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I. Stainless steel for pressure vessels and its welding characteristics. Stainless steel refers to steel into which a certain amount of chromium has been added, causing the steel to become passivated and thus resistant to rusting. To achieve this, its chromium content must be above 12%. To improve the passivation property of steel, elements such as nickel and molybdenum, which can passivate the steel, are often added to stainless steels as well. The stainless steel referred to generally is actually a general term for stainless steels and acid-resistant steels. Stainless steel is not necessarily acid-resistant; however, acid-resistant steel generally exhibits good corrosion resistance. Stainless steels can be classified into four categories based on their steel structure: austenitic stainless steels, ferritic stainless steels, martensitic stainless steels, and austenitic-ferritic duplex stainless steels. 1. Austenitic stainless steels and their welding characteristics. Austenitic stainless steels are the most widely used type of stainless steel, with the high Cr-Ni variants being the most common. Currently, austenitic stainless steels can be roughly divided into the Cr18-Ni8 type, the Cr25-Ni20 type, and the Cr25-Ni35 type. Austenitic stainless steels have the following welding characteristics: ① Welding hot cracks. Due to their low thermal conductivity and high linear expansion coefficient, austenitic stainless steels experience a longer period of high temperature at the weld joint during welding. This leads to the formation of coarse columnar crystal structures in the weld. If impurity elements such as sulfur, phosphorus, tin, antimony, and niobium are present in high amounts, low-melting-point eutectics can form between the crystals. When the weld joint is subjected to high tensile stresses, solidification cracks can form within the weld, while liquefaction cracks can occur in the heat-affected zone; all of these are considered welding hot cracks. The most effective way to prevent hot cracking is to reduce the impurity elements in steel and welding materials that are prone to forming low-melting-point eutectics, and to ensure that chromium-nickel austenitic stainless steels contain 4%–12% ferritic structure. ② Intergranular corrosion: According to the chromium-depletion theory, the precipitation of chromium carbides at grain boundaries leads to chromium depletion there, which is the main cause of intergranular corrosion. To this end, choosing 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 typically manifests as brittle failure, and the failure process occurs in a short time, making it highly hazardous. The main cause of stress corrosion cracking in austenitic stainless steels is welding residual stress. Changes in the microstructure of the welded joint or the presence of stress concentration, as well as the concentration of local corrosion agents, are also factors that contribute to stress corrosion cracking. ④ Sigma-phase embrittlement of welded joints. The sigma phase is a brittle and hard intermetallic compound that primarily precipitates at the grain boundaries of columnar crystals. Both the γ phase and the δ phase can undergo the σ-phase transformation. For example, when Cr25Ni20-type welds are heated to 800°C–900°C, a strong γ→δ transformation occurs. For chromium-nickel austenitic stainless steels, especially chromium-nickel-molybdenum stainless steels, the δ→σ phase transformation is prone to occur. This is mainly because chromium and molybdenum have a significant σ-forming effect. When the content of δ-ferrite in the weld exceeds 12%, this transformation becomes very pronounced, resulting in considerable embrittlement of the weld metal. This is precisely why the content of δ-ferrite in the surfacing layer on the inner wall of hot-wall hydrogenation reactors is kept within the range of 3% to 10%. 2. Ferritic stainless steels and their welding characteristics Ferritic stainless steels are divided into two main categories: ordinary ferritic stainless steels and ultra-pure ferritic stainless steels. Among them, the ordinary ferritic stainless steels include Cr12–Cr14 types, such as 00Cr12 and 0Cr13Al; Cr16–Cr18 types, such as 1Cr17Mo; and Cr25–30 types. Due to the relatively high contents of carbon and nitrogen in conventional ferritic stainless steels, their processing, forming, and welding are quite difficult; moreover, their corrosion resistance is hard to guarantee, thereby limiting their applications. In ultra-pure ferritic stainless steels, the total amounts of carbon and nitrogen are strictly controlled—typically at three levels: 0.035%–0.045%, 0.030%, and 0.010%–0.015%. Additionally, necessary alloying elements are added to further improve the steel’s corrosion resistance and overall properties. Compared with ordinary ferritic stainless steels, ultra-pure high-chromium ferritic stainless steels exhibit excellent resistance to uniform corrosion, pitting, and stress corrosion, and are widely used in petrochemical equipment. Ferritic stainless steels have the following welding characteristics: ① Under the effect of high welding temperatures, in the heat-affected zone where the heating temperature exceeds 1000°C, especially in the vicinity of the weld, the grains grow rapidly. Even after rapid cooling following welding, it is impossible to prevent a sharp decrease in toughness and a higher tendency toward intergranular corrosion caused by grain coarsening. ② Ferritic steel inherently has a high chromium content, as well as higher levels of harmful elements such as carbon, nitrogen, and oxygen. It has a high brittle transition temperature and is relatively sensitive to notches. Therefore, the post-welding embrittlement phenomenon is relatively severe. ③ During prolonged heating and slow cooling at temperatures between 400°C and 600°C, embrittlement occurs at 475°C, resulting in a significant decrease in toughness at room temperature. After prolonged heating at temperatures between 550°C and 820°C, the σ phase readily precipitates from ferrite, which also significantly reduces its plasticity and toughness. 3. Martensitic stainless steels and their welding characteristics. Martensitic stainless steels can be classified into Cr13-type martensitic stainless steels, low-carbon martensitic stainless steels, and super martensitic stainless steels. Type Cr13 possesses ordinary corrosion resistance. It is a martensitic stainless steel based on Cr12; by the addition of alloying elements such as nickel, molybdenum, tungsten, and vanadium, it not only has certain corrosion resistance but also high strength at high temperatures as well as resistance to high-temperature oxidation. Welding characteristics of martensitic stainless steels: Cr13-type martensitic stainless steels exhibit a high tendency to harden in their welds and heat-affected zones; under air cooling, the welded joints develop hard and brittle martensite. Under the influence of welding restraint stresses and diffused hydrogen, welding cold cracks can occur quite easily. When the cooling rate is low, coarse ferrite and intergranular precipitated carbides form in the near-suture zone and the weld metal, significantly reducing the plasticity and toughness of the joint. After cooling, the weld and heat-affected zones of low-carbon and super martensitic stainless steels completely transform into low-carbon martensite; however, there is no significant hardening phenomenon, and they exhibit good weldability. II. Selection of stainless steel welding materials for pressure vessels 1. Selection of austenitic stainless steel welding materials The principle for selecting austenitic stainless steel welding materials is to ensure, without the formation of cracks, that the corrosion resistance and mechanical properties of the weld metal are roughly equivalent to those of the base material, or even higher; generally, it is required that their alloy composition be similar to that of the base material. For corrosion-resistant austenitic stainless steels, it is generally desirable to have a certain amount of ferrite, as this ensures good crack resistance as well as excellent corrosion resistance. However, in certain special media, such as in the weld metal of urea equipment, the presence of ferrite is not allowed; otherwise, its corrosion resistance will be reduced. For heat-resistant austenitic steels, control of the ferrite content in the weld metal should be considered. For austenitic steel weldments that are operated at high temperatures for extended periods, the ferrite content in the weld metal should not exceed 5%. Readers can estimate the corresponding ferrite content based on the Schaeffler diagram, using the chromium equivalent and nickel equivalent in the weld metal. 2. Selection of ferritic stainless steel welding materials. There are basically three types of ferritic stainless steel welding materials: 1) welding materials whose composition is roughly compatible with that of the base material; 2) austenitic welding materials; 3) nickel-based alloy welding materials, which are rarely used due to their high cost. For ferritic stainless steel welding materials, materials equivalent to the base metal can be used; however, under conditions of high restraint, cracks are likely to occur. Post-weld heat treatment can be employed to restore corrosion resistance and improve the plasticity of the weld joint. The use of austenitic welding materials eliminates the need for preheating and post-weld heat treatment; however, for various steels lacking stabilizing elements, sensitization in the heat-affected zone still occurs. Type 309 and Type 310 chromium-nickel austenitic welding materials are commonly used. For Cr17 steel, Type 308 welding consumables can also be used; welding consumables with a high alloy content help improve the plasticity of the weld joint. Austenitic or austenite-ferrite weld metal has roughly the same strength as the ferritic base metal, but in certain corrosive environments, the corrosion resistance of the weld can differ significantly from that of the base metal; this factor should be taken into consideration when selecting welding materials. 3. Selection of welding materials for martensitic stainless steels Among stainless steels, martensitic stainless steels are those whose properties can be adjusted through heat treatment. Therefore, to meet the required service properties—especially in the case of martensitic stainless steels used for high-temperature applications—the composition of the weld metal should be as close as possible to that of the base metal. To prevent cold cracking, austenitic welding materials can also be used; in this case, the strength of the weld will inevitably be lower than that of the base material. When the composition of the weld is similar to that of the base material, both the weld and the heat-affected zone will harden and become brittle; simultaneously, a tempering softening zone will appear in the heat-affected zone. To prevent cold cracking, components with a thickness of 3 mm or more often require preheating, and post-weld heat treatment is also frequently necessary to improve the joint properties. Since the coefficients of thermal expansion of the weld metal and the base material are essentially the same, it is possible to completely eliminate welding stresses after heat treatment. When the workpiece cannot be preheated or heat-treated, an austenitic microstructure weld can be chosen. Since such welds possess high plasticity and toughness, they can relieve welding stresses and absorb more hydrogen, thereby reducing the tendency for cold cracking in the joint. However, in joints made from this material, differences in thermal expansion coefficients may lead to shear stresses in the fusion zone under cyclic temperature conditions, which can result in joint failure. For simple Cr13-type martensitic steels, when welds without austenitic structure are used, there is little room for adjusting the weld composition; it generally remains the same as that of the base metal. However, it is necessary to limit harmful impurities such as S, P, and Si. In Cr13-type martensitic steel welds, Si can promote the formation of coarse martensite. Reducing the carbon content helps to decrease hardenability. The presence of small amounts of elements such as Ti, N, or Al in the weld can also refine the grain structure and reduce hardenability. For Cr12-based martensitic heat-resistant steels with multi-element alloying, their main application is in high-temperature resistance. Generally, austenitic welding materials are not used; it is desirable for the composition of the weld metal to be similar to that of the base metal. When adjusting the composition, it is necessary to ensure that no ferrite phase appears in the weld, as it is highly detrimental to its properties. Since the main components of Cr13-based martensitic heat-resistant steels are ferrite-forming elements such as Mo, Nb, W, V, etc., in order to maintain a homogeneous martensitic structure, it is essential to balance this with austenite-forming elements; that is, appropriate amounts of C, Ni, Mn, N, and other elements are required. Martensitic stainless steels have a relatively high tendency to cold cracking; therefore, it is essential to maintain low hydrogen levels, or even ultra-low hydrogen levels. This must be taken into consideration when selecting welding consumables. III. Key points for welding stainless steel used in pressure vessels 1. Key points for welding austenitic stainless steel Generally speaking, austenitic stainless steel possesses excellent weldability. Almost all fusion welding methods can be used to weld austenitic stainless steels. The thermophysical properties and microstructural characteristics of austenitic stainless steels determine the key aspects of their welding processes. ① Due to the low thermal conductivity and high coefficient of thermal expansion of austenitic stainless steel, significant deformation and welding stress are likely to occur during welding. Therefore, welding methods that concentrate welding energy should be selected whenever possible. ② Due to the low thermal conductivity of austenitic stainless steel, a greater penetration depth can be achieved compared to low-alloy steel under the same current. At the same time, due to its high resistivity, in shielded metal arc welding, a lower welding current is required compared to carbon steel or low-alloy steel electrodes of the same diameter, in order to prevent the electrode from turning red. ③ Welding specifications. High wire energy is generally not used for welding. In shielded metal arc welding, it is advisable to use electrodes with a small diameter and perform welding in multiple rapid passes. For welds requiring high quality, cooling can even be accelerated by pouring cold water. For pure austenitic stainless steels and super austenitic stainless steels, which are highly susceptible to hot cracking, the welding heat input must be strictly controlled to prevent excessive grain growth in the weld and the occurrence of welding hot cracks. ④ To improve the heat-cracking resistance and corrosion resistance of welds, special attention must be paid to keeping the welding area clean during welding, in order to prevent harmful elements from penetrating the weld. ⑤ Generally, preheating is not required when welding austenitic stainless steel. To prevent grain growth and carbide precipitation in the weld and heat-affected zone, and to ensure the plasticity, toughness, and corrosion resistance of the welded joint, a lower interpass temperature should be maintained, generally not exceeding 150°C. 2. Key points for welding ferritic stainless steel: Ferritic stainless steel contains a relatively high amount of elements that promote the formation of ferrite, and a relatively low amount of elements that promote the formation of austenite; as a result, such materials have a lower tendency to harden or develop cold cracks. Under the influence of the welding heat cycle, the grain size in the heat-affected zone of ferritic stainless steels increases significantly, resulting in a sharp decline in the toughness and plasticity of the joint. The degree of grain growth in the heat-affected zone depends on the maximum temperature reached during welding and the duration for which it is maintained. Therefore, when welding ferritic stainless steels, it is advisable to use minimal heat input—that is, methods that concentrate energy, such as TIG welding with low current or manual welding using electrodes of small diameter. Additionally, measures like employing a narrow groove design, high welding speed, and multi-pass welding should be adopted as much as possible, while strictly controlling the interpass temperature. Due to the effect of the welding thermal cycle, ordinary ferritic stainless steels become sensitized in the high-temperature region of the heat-affected zone, leading to intergranular corrosion in certain media. After welding, annealing at 700–850°C to homogenize chromium can restore its corrosion resistance. Ordinary high-chromium ferritic stainless steels can be welded using fusion welding methods such as shielded metal arc welding, gas shielded welding, and submerged arc welding. Due to the inherent low plasticity of high-chromium steel, as well as grain growth in the heat-affected zone and the accumulation of carbides and nitrides at grain boundaries caused by the welding thermal cycle, both the plasticity and toughness of the welded joint are very low. Cracks are likely to occur when welding materials with a chemical composition similar to that of the base material are used under high restraint. To prevent cracks and improve the plasticity and corrosion resistance of joints, the following process measures can be adopted, taking shielded metal arc welding as an example. ① Preheat to around 100 ~ 150°C to weld the material in a tough state. The higher the chromium content, the higher the preheating temperature should be. ② Use low wire energy and weld without oscillation. During multi-layer welding, the interlayer temperature should be kept below 150°C, and continuous welding should be avoided to reduce the effects of high-temperature embrittlement and embrittlement at 475°C. ③ An annealing treatment at 750 ~ 800°C is carried out after welding; due to the spheroidization of carbides and uniform distribution of chromium, corrosion resistance is restored and the plasticity of the joint is improved. After annealing, rapid cooling should be performed to prevent the formation of the σ phase and 475°C brittleness. 3. Key points for welding martensitic stainless steels For Cr13-type martensitic stainless steels, when using electrodes made of the same material for welding, in order to reduce the susceptibility to cold cracking and ensure the plasticity and toughness of the weld joint, low-hydrogen electrodes should be selected, and the following measures must also be taken: ① Preheating. The preheating temperature increases with the increase of carbon content in steel; it generally falls within the range of 100°C to 350°C. ② Post-heating. For welded joints with a high carbon content or high restraint, post-weld heat treatment is applied to prevent welding-induced hydrogen cracking. ③ Post-weld heat treatment. To improve the plasticity, toughness, and corrosion resistance of the welded joint, the post-weld heat treatment temperature is generally between 650°C and 750°C, with the holding time calculated at 1 hour per 25 mm. For super and low-carbon martensitic stainless steels, preheating is generally not required. However, when the restraint is high or the hydrogen content in the weld is relatively high, preheating and post-heating measures should be taken. The preheating temperature is typically 100°C to 150°C, while the post-weld heat treatment temperature ranges from 590°C to 620°C. For martensitic steels with a high carbon content. Or in situations where preheating before welding and post-weld heat treatment are difficult to implement, as well as when there is high restraint on the joint, austenitic welding materials can also be used in engineering to improve the plasticity and toughness of the welded joint and prevent crack formation. However, when the weld metal is of austenitic structure or predominantly austenitic in composition at this time, its strength is actually low compared to that of the base material. Moreover, there are significant differences between the weld metal and the base material in terms of chemical composition, microstructure, thermophysical properties, and mechanical properties; as a result, welding residual stresses are inevitable, and these stresses can easily lead to stress corrosion or high-temperature creep failure. IV. Welding of Duplex Stainless Steels 1. Types of Duplex Stainless Steels Due to their austenite + ferrite dual-phase structure, with roughly equal amounts of these two phases, duplex stainless steels possess the characteristics of both austenitic and ferritic stainless steels. The yield strength can reach 400 Mpa to 550 MPa, which is twice that of ordinary austenitic stainless steels. Compared with ferritic stainless steels, duplex stainless steels possess higher toughness and a lower brittle transition temperature, and their resistance to intergranular corrosion as well as their weldability have been significantly improved. At the same time, they retain some of the characteristics of ferritic stainless steels, such as brittleness at 475°C, high thermal conductivity, low linear expansion coefficient, superplasticity, and magnetism. Compared to austenitic stainless steels, duplex stainless steels have higher strength, particularly with a significant increase in yield strength; their resistance to pitting corrosion, stress corrosion, and corrosion fatigue has also seen notable improvements. Based on their chemical composition, duplex stainless steels can be classified into four categories: the Cr18 type, the Cr23 (without Mo) type, the Cr22 type, and the Cr25 type. The Cr25-type duplex stainless steels can be further divided into standard and super duplex stainless steels. Among them, the Cr22-type and Cr25-type have been more widely used in recent years. The duplex stainless steels used in our country are mostly of Swedish origin; the specific grades include: 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), and SAF2507 (Cr25 type). 2. Welding characteristics of duplex stainless steels ① Duplex stainless steels have good weldability. They do not exhibit embrittlement in the heat-affected zone during welding, as is the case with ferritic stainless steels; nor are they prone to welding hot cracks, unlike austenitic stainless steels. However, due to their high ferrite content, hydrogen-induced cold cracks may occur when the structural rigidity is high or when the hydrogen content in the weld is elevated. Therefore, it is crucial to strictly control the sources of hydrogen. ② To preserve the characteristics of duplex stainless steels, ensuring an appropriate ratio of austenite to ferrite in the microstructure of the weld joint is crucial for welding this type of steel. When the cooling rate of the post-weld joint is slow, the secondary phase transformation from δ to γ occurs more completely; consequently, a duplex microstructure with an appropriate phase ratio can be obtained at room temperature. This necessitates a sufficiently large amount of heat input during welding. Otherwise, if the post-weld cooling rate is rapid, the amount of δ-ferrite phase increases, leading to a significant deterioration in the plasticity, toughness, and corrosion resistance of the joint. 3. Selection of welding materials for duplex stainless steel: Welding materials used for duplex stainless steel feature a weld microstructure that is predominantly austenitic in nature. The contents of the key corrosion-resistant elements (such as chromium and molybdenum) are similar to those in the base material, thereby ensuring corrosion resistance on par with that of the base material. To ensure the austenite content in the weld, it is common to increase the levels of nickel and nitrogen, that is, to raise the nickel equivalent by about 2% to 4%. In duplex stainless steel base metals, there is generally a certain amount of nitrogen content. It is also desirable for welding materials to have a certain nitrogen content; however, this level should not be too high, otherwise porosity may occur. Thus, the higher nickel content becomes a key difference between the welding material and the base metal. The welding electrode that matches the chemical composition of the base material is selected based on the requirements regarding corrosion resistance and joint toughness; for example, when welding Cr22-type duplex stainless steel, a Cr22Ni9Mo3-type welding electrode such as E2209 can be used. When using acidic electrodes, slag removal is excellent and the weld appearance is aesthetically pleasing; however, their impact toughness is relatively low. When it is required that the weld metal possess high impact toughness and all-position welding must be performed, basic electrodes should be used. When welding the root seal, alkaline electrodes are usually used. When there are special requirements for the corrosion resistance of the weld metal, alkaline electrodes with super duplex steel composition should also be used. For solid gas-shielded welding wires, while ensuring that the weld metal possesses good corrosion resistance and mechanical properties, attention should also be paid to its weldability. For flux-cored wires, rutile-type or titania-calcium-type flux-cored wires can be used when an aesthetically pleasing weld shape is required; whereas wires with a higher alkalinity are more suitable when higher impact toughness is needed or when welding under high constraint conditions. For submerged arc welding, it is advisable to use wire with a smaller diameter in order to enable multi-pass welding under medium and low welding parameters, thereby preventing the embrittlement of the weld heat-affected zone and the weld metal; meanwhile, a suitable basic flux should be used. 4. Key points for welding duplex stainless steel: ① Control of the welding heat process. Factors such as weld input energy, interpass temperature, preheating, and material thickness all affect the cooling rate during welding, which in turn influences the microstructure and properties of the weld and the heat-affected zone. Both too fast and too slow cooling rates can affect the toughness and corrosion resistance of duplex steel welded joints. Too fast a cooling rate can lead to an excessive amount of alpha phase and an increased precipitation of Cr2N. A too slow cooling rate can lead to severe grain coarsening; it may even result in the precipitation of some brittle intermetallic compounds, such as the σ phase. Table 1 lists some recommended ranges for welding line energy and interpass temperature. When selecting the wire energy, the specific material thickness should also be taken into account; the upper limit of wire energy in the table is suitable for thick plates, while the lower limit is suitable for thin plates. When welding duplex steels with a high alloy content and an ω(Cr) of 25% as well as super stainless steels, to achieve the best weld properties, it is recommended to keep the maximum interpass temperature at 100°C. When heat treatment is required after welding, the interlayer temperature can be unrestricted. ② Post-weld heat treatment: It is preferable not to carry out heat treatment on duplex stainless steel after welding. However, when the α-phase content in the as-welded state exceeds the required level or harmful phases such as the σ-phase precipitate, post-weld heat treatment can be used to improve the properties. The heat treatment method used is water quenching. The heating during heat treatment should be as rapid as possible; a holding time at the heat treatment temperature of 5 to 30 minutes should be sufficient to restore phase equilibrium. During heat treatment, metal oxidation is very severe; therefore, the use of inert gas protection should be considered. Dual-phase steels with an ω(Cr) of 22% should be heat-treated at temperatures between 1050°C and 1100°C, while dual-phase steels and super-dual-phase steels with an ω(Cr) of 25% require heat treatment at temperatures between 1070°C and 1120°C. V. Welding example for stainless steel pressure vessels: A flash drum with a diameter of 800 mm and a wall thickness of 10 mm; the shell material is 0Cr18Ni9. Note: ① The diameter of the cylinder is 800 mm; welders can enter the cylinder to perform welding. Therefore, shielded metal arc welding is used for double-sided welding of the longitudinal and circumferential seams of the cylinder. ② This equipment has no manholes; therefore, the closing welds can only be welded from the outside. To ensure welding quality, TIG welding is used for the root pass. However, during TIG welding of stainless steel, the metal on the backside gets oxidized. Previously, the only solution was to apply argon shielding on the backside. But when the equipment is large or when argon shielding cannot be applied to the backside, this method results in a significant waste of argon, and inadequate protection may still occur. To overcome this processing difficulty, the Welding Division of Japan Oil & Fats Co., Ltd. developed and manufactured a back-side self-protective stainless steel TIG welding wire. This is a welding wire with a special coating; upon melting, this coating (i.e., flux) penetrates to the back side of the weld pool, forming a dense protective layer that functions similarly to the flux coating on a welding electrode. The usage method of this welding wire is exactly the same as that of ordinary TIG welding wires; its coating has no effect on the arc and weld pool formation. **This reduces the welding costs associated with TIG welding of stainless steel.** In this device, using backside argon protection results in significant argon waste; therefore, self-shielding welding wire is employed. ③ For the fillet welds between the pipe fitting and the flat flange, as well as those between the pipe fitting and the shell, given the shape of these welds and the welding conditions, shielded metal arc welding is generally used. If the diameter of the stub is too small, TIG welding can also be used to reduce the difficulty of welding. ④ The fillet welds joining the supports to the shell are non-pressure-bearing welds. Gas metal arc welding with pure CO2 as the shielding gas is employed; this method offers high efficiency and excellent weld formation. TFW-308L is a welding material grade; its welding material type is E308LT1-1 (AWS A5.22).