Welding Technology of Stainless Steel Pressure Vessels
The pressure vessel with stainless steel and its welding characteristics
The so-called stainless steel is added to a certain amount of chromium in the steel, so that the steel in a passivated state, with no rust characteristics. In order to achieve this purpose, its chromium content must be more than 12%. In order to progress in the passivation of steel, stainless steel is often required to add nickel, molybdenum and other elements that can make steel passivation. The general reference to stainless steel is actually a general term for stainless steel and acid-resistant steel. Stainless steel is not necessarily acid-resistant, while acid-resistant steel generally has good stainless properties.
Stainless steel according to its steel organization can be divided into four categories, namely austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, austenitic-ferritic duplex stainless steel.
1. Austenitic stainless steel and its welding characteristics
Austenitic stainless steel is the most widely used stainless steel, with high Cr-Ni type is the most common. At present, austenitic stainless steel can be roughly divided into Cr18-Ni8 type, Cr25-Ni20 type, Cr25-Ni35 type. Austenitic stainless steel has the following welding characteristics.
- ① Welding thermal cracking austenitic stainless steel due to its small thermal conductivity, large coefficient of linear expansion, so in the welding process, the welded joint parts of the high temperature dwell time is longer, the weld is easy to form coarse columnar crystal organization, in the solidification crystallization process, if sulfur, phosphorus, tin, antimony, niobium and other impurity elements content is high, it will form a low melting point eutectic between the crystal, in the welded joint is subjected to high tensile stress, it is easy in the weld The formation of solidification cracks, the formation of liquefaction cracks in the heat-affected zone, which are welding thermal cracking. The most effective way to prevent thermal cracking is to reduce the steel and weld material is prone to low melting point eutectic impurity elements and make chromium-nickel austenitic stainless steel contains 4-12% of the ferrite organization.
- ② Intergranular corrosion according to the theory of poor chromium, chromium carbide precipitated on the intergranular, resulting in grain boundary poor chromium is the main cause of intergranular corrosion. For this reason, the choice of ultra-low carbon welding consumables or containing niobium, titanium and other stabilizing elements of the welding consumables is the main measure to prevent intergranular corrosion.
- ③ Stress corrosion cracking stress corrosion cracking is usually manifested as brittle damage, and the process of damage occurs in a short period of time, so the damage is serious. The main cause of stress corrosion cracking of austenitic stainless steel is welding residual stress. Organizational changes in the welded joint or the presence of stress concentrations, local corrosion medium concentration is also a cause of stress corrosion cracking.
- ④ σ-phase embrittlement of welded joints σ-phase is a brittle and hard intermetallic compound, mainly precipitated in the grain boundaries of columnar crystals. γ-phase and δ-phase can both undergo σ-phase transformation. For example, for Cr25Ni20 type welds in 800 ℃ – 900 ℃ when heated, a strong γ → δ transformation will occur. For chromium-nickel austenitic stainless steel, especially chromium-nickel-molybdenum stainless steel, prone to δ → σ phase transformation, which is mainly due to chromium, molybdenum elements have obvious σ effect, when the δ ferrite content of the weld more than 12%, δ → σ transformation is very obvious, resulting in the obvious embrittlement of the weld metal, which is why the hot wall hydrogenation reactor wall cladding layer will be δ ferrite content control at 3% -10% The reason.
2. Ferritic stainless steel and its welding characteristics
Ferritic stainless steel is divided into two categories: ordinary ferritic stainless steel and ultrapure ferritic stainless steel, of which ordinary ferritic stainless steel has Cr12-Cr14 type, such as 00Cr12, 0Cr13Al; Cr16-Cr18 type, such as 1Cr17Mo; Cr25-30 type.
Due to the high carbon and nitrogen content in ordinary ferritic stainless steel, so processing and forming and welding are more difficult, corrosion resistance is also difficult to ensure that the use is limited, in the ultra-pure ferritic stainless steel strictly control the total amount of carbon and nitrogen in the steel, generally controlled at 0.035%-0.045%, 0.030%, 0.010%-0.015% three levels, while also adding the necessary alloying elements to Further progress in the steel corrosion resistance and comprehensive performance. Compared with ordinary ferritic stainless steel, ultra-pure high chromium ferritic stainless steel has very good resistance to uniform corrosion, pitting and stress corrosion performance, more often used in petrochemical equipment. Ferritic stainless steel has the following welding characteristics.
- ① Welding under the action of high temperature, the heat-affected zone in the heating temperature of 1000 ℃ or more, especially in the near seam area of the grain will grow sharply, even after welding rapid cooling, but also can not avoid the rapid decline in toughness and higher intergranular corrosion tendency caused by the coarsening of the grain.
- ② Ferritic steel itself contains high chromium, harmful elements such as carbon, nitrogen, oxygen, etc., the brittle transition temperature is higher, the gap sensitivity is stronger. Therefore, the embrittlement phenomenon is more serious after welding.
- ③ In 400 ℃ – 600 ℃ long time heating slow cooling, there will be 475 ℃ embrittlement, so that the room temperature toughness is seriously reduced. After a long time heating at 550 ℃-820 ℃, it is easy to precipitate σ phase from the ferrite, but also significantly reduce its plasticity and toughness.
3. Martensitic stainless steel and its welding characteristics
Martensitic stainless steel can be divided into Cr13 type martensitic stainless steel, low carbon martensitic stainless steel and super martensitic stainless steel. cr13 type has general corrosion resistance, from Cr12-based martensitic stainless steel, due to the addition of nickel, molybdenum, tungsten, vanadium and other alloying elements, in addition to a certain degree of corrosion resistance, but also has a high temperature strength and high temperature oxidation resistance.
Martensitic stainless steel welding characteristics: Cr13 martensitic stainless steel weld and heat-affected zone hardening tendency is particularly large, welded joints in air-cooled conditions can be hard and brittle martensite, in the weld constraint stress and diffusion of hydrogen, it is easy to appear welded cold cracking. When the cooling rate is small, near the seam area and weld metal will form coarse ferrite and along the crystal precipitation carbide, so that the joint plasticity, toughness significantly reduced.
Low-carbon and super martensitic stainless steel weld and heat-affected zone after cooling, although all transformed into low-carbon martensite, but there is no obvious hardening phenomenon, with good welding performance.
The selection of stainless steel welding consumables for pressure vessels
1. Austenitic stainless steel welding consumables selection
Austenitic stainless steel welding material selection principle is in the absence of cracking conditions, to ensure that the corrosion resistance of the weld metal and mechanical properties and the parent material is basically equivalent, or higher than the parent material, the general requirements of its alloy composition roughly matched with the parent material composition. For corrosion-resistant austenitic stainless steel, generally hope to contain a certain amount of ferrite, so as to ensure good cracking resistance, but also good corrosion resistance. But in some special media, such as urea equipment weld metal is not promised to have ferrite present, otherwise it will reduce its corrosion resistance. For heat-resistant austenitic steel, should consider the control of ferrite content within the weld metal. For long-term operation at high temperatures of austenitic steel weldments, the ferrite content in the weld metal should not exceed 5%. Readers can be based on Schaeffler chart, according to the weld metal in the chromium equivalent and nickel equivalent estimated the corresponding ferrite content.
2. Ferritic stainless steel welding consumables selection
Ferritic stainless steel welding consumables basically have three categories: 1) the composition of the basic and the base material matching the welding consumables; 2) austenitic welding consumables; 3) nickel-based alloy welding consumables, due to its high price, so rarely used.
Ferritic stainless steel welding consumables can be used with the base material, but in the constraint degree, it is easy to produce cracks, after welding heat treatment can be used to restore corrosion resistance, and improve the plasticity of the joint. The use of austenitic welding consumables can be exempt from preheating and post-weld heat treatment, but for a variety of steels without stable elements, the heat-affected zone sensitization still exists, commonly used 309 and 310 type chromium-nickel austenitic welding consumables. For Cr17 steel, also available 308 type welding material, high alloy content of the welding material is conducive to the progress of welded joint plasticity. Austenitic or austenitic a ferrite weld metal basic and ferrite base material, such as strong, but in some corrosive media, the weld corrosion resistance may be very different from the base material, which should be noted in the selection of welding consumables.
3. Martensitic stainless steel welding material selection
In stainless steel, martensitic stainless steel can be used to adjust the performance of heat treatment, therefore, in order to ensure the use of performance requirements, especially heat-resistant martensitic stainless steel, weld composition should be as close as possible to the composition of the parent material. In order to prevent cold cracking, austenitic welding material can also be used, when the weld strength is necessarily lower than the base material.
When the weld composition is similar to that of the base material, the weld and heat-affected zone will be hardened and brittle at the same time, while the heat-affected zone appears in the tempered softening zone. In order to prevent cold cracking, the thickness of more than 3mm components often need to be preheated, after welding is also often required to heat treatment to improve the performance of the joint, due to the weld metal and the coefficient of thermal expansion of the base material is basically the same, after heat treatment is possible to completely eliminate the welding stress.
When the workpiece does not promise preheating or heat treatment, you can choose austenitic tissue weld, due to the weld has a high plasticity and toughness, can relax the welding stress, and can be more solid solution hydrogen, and thus can reduce the cold cracking tendency of the joint, but this metal material is not uniform joint, due to the different coefficients of thermal expansion, in the working environment of the cyclic temperature, in the fusion zone may produce shear stress, and lead to joint damage.
For simple Cr13 type martensitic steel, not austenitic organization of the weld, the weld composition of the adjustment margin is not much, generally the same as the base material, but must limit the harmful impurities S, P and Si, etc., Si in the Cr13 type martensitic steel weld can contribute to the formation of coarse martensite. Reduce the amount of C, in favor of reducing the hardenability, the presence of a small amount of Ti, N or Al elements in the weld, can also refine the grain and reduce the hardenability.
For multi-component alloying of Cr12-based martensitic hot steel, the main use is heat-resistant, usually without austenitic welding consumables, weld composition is close to the parent material. In adjusting the composition, must ensure that the weld does not appear a ferrite phase, because it is very harmful to the performance, due to Cr13-based martensitic hot strong steel, the main components are mostly ferrite elements (such as Mo, Nb, W, V, etc.), in order to ensure that all the organization is homogeneous martensite, must be balanced with austenitic elements, that is, to have the appropriate C, Ni, Mn, N and other elements.
Martensitic stainless steel has a fairly high tendency to cold cracking, so must be strictly maintained low hydrogen, or even ultra-low hydrogen, in the selection of welding consumables, must pay attention to this point.
The pressure vessel with stainless steel welding points
1. Austenitic stainless steel welding points
In general, austenitic stainless steel has excellent weldability. Almost all melt welding methods can be used to weld austenitic stainless steel, austenitic stainless steel thermophysical properties and organizational characteristics determine the main points of the welding process.
- ① Because the thermal conductivity of austenitic stainless steel is small and the coefficient of thermal expansion is large, welding is easy to produce large deformation and welding stress, so the welding energy should be used as much as possible to focus the welding method.
- ② As austenitic stainless steel thermal conductivity is small, in the same current, comparable to low-alloy steel to get a larger depth of melt. At the same time, due to its high resistivity, in the welding electrode arc welding, in order to avoid redness of the electrode, compared with the same diameter of carbon steel or low-alloy steel electrode, welding current is smaller.
- ③ Welding specification. Generally do not use large line energy for welding. When welding electrode arc welding, it is appropriate to use a small diameter welding rod, fast multi-pass welding, for the high requirements of the weld, and even the use of cold water to accelerate cooling, for pure austenitic stainless steel and super austenitic stainless steel, due to thermal cracking sensitivity, should be strictly controlled welding line energy, to prevent serious grain growth of the weld and the occurrence of welding thermal cracking.
- ④ In order to improve the weld resistance to thermal cracking and corrosion resistance, when welding, pay special attention to the cleanliness of the weld area, to avoid the penetration of harmful elements weld.
- ⑤ Austenitic stainless steel welding generally does not require preheating. In order to prevent the weld and heat-affected zone grain growth and carbide precipitation, to ensure the plasticity of the welded joint, toughness and corrosion resistance surname, should control the lower interlayer temperature, generally not more than 150 ℃.
2. Ferritic stainless steel welding points
Ferritic stainless steel ferrite forming elements are relatively more, austenite forming elements are relatively less, the material hardening and cold cracking tendency is smaller. Ferritic stainless steel in the role of welding heat cycle, the heat-affected zone grain growth is obvious, the joint toughness and plasticity drop sharply. The degree of grain growth in the heat-affected zone depends on the highest temperature reached when welding and its holding time, for this reason, when welding ferritic stainless steel, should try to use a small line energy, that is, the use of energy concentration methods, such as small-current TIG, small diameter electrode manual welding, etc., while using narrow gap bevels, high welding speed and multi-layer welding measures, and strict control of the interlayer temperature.
Due to the role of welding thermal cycle, the general ferritic stainless steel in the heat-affected zone of the high-temperature zone produced sensitization, intergranular corrosion in some media. After welding by 700-850 ℃ annealing treatment, so that the chromium homogenization, can restore its corrosion resistance.
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, as well as welding heat cycle caused by the heat-affected zone grain growth and carbides, nitrides in the grain boundary agglomeration, the plasticity and toughness of the welded joint are very low. In the use of similar chemical composition of the base material and the constraint degree is large, it is easy to produce cracks. In order to prevent cracking and improve the plasticity and corrosion resistance of the joint, the following process measures can be taken, for example, in electrode arc welding.
- ① Preheat about 100-150 ℃, so that the material in a ductile state of welding. The higher the chromium content, the higher the preheating temperature should be.
- ② Adopt a small line energy, not swing welding. Multi-layer welding, should control the interlayer temperature is not higher than 150 ℃, should not be continuous welding to reduce the impact of high temperature embrittlement and 475 ℃ brittleness.
- ③ After welding for 750-800 ℃ annealing treatment, due to carbide spheroidization and chromium distribution, can restore corrosion resistance, and improve the plasticity of the joint. After annealing should be fast cooling to prevent σ phase and 475 ℃ brittleness.
3. Martensitic stainless steel welding points
For Cr13 type martensitic stainless steel, when using the same material welding rod, in order to reduce cold crack sensitivity, to ensure that the welded joint plasticity, toughness, should be selected with low-hydrogen type welding rod and take the following measures.
- ① Preheating. Preheating temperature with the increase in carbon content of steel and progress, generally in the range of 100 ℃ -350 ℃.
- ② Post-heat. For higher carbon content or constraint degree of the welded joint, post-welding measures to prevent hydrogen cracking of the weld.
- ③ Post-weld heat treatment. To improve the plasticity, toughness and corrosion resistance of welded joints, post-weld heat treatment temperature is generally 650 ℃ -750 ℃, holding time of 1h/25mm.
For super and low-carbon martensitic stainless steel, generally may not take preheating measures, when the constraint degree is large or high hydrogen content in the weld, take preheating and post-heating measures, preheating temperature is generally 100 ℃ -150 ℃, post-weld heat treatment temperature of 590-620 ℃.
For high carbon content of martensitic steel. Or in the preheat before welding, post-weld heat treatment is difficult to implement, as well as the joint constraint is large, the project can also be used in austenitic welding material, in order to progress the plasticity of the welded joint, toughness, to prevent cracking. But at this time, the weld metal is austenite organization or austenite-based organization, compared with the parent material strength is really a low strength match, and the weld metal and the parent material in the chemical composition, metallurgical organization, thermophysical properties, mechanical properties are very different, welding residual stress is inevitable, easily triggered by stress corrosion or high temperature creep damage.
The welding of duplex stainless steel
1. Types of duplex stainless steel
Duplex stainless steel has austenite + ferrite duplex organization, and the content of the two phase organization is basically equivalent, so both austenitic stainless steel and ferrite stainless steel characteristics. Yield strength up to 400Mpa-550MPa, is two times the ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has high toughness, low brittle transition temperature, intergranular corrosion resistance and welding performance are significantly improved; while preserving some characteristics of ferritic stainless steel, such as 475 ℃ brittleness, high thermal conductivity, small linear expansion coefficient, superplasticity and magnetic properties. Compared with austenitic stainless steel, the strength of duplex stainless steel is high, especially the yield strength is significantly improved, and pore corrosion resistance, stress corrosion resistance, corrosion fatigue resistance and other properties have also been significantly improved.
Duplex stainless steel according to its chemical composition classification, can be divided into Cr18 type, Cr23 (without Mo) type, Cr22 type and Cr25 type four categories. For Cr25 type duplex stainless steel can be divided into ordinary and super duplex stainless steel, which in recent years more applications are Cr22 type and Cr25 type. China’s use of duplex stainless steel to the majority of Swedish production, the specific grades are: 3RE60 (Cr18), SAF2304 (Cr23), SAF2205 (Cr22), SAF2507 (Cr25).
2. Welding characteristics of duplex stainless steel
- ① Duplex stainless steel has good weldability, it is neither like ferritic stainless steel welding heat-affected zone is easy to embrittlement, nor like austenitic stainless steel easy to produce welding heat cracking, but because it has a large number of ferrite, when the rigidity is larger or the weld contains a high amount of hydrogen, it is possible to produce hydrogen cold cracking, so it is very important to strictly control the source of hydrogen.
- ② To ensure the characteristics of duplex steel, it is crucial to ensure that the organization of the welded joint has the right ratio of austenite and ferrite for welding this type of steel. When the welded joint cooling rate is slow, δ → γ of the secondary phase change is more adequate, so to room temperature can be obtained when the phase ratio is more appropriate duplex organization, which requires an appropriate amount of welding heat input, otherwise if the post-weld cooling rate is faster, it will make δ ferrite phase increase, resulting in a serious decline in joint plastic toughness and corrosion resistance.
3. Duplex stainless steel welding consumables selection
Duplex stainless steel with welding consumables, characterized by the weld organization for austenite predominant duplex organization, the main corrosion-resistant elements (chromium, molybdenum, etc.) content and the parent material, thus ensuring comparable corrosion resistance with the parent material. In order to ensure the content of austenite in the weld, usually is the progress of nickel and nitrogen content, that is, the progress of about 2-4% of the nickel equivalent. In the duplex stainless steel base material, there is generally a certain amount of nitrogen content, in the weld also want to have a certain amount of nitrogen content, but generally should not be too high, otherwise it will produce porosity. Thus the higher nickel content becomes a major difference between the weld material and the base material.
According to the different requirements of corrosion resistance, joint toughness to choose the chemical composition of the base material to match the electrode, such as welding Cr22 duplex stainless steel, Cr22Ni9Mo3 type electrode can be used, such as E2209 electrode. Acid electrode with excellent slag removal, beautiful weld seam formation, but low impact toughness, when the weld metal is required to have a high impact toughness, and the need for all-position welding, alkaline electrode should be used. When the root sealing welding, usually using alkaline welding rod. When there are special requirements for the corrosion resistance of the weld metal, the alkaline electrode of super duplex steel composition should also be used.
For solid gas shielded welding wire, in order to ensure that the weld metal has good corrosion resistance and mechanical properties at the same time, should also pay attention to its welding process performance, for flux-cored wire, when the requirements for beautiful weld seam formation, rutile or titanium calcium flux-cored wire can be used, when the requirement for higher impact toughness or welding in larger constraint conditions, it is appropriate to use the higher alkalinity of flux-cored wire.
For submerged arc welding should use smaller diameter wire to achieve multi-layer multi-channel welding under small and medium welding specifications to prevent the weld heat-affected zone and embrittlement of the weld metal, and the use of matching alkaline flux.
4. Duplex stainless steel welding points
① The control of the welding thermal process welding line energy, interlayer temperature, preheating and material thickness will affect the cooling rate when welding, 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 a cooling rate can cause severe grain coarsening and possibly even precipitation of brittle intermetallic compounds, such as σ-phase. Table 1 lists some recommended ranges of welding line energy and interlayer temperature. The specific material thickness should also be taken into account when selecting the line energy, the upper limit of the line energy in the table is suitable for thick plates and the lower limit is suitable 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 heat treatment is required after welding, the interlayer temperature can be unrestricted.
Recommended choice of duplex steel line energy and interlayer temperature
|Steel type||Linear energy/(kJ/cm)||Maximum interpass temperature/℃|
|Cr23% Mo free dual phase steel||5 – 25||150 – 200|
|Cr22% duplex steel||5 – 25||125 – 200|
|Cr25% (Cu0 – 2.5%) dual phase steel||2 – 15||100 – 150|
|Cr25% super duplex steel||2 – 15||100 – 150|
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, and the holding time of 5-30 min at the heat treatment temperature should be sufficient to restore the phase equilibrium. Oxidation of the metal during heat treatment is very severe and inert gas protection should be considered. For ω (Cr) for 22% duplex steel should be heat treated at 1050 ℃ – 1100 ℃ temperature, while ω (Cr) for 25% duplex steel and super duplex steel requires heat treatment at 1070 ℃ – 1120 ℃ temperature.
Stainless steel pressure vessel welding examples
Diameter of 800mm, wall thickness of 10mm flash tank, shell material for 0Cr18Ni9, its main pressure-bearing welds welding process is shown in Table 2.
Table.2 Flash tank welding process
|Weld No||Weld position||Welding method||Welding material||Explain|
|1A1、B1||Longitudinal and circumferential seams of shell||Double sided SMAW||A102||①|
|B2||Closure weld of shell||
Fillet weld between nozzle and flat welding flange
Fillet weld between nozzle and shell
|E1||Fillet weld of support and shell||GMAW (CO2 welding)||TFW-308L||④|
- ① Cylinder diameter of 800mm, welders can drill into the cylinder body welding, so the cylinder longitudinal, circumferential seam so using electrode arc welding for double-sided welding.
- ② No one holes in this equipment, so the closing weld can only be welded from the outside. In order to ensure the quality of welding, TIG welding is used for priming. However, the metal on the back side will be oxidized when welding stainless steel TIG welding, previously only by using the back side argon protection method, but when the equipment is large or the back side can not be implemented argon protection, will be a lot of wasted argon, and may still appear poor protection. In order to solve this process difficulty, the welding division of Japan Grease developed and manufactured a back side self-protection stainless steel TIG wire, which is a wire with a special coating, the coating (i.e., flux skin) will penetrate the back side of the molten pool after melting, forming a dense protective layer, equivalent to the role of the electrode flux. This wire is used in exactly the same way as ordinary TIG wire, the coating does not affect the front arc and melt pool form, greatly reducing the welding cost of stainless steel TIG welding. In this equipment, if the back side of the argon gas protection, argon gas waste serious, so the use of self-protection wire.
- ③ Receiver and flat welding flange fillet weld, receiver and shell fillet weld, given the shape of this part of the weld and welding conditions, the general choice of welding rod arc welding. If the diameter of the receiver is too small, in order to reduce the difficulty of welding, you can also use TIG welding.
- ④ Support and shell welding fillet weld is a non-pressure welding seam, the use of melting electrode gas shielded welding (shielding gas for pure CO2), high efficiency, good weld formation. TFW-308L for the welding material grade, the welding material model E308LT1-1 (AWSA5.22).
Source: China Pressure Vessels Manufacturer – www.secmachinery.com