The so-called stainless steel means that after a certain amount of chromium is added to the steel, the steel is in a passivated state and has the characteristics of no rust. For 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 the stainless steel. The stainless steel generally referred to 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 its 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 Crack - Austenitic stainless steel has low thermal conductivity and large linear expansion coefficient, so during the welding process, the high temperature residence time of the welded joint is long, and the weld is easy to form a coarse columnar grain structure. During the solidification and crystallization process If the content of sulfur, phosphorus, tin, antimony, niobium and other impurity elements is high, a low-melting eutectic will be formed between grains. When the welded joint is subjected to high tensile stress, it is easy to form solidification cracks in the weld. Liquefaction cracks are formed in the heat-affected zone, which belong to welding hot cracks. The most effective way to prevent hot cracks is to reduce impurity elements that are prone to low melting point eutectic in steel and welding consumables, and to make chromium-nickel austenitic stainless steel contain 4% to 12% of ferrite.
②Intergranular Corrosion - According to the theory of poor chromium, chromium carbide is precipitated on the intergranular, resulting in poor chromium at the grain boundary, which is the main reason for intergranular corrosion. For this reason, the selection of ultra-low carbon welding materials or welding materials containing stabilizing elements such as niobium and titanium is the main measure to prevent intergranular corrosion.
③Stress Corrosion Cracking - Stress corrosion cracking usually manifests as brittle failure, and the process time of failure is short, so the damage is serious. The main cause of stress corrosion cracking of austenitic stainless steel is welding residual stress. The structural 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.
④Embrittlement of σ phase - In welded joints σ phase is a kind of brittle and hard intermetallic compound, which mainly gathers in the grain boundaries of columnar grains. The sigma phase transition can occur in both the gamma phase and the delta phase. For example, when the Cr25Ni20 type weld is heated at 800 ℃ ~ 900 ℃, a strong γ→δ transformation will occur. For chromium-nickel-type austenitic stainless steel, especially chromium-nickel-molybdenum type stainless steel, δ→σ phase transformation is prone to occur, which is mainly due to the obvious σ effect of chromium and molybdenum elements. When the content of δ ferrite in the weld exceeds At 12%, the transformation of δ→σ is very obvious, causing obvious embrittlement of the weld metal, which is why the surfacing layer on the inner wall of the hot-wall hydrogenation reactor controls the content of δ-ferrite at 3%~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, form and weld, and it is difficult to ensure corrosion resistance, and the use is limited. In ultra-pure ferritic stainless steel, carbon and nitrogen in the steel are strictly controlled. The total amount of nitrogen is generally controlled at three levels of 0.035% ~ 0.045%, 0.030%, 0.010% ~ 0.015%, and necessary alloying elements are also added to further improve the corrosion resistance and comprehensive properties of the steel. Compared with ordinary ferritic stainless steel, ultra-pure high-chromium ferritic stainless steel has good resistance to uniform corrosion, pitting corrosion and stress corrosion, and is widely used in petrochemical equipment. Ferritic stainless steel has the following welding characteristics:
① Under the action of high temperature welding, the grains in the heat-affected zone, especially in the vicinity of the seam, will grow sharply when the heating temperature reaches above 1000 °C. Higher intergranular corrosion tendency.
② Ferritic steel itself contains high chromium content, and there are many harmful elements such as carbon, nitrogen, oxygen, etc., the brittle transition temperature is high, and the notch sensitivity is strong. Therefore, the post-weld embrittlement phenomenon is more serious.
③ When it is heated and cooled slowly at 400℃~600℃ for a long time, embrittlement will occur at 475℃, which will seriously reduce the normal temperature toughness. After being heated 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 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 certain corrosion resistance, it also has high high temperature strength and high temperature resistance. Oxidation properties.
Welding characteristics of martensitic stainless steel: Cr13 martensitic stainless steel has a particularly large tendency to harden the weld and heat affected zone. The welded joint can obtain hard and brittle martensite under the condition of air cooling. Under the action of , it is easy to appear welding cold cracks. When the cooling rate is small, coarse ferrite will be formed in the near seam area and the weld metal and carbides will be precipitated along the grain, which will significantly reduce the plasticity and toughness of the joint.
After the welding seam 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 it has 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 equal to or higher than the base metal under the condition of no cracks. Generally, the alloy composition is generally required to be roughly the same as that of the base metal. match. For corrosion-resistant austenitic stainless steel, it is generally desirable 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, otherwise its corrosion resistance will be reduced. For heat-resistant austenitic steels, the control of the ferrite content in the weld metal should be considered. For austenitic steel weldments operating at high temperatures for a long time, the ferrite content in the weld metal should not exceed 5%. The reader can estimate the corresponding ferrite content in terms of chromium equivalents and nickel equivalents in the weld metal from the Schaeffler diagram.
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 due to their high price.
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 easily generated. After welding, heat treatment can be used 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 without stabilizing elements, the sensitization of the heat-affected zone still exists, and 309-type and 310-type chromium-nickel austenitic welding materials are commonly used. For Cr17 steel, 308 type welding consumables can also be used. The welding consumables with high alloy content are beneficial to improve the plasticity of welded joints. 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 the base metal, which Be careful when choosing welding consumables.
3. Selection of Martensitic Stainless Steel Welding Consumables
Among the stainless steels, martensitic stainless steels can be adjusted by heat treatment. Therefore, in order to ensure the performance requirements, especially for heat-resistant martensitic stainless steels, 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 be hardened and 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 joint performance. Since the thermal expansion coefficient of the weld metal and the base metal is basically the same, it is possible to completely eliminate welding after heat treatment. stress.
When the workpiece does not allow preheating or heat treatment, the austenitic structure weld can be selected. Because the weld has high plasticity and toughness, it can relax the welding stress, and can dissolve more hydrogen, which can reduce the joint's strength. Cold cracking tendency, but this kind of joint with uneven material, due to the different coefficient of thermal expansion, under the working environment of cyclic temperature, shear stress may be generated in the fusion zone, resulting in joint failure.
For simple Cr13 martensitic steel, when the weld with austenitic structure is not used, there is little room for adjustment of the weld composition, which is generally the same as the base metal matrix, but the harmful impurities S, P and Si must be limited. Si can promote the formation of coarse martensite in the weld of Cr13 martensitic steel. Reducing the C content is conducive to reducing the hardenability. The presence 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 hot-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 primary ferrite phase does not appear in the weld, because it is very harmful to the performance, because the main components of the Cr13-based martensitic hot-strength steel are mostly ferrite elements (such as Mo, Nb, W, V, etc.), in order to ensure that the entire organization is uniform martensite, it must be balanced with austenite elements, that is, there must be appropriate C, Ni, Mn, N and other elements. Martensitic stainless steel has a relatively high tendency to cold crack, so it is necessary to strictly maintain low hydrogen, even ultra-low hydrogen, which must be paid attention to when selecting welding materials.
Stainless steel welding points 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. 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 generate 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, under the same current, a larger penetration depth can be obtained than low alloy steel. At the same time, due to its high resistivity, in order to avoid the 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, large line energy is not used for welding. In electrode arc welding, it is advisable to use small-diameter electrodes and fast 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 the sensitivity of hot cracks If it is large, the welding line energy should be strictly controlled to prevent the serious growth of the weld grains and the occurrence of welding hot cracks.
④ In order to improve the thermal crack resistance and corrosion resistance of the weld, special attention should be paid to the cleaning of the welding area during welding to prevent harmful elements from penetrating the weld.
⑤ Austenitic stainless steel generally does not need preheating when welding. In order to prevent the grain growth of the weld and the heat-affected zone and the precipitation of carbides, and to ensure the plasticity, toughness and corrosion resistance of the welded joint, the lower interlayer temperature should be controlled, generally not exceeding 150 °C.
2. Welding points of ferritic stainless steel
Ferritic stainless steel has relatively more ferrite-forming elements and relatively few 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 significantly, and the toughness and plasticity of the joint drop sharply. The degree of grain growth in the heat-affected zone depends on the highest temperature reached during welding and its holding time. Therefore, when welding ferritic stainless steel, the small line energy should be used as much as possible, that is, the method of energy concentration should be used, such as Small current TIG, small diameter electrode manual welding, etc. At the same time, measures such as narrow gap groove, high welding speed and multi-layer welding are adopted as much as possible, and the temperature between layers is strictly controlled.
Due to the effect of welding thermal cycle, general 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℃ 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 fusion welding methods. Due to the inherent low plasticity of high-chromium steels, as well as grain growth in the heat-affected zone and the accumulation of carbides and nitrides at the grain boundaries caused by welding thermal cycles, the plasticity and toughness of welded joints are very low. Cracks are easy 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 process measures can be taken.
① Preheat about 100 ~ 150 ℃, so that the material can be welded in a tough state. The higher the chromium content, the higher the preheating temperature should be.
② Use small line energy and do not swing welding. When multi-layer welding, the interlayer temperature should be controlled not to be higher than 150 °C, and continuous welding should not be performed to reduce the effect of high temperature embrittlement and 475 °C brittleness.
③ After welding, annealing at 750 ~ 800 ℃ can restore corrosion resistance and improve joint plasticity due to carbide spheroidization and uniform distribution of chromium. After annealing, it should be cooled quickly to prevent σ phase and brittleness at 475°C.
3. Welding points of martensitic stainless steel
For Cr13 martensitic stainless steel, when welding electrodes of the same material are used, 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 Warm up. The preheating temperature increases with the increase of the carbon content of the steel, generally in the range of 100°C ~ 350°C.
② After the heat. For welded joints with high carbon content or high restraint, post-heating measures should 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 steels, preheating measures are generally not required. When the degree of restraint is large or the hydrogen content in the weld is high, preheating and post-heating measures are adopted. The preheating temperature is generally 100°C ~ 150°C , the post-weld heat treatment temperature is 590 ~ 620 ℃.
For martensitic steels with higher carbon content. Or in the case that preheating before welding and post-welding heat treatment are difficult to implement, and the joint has a large degree of restraint, austenitic welding consumables can also be used in the project to improve the plasticity and toughness of the welded joint and prevent cracks. However, at this time, when the weld metal is austenite or austenite-based, it is actually a low-strength match compared with the strength of the base metal, and the weld metal and base metal are in chemical composition, metallographic structure, thermal The physical properties and mechanical properties are very different, and the welding residual stress is unavoidable, which can easily cause stress corrosion or high temperature creep damage.
Welding of Duplex Stainless Steels
1. Types of Duplex Stainless Steels
Duplex stainless steel has the characteristics of austenitic stainless steel and ferritic stainless steel because it has austenite + ferrite dual-phase structure, and the content of the two phase structures is basically the same. 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, and significantly improved intergranular corrosion resistance and welding performance. High conductivity, small coefficient of linear expansion, 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 the pitting corrosion resistance, stress corrosion resistance, corrosion fatigue resistance and other properties are also significantly improved.
2. Welding characteristics of duplex stainless steel
① Duplex stainless steel has good weldability. It is neither easy to embrittle the heat affected zone during welding like ferritic stainless steel, nor prone to welding hot cracks like austenitic stainless steel, but because of its 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 microstructure of the welded joint 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 more sufficient, so a duplex structure with a suitable phase ratio can be obtained at room temperature, which requires a suitable large welding heat transfer during welding. Otherwise, if the cooling rate after welding is fast, the delta ferrite phase will increase, resulting in a serious decrease in the plastic toughness and corrosion resistance of the joint.
3. Selection of duplex stainless steel welding consumables
Welding consumables for duplex stainless steel are characterized in that the weld structure is austenite-dominated duplex structure, and the content of main corrosion-resistant elements (chromium, molybdenum, etc.) is equivalent to that of the base metal, thereby ensuring equivalent corrosion resistance to 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 base metal of duplex stainless steel, there is generally a certain amount of nitrogen content, and a certain amount of nitrogen content is also expected in the welding material, but generally it should not be too high, otherwise pores will occur. The higher nickel content thus becomes a major difference between the welding consumable and the base metal.
According to the different requirements of corrosion resistance and joint toughness, the electrode that matches the chemical composition of the base metal should be selected, such as welding Cr22 duplex stainless steel, or Cr22Ni9Mo3 electrode, such as E2209 electrode. When acid electrodes are used, the slag removal is excellent, and the welding seam 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. Alkaline electrodes are usually used when root cap welding is performed. When there are special requirements for the corrosion resistance of the weld metal, a basic electrode of super duplex steel composition should also be used.
For solid gas shielded welding wire, while ensuring the weld metal has good corrosion resistance and mechanical properties, attention should also be paid to its welding process performance. For flux-cored welding wire, when the welding seam is required to be beautiful, rutile or titanium For calcium type flux-cored welding wire, when higher impact toughness is required or welding is performed under conditions of greater restraint, flux-cored welding wire with higher alkalinity should be used.
For submerged arc welding, a wire with a smaller diameter should be used to realize multi-layer multi-pass welding under small and medium welding specifications to prevent the embrittlement of the welding heat affected zone and the weld metal, and use the matching alkaline flux.
4. Welding points of duplex stainless steel
