After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?
What is Austenitic stainless steel?
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:
Weld hot crack
Austenitic stainless steel has low thermal conductivity and a high coefficient of linear expansion, 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 crystal structure. If the content of phosphorus, tin, antimony, niobium and other impurity elements is high, a low melting point of eutectic will be formed in the intergranular. Liquefaction cracks are formed, 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.
Intergranular corrosion
According to the theory of depletion of chromium, chromium carbide is precipitated on the intergranular, resulting in the depletion of chromium at the grain boundary, which is the main cause of intergranular corrosion. For this reason, the selection of ultra-low carbon welding consumables or welding consumables containing niobium, titanium and other stabilizing elements is the main measure to prevent intergranular corrosion.
Stress corrosion cracking
Stress corrosion cracking usually manifests as a brittle failure, and the processing 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 microstructure change of welded joints or the existence of stress concentration and local concentration of corrosive medium are also the reasons that affect stress corrosion cracking.
Sigma-phase embrittlement of welded joints
The σ phase is a kind of brittle and hard intermetallic compound, which mainly gathers at 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 austenitic stainless steel, especially chromium-nickel-molybdenum type stainless steel, δ→σ phase transformation is easy to occur. 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 content of δ-ferrite at 3%~10%. reason.
After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?
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.
5 welding points of austenitic stainless steel
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.
Welding current
Due to the small thermal conductivity of austenitic stainless steel, it can obtain a 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.
Welding Specifications
Generally, high 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 The welding line energy should be strictly controlled to prevent the serious growth of the weld grains and the occurrence of welding hot cracks.
Welding area cleaning
In order to improve the thermal crack resistance and corrosion resistance of the weld, special attention should be paid to the cleanliness of the weld area during welding to prevent harmful elements from penetrating the weld.
Warm-up
Preheating is generally not required when welding austenitic stainless steel. In order to prevent the grain growth and the precipitation of carbides in the weld and the heat-affected zone, 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.
How to choose 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.
- Corrosion-resistant austenitic stainless steel generally contains 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 should not exist, 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%.
After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?
What is ferritic stainless steel
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.
3 welding characteristics of ferritic stainless steel
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, the carbon in the steel is strictly controlled. The total amount of nitrogen and 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:
Intergranular corrosion
Under the action of high welding temperature, the grains in the heat affected zone, especially in the near seam zone, will grow sharply when the heating temperature reaches above 1000 °C. High intergranular corrosion tendency.
Post-weld embrittlement
Ferritic steel itself contains high chromium content, more harmful elements such as carbon, nitrogen, oxygen, etc., high brittle transition temperature, and strong notch sensitivity. Therefore, the post-weld embrittlement phenomenon is more serious.
Toughness
When it is heated and cooled slowly for a long time at 400 ℃ ~ 600 ℃, embrittlement at 475 ℃ will occur, which will seriously reduce the normal temperature toughness. After long-term heating at 550 ℃ ~ 820 ℃, the σ phase is easily precipitated from the ferrite, and its plasticity and toughness are also significantly reduced.
How to choose ferritic stainless steel welding consumables
There are basically three types of ferritic stainless steel welding consumables:
- Welding consumables whose composition basically matches the base metal;
- Austenitic welding consumables;
- Nickel-based alloy welding consumables are rarely used due to their high price.
Precautions
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 consumables are commonly used.
For Cr17 steel, 308 type welding consumables can also be used, and the welding consumables with high alloy content are beneficial to improve the plasticity of welded joints.
Note: Austenitic or austenitic-ferritic weld metal is basically as strong as the ferrite base metal, but in some corrosive media, the corrosion resistance of the weld may be very different from the base metal. This point should be paid attention to when choosing welding consumables.
Welding process focus of ferritic stainless steel
- Ferritic stainless steel has relatively more ferrite-forming elements, relatively few austenite-forming elements, and the material is less prone to hardening and cold cracking.
- Under the action of welding thermal cycle, the grains in the heat-affected zone of ferritic stainless steel grow significantly, and the toughness and plasticity of the joint decrease sharply.
- The degree of grain growth in the heat-affected zone depends on the highest temperature reached during welding and its holding time. For this reason, when welding ferritic stainless steel, 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., and 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.
Three measures to prevent cracks after welding
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 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 the welding thermal cycle, the plasticity and toughness of the 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.
- 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.
- Use small line energy and do not swing welding. When multi-layer welding, the temperature between layers 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.
- 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.
After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?
What is Duplex Stainless Steel?
Duplex stainless steel has the characteristics of both austenitic stainless steel and ferritic stainless steel because it has an austenite + ferrite dual-phase structure, and the content of the two-phase structures is 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 obvious improvement in 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.
What is the classification of duplex stainless steel?
Duplex stainless steels are classified according to their chemical composition and can be divided into four types: Cr18 type, Cr23 (without Mo) type, Cr22 type, and Cr25 type.
For Cr25 type duplex stainless steel, it can be divided into ordinary type and super duplex stainless steel, among which Cr22 type and Cr25 type have been used more in recent years. Most of the duplex stainless steel used in my country is made in Sweden. The specific grades are 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), and SAF2507 (Cr25 type).
2 major welding requirements for duplex stainless steel
Strictly control hydrogen
Duplex stainless steel has good weldability. It is neither easy to embrittle the heat-affected zone when welding ferritic stainless steel, nor prone to welding hot cracks like austenitic stainless steel, but because it has a large amount of ferrite, when 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.
Welding heat input
To ensure the characteristics of dual-phase steel, it is the key to the welding of this type of steel to ensure that the proportion of austenite and ferrite in the microstructure of the welded joint is appropriate. When the cooling rate of the post-weld joint is slow, the second phase change of δ→γ is sufficient, so a duplex structure with a 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 delta ferrite phase will increase, resulting in a serious decrease in the plastic toughness and corrosion resistance of the joint.
Duplex stainless steel welding consumables selection
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.) sex. 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.
Corrosion resistance, joint toughness
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 the acid electrode is 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, the basic electrode should be used. Alkaline electrodes are usually used when the root is welded. When there are special requirements for the corrosion resistance of the weld metal, a basic electrode with super duplex steel composition should also be used.
Flux cored wire
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 weld 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.
Alkaline Flux
For submerged arc welding, a wire with a smaller diameter should be used to achieve 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 a matching alkaline flux should be used.
Top 2 welding requirements for duplex stainless steels
Requirements 1#: Control of Welding Thermal Process
Welding line energy, inter pass temperature, preheating and material thickness will affect the cooling rate during welding, thus affecting the structure and properties of the weld and heat-affected zone.
- Cooling rates that are too fast or too slow can affect the toughness and corrosion resistance of dual-phase steel welded joints. Excessive α-phase content and increased Cr2N precipitation are caused when the cooling rate is too fast.
- Too slow a cooling rate will cause serious coarse grains, and may even precipitate some brittle intermetallic compounds, such as σ phase.
- The specific material thickness should also be considered when selecting the line energy.
- When welding duplex steels and super stainless steels with a high ω(Cr) content of 25 %, 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.
Requirements 2#: post weld heat treatment
- It is best not to carry out heat treatment after welding of duplex stainless steel, but when the content of α phase exceeds the requirements in the welding state 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 should be 5 to 30 min, which should be sufficient to restore the equilibrium of the phases.
- Oxidation of the metal during heat treatment is very serious, and inert gas protection should be considered.
- For dual-phase steel with ω(Cr) of 22%, heat treatment should be carried out at 1050℃ ~ 1100℃, while dual-phase steel and super duplex steel with ω(Cr) of 25% should be heat treated at 1070℃ ~ 1120℃ heat treatment.
