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What are the precautions for stainless steel plate processing?

Choose suitable processing methods and tools
1. Suitable processing methods should be used when processing stainless steel plates, such as cutting, forming, welding, etc. Different processing methods require the use of different tools and equipment, which should be selected according to specific circumstances.

2. Use special stainless steel knives and do not mix them to avoid iron powder contamination or accelerated tool wear.

3. Select the appropriate coolant to ensure the service life and processing effect of the tool.

Ensure the surface of the board is clean before processing
1. Before processing, the oil and dust on the surface of the plate should be cleaned to ensure the processing quality.

2. Drill small holes before cutting or drilling to prevent tool vibration from damaging the surface finish.

3. Do not touch the board directly with your hands during processing to avoid leaving fingerprints and stains.

Control processing temperature
1. The processing temperature of stainless steel plates should be controlled below 400°C. Excessive temperature may cause plate deformation, oxidation, annealing, and other problems.

2. The cutting speed of stainless steel plates should be moderate to avoid processing difficulties if it is too slow, and affecting the quality of the cut if it is too fast.

Ensure the surface quality after processing
1. Clean the oil stains and oxide scale on the surface of the plate promptly after processing to ensure surface finish and anti-corrosion.

2. For plates that require surface treatment, such as polishing, passivation, etc., they should be carried out before processing to avoid affecting the processing quality.

3. When processing stainless steel plates, attention should be paid to the surface quality after processing, and scratches or dents should be processed in time to ensure the appearance quality.

Everyone is familiar with the intergranular corrosion and stress corrosion cracking problems of austenitic stainless steel.

The intergranular corrosion tendency test of stainless steel is common content in design documents, and the relevant content in standards such as HG/T 20581 is also relatively clear. The hydrostatic test or the chloride ion content in the operating medium is also a basic concern when designing austenitic stainless steel equipment. In addition to chloride ions, wet hydrogen sulfide, polythionic acid, and other environments that may produce sulfides can also cause stress corrosion cracking of austenitic stainless steel.

It is worth mentioning that although austenitic stainless steel is not mentioned in the chapter on wet hydrogen sulfide corrosion in HG/T 20581, the reference literature points out that austenitic stainless steel has a much greater ability to dissolve atomic hydrogen than ferritic steel. , but hydrogen-induced wet hydrogen sulfide stress corrosion cracking will still occur, especially after the deformation martensitic structure transformation occurs during cold work hardening.

Cold work hardening increases stress corrosion cracking susceptibility

Austenitic stainless steel has excellent cold working properties, but its work hardening is very obvious. The greater the degree of cold working deformation, the higher the hardness rises. Increased hardness due to work hardening is also an important cause of stress corrosion cracking in stainless steels, especially those in the base metal rather than the weld.

There are some cases below:

The first type of case is after austenitic stainless steel is cold-spinning to process an elliptical or disc-shaped head, the cold deformation in the transition zone is the largest, and the hardness also reaches the highest. After it was put into use, chloride ion stress corrosion cracking occurred in the transition zone, causing equipment leakage.

The second type of case is a U-shaped corrugated expansion joint made by hydroforming after rolling stainless steel plates. The cold deformation is the largest at the wave crest, and the hardness is also the highest. The most stress corrosion cracking occurs along the wave crest, and cracks occur along a circle of wave crests. Explosion accident involving low-stress brittle fracture.

The third type of case is stress corrosion cracking of corrugated heat exchange tubes. Corrugated heat exchange tubes are cold extruded from stainless steel seamless tubes. The crests and troughs are subject to varying degrees of cold deformation and thinning. The crests and troughs may cause several stress corrosion cracks.

The essence of cold work hardening of austenitic stainless steel is the generation of deformation martensite. The greater the cold working deformation, the more deformation martensite is produced and the higher the hardness. At the same time, the internal stress inside the material is also greater. if solid solution heat treatment is performed after processing and forming, the hardness can be reduced and the residual stress can be greatly reduced. At the same time, the martensite structure can also be eliminated, thereby avoiding stress corrosion cracking.

Embrittlement problems caused by long-term service at high temperatures

Currently, Cr-Mo steel with higher high-temperature strength is the main material for containers and pipes at temperatures between 400 and 500°C, while various austenitic stainless steels are mainly used at temperatures between 500 and 600°C or even 700°C. In design, people tend to pay more attention to the high-temperature strength of austenitic stainless steel and require that its carbon content not be too low. The allowable stress at high temperatures is obtained by extrapolating the high-temperature endurance strength test, which can ensure that no creep rupture will occur during 100,000 hours of service under the design stress.

However, the problem of age embrittlement of austenitic stainless steel at high temperatures cannot be ignored. After long-term service at high temperatures, austenitic stainless steel will undergo a series of structural changes, which will seriously affect a series of mechanical properties of the steel, especially the brittleness It increased significantly and the toughness decreased significantly.

The embrittlement problem after long-term service at high temperatures is generally caused by two factors, one is the formation of carbides, and the other is the formation of σ phase. The carbide phase and σ phase continue to precipitate along the crystal after the material has been in service for a long time and even form continuous brittle phases on the grain boundaries, which can easily cause intergranular fracture.

The formation temperature range of the σ phase (Cr-Fe intermetallic compound) is approximately 600 to 980°C, but the specific temperature range is related to the alloy composition. The result of the precipitation of σ phase is that the strength of austenitic steel increases significantly (the strength may be doubled), and it also becomes hard and brittle. High chromium is the main reason for the formation of the high-temperature σ phase. Mo, V, Ti, Nb, etc. are alloy elements that strongly promote the formation of σ phase.

The formation temperature of carbide (Cr23C6) is in the sensitization temperature range of austenitic stainless steel, which is 400~850 ℃. Cr23C6 will dissolve above the upper limit of the sensitization temperature, but the dissolved Cr will promote the further formation of the σ phase.

Therefore, when austenitic steel is used as heat-resistant steel, the understanding and prevention of high-temperature aging embrittlement should be strengthened. Just like the metal monitoring in thermal power plants, the metallographic structure and hardness changes can be regularly inspected. If necessary, samples can be taken out for metallographic and hardness inspections, and even comprehensive mechanical properties and durable strength tests can be conducted.

The importance of preheating before welding and post-weld heat treatment

Preheat before welding

Preheating before welding and heat treatment after welding are very important to ensure welding quality. The welding of important components, the welding of alloy steel and the welding of thick parts all require preheating before welding. The main functions of preheating before welding are as follows:

(1) Preheating can slow down the cooling rate after welding, facilitate the escape of diffuse hydrogen in the weld metal, and avoid hydrogen-induced cracks. At the same time, it also reduces the hardening degree of the weld and heat-affected zone and improves the crack resistance of the welded joint.

(2) Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also called temperature gradient) between the workpieces to be welded in the welding area. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is helpful to avoid welding cracks.

(3) Preheating can reduce the constraint of welded structures, especially in corner joints. As the preheating temperature increases, the crack incidence rate decreases.

The selection of preheating temperature and interlayer temperature is not only related to the chemical composition of the steel and welding rod, but also related to the rigidity of the welding structure, welding method, ambient temperature, etc., and should be determined after comprehensive consideration of these factors. In addition, the uniformity of the preheating temperature in the thickness direction of the steel plate and the uniformity in the weld area have an important impact on reducing welding stress. The width of local preheating should be determined according to the restraint condition of the workpiece to be welded. Generally, it should be three times the wall thickness around the weld area, and should not be less than 150-200 mm. If the preheating is uneven, instead of reducing the welding stress, it will increase the welding stress.

Post weld heat treatment

The purpose of post-weld heat treatment is threefold: eliminate hydrogen, eliminate welding stress, and improve the weld structure and overall performance.

Post-weld hydrogen elimination treatment refers to the low-temperature heat treatment performed after the welding is completed and the weld has not yet cooled to below 100°C. The general specification is to heat to 200~350℃ and keep warm for 2-6 hours. The main function of post-weld hydrogen elimination treatment is to accelerate the escape of hydrogen in the weld and heat-affected zone, and is extremely effective in preventing welding cracks during welding of low-alloy steel.

During the welding process, due to the uneven heating and cooling, as well as the constraints or external constraints of the component itself, welding stress will always be generated in the component after the welding work is completed. The existence of welding stress in components will reduce the actual load-bearing capacity of the welded joint area and cause plastic deformation. In severe cases, it will also cause damage to the component.

Stress relief heat treatment is to reduce the yield strength of the welded workpiece under high temperature to achieve the purpose of relaxing the welding stress. There are two commonly used methods: one is overall high-temperature tempering, that is, placing the entire weldment into a heating furnace, slowly heating it to a certain temperature, then keeping it warm for a period of time, and finally cooling it in the air or in the furnace. This method can eliminate 80%-90% of welding stress. Another method is local high-temperature tempering, that is, only heating the weld and its surrounding area, and then slowly cooling it to reduce the peak value of the welding stress and make the stress distribution gentler, thereby partially eliminating the welding stress.

After welding of some alloy steel materials, the welded joints will have hardened structures, which will deteriorate the mechanical properties of the materials. In addition, this hardened structure may cause joint damage under the action of welding stress and hydrogen. If after heat treatment, the metallographic structure of the joint is improved, the plasticity and toughness of the welded joint are improved, thereby improving the comprehensive mechanical properties of the welded joint.

Things to note when bending stainless steel plates

1. The thicker the stainless steel plate, the greater the required bending strength. As the plate thickness increases, the bending strength must be adjusted accordingly when adjusting the bending machine.

2. In unit size, the greater the tensile strength of the stainless steel plate, the smaller the elongation, and the required bending strength and bending angle must also be greater.

3. The thickness of the stainless steel plate in the design drawing corresponds to the bending radius. Experience shows that the developed size of the bent product is the right-angled side minus the sum of the thicknesses of the two plates, which meets the design accuracy requirements.

4. The higher the yield strength of stainless steel, the stronger the elastic recovery. In order to achieve a 90° angle in the curved section, the required tableting angle must be reduced.

5. Compared with carbon steel, stainless steel with the same thickness has larger bending angles and requires special attention, otherwise bending cracking will occur and affect the strength of the workpiece.
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I wonder how much you gold fans know about seamless steel pipes? Seamless steel pipe is a round, square, or rectangular steel material with a hollow cross-section and no seams around it. Seamless steel pipes are made from steel ingots or solid tube blanks that are perforated into capillary tubes, and then hot-rolled, cold-rolled, or cold-drawn. Seamless steel pipes have hollow cross-sections and are widely used as pipes for transporting fluids. Compared with solid steel materials such as round steel, steel pipes are lighter in weight when the bending and torsional strength are the same. They are an economical cross-section steel and are widely used in manufacturing structures. parts and mechanical parts, such as steel scaffolding for oil drills, etc.

Development history of seamless steel pipe
Seamless steel pipe production has a history of nearly 100 years.
The German Mannesmann brothers first invented the two-roller cross-rolling piercing machine in 1885, and the cycle pipe rolling machine in 1891. In 1903, the Swiss R.C. Stiefel invented the automatic pipe rolling machine (also called the top rolling machine). pipe machine), and later various stretching machines such as continuous pipe rolling machines and pipe jacking machines appeared, and the modern seamless steel pipe industry began to form.

In the 1930s, the variety and quality of steel pipes were improved due to the adoption of three-roll pipe rolling machines, extruders, and periodic cold-rolled pipe machines. In the 1960s, due to the improvement of continuous pipe rolling machines and the emergence of three-roll piercing machines, especially the success of applying tension reducers and continuous casting billets, production efficiency was improved and the ability of seamless pipes to compete with welded pipes was enhanced. In the 1970s, seamless pipes and welded pipes were keeping pace with each other, and world steel pipe production was increasing at a rate of more than 5% per year.
After 1953, China attached great importance to the development of the seamless steel pipe industry and initially formed a production system for rolling various large, medium, and small pipes. Copper pipes also generally use ingot cross-rolling and perforation, pipe rolling machine rolling, and coil drawing processes.

The uses and classification of seamless steel pipes
Purpose: Seamless steel pipe is an economical cross-section steel that plays an important role in the national economy and is widely used in petroleum, chemical industry, boilers, power stations, ships, machinery manufacturing, automobiles, aviation, aerospace, energy, geology, construction and various sectors such as military industry.

Classification:
① According to the cross-section shape: circular cross-section pipe, special-shaped cross-section pipe

②According to material: carbon steel pipe, alloy steel pipe, stainless steel pipe, composite pipe

③ According to the connection method: threaded connection pipe, welded pipe

④According to production method: hot-rolled (extruded, topped, expanded) pipes, cold-rolled (drawn) pipes

⑤According to use: boiler pipes, oil well pipes, pipeline pipes, structural pipes, fertilizer pipes…

Seamless steel pipe production process
① The main production processes of hot-rolled seamless steel pipes (main inspection processes):

Preparation and inspection of tube blanks → Heating of tube blanks → Perforation → pipe rolling → Reheating of waste pipes → determining (reducing) diameter → Heat treatment → Straightening of finished pipes → Finishing → Inspection (non-destructive, physical and chemical, Taiwan inspection) → warehousing

②The main production processes of cold-rolled (drawn) seamless steel pipes

Blank preparation→pickling and lubrication→cold rolling (drawing)→heat treatment→straightening→finishing→inspection

The production process flow chart of hot-rolled seamless steel pipe is as follows: