Stainless Steel for Pressure Vessels and Its Welding Characteristics
The so-called stainless steel refers to adding a certain amount of chromium to the steel, so that the steel is in a passivated state and has the characteristics of not rusting. In order to achieve this purpose, its chromium content must be above 12%. In order to improve the passivation of steel, elements such as nickel and molybdenum that can passivate the steel are often added to stainless steel. Generally referred to as stainless steel is actually a general term for stainless steel and acid-resistant steel. Stainless steel is not necessarily acid-resistant, and acid-resistant steel generally has good stainless properties. Stainless steel can be divided into four categories according to the structure of the steel, namely austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and austenitic-ferritic duplex stainless steel.
1. Austenitic stainless steel and its welding characteristics
Austenitic stainless steel is the most widely used stainless steel, and the high Cr-Ni type is the most common. At present, austenitic stainless steel can be roughly divided into Cr18-Ni8 type, Cr25-Ni20 type, and Cr25-Ni35 type. Austenitic stainless steel has the following welding characteristics:
① Welding hot-cracked austenitic stainless steel has a small thermal conductivity and a large linear expansion coefficient, so during the welding process, the high-temperature residence time of the welded joint is longer, and the weld is easy to form a coarse columnar grain structure. If the content of impurity elements such as sulfur, phosphorus, tin, antimony, and niobium is high, a low melting point eutectic will be formed between the grains, and solidification cracks will easily form in the weld when the welded joint is subjected to high tensile stress. Liquefaction cracks are formed in the heat-affected zone, which all belong to welding heat cracks. The most effective way to prevent hot cracks is to reduce the impurity elements that are prone to produce low melting point eutectics in steel and welding consumables and to make the chromium-nickel austenitic stainless steel contain 4% to 12% ferrite structure.
② Intergranular corrosion According to the theory of chromium depletion, the precipitation of chromium carbide on the intergranular surface, resulting in the depletion of chromium in the grain boundary is the main cause of intergranular corrosion. Therefore, choosing ultra-low carbon welding consumables or welding consumables containing stabilizing elements such as niobium and titanium is the main measure to prevent intergranular corrosion.
③ Stress corrosion cracking Stress corrosion cracking usually manifests as brittle failure, and the damage process takes a short time, so the damage is serious. The main cause of stress corrosion cracking of austenitic stainless steel is welding residual stress. The structure change of welded joints or the existence of stress concentration, and the concentration of local corrosion medium are also the reasons that affect stress corrosion cracking.
④ σ phase embrittlement of welded joints σ phase is a kind of brittle and hard intermetallic compound, which mainly gathers in the grain boundaries of columnar grains. Both γ phase and δ phase can undergo σ phase transition. For example, when the Cr25Ni20 type weld is heated at 800 °C ~ 900 °C, a strong γ→δ transformation will occur. For chromium-nickel austenitic stainless steel, especially chromium-nickel-molybdenum stainless steel, δ→σ phase transformation is prone to occur, mainly because chromium and molybdenum elements have obvious sigma transformation, when the δ ferrite content in the weld exceeds At 12%, the transformation of δ→σ is very obvious, resulting in obvious embrittlement of the weld metal, which is why the surfacing layer on the inner wall of the hot wall hydrogenation reactor controls the δ ferrite content at 3% to 10%. reason.
2. Ferritic stainless steel and its welding characteristics
Ferritic stainless steel is divided into two categories: ordinary ferritic stainless steel and ultra-pure ferritic stainless steel. Among them, 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 content of carbon and nitrogen in ordinary ferritic stainless steel, it is difficult to process and weld, and the corrosion resistance is difficult to guarantee, so the use is limited. In ultra-pure ferritic stainless steel, the carbon and nitrogen in the steel are strictly controlled. The total amount of nitrogen is generally controlled at three levels of 0.035% to 0.045%, 0.030%, and 0.010% to 0.015%. At the same time, necessary alloying elements are added to further improve the corrosion resistance and comprehensive performance 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 where the heating temperature reaches above 1000°C, especially in the near seam area, will grow rapidly. Even if it is cooled rapidly after welding, the sharp decrease in toughness and High tendency to intergranular corrosion.
② Ferritic steel itself has a higher chromium content, more harmful elements such as carbon, nitrogen, oxygen, etc., a higher brittle transition temperature, and a stronger notch sensitivity. Therefore, post-weld embrittlement is more serious.
③ When heated and cooled slowly at 400°C ~ 600°C for a long time, embrittlement at 475°C will occur, which will seriously reduce the toughness at room temperature. After heating for a long time at 550 ° C ~ 820 ° C, the σ phase is easily precipitated from the ferrite, and its plasticity and toughness are also significantly reduced.
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 anti-corrosion performance. From Cr12-based martensitic stainless steel, due to the addition of nickel, molybdenum, tungsten, vanadium and other alloying elements, it not only has certain corrosion resistance, but also has high high-temperature strength and high-temperature resistance. Oxidation properties.
Welding characteristics of martensitic stainless steel: Cr13 type martensitic stainless steel weld seam and heat-affected zone have a particularly large hardening tendency, and the welded joint can obtain hard and brittle martensite under air-cooling conditions. Under the action of welding, it is easy to appear welding cold cracks. When the cooling rate is small, coarse ferrite and intergranular carbides will be formed in the near seam area and weld metal, which will significantly reduce the plasticity and toughness of the joint.
After the weld and heat-affected zone of low-carbon and super martensitic stainless steel are cooled, all of them are transformed into low-carbon martensite, but there is no obvious hardening phenomenon, and they have good welding performance.
Selection of Stainless Steel Welding Consumables for Pressure Vessels
1. Selection of austenitic stainless steel welding consumables
The selection principle of austenitic stainless steel welding consumables is to ensure that the corrosion resistance and mechanical properties of the weld metal are basically equivalent to or higher than those of the base metal under the condition of no cracks. match. For corrosion-resistant austenitic stainless steel, it is generally desired to contain a certain amount of ferrite, which can not only ensure good crack resistance, but also have good corrosion resistance. However, in some special media, such as the weld metal of urea equipment, ferrite is not allowed to exist, otherwise its corrosion resistance will be reduced. For heat-resistant austenitic steels, the control of ferrite content in the weld metal should be considered. For austenitic steel weldments operated at high temperature for a long time, the ferrite content in the weld metal should not exceed 5%. Readers can estimate the corresponding ferrite content according to the chromium equivalent and nickel equivalent in the weld metal according to the Schaeffler diagram.
picture
2. Selection of ferritic stainless steel welding consumables
There are basically three types of ferritic stainless steel welding consumables: 1) welding consumables whose composition basically matches the base metal; 2) austenitic welding consumables; 3) nickel-based alloy welding consumables, which are rarely used because of their high prices.
Ferritic stainless steel welding consumables can be made of materials equivalent to the base metal, but when the degree of restraint is large, cracks are easy to occur. Heat treatment can be used after welding to restore corrosion resistance and improve joint plasticity. The use of austenitic welding consumables can avoid preheating and post-weld heat treatment, but for various steels that do not contain stable elements, the sensitization of the heat-affected zone still exists, and 309 and 310 chromium-nickel austenitic welding consumables are commonly used. For Cr17 steel, 308 welding consumables can also be used. Welding consumables with high alloy content are beneficial to improve the plasticity of welded joints. The austenitic or austenitic-ferritic weld metal is basically as strong as the ferritic base metal, but in some corrosive media, the corrosion resistance of the weld may be very different from that of the base metal. Pay attention when choosing welding materials.
3. Selection of martensitic stainless steel welding consumables
In stainless steel, martensitic stainless steel can be adjusted by heat treatment. Therefore, in order to ensure the performance requirements, especially for heat-resistant martensitic stainless steel, the composition of the weld should be as close as possible to the composition of the base metal. In order to prevent cold cracks, austenitic welding consumables can also be used, and the weld strength at this time must be lower than that of the base metal.
When the composition of the weld is similar to that of the base metal, the weld and the heat-affected zone will harden and become brittle at the same time, and a temper-softening zone will appear in the heat-affected zone. In order to prevent cold cracking, components with a thickness of more than 3mm often need to be preheated, and heat treatment is often required after welding to improve the performance of the joint. Since the thermal expansion coefficient of the weld metal and the base metal are basically the same, it is possible to completely eliminate the weld after heat treatment. stress.
picture
When the workpiece is not allowed to be preheated or heat treated, austenitic weld seam can be selected. Because the weld seam has high plasticity and toughness, it can relax the welding stress and can dissolve more hydrogen, thus reducing the stress of the joint. Cold cracking tendency, but the joints with uneven materials, due to the different thermal expansion coefficients, may generate shear stress in the fusion zone under the working environment of circulating temperature, resulting in joint failure.
For the simple Cr13 type martensitic steel, when the weld with austenitic structure is not used, there is not much room for adjustment of the weld composition, which is generally the same as the base metal matrix, but harmful impurities such as S, P and Si must be limited. Si can promote the formation of coarse martensite in Cr13 martensitic steel welds. Reducing the content of C is beneficial to reduce the hardenability, and the existence of a small amount of elements such as Ti, N or Al in the weld can also refine the grains and reduce the hardenability.
For multi-component alloyed Cr12-based martensitic heat-strength steel, the main purpose is heat resistance, and austenitic welding consumables are usually not used, and the weld composition is expected to be close to the base metal. When adjusting the composition, it must be ensured that the weld does not appear a ferrite phase, because it is very harmful to the performance, because the main components of Cr13-based martensitic heat-strength steel are mostly ferrite elements (such as Mo, Nb, W, V, etc.), in order to ensure that the entire structure is uniform martensite, it must be balanced with austenite elements, that is, there must be appropriate elements such as C, Ni, Mn, and N.
Martensitic stainless steel has a very high tendency to cold cracking, so it is necessary to strictly maintain low hydrogen, even ultra-low hydrogen, and this must be paid attention to when selecting welding materials.
Key Points of Stainless Steel Welding for Pressure Vessels
1. Key points of austenitic stainless steel welding
In general, austenitic stainless steels have excellent weldability. Almost all fusion welding methods can be used to weld austenitic stainless steel, and the thermophysical properties and microstructure characteristics of austenitic stainless steel determine the key points of its welding process.
① Due to the small thermal conductivity and large thermal expansion coefficient of austenitic stainless steel, it is easy to produce large deformation and welding stress during welding, so the welding method with concentrated welding energy should be selected as much as possible.
② Due to the small thermal conductivity of austenitic stainless steel, it can obtain larger penetration depth than low alloy steel under the same current. At the same time, due to its high resistivity, in order to avoid redness of the electrode during arc welding, the welding current is smaller than that of carbon steel or low alloy steel electrodes of the same diameter.
③ Welding specifications. Generally do not use large input energy for welding. For electrode arc welding, it is advisable to use small-diameter electrodes for rapid multi-pass welding. For high-demand welds, even pour cold water to accelerate cooling. For pure austenitic stainless steel and super austenitic stainless steel, due to thermal crack sensitivity If it is large, the welding line energy should be strictly controlled to prevent the serious growth of weld grains and the occurrence of welding hot cracks.
④ In order to improve the thermal cracking resistance and corrosion resistance of the weld, special attention should be paid to the cleanliness of the welding area during welding to prevent harmful elements from penetrating the weld.
⑤ Austenitic stainless steel generally does not require preheating during welding. In order to prevent grain growth and carbide precipitation in the weld seam and heat-affected zone, and ensure the plasticity, toughness and corrosion resistance of the welded joint, a lower interlayer temperature should be controlled, generally not exceeding 150 °C.
2. Ferritic stainless steel welding points
Ferritic stainless steel has relatively more ferrite-forming elements, relatively less austenite-forming elements, and the material has less tendency to harden and cold crack. Under the action of welding thermal cycle of ferritic stainless steel, the grains in the heat-affected zone grow obviously, and the toughness and plasticity of the joint decrease sharply. The degree of grain growth in the heat-affected zone depends on the maximum temperature reached during welding and its holding time. Therefore, when welding ferritic stainless steel, a small line energy should be used as much as possible, that is, a method of energy concentration, such as Small current TIG, manual welding with small diameter electrodes, etc. At the same time, measures such as narrow gap groove, high welding speed and multi-layer welding should be adopted as much as possible, and the temperature between layers should be strictly controlled.
Due to the effect of welding heat cycle, generally ferritic stainless steel is sensitized in the high temperature zone of the heat affected zone, and intergranular corrosion occurs in some media. After welding, it is annealed at 700~850°C to homogenize the chromium and restore its corrosion resistance.
Ordinary high-chromium ferritic stainless steel can be welded by electrode arc welding, gas shielded welding, submerged arc welding and other welding methods. Due to the inherent low plasticity of high-chromium steel, as well as the grain growth in the heat-affected zone and the accumulation of carbides and nitrides at the grain boundaries caused by welding heat cycles, the plasticity and toughness of welded joints are very low. Cracks are likely to occur when welding consumables with similar chemical composition to the base metal are used and the degree of restraint is large. In order to prevent cracks and improve joint plasticity and corrosion resistance, taking electrode arc welding as an example, the following technological measures can be taken.
① Preheat at about 100 ~ 150°C to weld the material in a tough state. The higher the chromium content, the higher the preheating temperature should be.
② Welding with small input energy and no swing. During multi-layer welding, the temperature between layers should be controlled not to be higher than 150°C, and continuous welding should not be used to reduce the effects of high temperature embrittlement and 475°C embrittlement.
③ After welding, annealing at 750 ~ 800 °C can restore the corrosion resistance and improve the plasticity of the joint due to the spheroidization of carbides and uniform distribution of chromium. After annealing, it should be cooled quickly to prevent the occurrence of σ phase and brittleness at 475 °C.
3. Martensitic stainless steel welding points
For Cr13 type martensitic stainless steel, when using electrodes of the same material for welding, in order to reduce the sensitivity of cold cracks and ensure the plasticity and toughness of the welded joints, low-hydrogen electrodes should be selected and the following measures should be taken at the same time:
① Preheat. The preheating temperature increases with the increase of the carbon content of the steel, generally in the range of 100°C to 350°C.
② After heating. For welded joints with high carbon content or high restraint, post-heating measures shall be taken after welding to prevent welding hydrogen-induced cracks.
③ Post-weld heat treatment. In order to improve the plasticity, toughness and corrosion resistance of welded joints, the post-weld heat treatment temperature is generally 650 ° C ~ 750 ° C, and the holding time is calculated as 1h / 25mm.
For super and low-carbon martensitic stainless steel, preheating measures are generally not required. When the restraint degree is large or the hydrogen content in the weld is high, preheating and postheating measures are taken. The preheating temperature is generally 100 ° C ~ 150 ° C , post-weld heat treatment temperature is 590 ~ 620 ℃. For martensitic steels with higher carbon content. Or when pre-welding preheating and post-welding heat treatment are difficult to implement, and the joints are highly restrained, austenitic welding consumables can also be used in engineering to improve the plasticity and toughness of welded joints and prevent cracks. But at this time, when the weld metal is austenitic or austenite-based, it is actually a low-strength match compared with the strength of the base metal, and the weld metal and the base metal are different in chemical composition, metallographic structure, thermal The physical and mechanical properties are very different, and the welding residual stress is inevitable, which can easily cause stress corrosion or high temperature creep damage.
Welding of duplex stainless steel
1. Types of duplex stainless steel
Duplex stainless steel has austenite + ferrite duplex structure, and the content of the two phase structures
Basically the same, so it has the characteristics of austenitic stainless steel and ferritic stainless steel. The yield strength can reach 400Mpa ~ 550MPa, which is twice that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has high toughness, low brittle transition temperature, significantly improved intergranular corrosion resistance and welding performance; at the same time, it retains some characteristics of ferritic stainless steel, such as 475 °C brittleness, thermal High conductivity, small coefficient of linear expansion, superplasticity and magnetism. Compared with austenitic stainless steel, the strength of duplex stainless steel is high, especially the yield strength is significantly improved, and the performance of pitting corrosion resistance, stress corrosion resistance, and corrosion fatigue resistance is also significantly improved.
Duplex stainless steel is classified according to its chemical composition, and can be divided into four types: Cr18 type, Cr23 (excluding Mo), Cr22 type and Cr25 type. For Cr25 duplex stainless steel, it can be divided into common type and super duplex stainless steel, among which Cr22 type and Cr25 type have been widely used in recent years. Most of the duplex stainless steels used in my country are produced in Sweden, and the specific grades are: 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), SAF2507 (Cr25 type).
2. Welding characteristics of duplex stainless steel
① Duplex stainless steel has good weldability. It is not easy to embrittle the heat-affected zone during welding like ferritic stainless steel, nor is it easy to produce welding hot cracks like austenitic stainless steel. However, because it has a large amount of ferrite, When the rigidity is high or the hydrogen content of the weld is high, hydrogen cooling cracks may occur, so it is very important to strictly control the source of hydrogen.
② In order to ensure the characteristics of dual-phase steel, ensuring that the proportion of austenite and ferrite in the structure of the welded joint is appropriate is the key to welding this type of steel. When the cooling rate of the joint after welding is slow, the secondary phase change of δ→γ is relatively sufficient, so a duplex structure with a relatively suitable phase ratio can be obtained at room temperature, which requires a suitable large welding heat input during welding. Otherwise, if the cooling rate after welding is fast, the δ ferrite phase will increase, resulting in a serious decrease in the plasticity, toughness and corrosion resistance of the joint.
3. Selection of duplex stainless steel welding consumables
Welding consumables for duplex stainless steel, which are characterized in that the weld structure is a duplex structure dominated by austenite, and the content of main corrosion-resistant elements (chromium, molybdenum, etc.) is equivalent to that of the base metal, thereby ensuring the same corrosion resistance as the base metal sex. In order to ensure the content of austenite in the weld, the content of nickel and nitrogen is usually increased, that is, the nickel equivalent is increased by about 2% to 4%. In the duplex stainless steel base material, there is generally a certain amount of nitrogen content, and a certain amount of nitrogen content is also expected in the welding consumables, but generally it should not be too high, otherwise pores will occur. In this way, the high nickel content has become a major difference between the welding material and the base metal.
According to the different requirements of corrosion resistance and joint toughness, choose the electrode that matches the chemical composition of the base metal, such as welding Cr22 duplex stainless steel, you can choose Cr22Ni9Mo3 electrode, such as E2209 electrode. When acidic electrodes are used, the slag removal is good and 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, alkaline electrodes should be used. Basic electrodes are usually used when root backing is welded. When there are special requirements for the corrosion resistance of the weld metal, basic electrodes with super duplex steel components should also be used.
For solid gas shielded welding wire, while ensuring that the weld metal has good corrosion resistance and mechanical properties, attention should also be paid to its welding process performance. For flux cored wire, when the weld shape is required to be beautiful, rutile or titanium For calcium-type flux-cored wire, when higher impact toughness is required or welding under conditions of greater restraint, a flux-cored wire with higher alkalinity should be used.
For submerged arc welding, it is advisable to use welding wire with a smaller diameter to realize multi-layer and multi-pass welding under small and medium-sized welding specifications, so as to prevent embrittlement of the welding heat-affected zone and weld metal, and use matching alkaline flux.
4. Welding points of duplex stainless steel
① Control of welding heat process Welding heat energy, interlayer temperature, preheating and material thickness will all affect the cooling rate during welding, thus affecting the structure and performance of the weld and heat-affected zone. Too fast or too slow a cooling rate will affect the toughness and corrosion resistance of duplex steel welded joints. When the cooling rate is too fast, it will cause excessive α phase content and increase the precipitation of Cr2N. If the cooling rate is too slow, the crystal grains will be severely coarsened, and even some brittle intermetallic compounds, such as σ phase, may be precipitated. Table 1 lists some recommended welding line energies and interpass temperature ranges. When selecting the line energy, the specific material thickness should also be considered. 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 steel with 25% ω(Cr) and super stainless steel with high alloy content, in order to obtain the best weld metal properties, it is recommended that the maximum interpass temperature be controlled at 100°C. When heat treatment is required after welding, the interpass temperature may not be limited.
② Post-weld heat treatment It is best not to heat-treat duplex stainless steel after welding, but when the content of α phase in the as-welded state exceeds the requirement or when harmful phases, such as σ phase, are precipitated, post-weld heat treatment can be used to improve. The heat treatment method used is water quenching. During heat treatment, the heating should be as fast as possible, and the holding time at the heat treatment temperature is 5 ~ 30min, which should be sufficient to restore the equilibrium of the phases. Metal oxidation is very serious during heat treatment, and inert gas protection should be considered. For the dual-phase steel with 22% ω (Cr), heat treatment should be carried out at the temperature of 1050 ° C ~ 1100 ° C, while the dual-phase steel and super dual-phase steel with 25 % ω (Cr) require heat treatment at the temperature of 1070 ° C ~ 1120 ° C Carry out heat treatment.
Welding example of stainless steel pressure vessel
The flash tank with a diameter of 800mm and a wall thickness of 10mm is made of 0Cr18Ni9.
illustrate:
① The diameter of the cylinder is 800mm, and the welder can drill into the cylinder for welding. Therefore, the longitudinal and circular seams of the cylinder are welded on both sides by electrode arc welding.
② There is no hole in this equipment, so the closing weld can only be welded from the outside. In order to ensure the welding quality, TIG welding is used as the backing. However, the back metal will be oxidized during argon arc welding of stainless steel. In the past, only the method of filling argon on the back can be used for protection. not good. In order to solve this process difficulty, the Welding Division of Nippon Oil & Fat Company developed and manufactured a back self-protecting stainless steel TIG welding wire, which is a welding wire with a special coating, and the coating (that is, the coating) will penetrate into the molten pool after melting On the back, a dense protective layer is formed, which is equivalent to the role of the electrode coating. The use of this welding wire is exactly the same as that of ordinary TIG welding wire, and the coating will not affect the front arc and molten pool shape, which greatly reduces the welding cost of stainless steel argon arc welding. In this equipment, if the rear argon protection is used, the argon waste is serious, so the self-shielding welding wire is used.
③ For the fillet welds between the connecting pipe and the flat welding flange, and between the connecting pipe and the shell, in view of the shape and welding conditions of the welds at this part, electrode arc welding is generally used. If the diameter of the connecting pipe 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, and the gas shielded welding is used (the shielding gas is pure CO2), which has high efficiency and good weld shape. TFW-308L is the welding consumable grade, and its welding consumable model is E308LT1-1 (AWS A5.22).
