Welding technology for stainless steel pressure vessels

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I. Stainless steels for pressure vessels and their welding characteristics. Stainless steel refers to steel to which a certain amount of chromium is added, enabling it to enter a passivated state and thus possessing rust-resistant properties. To achieve this, its chromium content must be over 12%. To improve the passivation property of steel, elements such as nickel and molybdenum, which can passivate the steel, are often added to stainless steel 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, while acid-resistant steels generally have good rust resistance. Stainless steel can be divided into four categories based on its microstructure: austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and austenitico-ferritic duplex stainless steel. 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 occur in the weld, while liquefaction cracks can form in the heat-affected zone; all of these are considered welding hot cracks. The most effective way to prevent thermal cracking is to reduce the impurity elements in steel and welding materials that tend to form low-melting eutectics, and to include 4% to 12% ferritic structure in chromium-nickel austenitic stainless steels. ② Intergranular corrosion: According to the chromium-depletion theory, the precipitation of chromium carbide at the grain boundaries leads to chromium depletion in those areas, which is the main cause of intergranular corrosion. To this end, selecting 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 σ-phase embrittlement in welded joints: The σ-phase is a brittle and hard intermetallic compound that primarily accumulates at the grain boundaries of columnar grains. Both the γ phase and the δ phase can undergo a σ phase transformation. For example, when Cr25Ni20-type welds are heated at 800°C to 900°C, a strong γ→δ transformation occurs. In chromium-nickel-type austenitic stainless steels, especially those of the chromium-nickel-molybdenum type, a δ→σ phase transformation tends to occur. This is primarily due to the σ-forming effect of chromium and molybdenum elements; when the content of δ-ferrite in the weld exceeds 12%, the δ→σ transformation becomes significant, resulting in a marked increase in the brittleness of the weld metal. This is why, in the lining layers of hot-wall hydrogenation reactors, the content of δ-ferrite is kept between 3% and 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. The ordinary ferritic stainless steels include the Cr12 ~ Cr14 series, such as 00Cr12 and 0Cr13Al ； Cr16 ~ Cr18 types, such as 1Cr17Mo ； Type Cr25 ~ 30. Due to the high levels of carbon and nitrogen in conventional ferritic stainless steels, they are difficult to process and weld, and their corrosion resistance is hard to ensure, which limits their use. In ultra-pure ferritic stainless steels, the total amounts of carbon and nitrogen in the steel are strictly controlled, typically within the ranges of 0.035%–0.045%, 0.030%, and 0.010%–0.015%. Additionally, necessary alloying elements are incorporated to further improve the steel’s corrosion resistance and overall performance. 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 caused by grain coarsening, as well as a higher tendency toward intergranular corrosion. 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 400°C to 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 divided into Cr13-type martensitic stainless steels, low-carbon martensitic stainless steels, and supermartensitic stainless steels. Type Cr13 possesses general corrosion resistance. It is a martensitic stainless steel based on Cr12; by adding 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-seam area and the weld metal, significantly reducing the plasticity and toughness of the joint. After cooling, the welds and heat-affected zones of low-carbon and supermartensitic stainless steels do transform entirely into low-carbon martensite, but there is no significant hardening, resulting in 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 the weld metal of urea plants, the presence of ferrite is not allowed, as it otherwise reduces its corrosion resistance. 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 consistent with that of the base material ； 2) Austenitic welding materials ； 3) Nickel-based alloy welding materials are rarely used due to their high cost. Ferritic stainless steel welding materials can use materials equivalent to those of the base metal, but cracks can easily occur under high restraint; heat treatment can be applied after welding to restore corrosion resistance and improve the ductility of the joint. The use of austenitic welding materials eliminates the need for preheating and post-weld heat treatment, but sensitization in the heat-affected zone still occurs in various steels that lack stabilizing elements; 309 and 310 chromium-nickel austenitic welding materials are commonly used. For Cr17 steel, 308-type welding materials can also be used; welding materials with a higher alloy content help to improve the plasticity of the welded joint. Austenitic or austenite-ferritic weld metal has essentially 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 steel: Among stainless steels, martensitic stainless steel can have its properties adjusted through heat treatment; therefore, to meet the requirements regarding performance, especially in the case of martensitic stainless steel used in high-temperature applications, the composition of the weld should be as similar as possible to that of the base material. 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, while a tempering softening zone appears in the heat-affected zone. To prevent cold cracking, components with a thickness of 3 mm or more often require preheating, and heat treatment is also usually necessary after welding to improve the properties of the joint. Since the thermal expansion coefficients of the weld metal and the base material are essentially the same, welding stresses can potentially be completely eliminated through 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, harmful impurities such as S, P, and Si must be kept under control. 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 multi-component alloyed Cr12-based martensitic heat-resistant steels, the main application is heat resistance; austenitic welding materials are generally not used, and it is desirable that the composition of the weld be similar to that of the base material. 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, austenite-forming elements must be used to balance this out; that is, appropriate amounts of C, Ni, Mn, N, and other elements are required. Martensitic stainless steels have a fairly high tendency to cold cracking; therefore, it is essential to maintain low hydrogen levels, or even ultra-low hydrogen levels. This aspect must be taken into consideration when selecting welding materials. III. Key points for welding stainless steel used in pressure vessels 1. Key points for welding austenitic stainless steel Generally speaking, austenitic stainless steel has excellent weldability. Almost all fusion welding methods can be used to weld austenitic stainless steels, and the thermophysical properties and microstructural characteristics of these steels determine the key aspects of their welding processes. ①Due to the low thermal conductivity and high coefficient of thermal expansion of austenitic stainless steels, significant deformation and welding stresses tend to occur during welding; therefore, welding methods that concentrate welding energy should be used as much as possible. ②Due 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. ③Welding specifications. High wire energy is generally not used for welding. In shielded metal arc welding, it is advisable to use electrodes of small diameter and carry out rapid multi-pass welding. For welds with high quality requirements, cold water may even be used to accelerate cooling. In the case of pure austenitic stainless steels and super-austenitic stainless steels, due to their high sensitivity to thermal cracking, it is essential to strictly control the welding heat input in order to prevent excessive growth of weld grains and the occurrence of welding thermal cracks. ④To 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. ⑤Austenitic stainless steel generally does not require preheating during welding. 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, it has 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 ductility of the joint. The degree of grain growth in the heat-affected zone depends on the highest temperature reached during welding and the duration for which it is maintained. Therefore, when welding ferritic stainless steels, it is advisable to use a low wire energy, that is, to employ methods that concentrate energy, such as TIG welding with low current or manual welding using electrodes of small diameter. At the same time, it is important to use narrow-gap grooves, high welding speeds, and multi-layer welding, while strictly controlling the interlayer 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 is carried out to homogenize the chromium, thereby restoring 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 caused by the welding heat cycle and the accumulation of carbides and nitrides at grain boundaries, the weld joints exhibit very low plasticity and toughness. 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. ①Preheat 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. ②Use 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 brittleness and brittleness at 475°C. ③An 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 applied to prevent the formation of the σ phase and brittleness at 475°C. 3. Key points for welding martensitic stainless steel: For Cr13-type martensitic stainless steel, when welding with electrodes of the same material, in order to reduce susceptibility to cold cracking and ensure the plasticity and toughness of the welded joint, low-hydrogen electrodes should be used, along with the following measures: 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. ②Post-heating. For welded joints with a high carbon content or high restraint, post-weld heat treatment should be applied to prevent welding hydrogen-induced cracking. ③Post-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 necessary. When there is high constraint or a high hydrogen content in the weld, preheating and post-heating measures should be taken; the preheating temperature is usually between 100°C and 150°C, while the post-weld heat treatment temperature is between 590°C and 620°C. For martensitic steels with a high carbon content. Alternatively, in situations where pre-welding heating and post-welding heat treatment are difficult to carry out, or when the joint has a high degree of restraint, austenitic welding materials can also be used in engineering applications to improve the plasticity and toughness of the weld 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, which 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 each phase, 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 exhibit higher toughness and a lower brittle transition temperature, as well as significantly improved resistance to intergranular corrosion and weldability ； At the same time, it retains 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 exhibit higher strength; in particular, their yield strength shows a significant improvement. Additionally, their resistance to pitting corrosion, stress corrosion, and corrosion fatigue has also been markedly enhanced. 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. Cr25-type duplex stainless steels can be further divided into ordinary types and super duplex stainless steels; among them, 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 steel: ① Duplex stainless steel has good weldability; it does not suffer from brittle formation in the heat-affected zone during welding like ferritic stainless steel, nor does it prone to weld heat cracks like austenitic stainless steel. However, due to its high content of ferrite, hydrogen-induced cold cracking may occur when there is high rigidity or a high hydrogen content in the weld, so it is very important to strictly control the sources of hydrogen. ②To maintain the properties of duplex steel, ensuring an appropriate ratio of austenite to ferrite in the microstructure of the welded joint is key to welding such steel. When the cooling rate of the weld joint after welding is slow, the secondary phase transformation from δ to γ occurs more thoroughly; as a result, a biphase structure with a favorable phase ratio can be obtained at room temperature. This requires an appropriately high amount of heat input during welding. Otherwise, if the cooling rate after welding is fast, the amount of δ-ferrite phase increases, leading to a significant decline 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 main corrosion-resistant elements (such as chromium and molybdenum) are similar to those in the base material, thereby ensuring corrosion resistance comparable to 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 the base metal of duplex stainless steel, there is generally a certain amount of nitrogen, and it is also desirable for the welding material to contain a certain amount of nitrogen as well; however, this level should not be too high, otherwise pores may form. Thus, the higher nickel content becomes a key difference between the welding material and the base metal. The welding rod 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 rod such as E2209 can be used. When acidic electrodes are used, slag removal is excellent and the weld shape is good, but the impact toughness is low. When high impact toughness of the weld metal is required and welding in all positions is necessary, alkaline electrodes should be used. When welding the root seam, 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 attractive weld appearance 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; additionally, an appropriate 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 welded joints in duplex steel. Too fast a cooling rate can lead to an excessive amount of α phase and an increased precipitation of Cr2N. Too slow a cooling rate can cause the grains to become significantly coarse, and it may even lead to the formation of some brittle intermetallic compounds, such as the σ phase. Table 1 lists some recommended ranges for welding wire 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 ω(Cr) of 25% as well as super stainless steels, to achieve optimal 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. Image 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. During heat treatment, heating should be carried out as quickly as possible, and the 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; 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, whereas 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.