferior product of the steel plant. The main difference is that the second grade material is cheaper than the first grade material in terms of its surface, nickel content and uneven thickness. The steel works enter the market after a little treatment, but the quality is not stable.
(II) Influence of forming process on the performance and service life of pipe fittings
“Pipe forming” is the most important step in pipe production process. With the increasing of stress in the process of pipe processing, the accuracy of pipe is easy to change under the action of stress.
At present, most of the tube manufacturers mainly adopt the “cold extrusion” and “bulging” processes in the forming process. There are differences in the size, wall thickness and function of pipe fittings due to different production processes. Therefore, it is very important to adopt what kind of molding process to maintain the stability of quality and improve the production efficiency. Here are the characteristics of the two processes.
1. “Cold extrusion” process: put the stainless steel blank tube after blanking into the die cavity, apply pressure to the blank through the fixed punch on the press, so as to deform the stainless steel blank tube, and obtain the size requirements of the pipe fittings. Main features:
- (1) The surface is smooth. Generally, the size accuracy of extrusion parts can reach 8-9 grades, if lubrication is used, it is next to polished surface. Generally, the surface of parts manufactured by extrusion does not need to be reprocessed.
- (2) The auxiliary means of cold extrusion process configuration can meet the product quality requirements. For example, the “U-groove” and “three-way pull” of pipe fittings are formed, and the wall thickness of the “U-groove” and “three-way pull” is not reduced through the way of feeding during the extrusion process, so as to ensure the quality of products.
- (3) Cold extrusion will cause tensile stress of pipe fittings. The cold extrusion process has the characteristics of cold deformation. In the process of extrusion, the metal blank is in the state of three-dimensional pressure, and the metal structure is modified after deformation.
2. “Bulging” process: the bulging process is to apply high-pressure liquid, gas or solid soft die inside the tube blank to force the plastic change of the tube, so as to meet the requirements of the external dimension of the workpiece. Main features:
- (1) During bulging, the deformation of the material is limited to the fixed mold, and the quality is stable. The workpiece will not wrinkle, and the surface of the formed workpiece is smooth and the rebound is small.
- (2) When bulging, the material is in the stress state of two-way tension. Under the action of the die, the wall thickness of the tube is reduced and the surface area is increased to obtain the required geometry.
- (3) During bulging, the deformation area of the tube is limited in the deformation area, and the tube does not transfer to the outside of the deformation area. For example, the formation of U-shaped groove is realized by local thinning of pipe.
(III) Influence of wall thickness on pipe connection and service life
The wall thickness of thick walled stainless steel pipe is the key to the connection strength of pipe fittings. When the wall thickness is reduced below the safety factor, the reliability must be reduced by several grades. In fact, the wall thickness of pipe fittings is to ensure the strength of the connection of pipe fittings. At the same time, the wall thickness also has the function of anti vibration in various environments, the performance of anti-collision of pipe fittings against external forces, and the service life and anti-corrosion ability in concealed works. This paper focuses on the relationship between the wall thickness of stainless steel pipe fittings and their service life.
1. Influence of wall thickness change on quality during production
In the production process of pipe fittings, there are many processes such as blanking, bending, forming (cold extrusion, bulging), etc. First of all, the main factors affecting the wall thickness are bending, forming and other processes. The wall thickness of pipe fittings will be greatly influenced by the different molding technology.
(1) The influence of elbow on the wall thickness of pipe fittings. In tube bending, the shape change of tube bending is material displacement, which is the surface phenomenon of plastic deformation. The essence of deformation lies in the elongation and thinning outside the tube wall, and the shortening and thickening inside the tube. The thinning amount of tube wall thickness increases with the increase of hardening index.
Therefore, when designing and selecting the material wall thickness, we should consider the elongation of the pipe and the important factors of the thinning of the outer side of the elbow, and use a reasonable wall thickness pipe, otherwise the outer wall thickness after bending and stretching is difficult to ensure the size requirements, and the size of the elbow radius will also have a greater impact on the wall thickness.
(2) The influence of bulging on the wall thickness of pipe fittings. Tube bulging is the forming process of expanding tube along radial direction under the action of pressure. It can not only complete the local expansion of tube blank, but also complete the overall expansion of tube blank. The bulging process can be divided into two deformation modes: “axial shrinkage free” and “axial shrinkage”.
(a) Axial non shrinkage deformation: the bulging part is completely formed by local thinning of the wall thickness of the tube blank. The deformation property is local forming at the upper end of the tube blank, and the wall thickness at the lower end of the tube remains unchanged. For example, the expansion of pipe tee is “axial non shrinkage deformation”.
(b) There is shrinkage deformation in the axial direction: when the wall thickness of the tube blank at the bulging position locally thins, there is also a certain axial free shrinkage of the tube blank. For example, the U-shaped groove of pipe fittings is formed by local thinning, which is “axial shrinkage deformation”.
2. Influence of wall thickness on pipe connection
Clamping connection principle of pipe fittings: make full use of the principle of elastic compression ratio of stainless steel and sealing materials, use clamping tools to press on both sides of U-groove of pipe fittings to make pipe fittings and pipes shrink and deform (compress into hexagon) at the same time, play a role of positioning and fixing, so as to improve the ability of anti drawing and anti rotation, and realize the effective connection of stainless steel pipe fittings.
It can be seen from this that it is connected by crimping shrinkage according to the steel property of stainless steel itself, so the wall thickness of pipe and pipe fittings plays an important role. If the wall thickness of pipe and pipe is not the same, the steel property will be different, so the pressure acceptance force will be uneven.
3. Confirmation and selection of wall thickness in use
The function of pipe fittings is to connect pipes, so that pipe fittings can change direction and diameter. Therefore, in general, pipes and pipe fittings are matched, and the requirements for wall thickness are the same.
- (1) First of all, the specification of pipes and fittings shall be matched;
- (2) The wall thickness of pipes and fittings shall be consistent, which plays an important role in the safety and reliability of crimping and welding;
- (3) The wall thickness outside the elbow shall be checked to see whether it is within the standard range;
- (4) The wall thickness of “three-way pull” and “U-groove” shall be checked. Too thin pipe fitting wall can not meet the strength requirements of crimping, which will leave a hidden danger.
Because the compression type connection relies on the wall thickness to deform and form a seal with the fastener, the thin pipe wall can not meet the strength requirements of the surface compression deformation, and it is easy to produce water leakage or seepage accidents.
(IV) Anti corrosion function of solid solution process and influence of service life
The processing of pipe fittings will affect the fatigue properties of metal parts. The processes that affect the fatigue properties of metal parts include: welding, grinding, forming, etc., which will generate the tensile stress of pipe fittings, thus accelerating the occurrence of metal fatigue failure and reducing the service life of stainless steel. Therefore, the solution (heat treatment) process must be carried out.
- 1. After forming and welding, when the temperature is about 1050 ℃, the stress can be released to restore the stress and intergranular change of stainless steel pipe fittings in the production process, and enhance the resistance to stress and intergranular corrosion.
- 2. Reduce the hardness of stainless steel to below 220hv, improve the fatigue property, improve the plasticity and toughness of stainless steel, and improve the safety performance of pipe fittings in the clamping installation.
- 3. Reduce the natural brightness of stainless steel surface, and make it more beautiful and natural.
1. Control the quality of stainless steel materials in procurement
At present, the quality of raw materials in the market is different and inferior, which should be strictly controlled in the process of purchasing raw materials. First, evaluate and select suppliers, establish evaluation reports and quality records of suppliers, and purchase information shall be clear and complete; second, quality inspection reports of manufacturers shall be available. Third, the materials purchased back to the factory shall be tested and analyzed. It is suggested to go to a large steel plant for purchase.
2. Reasonable process and technical measures to control pipe wall thickness
The biggest advantage of extrusion forming is that the wall thickness of “U-groove” and “three-way draw” is not reduced by feeding during the extrusion process, so as to meet the product quality requirements. The advantages of bulging process are stable processing performance, smooth surface and small rebound, and the disadvantages are the thinning of pipe wall thickness.
Whether it is extrusion or bulging process, to ensure the thickness and quality of the pipe wall is the premise, but also the embodiment of the cost, quality and technical requirements of the enterprise. In the domestic market, there are often manufacturers selling too cheap stainless steel pipes to customers, most of which sacrifice the thickness and strength of the pipe, which brings hidden dangers to the connection and safety of thick walled stainless steel pipe fittings.
3. Solid solution is an indispensable process to improve the quality of pipe fittings
The solution process can eliminate the stress and intergranular change in the production process. It is clear to all that although the solution will increase the cost by 2600-2800 yuan per ton, it is an important measure to improve the corrosion resistance. The solution should be carried out after the pipe forming and processing is completed, so as to really play a protective role, which cannot be omitted.
In the future, China’s stainless steel pipe fitting market will have a rapid growth and develop to many fields. This is the current development trend. There is no doubt that as an enterprise, how to deal with the development, while doing a good job in quality management of stainless steel materials, forming process, wall thickness change and solution (heat treatment) and other quality management, it is also necessary to do a good job in the implementation and guidance of product quality and technical specifications, so that more users It is a systematic project to understand the function, use and maintenance of the products, accelerate the popularization of stainless steel water supply pipe fittings and increase the market share, and promote the application of stainless steel pipe fittings in various fields. It also needs the cooperation and efforts of all links and all aspects.
- 1. Cost and cost performance. The one-time investment cost of stainless steel pipe fitting is about twice that of common galvanized pipe, about 50% higher than that of PPR composite pipe fitting. But if the whole life cycle cost of the building is taken into account, the average cost advantage of stainless steel pipe fitting is very obvious.
- The service life of stainless steel pipe fitting is generally 70 years, the service life of carbon steel pipe fitting is about 10 years, and the cost of replacing the pipeline is much higher than the original investment. Stainless steel pipe fitting is the most suitable pipe for water delivery.
- 2. Wall thickness of stainless steel pipe fittings. From the perspective of safety factors and economy, we suggest that the wall thickness of Series II specified in GB∕T 12459-2017 should be selected for dn15-dn50 stainless steel pipe fitting, and series I specified in GB∕T 12459-2017 should be selected for dn65-dn100 stainless steel pipe fitting, which is the best choice for thick wall stainless steel pipe.
- 3. Protection after installation and use. Stainless steel has very good corrosion resistance. When stainless steel products are placed in micro acid and micro alkali environment for a long time, stainless steel will also be corroded. This kind of corrosion not only corrodes the appearance of stainless steel products, but also corrodes the intergranular structure of stainless steel products. Secondly, high humidity environment also produces corrosion to stainless steel. If stainless steel is in high humidity environment for a long time, it will also cause corrosion to stainless steel products. Third, in the dust environment will also cause product corrosion, reducing the service life of stainless steel pipe fittings.
- 4. Pay attention to water for pipe fitting pressing test. Clean tap water must be used for pressure test. It is strictly prohibited to use engineering sewage or unclean river water, well water or groundwater, which will cause corrosion of stainless steel pipe fittings.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.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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What is the difference between 304 and 316 stainless?
What’s the difference between 304 and 316 stainless steel? The simple answer is304 contains 18% chromium and 8% nickel while 316 contains 16% chromium, 10% nickel and 2% molybdenum. The molybdenum is added to help resist corrosion to chlorides (like sea water and de-icing salts).
What are the different types of fittings?
Fittings are generally used in mechanical and plumbing operations for a number of different purposes. There are many different kinds of fittings, made from a variety of materials: some of the most common types are elbows, tees, wyes, crosses, couplings, unions, compression fittings, caps, plugs and valves.
How do I order a pipe tee?
Pipe tees are used to connect (3) pipe pipes and can be used to branch off the main pipe pipe with equal or smaller pipe sizes. The proper way to read a pipe tee is Side x Side x Center (Top). For example, a 3/4″ x 1/2″ x 1″ pipe tee fitting is the same as 1/2″ x 3/4″ x 1″.
How do you size a tee fitting?
Tees are generally measured from the “Run” to the “Bull”. The “Run” is the left-to-right or the length of the Tee. The “Bull” is the top part of the Tee. For example if a Tee is 3/4″ X 3/4″ X 1/2″, then the left inside diameter is 3/4″, the right inside diameter is 3/4″ and the top inside diameter part is 1/2″.
What is reducing tee?
Reducing tee is another type of pipe tee used in pipe and tube fittings. Reducing tee, like all pipe tees, is in the shape of T with two ends of equal size and the third one slightly smaller. In other words, reducing tee is a type of pipe fitting in which the branch port size is smaller than the ports of the run.
What is a branch tee?
Branch tee (plural branch tees) A tee pipe fitting with female connections on the two straight ends and a male connection on the branch.
How many types of elbow are there?
Elbow mainly available in two standard types 90° and 45°. However, it Can be cut to any other degree. Elbows are available in two radius types, Short radius (1D) and Long radius (1.5D).
How do I identify a pipe fitting?
Measure the Outside Diameter (OD) of your pipe or pipe fitting:
1. Wrap a string around the pipe.
2. Mark the point where the string touches together.
3. Use a ruler or measuring tape to find the length between the tip of The string and the mark you made (circumference).
4. Divide the circumference by 3.14159.