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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.
Austenitic stainless steel, as its name implies, has an austenite structure. The heat treatment of austenitic stainless steel is very important because the important task of austenitic stainless steel is corrosion resistance. If the heat treatment is improper, its corrosion resistance will be greatly reduced. This article mainly tells you about it. Heat treatment of austenitic stainless steels.
Austenitic stainless steel is a common stainless steel (18-8 steel). For example, many tableware in the kitchen are made of austenitic stainless steel. Austenitic stainless steel, as its name implies, has an austenite structure. It is non-magnetic and has no hardenability.
Austenitic stainless steel has very strong corrosion resistance in oxidizing environments. The so-called oxidizing environment can be simply understood as an environment containing more oxygen. Austenitic stainless steel has good toughness and is easy to process and form, so it has a wide range of uses.
Austenitic stainless steel is mainly used for corrosion resistance purposes, and heat treatment has a great influence on it. The corrosion resistance and acid resistance of austenitic stainless steel mainly depend on surface passivation. If surface passivation cannot be maintained, it will corrode.
Therefore, austenitic stainless steel is not completely stainless, it is only suitable for oxidizing environments and acidic environments. It does not have strong resistance to special ions. The heat treatment of austenitic stainless steel mainly affects the passivation ability of the surface layer, thus affecting its corrosion performance.
304 stainless steel polarization curve, anode passivation zone appears
Uniform corrosion is the most common corrosion phenomenon, and uniform corrosion depends on the uniform distribution of chromium elements. Heat treatment affects the distribution of chromium elements, which naturally affects the uniform corrosion resistance of austenitic stainless steel.
Intergranular corrosion is also one of the important corrosion properties for evaluating austenitic stainless steel. Generally speaking, if austenitic stainless steel is sensitized and a large number of bead-like carbides precipitate at the grain boundaries, its intergranular corrosion performance will be greatly reduced.
If austenitic stainless steel is sensitized, severe intergranular corrosion will occur even in a very ordinary electrochemical environment.
Stress corrosion cracking is the most common failure mode of austenitic stainless steel. Everyone needs to note that stress corrosion cracking depends on two main factors:
First, there must be stress, which may be applied stress or residual stress;
Second, stress corrosion cracking-sensitive ions, such as halogen ions, especially chloride ions, are the most common.
Where austenitic stainless steel is used, its ability to withstand stress is often not used, so special attention should be paid to residual stress because in an environment containing chloride ions, residual stress will cause stress corrosion cracking. The method to remove residual stress is stress relief annealing.
Pitting corrosion is the most terrifying form of corrosion. It is said to be the most terrifying corrosion, and it is most appropriate to use a saying from the ancients to describe this problem: “A dike of a thousand miles collapses in an ant nest.”
There are two main reasons why pitting corrosion occurs:
First, if the material composition is uneven, such as sensitization, austenitic stainless steel is particularly prone to pitting corrosion;
Second, the concentration of environmental corrosive media is uneven, which is also the cause of pitting corrosion.
Once pitting corrosion occurs, the local passivation film layer will be destroyed, and there will be competition between the active and passivation states. Once passivation cannot occur, pitting corrosion will continue until the component is perforated.
Austenitic stainless steel has no solid phase transformation point at room temperature to high temperature. The main purpose of heat treatment is to dissolve the carbides generated during processing into the matrix, thereby making the distribution of alloy elements more uniform.
Heating the austenitic stainless steel to a high temperature to dissolve the carbides into the matrix, and then quickly cooling it to room temperature. During this process, the austenitic stainless steel will not harden because there is no phase transformation and the austenitic state will remain at room temperature. This process It’s called solid solution treatment.
In solid solution treatment, the purpose of rapid cooling is only to make the distribution of carbon atoms and alloying elements more uniform.
During solid solution treatment of austenitic stainless steel, if the cooling rate is too slow, as the temperature drops, the solubility of carbon atoms in the matrix decreases, and carbides will precipitate. Moreover, carbon atoms are particularly easy to combine with chromium to form M23C6 carbides, which are distributed on the grain boundaries. Chromium depletion occurs in the grain boundaries and sensitization occurs.
After sensitization occurs in austenitic stainless steel, it should be heated above 850ºC. The carbides will dissolve into a solid solution, and then rapid cooling can solve the sensitization problem.
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.
Seamless steel pipe insulation work is more common in the refrigeration system in a project, doing a good job of this work can better protect the normal work of refrigeration equipment, and improve the energy efficiency of the system. So what details should be noted in the refrigeration seamless steel pipe insulation project?
Seamless steel pipe insulation project preparation work includes:
1. In the material preparation Purchase insulation materials that meet the requirements, such as polyurethane insulation steel pipe, insulation elbow tee, and other pipe fittings, valves, and so on.
2. Tool preparation Need to prepare power supply, welding machine, insulation thickness measurement tools, etc..
3. Construction environment preparation, including cleaning up the construction area, to ensure that the construction site is clean and tidy, to avoid adverse effects on the insulation construction.
The seamless steel pipe insulation project includes the following steps:
1. Seamless steel pipe lifting. Need to be according to the actual situation of the site, according to the requirements of the seamless steel pipe lifting up, so that the welding workers in the welding group will be more convenient.
2. Welding of insulation steel pipe. Before welding, we should fully understand whether the pipe needs to be ultrasonic, flaw detection, etc. In the absence of ultrasonic and flaw detection welding process will be much simpler, but if it is necessary for ultrasonic or flaw detection, it is also necessary to pipe sub-arc welding priming.
In refrigeration throughout the seamless steel pipe insulation insulation project, the safety issue is crucial. Construction personnel must wear work clothes helmets and other items according to the requirements, and strictly do a good job of protective measures, according to the relevant operational specifications for implementation. And to be regularly on the state of the insulation layer and insulation layer of all-round inspection, in order to timely maintain and repair, so that the refrigeration system achieves a longer operating cycle.
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:
Aluminum-titanium alloy profiles add alloy elements to industrial pure titanium to improve the strength of titanium. Titanium alloys can be divided into three types: titanium alloy, b titanium alloy, and a+b titanium alloy. ab titanium alloy is composed of a and b dual phases. This type of alloy has a stable structure, good high-temperature deformation performance, toughness, and plasticity. It can be quenched and aged to strengthen the alloy.
The performance characteristics of titanium alloy are mainly reflected in:
1)High specific strength. Aluminum-titanium alloy profiles have a low density (4.4kg/dm3) and are light in weight, but their specific strength is greater than ultra-high-strength steel.
2) High thermal strength. Aluminum-titanium alloy profiles have good thermal stability, and their strength is about 10 times higher than that of aluminum alloys at 300 to 500°C.
3) High chemical activity. Titanium can produce strong chemical reactions with oxygen, nitrogen, carbon monoxide, water vapor, and other substances in the air, forming TiC and TiN hardened layers on the surface.
Poor thermal conductivity. Titanium alloy has poor thermal conductivity. The thermal conductivity of titanium alloy TC4 at 200℃ is l=16.8W/m·℃, and the thermal conductivity is 0.036 cal/cm·s·℃.
Analysis of machining characteristics of aluminum-titanium alloy profiles
First of all, the thermal conductivity of titanium alloy is low, only 1/4 of steel, 1/13 of aluminum, and 1/25 of copper. Because the heat dissipation in the cutting area is slow, it is not conducive to thermal balance. During the cutting process, the heat dissipation and cooling effect are very poor, and it is easy to form high temperatures in the cutting area. After processing, the parts deform and rebound greatly, resulting in increased cutting tool torque and rapid edge wear. Durability reduced. Secondly, the thermal conductivity of titanium alloy is low, which makes the cutting heat accumulate in a small area around the cutting tool and is not easy to dissipate. The friction on the rake face increases, making it difficult to remove chips. The cutting heat is not easy to dissipate, which accelerates tool wear. Finally, titanium alloys are highly chemically active and tend to react with tool materials when processed at high temperatures, forming coatings and diffusions, resulting in phenomena such as sticking, burning, and breakage.
The selection of tool materials should meet the following requirements:
Sufficient hardness. The hardness of the tool must be much greater than the hardness of the aluminum-titanium alloy.
Sufficient strength and toughness. Since the cutting tool is subjected to large torque and cutting force when cutting aluminum-titanium alloy, it must have sufficient strength and toughness.
Sufficient wear resistance. Due to the good toughness of titanium alloy, the cutting edge must be sharp during processing, so the tool material must have sufficient wear resistance to reduce work hardening. This is an important parameter when selecting cutting tools for processing titanium alloys.
The affinity between tool materials and titanium alloys is poor. Due to the high chemical activity of aluminum-titanium alloys, it is necessary to prevent the tool material from forming an alloy with the aluminum-titanium alloys by dissolving and diffusing, causing sticking and burning of the tools.
When it comes to 904L stainless steel, the first thing that comes to mind is Rolex. Because in the industry, Rolex is the only one all-steel model are uses a 904L stainless steel enterprise, today we will come together to explore the following magic!
“Rolex Steel” 904L.
In fact, in today’s watch world, the main use of 316L stainless steel and 904L stainless steel for watch case production steel, the biggest difference between the two lies in the material content of chromium, 904L stainless steel chromium content is higher!
The 904L stainless steel contains a certain amount of copper, we all know that chromium can help the surface of the metal material to form a passivation film, thereby protecting the surface of the steel from corrosion of external media
We all know that chromium can help the surface of metal materials to form a passivation film, thereby protecting the surface of steel from external media corrosion, to improve the corrosion resistance of steel, and the addition of copper and other rare elements, not only can significantly improve the abrasion and corrosion resistance of steel but also to facilitate the surface of the high degree of polishing so that it can be used with other precious metals.
The gloss of the metal wants to fit; therefore, the price of 904L stainless steel is also much more expensive.
What is so special about 904L stainless steel?
Rolex first produced this 904L stainless steel case in 1985 and gradually replaced it with the brand’s full range of standard equipment. Let’s talk about the special features of 904L stainless steel.
Currently, 316L stainless steel is commonly used in the watch industry. 316L stainless steel is commonly known as “medical steel”, due to its hypoallergenic properties, not only for the production of watch cases but also used to make personal jewelry and medical scalpels. 904L stainless steel is the most common stainless steel used in the watch industry.
904L stainless steel is based on 316L stainless steel to make some changes, in composition, 904L stainless steel in chromium, nickel, and molybdenum content than 316L stainless steel content of 1.6 times more, while 904L stainless steel
More copper content. Therefore, 904L stainless steel is more wear-resistant, more corrosion-resistant and heavier. But there is not much difference in hardness. Designed for environments with harsh corrosive conditions, the alloy was originally developed for
developed to resist corrosion in dilute sulfuric acid. I don’t think any watch enthusiast would throw their watch in a dilute sulfuric acid bath!
For everyday seawater corrosion, 316L stainless steel is perfectly adequate 904L stainless steel is indeed superior in terms of corrosion resistance compared to 316L stainless steel, but that doesn’t mean 316L stainless steel isn’t superior. The simplest proof
The simplest proof is that, before Rolex also used 316L stainless steel, just later replaced by 904L stainless steel, while other watch brands in the past and now have used 316L stainless steel, after all, the general brand even if you want to use
After all, even if the general brand wants to use 904L stainless steel can not manage the high cost of manufacturing.
What are the performance characteristics of S38815 high silica stainless steel?
S38815 stainless steel is a special high alloy stainless steel with unique properties and wide applications. In this article, we will introduce the characteristics of S38815 stainless steel and application areas.
First of all, S38815 stainless steel has excellent corrosion resistance. It can resist the corrosion of a variety of media, including acids, alkalis, chloride ions, and so on. This corrosion resistance makes it widely used in the chemical industry, marine engineering, oil extraction, and other fields. Whether in harsh environmental conditions or in high-temperature and high-pressure working environments, S38815 stainless steel can maintain its stable performance.
Secondly, S38815 stainless steel has excellent oxidation resistance. It can resist high-temperature oxidation and maintain a low rate of steel corrosion. This property makes S38815 stainless steel very useful in working environments at high temperatures. It is widely used in heat exchangers, furnaces, burners, and other fields, and can ensure the long-term reliable operation of equipment.
In addition, S38815 stainless steel has good strength and toughness. It is able to withstand high loads and impacts and maintain a low deformation rate. This makes S38815 stainless steel widely used in the manufacturing field. It is commonly used to make a variety of parts and structural assemblies, such as in aerospace, automotive, and machinery, and is able to meet the requirements of complex working environments.
In addition, S38815 stainless steel has wear resistance and heat resistance properties. It is able to maintain good mechanical properties at high temperatures and is less susceptible to heat and wear. This makes S38815 stainless steel widely used in metallurgy, coal mining, and the chemical industry. For example, in coal mining, S38815 stainless steel can withstand high temperatures and abrasive environments to ensure the reliability and life of the equipment.
To summarize, S38815 stainless steel is a unique high-alloy stainless steel with excellent corrosion resistance, oxidation resistance, strength, and toughness. It is widely used in the chemical industry, marine engineering, oil extraction, aerospace, metallurgy, coal mining, and chemical industry. With the advancement of technology, the application areas of S38815 stainless steel will continue to expand, bringing more innovation and possibilities to various industries.
Often users call to consult the similarities and differences between Hastelloy C276 (N10276) and C22 (N06022) alloys, the two simple, largely similar, slightly different.
First of all, we look at the difference between the two materials:
C276 main components: 57NI-16MO-16CR-5FE-4W-2.5CO-1MN-0.35V-0.08SI-0.01C
C22 main components: 56NI-13MO-22CR-3FE-3W-2.5CO-0.5MN-0.35V-0.08SI-0.01C
As can be seen from the above, the main elements do not differ much, especially in NI, FE, CO content, Mo and Cr are slightly different.
C22 is more resistant to localized corrosion than 276 and can be understood as an upgraded version of 276.
In addition, “special steel 100 seconds” also lists the difference between the two welding consumables:
C276 welding consumables are wire ERnicrmo-4 and electrode Enicrmo-4.
C22 welding consumables are wire ERnicrmo-10 and electrode Enicrmo-10.
1, from the ease of procurement: C276 market spot is easier to purchase some of the more complete specifications, while C22 alloy stockists only one or two.
2, from the use of: C22 is better, but generally speaking, both can be used in most working conditions.
3, from the cost of materials: the main components are not very different, according to costing should be about the same. Even if C22 alloy (8.7) density is less than C276 alloy (8.9), should be cheaper than C276 cost, but because C22 can not form a large-scale production and supply, the cost is higher than C276.
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