Key points of welding process for various stainless steel 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 passivated and has the characteristics of no rust. In order to achieve this goal, the chromium content must be more than 12%. In order to improve the passivation of steel, nickel, molybdenum and other elements which can passivate steel are often added to stainless steel. Generally speaking, stainless steel is the general name of stainless steel and acid resistant steel. Stainless steel is not necessarily acid resistant, and acid resistant steel generally has good rust resistance.

Stainless steel can be divided into four types according to its microstructure, namely austenitic stainless steel, ferritic stainless steel, martensitic stainless steel and austenite ferrite duplex stainless steel.


20210312225916 17726 - Key points of welding process for various stainless steel pressure vessels

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 divided into cr18-ni8, cr25-ni20 and cr25-ni35. Austenitic stainless steel has the following welding characteristics:

  • ① Because of its low thermal conductivity and high coefficient of linear expansion, austenite stainless steel with welding hot crack is easy to form coarse columnar crystal structure in the welding seam due to its long residence time at high temperature in the welding process. If the content of impurity elements such as sulfur, phosphorus, tin, antimony and niobium is high in the solidification crystallization process, eutectic with low melting point will be formed in the intergranular, 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 are all welding hot cracks.
  • The most effective way to prevent hot cracking is to reduce the impurity elements which are easy to produce eutectic at low melting point in steel and welding materials and to make Cr Ni austenitic stainless steel contain 4% – 12% ferrite.
  • ② Intergranular corrosion according to the theory of chromium deficiency, chromium carbide precipitates on the intergranular surface, resulting in chromium deficiency on the grain boundary, which is the main cause of intergranular corrosion. Therefore, the main measure to prevent intergranular corrosion is to select ultra-low carbon welding materials or welding materials containing niobium, titanium and other stabilizing elements.
  • ③ Stress corrosion cracking (SCC) is usually characterized by brittle failure, and the process of failure is short, so the damage is serious. The main cause of stress corrosion cracking of austenitic stainless steel is welding residual stress. The change of microstructure or the existence of stress concentration and the concentration of local corrosion medium are also the causes of stress corrosion cracking.
  • ④ The σ phase embrittlement of welded joint is a kind of brittle and hard intermetallic compound, which mainly segregates at the grain boundary of columnar crystal. Both γ phase and δ phase can undergo σ phase transformation. For example, for cr25ni20 type weld, when heated at 800 ℃ ~ 900 ℃, there will be a strong γ → δ transformation.
  • For Cr Ni austenitic stainless steel, especially Cr Ni Mo stainless steel, δ → σ phase transformation is easy to occur, which is mainly due to the obvious σ effect of Cr and Mo elements. When the δ ferrite content in the weld exceeds 12%, the δ → σ transformation is very obvious, resulting in the obvious embrittlement of the weld metal, This is why the δ ferrite content of the hardfacing layer on the inner wall of the hot wall hydrogenation reactor is controlled at 3% ~ 10%.

Ferritic stainless steel and its welding characteristics

Ferritic stainless steel can be divided into ordinary ferritic stainless steel and ultra pure ferritic stainless steel. Among them, ordinary ferritic stainless steel has Cr12 ~ cr14 type, 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 and weld, the corrosion resistance is difficult to guarantee, and the use is limited. In ultra pure ferritic stainless steel, the total amount of carbon and nitrogen in steel is strictly controlled, which is generally controlled at three levels of 0.035% ~ 0.045%, 0.030%, 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 widely 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 (HAZ) with the heating temperature above 1000 ℃, especially in the near seam zone, will grow rapidly. Even if the grains are cooled rapidly after welding, the sharp decrease 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, higher brittle transition temperature and stronger notch sensitivity. Therefore, post weld embrittlement is more serious.
  • ③ When heated at 400 ℃ to 600 ℃ for a long time and cooled slowly, the brittleness at 475 ℃ will appear, and the toughness at room temperature will decrease seriously. When heated at 550 ℃ to 820 ℃ for a long time, it is easy to precipitate σ phase from ferrite, and the plasticity and toughness of ferrite are obviously reduced.

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 type has general corrosion resistance. Martensitic stainless steel based on Cr12 has high high temperature strength and high temperature oxidation resistance in addition to certain corrosion resistance due to the addition of nickel, molybdenum, tungsten, vanadium and other alloy elements.
Welding characteristics of martensitic stainless steel: the hardening tendency of weld and heat affected zone of Cr13 type martensitic stainless steel is particularly large, and 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 diffusive hydrogen, the welding cold crack is easy to appear. When the cooling rate is low, coarse ferrite and intergranular carbide precipitate in the weld metal and near the weld metal, resulting in the decrease of the plasticity and toughness of the joint.
The weld and heat affected zone of low carbon and super martensitic stainless steels are all transformed into low carbon martensite after cooling, but there is no obvious hardening phenomenon, so they have good weldability.

Selection of stainless steel welding materials for pressure vessels

Selection of austenitic stainless steel welding materials

The selection principle of austenitic stainless steel welding material 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, and the alloy composition is generally required to match the base metal composition. For corrosion-resistant austenitic stainless steel, it is generally expected to contain a certain amount of ferrite, which can ensure good crack resistance and corrosion resistance. But in some special media, such as urea equipment weld metal is not allowed to have ferrite, otherwise it will reduce its corrosion resistance. For heat resistant austenitic steel, the control of ferrite content in weld metal should be considered. For Austenitic Steel Weldments which are operated at high temperature for a long time, the ferrite content in weld metal should not exceed 5%. According to Schaeffler diagram, the reader can estimate the corresponding ferrite content according to chromium equivalent and nickel equivalent in weld metal.

Selection of welding materials for ferritic stainless steel

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, which are rarely used because of their high price.

Ferritic stainless steel welding material can use the same material as the base metal, but it is easy to crack when the restraint is large. After welding, heat treatment can be used to restore the corrosion resistance and improve the joint plasticity. Preheating and post weld heat treatment can be avoided by using austenitic welding materials, but for various steels without stable elements, the sensitization of heat affected zone still exists. 309 type and 310 type chromium nickel austenitic welding materials are commonly used. For Cr17 steel, 308 type welding material can also be used. Welding material with high alloy content is beneficial to improve the plasticity of welded joint. Austenite or austenite ferrite weld metal is basically as strong as ferrite base metal, but in some corrosive media, the corrosion resistance of weld metal may be very different from that of base metal, which should be paid attention to when selecting welding material.

Selection of welding materials for martensitic stainless steel

In stainless steel, martensitic stainless steel can use heat treatment to adjust the performance. Therefore, in order to ensure the performance requirements, especially for heat-resistant martensitic stainless steel, the composition of weld should be as close as possible to that of base metal. In order to prevent cold cracking, austenitic welding material can also be used, when the strength of the weld 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 the tempering softening zone will appear in the heat affected zone. In order to prevent cold cracking, components with a thickness of more than 3 mm often need to be preheated, and heat treatment after welding is often needed to improve the joint performance. Because the thermal expansion coefficient of weld metal and base metal is basically the same, the welding stress may be completely eliminated after heat treatment.

20210312230452 92064 - Key points of welding process for various stainless steel pressure vessels

Selection of welding materials for ferritic stainless steel

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, which are rarely used because of their high price.

Ferritic stainless steel welding material can use the same material as the base metal, but it is easy to crack when the restraint is large. After welding, heat treatment can be used to restore the corrosion resistance and improve the joint plasticity. Preheating and post weld heat treatment can be avoided by using austenitic welding materials, but for various steels without stable elements, the sensitization of heat affected zone still exists. 309 type and 310 type chromium nickel austenitic welding materials are commonly used. For Cr17 steel, 308 type welding material can also be used. Welding material with high alloy content is beneficial to improve the plasticity of welded joint. Austenite or austenite ferrite weld metal is basically as strong as ferrite base metal, but in some corrosive media, the corrosion resistance of weld metal may be very different from that of base metal, which should be paid attention to when selecting welding material.

Selection of welding materials for martensitic stainless steel

In stainless steel, martensitic stainless steel can use heat treatment to adjust the performance. Therefore, in order to ensure the performance requirements, especially for heat-resistant martensitic stainless steel, the composition of weld should be as close as possible to that of base metal. In order to prevent cold cracking, austenitic welding material can also be used, when the strength of the weld 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 the tempering softening zone will appear in the heat affected zone. In order to prevent cold cracking, components with a thickness of more than 3 mm often need to be preheated, and heat treatment after welding is often needed to improve the joint performance. Because the thermal expansion coefficient of weld metal and base metal is basically the same, the welding stress may be completely eliminated after heat treatment.

Welding points of stainless steel for pressure vessel

Welding points of austenitic stainless steel

In general, austenitic stainless steel has excellent weldability. Almost all fusion 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 of the low thermal conductivity and high thermal expansion coefficient of austenitic stainless steel, 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 Because the thermal conductivity of austenitic stainless steel is small, the penetration of austenitic stainless steel is larger than that of low alloy steel under the same current. At the same time, because of its high resistivity, in order to avoid electrode redness, 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. For SMAW, it is better to use small diameter electrode and fast multi pass welding. For the weld with high requirements, even the method of pouring cold water is used to accelerate the cooling. For pure austenitic stainless steel and super austenitic stainless steel, due to the high sensitivity of hot crack, the welding line energy should be strictly controlled to prevent the serious growth of weld grain and the occurrence of hot crack.
  • ④ In order to improve the hot cracking resistance and corrosion resistance of the weld, special attention should be paid to the cleaning of the welding area during welding to avoid the penetration of harmful elements into the weld.
  • ⑤ Generally, preheating is not required for austenitic stainless steel welding. In order to prevent the grain growth and carbide precipitation of weld and heat affected zone, and ensure the plasticity, toughness and corrosion resistance of welded joint, lower interlayer temperature should be controlled, generally not more than 150 ℃.

Welding points of ferritic stainless steel

The ferrite forming elements of ferritic stainless steel are relatively more, while the austenite forming elements are relatively less, and the hardening and cold cracking tendency of the material is less. Under the effect of welding thermal cycle, the grains in HAZ of ferritic stainless steel 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 and holding time during welding. Therefore, when welding ferritic stainless steel, small line energy should be used as far as possible, that is, energy concentration method, such as small current TIG, small diameter electrode manual welding, etc. at the same time, narrow gap groove, high welding speed, multi-layer welding and other measures should be adopted as far as possible, and the interlayer temperature should be strictly controlled.
Due to the effect of welding thermal cycle, general ferritic stainless steels are sensitized in the high temperature zone of heat affected zone, and intergranular corrosion occurs in some media. After annealing at 700 ~ 850 ℃, the chromium can be homogenized and the corrosion resistance can be recovered.
Common high chromium ferritic stainless steel can be welded by welding rod electric arc welding, gas shielded welding, submerged arc welding and other fusion welding methods. Due to the inherent low plasticity of high chromium steel, the grain growth in heat affected zone and the concentration of carbides and nitrides at grain boundaries caused by welding thermal cycle, 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 shielded metal 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 Small linear energy and no swing welding are adopted. In order to reduce the influence of high temperature embrittlement and 475 ℃ brittleness, the interlaminar temperature should not be higher than 150 ℃.
  • ③ After annealing at 750 ~ 800 ℃, the corrosion resistance can be restored and the joint plasticity can be improved due to carbide spheroidization and uniform distribution of chromium. After annealing, it should be cooled quickly to prevent σ phase and brittleness at 475 ℃.

Welding points of martensitic stainless steel

For Cr13 martensitic stainless steel, in order to reduce the sensitivity of cold crack and ensure the plasticity and toughness of welded joint, low hydrogen electrode should be used when welding with the same material electrode, and the following measures should be taken at the same time:

  • ① Warm up. The preheating temperature increases with the increase of carbon content in steel, generally in the range of 100 ℃ ~ 350 ℃.
  • ② Afterheat. For the welded joints with high carbon content or high restraint, post heating measures should be taken after welding to prevent hydrogen induced cracking.
  • 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 1 h / 25 mm.

For super and low carbon martensitic stainless steels, preheating measures are generally not taken. When the restraint degree 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 in the case that preheating before welding and heat treatment after welding are difficult to implement, and the joint has a large degree of restraint, austenite type welding materials can also be used in engineering to improve the plasticity and toughness of welded joints and prevent cracks. However, when the weld metal is austenite or mainly austenite, it is low strength matching compared with the base metal. Moreover, there are great differences between the weld metal and the base metal in chemical composition, metallographic structure, thermophysical properties and mechanical properties. The welding residual stress is inevitable, which easily leads to stress corrosion or high temperature creep failure.

Welding of duplex stainless steel

Types of duplex stainless steel

Duplex stainless steel has the characteristics of austenitic stainless steel and ferritic stainless steel because of its dual phase structure of austenite and ferrite, and the content of the two phases is almost the same. The yield strength can reach 400MPa ~ 550MPa, which is twice of that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher toughness, lower brittle transition temperature, better intergranular corrosion resistance and weldability. Meanwhile, 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, the strength of duplex stainless steel is higher, especially the yield strength, and the pitting corrosion resistance, stress corrosion resistance and corrosion fatigue resistance are also improved.
According to the chemical composition, duplex stainless steels can be divided into four types: 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 of the duplex stainless steels used in China are made in Sweden. The specific grades 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 as brittle as ferritic stainless steel in heat affected zone during welding, nor as easy to produce welding hot cracks as austenitic stainless steel. However, due to its large amount of ferrite, hydrogen cooling cracks may occur when the rigidity is large or the hydrogen content in weld is high. Therefore, it is very important to strictly control the source of hydrogen.
  • ② In order to ensure the characteristics of dual phase steel and to ensure the proper proportion of austenite and ferrite in the welded joint is the key to the welding of this kind of steel. When the cooling rate of the welded joint is slow, the secondary phase of δ→γ changes more fully, so the two-phase structure with appropriate phase ratio can be obtained at room temperature, which requires a large amount of welding heat input during welding. Otherwise, if the cooling rate of the welded joint is fast, the δ ferrite phase will increase, resulting in a serious decline in the ductility and corrosion resistance of the joint.

Welding material selection of duplex stainless steel

The welding material for duplex stainless steel is characterized by austenite dominated duplex structure, 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, it is usually to improve the content of nickel and nitrogen, that is, to improve the nickel equivalent by about 2% – 4%. Generally, there is a certain amount of nitrogen content in the base metal of duplex stainless steel, and a certain amount of nitrogen content is also expected in the welding material, but generally it should not be too high, otherwise there will be pores. In this way, the high nickel content becomes a main difference between the welding material and the base metal.
According to the different requirements of corrosion resistance and joint toughness, the welding rod matching the chemical composition of base metal can be selected, such as Cr22 duplex stainless steel, cr22ni9mo3 welding rod, such as E2209 welding rod. When the acid electrode is used, the slag removal is good, the weld shape 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, the alkaline electrode should be used. When the root seal welding, usually using alkaline electrode. When there are special requirements for the corrosion resistance of weld metal, the alkaline electrode with super dual phase steel composition should also be used.
For solid gas shielded welding wire, while ensuring the weld metal has good corrosion resistance and mechanical properties, we should also pay attention to its welding process performance. For flux cored wire, when the weld shape is required to be beautiful, rutile type or titanium calcium type flux cored wire can be used. When the impact toughness is required to be higher or the welding is under the condition of larger restraint, the 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 welding heat affected zone and embrittlement of weld metal, and the matching alkaline flux should be used.

Welding points of duplex stainless steel

  • ① The control of welding thermal process, such as welding line energy, interlayer temperature, preheating and material thickness, will affect the cooling rate during welding, thus affecting the microstructure and properties of weld and heat affected zone. Too fast and too slow cooling rate will affect the toughness and corrosion resistance of dual phase steel welded joint. If the cooling rate is too fast, the content of α phase and the precipitation of Cr2N will increase. If the cooling rate is too slow, the grains will coarsen seriously, and even some brittle intermetallic compounds, such as σ phase, may precipitate. Table 1 lists some recommended ranges of welding line energy and interpass temperature. The upper limit of line energy is suitable for thick plate, and the lower limit is suitable for thin plate. In order to obtain the best weld metal properties, it is suggested that the maximum interlayer temperature should be controlled at 100 ℃ when welding dual phase steel and super stainless steel with high alloy content and 25% ω (CR). When heat treatment is required after welding, the interlayer temperature can not be limited.
  • ② However, when the content of α phase exceeds the requirements or harmful phase precipitates, such as σ phase, it can be improved by post weld heat treatment. The heat treatment method used is water quenching. The heat treatment should be as fast as possible, and the holding time at the heat treatment temperature should be 5 ~ 30min, which should be enough to restore the phase equilibrium. During heat treatment, metal oxidation is very serious, so inert gas protection should be considered. The dual phase steel with 22% ω (CR) should be heat treated at 1050 ℃ ~ 1100 ℃, while the dual phase steel with 25% ω (CR) and super dual phase steel should be heat treated at 1070 ℃ ~ 1120 ℃.

Welding example of stainless steel pressure vessel

The flash tank with diameter of 800mm and wall thickness of 10mm is made of 0Cr18Ni9.
Explain:
① The diameter of the cylinder is 800mm, and the welder can drill into the cylinder for welding, so the longitudinal and circumferential seams of the cylinder are welded on both sides by shielded metal 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 method of back argon filling protection can only be used. However, when the equipment is large or the back can not implement argon protection, a lot of argon will be wasted, and the protection may not be good.
In order to solve this problem, a kind of Stainless Steel TIG welding wire with back self-protection was developed and manufactured by the welding division of Japan oil company. It is a kind of welding wire with special coating. After the coating (namely coating) is melted, it will penetrate into the back of the molten pool to form a dense protective layer, which is equivalent to the effect of coating.
This welding wire is used in the same way as the common TIG welding wire. The coating will not affect the front arc and molten pool shape, greatly reducing the welding cost of stainless steel argon arc welding. In this equipment, if the back argon protection is used, the argon waste is serious, so the self shielded welding wire is used.
③ In view of the weld shape and welding conditions, the welding rod arc welding is generally used. If the nozzle diameter is too small, in order to reduce the difficulty of welding, TIG welding can also be used.
④ The fillet weld between the support and the shell is a non pressure bearing weld, which adopts the GMAW (the shielding gas is pure CO2), with high efficiency and good weld formation. Tfw-308l is the welding material grade, and its welding material model is E308lt1-1 (AWS a5.22).

Source: Network Arrangement – China Pressure Vessels Manufacturer  www.secmachinery.com

www.secmachinery.com is one of the leading china filtration equipment & sanitary stainless steel pressure vessels manufacturer, with professional factory. We focus on the sanitary pressure vessels, filtration equipment research and development, manufacturing, sales and service since the company been established. Our products is widely used on bio-pharmaceutical, food and beverage, fine chemical industry. 

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

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