Selection of stainless steel welding materials for pressure vessels
Stainless steel for pressure vessel and its welding characteristics
The so-called stainless steel means that after adding a certain amount of chromium into the steel, the steel is in a passive state and has the characteristics of no rust. To achieve this, the chromium content must be more than 12%. In order to improve the passivation of steel, nickel, molybdenum and other elements that can passivate steel often need to be added to stainless steel. Generally speaking, stainless steel is actually the general name of stainless steel and acid resistant steel. Stainless steel is not necessarily acid resistant, but acid resistant steel generally has good rust resistance.
Stainless steel can be divided into four categories according to its steel structure, namely austenitic stainless steel, ferritic stainless steel, martensitic stainless steel and austenitic – ferritic duplex stainless steel.
Austenitic stainless steel and its welding characteristics
Austenitic stainless steel is the most widely used stainless steel, and high Cr Ni type is the most common. At present, austenitic stainless steel can be roughly divided into Cr18-Ni8, Cr25-Ni20 and Cr25-Ni35. Austenitic stainless steel has the following welding characteristics:
① Welding hot crack
Austenitic stainless steel has small thermal conductivity and large linear expansion coefficient. Therefore, in the welding process, the high temperature residence time at the welded joint is long, and the weld is easy to form coarse columnar crystal structure. In the solidification crystallization process, if the content of impurity elements such as sulfur, phosphorus, tin, antimony and niobium is high, low melting point eutectic will be formed between the crystals. When the welded joint bears high tensile stress, It is easy to form solidification cracks in the weld and liquefaction cracks in the heat affected zone, which belong to welding thermal cracks.
The most effective way to prevent hot crack is to reduce the impurity elements that are easy to produce low melting point eutectic in steel and welding materials and make chromium nickel austenitic stainless steel contain 4% – 12% ferrite structure.
② Intergranular corrosion
According to the theory of poor chromium, chromium carbide precipitated on the intergranular, resulting in poor chromium at the grain boundary is the main reason for intergranular corrosion. Therefore, the selection of ultra-low carbon welding materials or welding materials containing stabilizing elements such as niobium and titanium is the main measure to prevent intergranular corrosion.
③ Stress corrosion cracking
Stress corrosion cracking is usually characterized by brittle failure, and the failure process time is short, so the harm is serious. The main cause of stress corrosion cracking of austenitic stainless steel is welding residual stress. The microstructure change or stress concentration of welded joints and the concentration of local corrosion medium are also the reasons affecting stress corrosion cracking.
④ Of welded joints σ Phase embrittlement
σ Phase is a brittle and hard intermetallic compound, which mainly precipitates at the grain boundary of columnar crystal. γ Phase harmony δ Both phases can occur σ Phase transition. For example, when cr25ni20 weld is heated at 800 ℃ ~ 900 ℃, strong corrosion will occur γ → δ transformation.
For chromium nickel austenitic stainless steel, especially chromium nickel molybdenum stainless steel, it is easy to occur δ → σ. This is mainly due to the obvious phase transformation of chromium and molybdenum σ Chemical action, when in the weld δ When the ferrite content exceeds 12%, δ → σ: the transformation is very obvious, resulting in obvious embrittlement of weld metal, which is why the surfacing layer on the inner wall of hot wall hydrogenation reactor will δ. The reason why the ferrite content is controlled at 3% – 10%.
Ferritic stainless steel and its welding characteristics
Ferritic stainless steel is divided into ordinary ferritic stainless steel and ultra-pure ferritic stainless steel. Ordinary ferritic stainless steel has Cr12 – Cr14 types, such as 00Cr12 and 0Cr13Al; Cr16 – Cr18 type, such as 1Cr17Mo; Cr25 – 30 type.
Due to the high content of carbon and nitrogen in ordinary ferritic stainless steel, it is difficult to process, shape and weld, and the corrosion resistance is difficult to ensure. The use is limited. In ultra-pure ferritic stainless steel, the total amount of carbon and nitrogen in steel is strictly controlled, generally controlled at three levels of 0.035% – 0.045%, 0.030% and 0.010% – 0.015%, At the same time, necessary alloying elements are added to further improve the corrosion resistance and comprehensive properties of the steel.
Compared with ordinary ferritic stainless steel, ultra-pure high chromium ferritic stainless steel has good resistance to uniform corrosion, pitting corrosion and stress corrosion, and is more used in petrochemical equipment. Ferritic stainless steel has the following welding characteristics:
- ① Under the action of high welding temperature, the grains in the heat affected zone with heating temperature above 1000 ℃, especially in the near seam zone, will grow rapidly. Even if the grains are cooled rapidly after welding, the sharp decline of toughness and high tendency of intergranular corrosion caused by grain coarsening cannot be avoided.
- ② Ferritic steel has high chromium content, more harmful elements such as carbon, nitrogen and oxygen, high brittle transition temperature and strong notch sensitivity. Therefore, post weld embrittlement is more serious.
- ③ 475 ℃ embrittlement will occur when heating and slow cooling at 400 ℃ – 600 ℃ for a long time, which will seriously reduce the toughness at room temperature. After heating at 550 ℃ – 820 ℃ for a long time, it is easy to precipitate from ferrite σ Phase, but also significantly reduce its plasticity and toughness.
Martensitic stainless steel and its welding characteristics
Martensitic stainless steel can be divided into Cr13 martensitic stainless steel, low-carbon martensitic stainless steel and super martensitic stainless steel. Cr13 has general corrosion resistance. Martensitic stainless steel based on Cr12 has not only certain corrosion resistance, but also high temperature strength and high temperature oxidation resistance due to the addition of nickel, molybdenum, tungsten, vanadium and other alloy elements.
Welding characteristics of martensitic stainless steel: the hardening tendency of Cr13 martensitic stainless steel weld and heat affected zone is particularly large. The hard and brittle martensite can be obtained in the welded joint under the condition of air cooling. Under the action of welding restraint stress and diffusing hydrogen, it is easy to have welding cold Cracks. When the cooling rate is small, coarse ferrite and intergranular carbide will be formed in the near seam area and weld metal, which will significantly reduce the plasticity and toughness of the joint.
After cooling, the weld and heat affected zone of low-carbon and super martensitic stainless steel are all transformed into low-carbon martensite, but there is no obvious hardening phenomenon and has good welding properties.
Selection of stainless steel welding materials for pressure vessels
Selection of austenitic stainless steel welding materials
The selection principle of austenitic stainless steel welding materials is to ensure that the corrosion resistance and mechanical properties of the weld metal are basically equal to or higher than the base metal under the condition of no Crack. Generally, the alloy composition is required to roughly match the base metal composition. For corrosion-resistant austenitic stainless steel, it is generally expected to contain a certain amount of ferrite, which can not only ensure good Crack resistance, but also have good corrosion resistance. However, in some special media, such as the weld metal of urea equipment, ferrite is not allowed, otherwise its corrosion resistance will be reduced. For heat-resistant austenitic steel, the control of ferrite content in weld metal shall be considered. For Austenitic Steel Weldments operating at high temperature for a long time, the ferrite content in the weld metal shall not exceed 5%. The reader can estimate the corresponding ferrite content according to the chromium equivalent and nickel equivalent in the weld metal according to the Schaeffler diagram.
Selection of ferritic stainless steel welding materials
There are basically three types of ferritic stainless steel welding materials:
- 1) Welding materials whose composition basically matches the base metal;
- 2) Austenitic welding materials;
- 3) Nickel base alloy welding materials are rarely used because of their high price.
The welding material of ferritic stainless steel can be the same material as the base metal, but when the restraint is large, it is easy to produce Cracks. After welding, heat treatment can be used to restore corrosion resistance and improve joint plasticity. The use of austenitic welding materials can avoid preheating and post weld heat treatment, but for various steels without stable elements, the sensitization of heat affected zone still exists. 309 and 310 chromium nickel austenitic welding materials are commonly used. For Cr17 steel, 308 welding materials can also be used. Welding materials with high alloy content are conducive to improving the plasticity of welded joints. Austenitic or austenitic ferritic weld metal is basically as strong as ferritic base metal, but in some corrosive media, the corrosion resistance of weld may be very different from that of base metal, which should be paid attention to when selecting welding materials.
Selection of martensitic stainless steel welding materials
In stainless steel, martensitic stainless steel can adjust its performance by heat treatment. Therefore, in order to ensure the requirements of service performance, especially martensitic stainless steel for heat resistance, the composition of weld should be as close as possible to that of base metal. In order to prevent cold Cracks, austenitic welding materials can also be used. At this time, the weld strength must be lower than that of the base metal.
When the composition of the weld is similar to that of the base metal, the weld and the heat affected zone will harden and become brittle at the same time, and a tempering softening zone will appear in the heat affected zone. In order to prevent cold Cracking, components with a thickness of more than 3mm often need to be preheated, and heat treatment is often required after welding to improve the joint performance. Since the thermal expansion coefficient of weld metal is basically the same as that of base metal, it is possible to completely eliminate the welding stress after heat treatment.
When the workpiece is not allowed to be preheated or heat treated, the austenitic structure weld can be selected. Because the weld has high plasticity and toughness, it can relax the welding stress and more solid solution hydrogen, it can reduce the cold Crack tendency of the joint. However, the joint with uneven material may produce shear stress in the fusion zone under the working environment of circulating temperature due to different thermal expansion coefficient, And cause joint damage.
For simple Cr13 martensitic steel, when the weld with austenitic structure is not used, there is little room for adjustment of weld composition, which is generally the same as the base metal matrix, but harmful impurities S, P and Si must be limited. Si can promote the formation of coarse martensite in the weld of Cr13 martensitic steel. Reducing the content of C is conducive to reducing the hardenability. There are a small amount of Ti, n or Al in the weld, which can also refine the grain and reduce the hardenability.
For multi-component alloyed Cr12 based martensitic hot strength steel, the main purpose is heat resistance. Generally, austenitic welding materials are not used, and the weld composition is expected to be close to the base metal. When adjusting the composition, it must be ensured that the primary ferrite phase does not appear in the weld, because it is very harmful to the performance. Since the main components of Cr13 based martensitic hot strength steel are mostly ferrite elements (such as Mo, Nb, W, V, etc.), in order to ensure that all structures are uniform martensite, austenite elements must be balanced, that is, appropriate elements such as C, Ni, Mn, N, etc.
Martensitic stainless steel has a high tendency of cold cracking, so it must strictly maintain low hydrogen or even ultra-low hydrogen, which must be paid attention to when selecting welding materials.
Key points of stainless steel welding for pressure vessels
Key points of austenitic stainless steel welding
In general, austenitic stainless steel has excellent weldability. Almost all melting welding methods can be used to weld austenitic stainless steel. The key points of welding process are determined by the thermophysical properties and miCrostructure characteristics of austenitic stainless steel.
- ① Because austenitic stainless steel has small thermal conductivity and large thermal expansion coefficient, it is easy to produce large deformation and welding stress during welding. Therefore, the welding method with concentrated welding energy should be selected as far as possible.
- ② Due to the low thermal conductivity of austenitic stainless steel, it can obtain greater penetration than low alloy steel under the same current. At the same time, due to its high resistivity, in order to avoid electrode redness during electrode arc welding, the welding current is smaller than that of carbon steel or low alloy steel electrode with the same diameter.
- ③ Welding specification. Generally, large line energy is not used for welding. During electrode arc welding, small diameter electrode and fast multi pass welding should be used. For welds with high requirements, cold water is even used to accelerate cooling. For pure austenitic stainless steel and super austenitic stainless steel, due to the high sensitivity of thermal Crack, the welding linear energy should be strictly controlled to prevent serious growth of weld grain and occurrence of welding thermal Crack.
- ④ In order to improve the hot Crack resistance and corrosion resistance of the weld, pay special attention to the cleanness of the welding area during welding to avoid harmful elements penetrating the weld.
- ⑤ Preheating is generally not required for austenitic stainless steel welding. In order to prevent grain growth and carbide precipitation in the weld and heat affected zone and ensure the plasticity, toughness and corrosion resistance of the welded joint, the lower interlayer temperature shall be controlled, generally no more than 150 ℃.
Key points of ferritic stainless steel welding
Ferritic stainless steel has more ferrite forming elements, less austenite forming elements, and less hardening and cold cracking tendency. Under the action of welding thermal cycle, the grains in the heat affected zone grow obviously, and the toughness and plasticity of the joint decrease sharply. The degree of grain growth in heat affected zone depends on the maximum temperature reached during welding and its holding time. Therefore, when welding ferritic stainless steel, try to use small linear energy, that is, use energy concentration methods, such as low current TIG, manual welding with small diameter electrode, etc. at the same time, try to use measures such as narrow gap groove, high welding speed and multi-layer welding, and strictly control the interlayer temperature.
Due to the effect of welding thermal cycle, general ferritic stainless steel is sensitized in the high temperature zone of heat affected zone and intergranular corrosion is produced in some media. After annealing at 700 – 850 ℃ after welding, chromium can be homogenized and its corrosion resistance can be restored.
Ordinary high chromium ferritic stainless steel can be welded by welding rod arc welding, gas shielded welding, submerged arc welding and other fusion welding methods. Due to the inherent low plasticity of high chromium steel, grain growth in heat affected zone caused by welding thermal cycle and the agglomeration of carbides and nitrides at grain boundaries, the plasticity and toughness of welded joints are very low. When the welding material with similar chemical composition to the base metal is used and the restraint is large, it is easy to produce Cracks. In order to prevent Cracks and improve joint plasticity and corrosion resistance, the following process measures can be taken by taking electrode arc welding as an example.
- ① Preheat about 100 – 150 ℃ to weld the material in a ductile state. The higher the chromium content, the higher the preheating temperature.
- ② Welding with small linear energy and no swing. During multi-layer welding, the interlayer temperature shall be controlled not higher than 150 ℃, and continuous welding is not suitable to reduce the influence of high temperature embrittlement and 475 ℃.
- ③ Annealing at 750 – 800 ℃ after welding can restore corrosion resistance and improve joint plasticity due to carbide spheroidization and uniform distribution of chromium. After annealing, it shall be cooled quickly to prevent σ Phase and brittleness at 475 ℃.
Key points of martensitic stainless steel welding
For Cr13 martensitic stainless steel, when welding with electrodes of the same material, in order to reduce cold Crack sensitivity and ensure the plasticity and toughness of welded joints, low hydrogen electrodes shall be selected and the following measures shall be taken at the same time:
- ① Warm up. The preheating temperature inCreases with the increase of carbon content of steel, generally in the range of 100 ℃ – 350 ℃.
- ② Afterheat. For welded joints with high carbon content or high restraint, post heating measures shall be taken after welding to prevent hydrogen induced Cracks.
- ③ Post weld heat treatment. In order to improve the plasticity, toughness and corrosion resistance of welded joints, the post weld heat treatment temperature is generally 650 ℃ – 750 ℃, and the holding time is calculated as 1H / 25mm.
For super and low carbon martensitic stainless steel, preheating measures are generally not required. When the restraint is large or the hydrogen content in the weld is high, preheating and post heating measures are taken. The preheating temperature is generally 100 ℃ – 150 ℃ and the post weld heat treatment temperature is 590 – 620 ℃.
For martensitic steel with high carbon content. Or when preheating before welding and post welding heat treatment are difficult to implement, and the constraint of the joint is large, austenitic welding materials can also be used in the project to improve the plasticity and toughness of the welded joint and prevent Cracks. However, when the weld metal is austenitic structure or mainly austenitic structure, it is actually a low strong match compared with the strength of the base metal, and there are great differences in chemical composition, metallographic structure, thermophysical properties and mechanical properties between the weld metal and the base metal. The welding residual stress is inevitable, which can easily lead to stress corrosion or high-temperature creep failure.
Duplex stainless steel has the characteristics of both austenitic stainless steel and ferritic stainless steel because it has austenitic + ferritic duplex structure and the content of the two phases is basically the same. The yield strength can reach 400MPa – 550MPa, which is twice that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher toughness, lower brittle transition temperature, obvious improvement in intergranular corrosion resistance and welding performance; At the same time, some characteristics of ferritic stainless steel are preserved, such as brittleness at 475 ℃, high thermal conductivity, low coefficient of linear expansion, superplasticity and magnetism. Compared with austenitic stainless steel, duplex stainless steel has high strength, especially the yield strength has been significantly improved, and the properties of pitting corrosion resistance, stress corrosion resistance and corrosion fatigue resistance have also been significantly improved.
According to their chemical composition, duplex stainless steel can be divided into Cr18, Cr23 (excluding Mo), Cr22 and Cr25. Cr25 duplex stainless steel can be divided into ordinary and super duplex stainless steel, among which Cr22 and Cr25 are widely used in recent years. Most duplex stainless steels used in China are made in Sweden, and the specific brands are: 3re60 (Cr18), saf2304 (Cr23), SAF2205 (Cr22) and SAF2507 (Cr25).
Welding characteristics of duplex stainless steel
- ① Duplex stainless steel has good weldability, it is not like ferrite stainless steel welding heat affected zone is easy to embrittlement, and not like austenitic stainless steel easy to produce welding heat cracking, but because it has a large number of ferrite, when the rigidity or weld contains high hydrogen, there is a possibility of hydrogen cold cracking, so strict control of the source of hydrogen is very important.
- ② In order to ensure the characteristics of dual-phase steel, to ensure that the organization of the welded joint in the austenite and ferrite ratio is the key to welding this type of steel. When the cooling rate of the welded joint is slow, δ → γ of the secondary phase changes more fully, so to room temperature can be obtained when the phase ratio is more appropriate dual-phase organization, which requires an appropriate amount of welding heat input, otherwise, if the post-weld cooling rate is faster, it will make the δ ferrite phase increases, resulting in a serious decline in the plastic toughness and corrosion resistance of the joint.
Selection of welding materials for duplex stainless steel
The welding material for duplex stainless steel is characterized by the duplex structure dominated by austenite, and the content of main corrosion-resistant elements (chromium, molybdenum, etc.) is equivalent to that of the base metal, so as to ensure the corrosion resistance equivalent to that of the base metal. In order to ensure the content of austenite in the weld, the content of nickel and nitrogen is usually inCreased, that is, the nickel equivalent of about 2% – 4%. In the base metal of duplex stainless steel, there is generally a certain amount of nitrogen content, and it is also expected to have a certain nitrogen content in the welding material, but generally it should not be too high, otherwise pores will occur. In this way, the high nickel content has become a main difference between welding material and base metal.
According to the different requirements of corrosion resistance and joint toughness, select the welding rod matching the chemical composition of the base metal, such as welding Cr22 duplex stainless steel, and Cr22Ni9Mo3 welding rod, such as E2209 welding rod. When acid electrode is used, the slag removal is excellent and the weld formation is beautiful, but the impact toughness is low. When the weld metal is required to have high impact toughness and all position welding is required, alkaline electrode shall be used. Alkaline electrode is usually used for root back sealing welding. When there are special requirements for the corrosion resistance of weld metal, alkaline electrode with super dual phase steel composition shall also be used.
For solid gas shielded welding wire, while ensuring that the weld metal has good corrosion resistance and mechanical properties, attention should also be paid to its welding process performance. For flux cored wire, rutile or titanium calcium flux cored wire can be used when beautiful weld formation is required. When higher impact toughness is required or welding under greater restraint conditions, flux cored wire with higher alkalinity should be used.
For submerged arc welding, the welding wire with smaller diameter should be used to realize multi-layer and multi pass welding under medium and small welding specifications, so as to prevent the embrittlement of welding heat affected zone and weld metal, and the supporting alkaline flux should be used.
Welding points of duplex stainless steel
- ① Control of the welding thermal process welding line energy, interlayer temperature, preheating and material thickness will affect the cooling rate of the weld, thus affecting the organization and properties of the weld and heat affected zone. Cooling rate too fast and too slow will affect the toughness and corrosion resistance of duplex steel welded joints. Too fast cooling rate will cause excessive alpha phase content and increased Cr2N precipitation. Too slow cooling rate will cause serious grain coarsening, and may even precipitate some brittle intermetallic compounds, such as σ-phase. In the choice of line energy should also take into account the specific material thickness, the table in the upper limit of line energy for thick plates, the lower limit for thin plates. When welding duplex and super stainless steels with a high alloy content of ω(Cr) of 25%, it is recommended that the maximum interlayer temperature be controlled at 100°C in order to obtain the best weld metal properties. When the post-weld heat treatment requirements can not limit the interlayer temperature.
- ② Post-weld heat treatment duplex stainless steel after welding preferably without heat treatment, but when the weld state under the α-phase content exceeds the requirements or precipitation of harmful phases, such as σ-phase, post-weld heat treatment can be used to improve. The heat treatment method used is water quenching. Heat treatment should be heated as fast as possible, the holding time at the heat treatment temperature of 5-30min, should be sufficient to restore the phase equilibrium. The oxidation of the metal during heat treatment is very serious, and inert gas protection should be considered. For ω (Cr) for 22% of the duplex steel should be at 1050 ℃ ~ 1100 ℃ temperature heat treatment, while ω (Cr) for 25% of the duplex steel and super duplex steel requires heat treatment at 1070 ℃ ~ 1120 ℃ temperature.
Welding example of stainless steel pressure vessel
The flash tank with diameter of 800mm and wall thickness of 10mm is made of 0Cr18Ni9.
① The diameter of the cylinder is 800mm, and the welder can drill into the cylinder for welding. Therefore, the longitudinal and circumferential seams of the cylinder are welded on both sides by electrode arc welding.
② There is no hole in this equipment, so the closing weld can only be welded from the outside. In order to ensure the welding quality, TIG welding is used for backing. However, the back metal of stainless steel will be oxidized during argon arc welding. In the past, the back argon filling protection can only be used. However, when the equipment is large or the back cannot be protected by argon, a lot of argon will be wasted and poor protection may still occur.
In order to solve this process difficulty, the welding division of Nippon grease company has developed and manufactured a back self protective Stainless Steel TIG welding wire, which is a welding wire with special coating. After melting, the coating (i.e. coating) will penetrate into the back of the molten pool to form a dense protective layer, which is equivalent to the role of electrode coating.
The use method of this welding wire is exactly the same as that of ordinary TIG welding wire. The coating will not affect the front arc and molten pool shape, which greatly reduces the welding cost of stainless steel argon arc welding. In this equipment, if the back argon protection is adopted, the argon waste is serious, so the self-protection welding wire is adopted.
③ For the fillet weld between connecting pipe and flat welding flange, and the fillet weld between connecting pipe and shell, in view of the weld shape and welding conditions of this part, electrode arc welding is generally selected. If the nozzle diameter is too small, TIG welding can also be used to reduce the welding difficulty.
④ The fillet weld between the support and the shell is a non pressure weld, which adopts the gas metal arc welding (the shielding gas is pure CO2), with high efficiency and good weld formation. TFW-308l is the brand of welding material, and its welding material model is E308LT1-1 (AWS A5.22).
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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