Introduction and analysis of stainless steel pipe fittings

Stainless steel pipe fittings are mainly used to connect various pipes into pipes. With the continuous development of national economic construction, the application of stainless steel pipe fittings is becoming more and more extensive. Through the analysis of the corrosion resistance of the non-rust surface passivation layer, combined with advanced Production process and equipment application, producing high-quality stainless steel clamps to meet the needs of production and life, to fill the domestic gap.
20180809120907 68777 - Introduction and analysis of stainless steel pipe fittings
Stainless steel pipe fittings are one kind of pipe fittings. They are made of stainless steel, so they are called stainless steel pipe fittings. They include: stainless steel elbows, stainless steel tees, stainless steel crosses, stainless steel reducers, stainless steel caps, etc., according to the coupling method. It is divided into four types: socket type stainless steel pipe fittings, threaded stainless steel pipe fittings, flanged stainless steel pipe fittings and welded stainless steel pipe fittings. The stainless steel elbow is used for the turning of the pipe; the flange is used to connect the pipe to the pipe, the pipe is connected to the pipe end, the stainless steel tee is used for the collection of the three pipes; the stainless steel pipe is used for the collection of the four pipes. Place; stainless steel reducer for the connection of two pipes of different pipe diameters.
Stainless steel joints have been widely used in various fields of national economy construction such as pharmaceutical, food, beer, drinking water, bioengineering, chemical engineering, air purification, aviation, nuclear industry, etc., and have had a major impact on the production and life of the people.
Stainless steel pipe fittings materials are recognized as healthy materials and are widely used in food processing conveyors, including the pharmaceutical industry, beverages, brewing, etc. The stainless steel pipe material has superior performance and effectively reduces the transportation cost. It will not scale during the long-term transportation process, and the inner wall is smooth and clean, and the transportation energy consumption is low. It is the water pipe material with the lowest transportation cost so far. In addition, it is also used in medicine, such as various human steel brackets, steel nails in human bodies, artificial bones, artificial teeth, etc. Stainless steel pipe fitting materials are almost the only healthy materials that can be implanted into the human body.

Stainless steel has a unique corrosion resistance that reacts with the oxidant to form a thin, dense oxide film that prevents oxidation from proceeding further. For other types of metal pipes, almost no passivation function, which is one of the main reasons why the corrosion resistance of stainless steel pipe materials exceeds that of ordinary metal pipes. At the same time, stainless steel is also an environmentally friendly material. Water pipes made of stainless steel will not cause secondary pollution to water quality and meet health requirements. The stainless steel water pipe is a water pipe that can be completely recycled, leaving no untreated garbage to the children and grandchildren. The strength of the stainless steel water pipe is higher than that of any water pipe material, which greatly reduces the water leakage caused by the external force of the water pipe and effectively saves water resources.

Introduction and analysis of the use of stainless steel pipe fittings
Stainless steel pipe fittings are made of stainless steel products made of SUS310S, SUS316, 316L, 304, 304L, 321, 303, 303CU, 301, 202, 201, etc. It is a product of pipe fittings.
Stainless steel pipe fittings are mainly made of socket welding, butt welding and clamping.
First: socket welded stainless steel pipe fittings
The socket welded stainless steel pipe fitting is a pipe fitting which inserts a stainless steel pipe into the pipe socket portion and then performs argon arc welding along the edge of the pipe member, and is the most reliable connection method among all the connection modes.
Second: butt welded stainless steel pipe fittings
Butt welding stainless steel pipe fittings is a pipe fitting that makes a argon arc welding of the stainless steel pipe and the pipe fittings opposite to each other. Generally, welding welding is required.
Third: the card type stainless steel pipe fittings
The working principle of the clamped pipe fitting is to insert the thin-walled stainless steel pipe into the socket of the snap-in pipe fitting, and the stainless steel pipe is stuck in the pipe fitting by using a special pressing tool, and the cross-sectional shape of the pressing portion is hexagonal, and the stainless steel pipe is combined with There is an O-ring seal between the pipe fittings to make it leak-proof, pull-resistant, shock-resistant and high-pressure resistant.


Characteristics of stainless steel pipe fittings
1, life
Stainless steel tubing has the longest life. From the analysis of the use of stainless steel abroad, the service life of stainless steel water pipes can reach 100 years, at least 70 years, as long as the life of buildings.
2, corrosion resistance
One of the most outstanding advantages of stainless steel tubing is its excellent corrosion resistance, which is the best among all kinds of tubing. Because stainless steel can passivate with the oxidant, a tough and dense chromium-rich oxide protective film Dr2O3 is formed on the surface, which effectively prevents the further oxidation reaction. Other metal pipes, such as galvanized water pipes and copper pipes, have little passivation ability. This is the key reason why the corrosion resistance of copper pipes of galvanized pipes is far less than that of stainless steel pipes.
Stainless steel does not have uniform corrosion like carbon steel, no need to protect the coating when used; stainless steel water pipes have no restrictions on the chemical composition of water, because stainless steel is very good in water with various oxygen content, temperature, pH and hardness. Corrosion resistance; stainless steel water pipes can withstand high flow rates, even at flow rates greater than 40 m / s, still maintain a very low corrosion rate, no more than 0.003 mm / year, especially suitable for high-rise water supply.
3, heat and insulation
The thermal conductivity of stainless steel pipes is 1/25 of that of copper pipes, which is 1/4 of ordinary steel pipes, and is especially suitable for hot water transportation. The most commonly used stainless steels in the water industry are 304 and 316 stainless steels, which are suitable for most water treatment and transfer conditions.
4, strength
The strength of the material determines whether the water pipe is strong and crash resistant, safe and reliable. Safety and reliability are the most important requirements for building water supply. In the case of impact by external force, the possibility of water leakage from stainless steel pipes is extremely small. The working pressure of high-rise water supply system is generally greater than 0.6Mpa; the pressure on pipes is higher. Due to its excellent mechanical properties, thin-walled stainless steel pipes can withstand high water supply pressures up to 10Mpa, which is especially suitable for high-rise water supply.


Processing methods of stainless steel pipe fittings
20180809123008 35671 - Introduction and analysis of stainless steel pipe fittings

1. Forging method: The end or part of the pipe is punched out by a swaging machine to reduce the outer diameter. The common forging machine has a rotary type, a link type and a roller type.
2. Stamping method: The pipe end is expanded to the required size and shape with a tapered core on the punching machine.
3. Roller method: The core is placed in the tube, and the outer circumference is pressed by the roller for round edge processing.
4. Rolling method: Generally, the mandrel is not used, and it is suitable for the inner edge of the thick-walled pipe.
5. Bending forming method: There are three methods that are more commonly used. One method is called stretching method, the other method is called pressing method, the third method is roller method, there are 3-4 rollers, two fixed rollers, one adjusting roller, and adjustment. With a fixed roll gap, the finished elbow is curved.
6. Inflating method: one is to place rubber in the tube, and the upper part is compressed by a punch to make the tube protruding and formed; the other method is to form a hydraulic bulge, fill the middle of the tube with liquid, and the liquid pressure drums the tube into the required The shape, the production of bellows is mostly used in this way.


Description of passivation process for stainless steel pipe fittings:
As long as the items belonging to the stainless steel pipe, such as electroless plating or other coating requirements, are generally passivated before pretreatment (including pickling, black skin, polishing, etc.), in order to be used or assembled into parts.
Passivation improves the thermodynamic stability of stainless steel in environmental media, prevents localized corrosion of stainless steel, provides sufficient cleanliness on stainless steel surfaces, and eliminates hot work oxides on stainless steel surfaces.
Austenitic and ferritic stainless steels have a relatively uniform structure and good passivation. The martensitic stainless steel is strengthened by heat treatment, and the metallographic structure is multi-phase structure, so the passivation is not strong.
The smooth surface of the polished surface of stainless steel is better, and the surface of the workpiece obtained by casting, sand blasting, etc. is rough, which is not conducive to passivation.
The passivation process of stainless steel pipe fittings can be divided into wet and dry processes. The passivation effect of stainless steel depends on both the passivation process and the stainless steel material itself. The specific influencing factors are the elements contained in stainless steel, the stainless steel metallographic structure, and the processing state of stainless steel.
Among the constituent elements, chromium and nickel are elements with strong passivation, and the passivation of iron is second. Therefore, the higher the content of chromium and nickel, the stronger the passivation of stainless steel.


Effective measures to prevent corrosion of stainless steel pipe fittings
1. Start with the material of the pipeline and strictly control the quality of raw materials.
Strict entry inspection of stainless steel pipe and stainless steel pipe fittings. To ensure that the materials meet the requirements. In terms of material selection, from the perspective of economy and function, under special operating conditions, nickel-containing austenitic stainless steel, duplex stainless steel, titanium and titanium alloy, glass steel, etc. with stronger corrosion resistance can be considered.
When selecting materials, increase the corrosion margin of pipe and stainless steel tee fittings to meet the service life requirements of the piping system.
2, surface coating protection
At present, in view of the corrosion of stainless steel elbow pipes in the environment, in addition to the selection of materials with strong corrosion resistance, it is also possible to improve the corrosion resistance by coating the surface of stainless steel pipes. In view of the relatively smooth surface of stainless steel pipe substrate, the conventional stainless steel reducer anti-corrosion coating has poor adhesion on the surface. At present, an effective coating for stainless steel anticorrosion for marine environment has been developed – epoxy powder coating. . The coating is a new coating that is completely solvent-free and spray-formed into a film.
3. Electrochemical protection
Electrochemical protection methods mainly include protection of the protector, cathodic protection and anodic protection.
(1) The protector protection method refers to connecting a metal having a lower electrode potential to a metal to be protected to form a primary battery. A metal having a lower electrode potential is dissolved as an anode, and a protected metal serves as a cathode to avoid corrosion.
(2) Cathodic protection method refers to the application of a DC power supply, connecting the protected metal to the negative electrode as a cathode, so that the potential of the protected stainless steel flange metal is negatively adjusted to enter the non-corrosive zone of the E-pH diagram; On the positive electrode, a method of protecting the cathode by etching the scrap metal as an anode.
(3) The anode protection method refers to the use of an external DC power supply to connect the protected metal to the anode on the positive electrode, and the potential of the electrode moves in the positive direction (ie, the anode is polarized), and enters the passivation region of the E-pH diagram to be protected. The metal is “passivated” to be protected.


Analysis of electrochemical corrosion of stainless steel pipe fittings
Electrochemical corrosion mainly occurs in industrial stainless steel pipe fittings. Since these pipe fittings are often in contact with electrochemical articles, the following conditions are mainly caused:
1. Carbon steel pollution: Scratches and corrosive media caused by contact with carbon steel parts form galvanic cells and cause electrochemical corrosion.
2. Cutting: adhesion and corrosion of rust-prone substances such as slag and splashing form a galvanic cell to cause electrochemical corrosion.
3. Baking school: The composition and metallographic structure of the flame heating area change and are not uniform, and the original battery is formed with the corrosive medium to produce electrochemical corrosion.
4. Welding: physical defects (biting, pores, cracks, unfused, incomplete penetration, etc.) and chemical defects (grain coarse, grain boundary chromium deficiency, segregation, etc.) and corrosive medium forming the primary battery to generate electricity Chemical corrosion.
5. Material: Chemical defects in stainless steel (uneven composition, S, P impurities, etc.) and physical defects on the surface (loose, blisters, cracks, etc.) are conducive to electrochemical corrosion caused by the formation of galvanic cells with corrosive media.
6. Passivation: The acid passivation passivation effect is not good, resulting in uneven or thin passivation film on the surface of stainless steel, which is easy to form electrochemical corrosion.
7. Cleaning: The acid-washed passivation residue remaining and the chemically corroded product of stainless steel form electrochemical corrosion with the stainless steel piece.


Common defects in the quenching process of stainless steel pipe fittings
In the heat treatment process of stainless steel pipe fittings, a careless, stainless steel pipe fittings often have some defects, thus affecting the quality of the heat treatment of the product, and even appear to be scrapped, so effective measures must be taken to prevent it, and the existing defects must be saved. The following focuses on the analysis of common defects in the quenching process of stainless steel pipe fittings, and the corresponding preventive and remedial measures.
Defect: overheating and overburning
Cause: The heating temperature is too high or the heating time is too long


Preventive and remedial measures:
1. Correct selection of quenching heating temperature and heating time
2. Regularly check the temperature of the side temperature meter to prevent the instrument from malfunctioning and overheating.
3. Reasonably install the furnace to prevent the workpiece from being too close to the heating body


Defect: quenching deformation
cause:
1. The shape of the stainless steel casting workpiece is asymmetrical or thick and disparity
2. Mechanical processing stress is large, not eliminated before quenching
3. Uneven heating and cooling
4. The workpiece is heated and clamped and improperly cooled
5. Transformation of quenched structure
Preventive and remedial measures:
1. Improve the structure of the workpiece, reasonably select materials, adjust the machining allowance, etc.
2. Increase the preheating or stress relief annealing process and carry out reasonable preliminary heat treatment
3. Multiple preheating, pre-cooling quenching, two-liquid quenching, step quenching, isothermal quenching, etc.
4. Proper operation, reasonable support, bundling and heating of the workpiece, dispersion cooling
5. Perform the school on the deformed workpiece.


Passivation treatment of stainless steel pipe fittings
In our current life, stainless steel pipe fittings can be said to be used very much, but you know that the stainless steel pipe fittings that you just got can’t be used right away. Do you know how to passivate advanced? For stainless steel parts, such as electroless plating or other coating requirements.
It must be passivated before pretreatment (including pickling, black skin, polishing, etc.) before it can be used or assembled into parts. After passivation, it can improve the thermodynamic stability of stainless steel in environmental media, prevent local corrosion of stainless steel, and make stainless steel surface with sufficient cleanliness.
It also eliminates hot working oxides on stainless steel surfaces. The passivation process of stainless steel can be divided into wet method and dry method. Specifically, it can be divided into many types.
Here are the Borden stainless steel fittings to introduce the operation method of pickling passivation in the wet method:
Washing is a method of dissolving rust, oxide film and other products on the surface of the workpiece by chemical reaction. This method does not affect the base metal, and the purpose is to decontaminate the surface of the workpiece to achieve the purpose of purification. Passivation is the use of chemical reactions to form a dense oxide film on the surface of the workpiece.
Methods. The purpose is to establish an oxide film or an oxygen adsorption layer on the surface of the workpiece, thereby preventing the electrochemical corrosion from proceeding and improving the corrosion resistance (electrochemical corrosion resistance) of the metal.
I. Construction procedures
1, preparation work
(1) Before pickling and passivation, the welding scale, splash, burrs, dirt, etc. of the pipe fittings must be cleaned.
(2) The oil on the surface of the pipe can be wiped clean with organic solvents such as gasoline and acetone. For safety reasons, a large area of oil can be cleaned with steam or a 3 to 5% caustic soda (NaOH) solution, then rinsed with water and allowed to dry. Note that the CL-ion concentration of the used water should not exceed
Over 25mg/l.
(3) Prepare the equipment, tools and labor insurance products required for pickling and passivation operations (take the three people as an example).
1 equipment and tools
A. Acid-resistant pickling and passivation tank;
B. 3 stainless steel wire brush or hard plastic nylon brush;
C. One measuring cup, measuring cylinder, table scale and stirring rod;
D. 3 acid-resistant mops.
2 labor protection supplies
A. 3 pairs of acid-resistant long-sleeve rubber shoes;
B. Acid-resistant rubber gloves 3 pay;
C. 3 sets of acid-resistant overalls, work caps and masks;
D. Glasses 3 pay.
2, pickling, passivation materials and preparation of pickling, passivation solution
(1) Pickling and passivation material preparation
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 are the elements contained in stainless steel, the stainless steel metallographic structure, and the processing state of stainless steel. Among the constituent elements, chromium and nickel are elements with strong passivation, and the passivation of iron is second. 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. The martensitic stainless steel is strengthened by heat treatment, and the metallographic structure is multi-phase structure, so the passivation is not strong.
When we use stainless steel pipe fittings, in many cases, special process installation is required, for example, partial welding is required. Depending on the requirements, the method of connection may be different, so that it is more conducive to the use of stainless steel pipe fittings. The standard parts network shares the common connection methods for stainless steel pipe fittings.
1. The stainless steel pipe is arc-returned by the reflow method, and the arc is used to talk about the full arc pit. It should be completed in the slope of the arcing, and it is not possible to arc and arc on the surface of the pipe and the pipe. Cracks, pores and other phenomena are found during the arcing process and should be disposed of in a timely manner.
2. Stainless steel pipe fittings should be separated from other metals during lifting to avoid direct contact. It can be isolated by non-metallic materials such as padded wood and rubber sheets.
3. When the stainless steel pipe fittings are continuously welded, the temperature should not exceed 60 degrees Celsius. During tack welding, the inside of the weld should be protected by argon. When cutting, you can’t use ordinary grinding wheel cutting. You should use stainless steel pipe special grinding wheel or plasma cutting.
4. When connecting stainless steel pipe fittings and non-stainless steel pipes and fittings, the butt joints of the welded joints should be filled with argon gas before the bottom welding, and then the tungsten argon arc welding can be performed. The welds at the joints are to be pickled and passivated after welding.

Analyze the combination method and development situation of stainless steel pipe fittings
Analyze the method of combining stainless steel pipe fittings
(1) Threaded joint of stainless steel clamp pipe fittings (50mm and below)
Stainless steel pipe fittings cut the pipe end flat, trim the burrs, and remove the dust. Use the appropriate thread cutting tool to cut into the pipe thread with the bevel. When joining, first clean the thread surface, in the male thread part. PTFE tape] [coating lead oxide and glycerin mixture, adding linseed] [coated graphite-containing lubricant] [other approved thread joint agent] [other approved method], screwed into the female thread Tighten to prevent water leakage. The depth of the thread and the length shall be in accordance with the standard regulations. The number of threads outside the tube after the pipe is joined shall not exceed three.
(2) Stainless steel clamping pipe fittings for welding (65mm or more)
Stainless steel pipe welding shall be performed by argon (TIG) welding and construction according to welding specifications. In addition to careful selection of welders and welding rods, attention should be paid to pre-weld treatment of pipes. For pipe wall thickness [3mm] [] and above, V-shaped welds shall be opened. The butt weld depth is about 1/2 of the plate thickness. The V-shaped opening has the same welding depth as the plate thickness. Welds shall be continuous and shall not be interrupted, and the first and last joints shall overlap [10mm][]. When welding the flange, the pipe insertion flange should have the same thickness as the pipe end and the bottom of the pipe. The flange face and the pipe contact part should be open, welded on both sides, one side of the flange is welded to the pipe end, and the other side is welded to the pipe end. The outer wall of the tube.
(3) stainless steel clamp pipe fitting flange joint joint
Analysis of the development of stainless steel pipe manufacturing industry
Stainless steel tubes are available in a variety of ways. Common types of fittings are compression, compression, union, push, push thread, socket weld, flange connection, welding and welding combined with traditional connections. Series connection method. These connection methods vary according to their principles, but most of them are easy to install, reliable and reliable. The sealing ring or gasket material used for the connection is mostly made of silicone rubber, nitrile rubber and EPDM rubber which meet the requirements of national standards, which eliminates the worries of users.
Stainless steel pipes have been developed from single austenitic stainless steels to martensitic and ferritic duplex steels and high alloy steels; the products range from seamless stainless steel pipes, welded pipes, thin-walled stainless steel pipes, etc., depending on the classification. Stainless steel tube. The stainless steel pipe basically covers the specifications of various countries in the world, with more than 100 varieties, and the products are used in many fields such as industrial and civil.
Through statistical analysis and probabilistic analysis, 25% of the 70,000 tons produced last year can be reliably used in petroleum, chemical, fertilizer, boiler and aerospace, nuclear power and other industries. About 60% can be used in food, brewing, medicine, paper, printing and dyeing, urban environmental protection and other industries. There are still about 20% of non-conforming products or available products. The main problem is that the chemical composition of the steel pipe exceeds the allowable deviation; the purity of the steel is not enough, and the impurities are exceeded, resulting in unqualified grain size; Intergranular corrosion is unqualified; internal and external surface defects are serious, heavy skin, slag inclusions, internal folds are not removed, and severe walling. Industrial welded pipes have fewer defects; large and medium-sized industrial pipes have unsatisfactory roundness and straightness; decorative welded pipes have serious problems such as excessive component difference, pipe diameter and excessive wall diameter.
The automotive industry has a lot of demand for stainless steel pipes. According to estimates, only the Sino-US joint venture BUICK sedan and Shanghai Volkswagen’s Santana B5 luxury sedan consume five or six thousand tons of stainless steel tubes. The aerospace industry needs 200 tons of precision stainless steel tubes and high temperature full gold tubes. The annual demand for stainless steel pipes in electronics, instrumentation, metallurgy and other industries is around 5,000 tons.
Stainless steel elbow beverage, alcohol, food, textile, printing and dyeing, light industry and medical and pharmaceutical industries, used in equipment manufacturing and maintenance, etc., consumes more than 20,000 tons of stainless steel pipes per year.
Stainless steel pipes and their water delivery equipment are the most advanced basic water purification materials in the world. Some cities have begun to use stainless steel pipes for fluid transportation such as water and gas. Wuhan City has invested heavily in the extensive use of stainless steel pipes and equipment, and has upgraded the main water purification pipes and water collection tanks of 13 water plants, and will also be in new high-rise residential buildings and office buildings in the shortest possible time. Water is supplied by stainless steel pipes. Stainless steel pipes have also been used in the water supply pipes of some residents in Ningbo and Daqing. The future of civilian stainless steel pipe field will be a huge market.


Calculation method of stainless steel pipe fittings:
(1): Calculation method of cold drawn seamless rectangular steel pipe: W (kg / m) = side length * 4 * thickness * 0.00785
(2): The density of steel is 1.85kg/dm2, and the formula for calculating the weight per metre of steel pipe is: W(kg/m)=0.02466*wall thickness*(outer diameter-wall thickness)
(3): Formula for calculating the weight per metre of stainless steel pipe: W (kg / m) = 0.02491 * wall thickness * (outer diameter – wall thickness)

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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

Please notice that you might be interested in the other technical articles we’ve published:

  • Dimensions of Pipe Fittings

  • Specifications of Steel Pipe Fittings

  • Development trend of high pressure pipe fittings

  • How to test pipe fitting

  • How to Calculate a Pipe Bend

  • Stainless steel pipe bend

  • How to get high quality pipe fittings

References:

  1. https://www.yaang.com

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