Materials and classification of metal pipe fittings

Materials and classification of metal pipe fittings

Pipe fittings play the role of changing the direction, elevation or diameter, closing the pipe end and leading the branch pipe from the main pipe in the pipe system. In petrochemical plants, there are many types of pipelines, complex pipe systems, various shapes, and varying degrees of simplicity and complexity. There are also many types, materials, and quantities of pipe fittings used. When selecting, it is necessary to consider the specific usage.

Classification of metal pipe fittings

There are many types of pipe fittings. Here we introduce the classification method based on purpose, connection method, processing, and material.

(1) Classification by purpose ① Pipe fittings used to connect pipes to each other include: flanges, unions, pipe clamps, ferrules, throat clamps, etc. ② Pipe fittings that change the direction of pipes: elbows, elbows. ③ Pipe fittings that change the diameter of pipes: reducers (reducing pipes), reducing elbows, branch platforms, and reinforcement pipes. ④ Pipe fittings that increase pipeline branches: tees and crosses. ⑤ Pipe fittings used for pipeline sealing: gaskets, pipe plugs, blind plates, heads, and welding plugs. ⑥ Pipe fittings used for pipeline fixing: clamps, towing hooks, lifting rings, brackets, brackets, pipe clamps, etc.

(2) According to the connection method, they are divided into threaded pipe fittings, ferrule pipe fittings, clamp pipe fittings, socket pipe fittings, adhesive pipe fittings, hot-melt pipe fittings, and rubber ring connection pipe fittings.

(3) According to the processing method, they are divided into push-made pipe fittings, pressed pipe fittings, forged pipe fittings, and cast pipe fittings.

(4) According to the material classification, they are divided into cast steel pipe fittings, cast iron pipe fittings, stainless steel pipe fittings, plastic pipe fittings, PVC pipe fittings, rubber pipe fittings, graphite pipe fittings, forged steel pipe fittings, PPR pipe fittings, alloy pipe fittings, PE pipe fittings, and ABS pipe fittings. This article classifies pipe fittings according to the material from the perspective of material welding performance: carbon steel pipe fittings, alloy steel pipe fittings, stainless steel pipe fittings, non-ferrous metal pipe fittings, and special purpose pipe fittings.

Carbon steel

Carbon steel pipe fittings are the most widely used pipe fittings products at present. Iron alloys with a carbon content (mass fraction) between 0.02% and 2.04% are collectively referred to as steel. The commonly used steel is carbon steel, and its main materials are 10, 20, Q235-A, 20g, 20G, 16Mn, ASTM A234, ASTMA105, and ASTM A403. The main products include carbon steel elbows, carbon steel flanges, carbon steel tees, carbon steel crosses, carbon steel reducers (large and small heads), carbon steel heads, etc.

Cast steel

Carbon steel pipe fittings are mostly cast steel parts, and cast steel (caststeel) is a general term for iron-based alloys used to produce castings that do not undergo eutectic transformation during the solidification process. Cast steel is an alloy with iron and carbon as the main elements, and the carbon content is 0-2%. 3.2.3 Cast carbon steel Cast steel with carbon as the main alloying element and a small amount of other elements is cast carbon steel. Cast low carbon steel has a carbon content of less than 0.25%, cast medium carbon steel has a carbon content of 0.2% to 0.6%, and cast high carbon steel has a carbon content of more than 0.6%. As the carbon content increases, the strength of cast carbon steel increases and the hardness increases. Cast carbon steel has high strength, plasticity and toughness, and low cost.

Advantages of steel castings

(1) Greater design flexibility This design has the greatest freedom of choice for the shape and size of the casting, especially complex shapes and hollow parts. In addition, steel castings can be manufactured by the unique process of core castings, which are easy to form and change shape and can be quickly produced according to drawings. Finished products can provide fast response and shorten delivery time.

(2) The strongest flexibility and variability in metallurgical manufacturing. Different chemical compositions and organizational structures can be selected to meet the needs of different projects. The required mechanical properties and process properties can be obtained through different heat treatment processes.

(3) Improve overall structural strength Due to the high reliability of the project, coupled with weight reduction design and shorter delivery time, it can improve competitive advantages in terms of price and economy.

(4) Wide range of weight variation Small steel castings may weigh only 10g, while large steel castings may weigh several tons, tens of tons or even hundreds of tons. Compared with forged steel parts: The mechanical properties of steel castings are similar in all directions, which is superior to forged steel parts. Designers must consider the performance of materials in three directions when designing some high-tech products, which highlights the advantages of castings. Regardless of weight, volume and the amount produced at one time, steel castings can easily make complex shapes and non-stress concentration parts. Compared with welded structures, welded structures are more flexible than forged steel parts in terms of shape and size, but compared with steel castings, they have the following disadvantages. ① Easy to deform during welding. ② It is difficult to form a streamlined structure. ③ High internal stress during welding. ④ The weld affects the appearance and reliability of the part. Compared with iron castings and other alloy castings: Cast steel parts can be used in a variety of working conditions, and their mechanical properties are worse than other alloy castings. The vibration absorption, wear resistance and mobility of steel castings are not as good as those of iron castings, and the cost is also higher than that of iron castings.

Stainless steel pipe fittings

Stainless steel pipe fittings can also be called stainless steel pipe joints. They are commonly used connecting parts in pipelines, which play the role of connecting pipelines, changing the direction of pipelines, and connecting pipelines. When pipe joints are connected by threads, the specifications of threaded pipe joints are nominally marked with the outer diameter of the steel pipe, because the connection of the pipe joint is based on the outer diameter. Under various pressures, the outer diameter of the steel pipe and the outer dimensions of the pipe joint are the same, while the wall thickness and inner diameter are different. Under low pressure, the wall thickness of the pipe joint is relatively thin and the inner diameter is relatively large; under high pressure, the wall thickness of the pipe joint is relatively thick and the inner diameter is relatively small. Under the same flow rate, the flow velocity is slightly higher at high pressure than at low pressure. Although the internal pressure loss of the pipe increases slightly at high pressure, the relative value of the pressure loss does not increase, and in most cases it decreases. Therefore, it is based on the outer diameter and is called a pipe joint. Stainless steel pipe fittings include elbows, tees, crosses, reducers (tapered joints), flanges, flanged joints and pipe clamps. Except for flanges, these pipe fittings can be made of stainless steel plates, which are welded after blanking (cutting). The pipe fittings processed in this way are called pressed welded pipe fittings. This method can be used to process and produce pipe fittings such as elbows, tees, crosses, reducers (tapered joints), etc. Its biggest advantage is uniform wall thickness, short weld length, and the same service life as steel pipes. It is a product developed for the recycling system pipelines and main steam pipelines of atomic power generation to reduce welding points, improve reliability, and save inspection time. It is made of seamless pipes or welded pipes by high-frequency heating and bending. If a rough pipe is used, the elbow is processed by simmering under heating or normal temperature conditions, and the wall thickness of the back of the elbow will be reduced, affecting the use effect. In order to prevent the back wall of the elbow from thinning, rough pipes with different wall thicknesses of special shapes can be used for bending. Obviously, other integrated pipe fittings are welded by welding pipe fittings and steel pipes. With the development of stainless steel pipes, stainless steel pipelines are being used in the fields of drinking water purification, drinking water, cold water, hot water, seawater, gas, medical gas, etc. As a result, stainless steel compression pipe fittings have also been widely used. For this reason, the design, manufacture and acceptance of pipe fittings also have corresponding standards, such as GB/T 19288.1-2003 "Stainless steel compression fittings", GB/T19228.2-2003 "Thin-walled stainless steel pipes for stainless steel compression fittings" and CJ/T152-2001 "Stainless steel compression fittings". Classification of stainless steel pipe fittings Stainless steel pipe fittings are a type 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 heads, etc. The main stainless steel materials are 304, 304L, 316L, 316L, etc.

According to the connection method, they can be divided into three categories

(1) Socket welding stainless steel pipe fittings Socket welding stainless steel pipe fittings (stainless steel pipe fittings) are pipe fittings that insert stainless steel pipes into the socket part of the pipe fittings and then perform argon arc welding along the edge of the pipe fittings. They are the most reliable connection method among all connection methods. According to the different welding parts of the pipe fittings, they can be further divided into three types: bevel edge extension, step edge extension and no extension. The specifications of socket welding stainless steel pipe fittings are generally DN15~DN100.

(2) Butt welding stainless steel pipe fittings Butt welding stainless steel pipe fittings are pipe fittings that are welded along the seam of the stainless steel pipe and the pipe fitting with a circle of argon arc welding. Generally, the welding groove needs to be processed. Butt welding pipe fittings are generally used for welding stainless steel pipeline systems with larger specifications.

(3) Pressed stainless steel pipe fittings Pressed pipe fittings have the characteristics of reliable and safe connection, convenient and fast construction, suitable for embedded installation, maintenance-free update, and relatively superior economy. The working principle of press fittings is to insert a thin-walled stainless steel pipe into the socket of the press fittings, and use a special press tool to clamp the stainless steel pipe in the pipe fittings. The cross-sectional shape of the clamping part is hexagonal. The stainless steel pipe and the pipe fitting are sealed by an O-ring, which makes it have the characteristics of anti-leakage, anti-pulling, anti-vibration and high pressure resistance. Therefore, it is a more advanced connector in direct drinking water system, tap water system, heating system, steam system, industrial oil pipe system and industrial gas pipe system. Suitable for water, oil, gas and other pipeline connections.

Uses of stainless steel pipes

① Stainless steel elbows are used where pipes turn.

② Stainless steel flanges are used to connect pipes to each other and connect to the pipe ends.

③ Stainless steel tees are used where three pipes converge.

④ Stainless steel four-way pipes are used where four pipes converge.

⑤ Stainless steel reducers are used where two pipes of different diameters are connected.

Precautions when welding stainless steel pipe fittings

① Be sure to use a power supply with vertical external characteristics, and use positive polarity for direct current (welding wire connected to negative pole).

② Generally suitable for welding thin pipes below 6mm, with beautiful weld formation and small welding deformation.

③ The shielding gas is argon with a purity of 99.99%. When the welding current is 10-50A, the argon flow rate is 8-15L/min; when the current is 50-250A, the argon flow rate is 12-15L/min.

④ The length of the tungsten electrode extending from the gas nozzle is preferably 4-5mm, 2-3mm in places with poor shielding such as corner welding, and 5-6mm in places with deep grooves. The distance from the nozzle to the workpiece generally does not exceed 5mm.

⑤ To prevent the appearance of welding pores, the welding parts must be cleaned if there is rust, oil, etc.

⑥ The welding arc length is preferably 2-4mm when welding ordinary steel, and 3mm when welding stainless steel. If it is too long, the protection effect is not good.

⑦ When butt-bonding, in order to prevent the back of the bottom weld from being oxidized, gas protection is also required on the back.

⑧ In order to make the argon gas protect the welding molten pool well and facilitate welding operations, the center line of the tungsten electrode and the weldment should generally maintain an angle of 80°-85°, and the angle between the filler wire and the workpiece surface should be as small as possible, generally around 0°. ⑨ Windproof and ventilation. In windy places, screens should be set up, and appropriate ventilation measures should be set up indoors.

Nonferrous metal pipe fittings

Nonferrous metal pipe fittings are mostly made of pipes or steel plates bent and welded on the spot. For steel pipes below DN125, when the working pressure is less than 0.6MPa (gauge pressure) and no disassembly is required, independent pipe fittings are mostly not used, but short pipes are bent and directly welded to the pipeline; if the pressure is high or frequent disassembly and cleaning is required, independent pipe fittings should be made and then connected to the pipeline with flanges. Nonferrous metal pipe fittings mainly include copper pipe fittings, aluminum pipe fittings, titanium pipe fittings, nickel pipe fittings, hot-dip galvanized pipe fittings, etc.

Copper pipe fittings

Copper pipe fittings refer to pipe fittings such as joints, flanges, and tees made of copper metal materials. As the zinc content of brass pipe fittings increases, its color gradually changes from golden red to yellow, golden yellow, and white. The zinc content is generally not more than 50%. It has high plasticity and strength, good formability, and is widely used. It can be used to make various complex cold-stamped and deep-stamped parts, waveguide and condenser pipe fittings, and mechanical and electrical parts.

(1) Classification ① According to function. It is divided into sleeve joints, reducers, 90° elbows, 45° elbows, tees, reducing tees, crosses, reducing crosses, unions, flanges, and pipe clamps. ② According to connection method. It is divided into socket welding, threaded connection, and flange connection. ③ According to material. It is divided into red copper, brass, and bronze. ④ According to purpose. It is divided into water pipe fittings, air conditioning pipe fittings, refrigeration pipe fittings, and water heating pipe assemblies.

(2) Inspection of the positive and negative tolerances of the diameter of standard pipe fittings, wall thickness standards, socket length, inner and outer wall finish, pressure and temperature.

(3) Material composition Pipe fittings should be made of copper with the grade of UNS C10200, C12000 or C12200, or copper alloy with the grade of UNS C23000. The allowable stress can be found in the corresponding ASME standard. Other copper and copper alloys are allowed as long as the chemical composition meets the following requirements: w (Cu) ≥ 84%, w (Zn) ≤ 16%, and the pipe fittings produced by copper alloy meet all mechanical and corrosion resistance requirements of the final use of the pipe fittings. Copper alloys should not contain components that are prohibited from being combined with pipes or other pipe fittings.

Aluminum pipe fittings The relative advantages of aluminum pipe fittings are as follows.

(1) Light weight: Aluminum pipe fittings are lighter than other materials, which can save transportation and processing costs.

(2) Good strength: The mechanical properties of aluminum pipe fittings are not as good as steel, but the strength is higher. Alloy elements such as copper, magnesium, and manganese can be added to make aluminum alloys and then heat treated to obtain high strength.

(3) Beautiful, suitable for various surface treatments: Aluminum pipe fittings and their alloy pipe fittings have an oxide film on the surface, which is silvery white and relatively beautiful. If oxidized, the oxide film on the surface will be stronger, and it can also be dyed and painted to create a surface of various colors and gloss.

(4) Good corrosion resistance: Aluminum and aluminum alloy pipe fittings can form a hard and dense oxide film on the surface, and many substances do not corrode it.

(5) Chemical resistance: Aluminum pipe fittings do not react with chemicals such as nitric acid, glacial acetic acid, and hydrogen peroxide, and have very good chemical resistance.

(6) No magnetism: Aluminum pipe fittings are non-magnetic.

(7) Non-toxic: Aluminum itself is non-toxic, so aluminum pipe fittings are more suitable for industries such as medical machinery and food processing.

(8) Low temperature resistance: When the temperature is low, the strength of aluminum pipe fittings increases instead of being brittle, so it is an ideal material for low-temperature devices.

Titanium pipe fittings

(1) Classification of titanium pipe fittings Titanium pipe fittings, titanium welded pipe fittings, titanium spliced tees, titanium spliced elbows, titanium welded rings, titanium reducers, titanium tees, titanium elbows, titanium flanged butt welding rings.

(2) Working principle of titanium pipe fittings Titanium pipe fittings are pipes that connect various titanium equipment and are used for material flow between various equipment. Titanium pipe fittings have the corrosion resistance of titanium itself, so they can replace general pipes and be used in places where ordinary pipes cannot meet the needs. Generally, pipes above ф108mm are titanium welded pipes.

(3) Characteristics of titanium pipe fittings ① Excellent corrosion resistance in many media. ② Low density, high strength, and light equipment weight. ③ Smooth surface, no dirt, and greatly reduced dirt coefficient.

(4) Uses of titanium pipe fittings are widely used in chemical, petroleum, metallurgy, light industrial machinery, instrumentation, power generation, seawater desalination, medical equipment, chlor-alkali salt production, electroplating, environmental protection, copper industry and other industries.

(5) Material TA1/TA2/TA9/TA10/TC4.

(6) Advantages The biggest advantage of titanium pipe fittings is that they can be 100% recycled, which is environmentally friendly, energy-saving and resource-saving. In addition, the density of titanium alloy pipe fittings is low, which greatly reduces the actual weight of the product and is convenient for use in many industries.

Nickel pipe fittings

(1) Characteristics Excellent corrosion resistance to hot concentrated alkali solutions; excellent corrosion resistance to alkaline and neutral solution media of salts such as carbonates, nitrates, oxides and acetates; resistant to corrosion by fatty acids.

(2) Uses Applied to the alkali industry, chlor-alkali chemicals and organic chloride production, food processing industry, high-temperature halogen and salt corrosion environment, electronic instrument parts, water treatment, and various strong alkali-resistant equipment.

Hot-dip galvanized pipe fittings 

Hot-dip galvanized pipe fittings refer to pipe fittings that have been galvanized.

(1) Classification Hot-dip galvanized pipe fittings include galvanized elbows, galvanized flanges, galvanized elbows, galvanized reducers, and galvanized heads.

(2) Characteristics Thick galvanized layer, with advantages such as uniform coating, strong adhesion and long service life. The pipe base undergoes complex physical and chemical reactions with the molten plating solution to form a corrosion-resistant zinc-iron alloy layer with a tight structure. The alloy layer is integrated with the pure zinc layer and the pipe base, so it has strong corrosion resistance. Electrogalvanizing has low cost, but the surface is not very smooth, and its corrosion resistance is much worse than that of hot-dip galvanized pipes. The main purpose of galvanizing is to enhance the corrosion resistance of the pipe itself.

(3) Uses Hot-dip galvanized pipes are mainly used in pipeline projects such as gas and heating. If the steel pipe body needs to be galvanized for corrosion protection, its connecting pipes must also be galvanized.

(4) Production process The main production process of hot-dip galvanized pipes: degreasing → rinsing → pickling → cleaning → dipping in plating agent → hot air drying → hot-dip galvanizing → internal and external blowing → cooling → passivation and rinsing.