Selection of welding wire for austenitic stainless steel
Understanding of stainless steel
Stainless steel refers to the steel with the main additive element Cr higher than 12%, which can make the steel in the passivation state and has the characteristics of stainless steel. Stainless steel is actually the general name of stainless steel (resistant to weak corrosive media such as atmosphere and water) and acid resistant steel (resistant to strong corrosive media such as acid, alkali and salt). Stainless steel is not necessarily acid resistant, but acid resistant steel generally has good rust resistance.
Classification of stainless steel
According to its microstructure, stainless steel can be divided into Ferritic, martensitic, Austenitic, austenitic+ Ferritic and precipitation hardening stainless steels.
Stainless steel can be roughly divided into high chromium stainless steel and high chromium nickel stainless steel according to the main alloy elements.
High chromium stainless steel includes martensitic stainless steel and ferritic stainless steel.
High chromium nickel stainless steels include Austenitic, austenitic+ Ferritic and precipitation hardening stainless steels.
Martensitic stainless steel: commonly represented by Cr13 Stainless steel, it has martensitic structure at room temperature, such as 1Cr13, 2Cr13, 3Cr13, 1Cr17Ni2 and 9Cr18MoV.
Ferritic stainless steel: commonly represented by Cr13 and Cr17, it has ferritic structure at room temperature, such as 0Cr13, 0Cr13Al, 1Cr17, 1cr17ti and 1Cr25Ti.
Austenitic stainless steel: roughly divided into:
- Cr18-Ni8 type, such as 0Cr18Ni9, 0Cr17Ni12Mo2, etc;
- Cr25-Ni13 type, such as 0Cr23Ni13;
- Cr25-Ni20 type, such as 0Cr25Ni20;
- Cr25-Ni35 type, such as 4Cr25Ni35 (foreign cast stainless steel);
- Super austenitic type, such as 254SMO (20Cr-18Ni-6Mo).
Cr18Ni9 stainless steel is a basic chromium nickel austenitic stainless steel.
The stainless and corrosion resistance of stainless steel are relative and conditional. At present, there is no stainless steel with corrosion resistance to any corrosive environment, so the selection of stainless steel should be reasonably considered according to the specific service conditions. Technicians should pay attention to when replacing materials.
Structural characteristics of austenitic stainless steel:
Generally, the structure at room temperature is pure austenite, and some are austenite+ a small amount of ferrite, which helps to prevent hot cracks.
It cannot be strengthened by heat treatment. However, it has significant cold work hardening, and the strength can be improved by cold deformation.
The work hardening produced by cold deformation can be softened by solution treatment.
Welding characteristics of austenitic stainless steel
(1) Hot crack.
Try to make the weld metal show duplex structure, and the content of ferrite shall be controlled below 3-5%. Because ferrite can dissolve a large amount of harmful s and P impurities.
High quality electrode with alkaline coating shall be selected as far as possible to limit the content of S, P, C, etc. in weld metal.
(2) Intergranular corrosion: according to the chromium poor theory, chromium carbide precipitates on the grain boundary when the weld and heat affected zone are heated to 450-850 ℃ sensitization temperature zone, resulting in chromium poor grain boundary, which is not enough to resist corrosion.
Use low carbon or ultra-low carbon welding materials, such as A002; Electrodes containing titanium, niobium and other stabilizing elements are used, such as A137, A132, etc.
A certain amount of ferrite forming elements are melted into the weld by welding wire or electrode, so that the weld metal becomes a duplex structure of austenite+ ferrite (ferrite is generally controlled at 4-12%).
Reduce the overheating of welding pool, select smaller welding current and faster welding speed to speed up the cooling speed.
Post weld stabilization annealing treatment (for weldments with high requirements for intergranular corrosion resistance): 850 ℃/2-3H, air cooling.
(3) Stress corrosion cracking:
Stress corrosion cracking- delayed cracking of welded joints under tensile stress in a specific corrosive environment.
Stress corrosion cracking of austenitic stainless steel welded joints is a serious failure form of welded joints, which is characterized by brittle failure without plastic deformation.
Macroscopic characteristics of stress corrosion cracking: the crack begins to expand from the surface to the inside, and pitting corrosion is often the root of the crack. Various corrosion products and oxidation phenomena are often attached to the fracture.
Three factors affecting stress corrosion cracking: chemical composition, tensile stress and working medium.
Chemical composition: different materials have different sensitivity to stress corrosion.
Working medium: mainly the influence of medium concentration and temperature:
① For stress corrosion cracking of carbon steel and low alloy steel:
- Existence of H2S medium: H2S concentration reaches saturation state; The temperature of H2S aqueous solution has the greatest cracking tendency near room temperature.
- Existence of NaOH medium: alkali embrittlement can be produced in almost all concentration ranges of more than 5% NaOH, and it is most dangerous near 30% NaOH. The critical temperature of alkali embrittlement is about boiling point, and the minimum temperature of alkali embrittlement is about 60 ℃.
② For stress corrosion cracking of austenitic stainless steel:
- Existence of chloride medium: stress corrosion cracking can occur almost as long as Cl- exists; With the increase of temperature, stress corrosion cracking accelerates. In dilute solution with low Cl- concentration, there is a SCC sensitive temperature range, generally 150-300 ℃.
- Existence of NaOH medium: SCC occurs when the concentration of OH- is higher than 0.1%, and 40-42% NaOH is the most dangerous concentration; The lowest temperature for stress corrosion cracking is about 115 ℃ (for 40-42% NaOH).
- Existence of high temperature and high pressure water medium: the concentration of dissolved oxygen (o) has a great influence on the stress corrosion cracking tendency of stainless steel, and there is a sensitive temperature range of 150-300 ℃, which is most prone to stress corrosion cracking near 300 ℃.
- Joint tensile stress: the existence of tensile stress is a prerequisite for SCC. Compressive stress does not cause SCC. According to the investigation, the stress causing SCC is mainly residual stress, accounting for about 80%, of which the residual stress caused by welding accounts for about 30%.
Stress corrosion cracking prevention measures:
- ① Reasonable selection of corrosion-resistant materials. In practice, it is reasonable to select austenitic stainless steel with high Cr, Ni and high mo. duplex stainless steel has the best SCC resistance, and super austenitic stainless steel shows obvious stress corrosion resistance;
- ② Minimize stress concentration and reduce high stress areas.
Reasonably formulate the forming processing and assembly process, reduce the cold work deformation as much as possible, avoid forced assembly, and prevent various scars in the assembly process (various assembly scars and arc burns will become the crack source of SCC and easy to cause corrosion pits).
- Reasonable selection of welding materials: the weld shall be well matched with the base metal without any adverse structure, such as grain coarsening and hard brittle martensite;
- Adopt appropriate welding process: ensure good weld formation and no defects such as stress concentration or pitting corrosion, such as undercut; Adopt reasonable welding sequence to reduce the level of welding residual stress;
- Stress relief treatment: post weld heat treatment, such as complete annealing or annealing after welding; When it is difficult to carry out heat treatment, post weld hammering or shot peening shall be adopted.
Production management measures: control of impurities in medium, such as O2, N2, H2O, etc. in liquid ammonia medium; H2S in LPG; O2, FE3+, Cr6+ in chloride solution; Anti corrosion treatment: such as coating, lining or cathodic protection; Add corrosion inhibitor.
(4) Low temperature embrittlement of weld metal:
For austenitic stainless steel welded joints, the plastic toughness of weld metal is the key problem when used at low temperature. At this time, the existence of ferrite in the weld microstructure always worsens the low-temperature toughness.
Therefore, except for single-phase austenitic steel, other types of stainless steel are not suitable for low temperature conditions.
Preventive measures: single austenitic weld can be obtained by selecting pure austenitic welding materials and adjusting welding process.
(5) σ phase embrittlement of welded joints:
The weldment will precipitate a brittle in the weld after being heated at high temperature for a certain time σ Phase, resulting in embrittlement of the whole joint and significant decrease in plasticity and toughness.
The precipitation temperature of the σ phase ranges from 650 ℃ to 850 ℃. During high temperature heating, the σ phase is mainly transformed from ferrite. The longer the heating time, The more σ phase precipitates.
Limit the ferrite content in the weld metal (less than 15%); Super alloyed welding materials, i.e. high nickel welding materials, are adopted.
- Small specification is adopted to reduce the residence time of weld metal at high temperature;
- For precipitated σ phase shall be subject to solid solution treatment when conditions permit σ Phase dissolves into austenite.
Selection of welding wire for austenitic stainless steel
Stainless steel is mainly used for corrosion resistance, but it is also used as heat-resistant steel and low-temperature steel. Therefore, when welding stainless steel, the performance of the electrode must be consistent with the purpose of stainless steel. Stainless steel electrode must be selected according to base metal and working conditions (including working temperature and contact medium, etc.).
- (1) Generally speaking, the selection of welding wire can refer to the material of base metal, and the welding wire with the same or similar composition with the base metal can be selected. For example, A102 corresponds to 0Cr19Ni9; A137 corresponds to 1Cr18Ni9Ti.
- (2) Because the carbon content has a great iMpact on the corrosion resistance of stainless steel, stainless steel electrodes with carbon content of deposited metal not higher than that of base metal are generally selected. A022 electrode must be selected for 316L.
- (3) The weld metal of austenitic stainless steel shall ensure mechanical properties. It can be verified by process qualification.
- (4) For heat-resistant stainless steel (austenitic heat-resistant steel) working at high temperature, the selected electrode shall mainly meet the hot crack resistance of weld metal and the high temperature performance of welded joint.
Austenitic ferritic stainless steel electrodes are generally used for austenitic heat-resistant steels with Cr/Ni ≥ 1, such as 1Cr18Ni9Ti. It is appropriate that the weld metal contains 2-5% ferrite. When the ferrite content is too low, the crack resistance of weld metal is poor; If it is too high, it is easy to form during long-term use or heat treatment at high temperature σ Brittle phase, resulting in cracks. Such as A002, A102 and A137.
In some special applications, when full austenitic weld metal may be required, such as A402, A407 welding wire, etc.
For stable austenitic heat-resistant steels with Cr/Ni < 1, such as cr16Ni25Mo6, it is generally necessary to increase the content of Mo, W, Mn and other elements in the weld metal while ensuring that the chemical composition of the weld metal is roughly similar to that of the base metal, so as to improve the crack resistance of the weld metal while ensuring the thermal strength of the weld metal. If A502 and A507 are used.
(5) For corrosion-resistant stainless steel working in various corrosive media, the welding wire shall be selected according to the medium and working temperature, and its corrosion resistance shall be guaranteed (the corrosion performance test of welded joints shall be conducted).
For the medium with strong corrosivity and working temperature above 300 ℃, Ti or Nb stabilizing element or ultra-low carbon stainless steel electrode must be used. Such as A137 or A002.
For the medium containing dilute sulfuric acid or hydrochloric acid, stainless steel electrodes containing Mo or Mo and Cu are often selected, such as A032, A052, etc.
Stainless steel electrodes without Ti or Nb can be used for equipment working at room temperature with weak corrosivity or only to avoid corrosion pollution.
In order to ensure the stress corrosion resistance of weld metal, superalloyed welding materials are used, that is, the content of corrosion-resistant alloy elements (Cr, Mo, Ni, etc.) in weld metal is higher than that of base metal. If 00cr18Ni12Mo2 welding material (such as A022) is used to weld 00Cr19Ni10 weldment.
(6) For austenitic stainless steel working at low temperature, the low-temperature iMpact toughness of welded joints at service temperature shall be ensured, so pure austenitic electrode shall be used. Such as A402 and A407.
(7) Nickel base alloy electrode can also be selected. For example, nickel base welding materials with Mo up to 9% are used to weld Mo6 super austenitic stainless steel.
(8) Selection of electrode coating type:
Because the weld metal of Duplex Austenitic steel contains a certain amount of ferrite and has good plasticity and toughness, the difference between alkaline coated electrode and titanium calcium coated electrode is not as significant as that of carbon steel electrode from the perspective of crack resistance of weld metal. Therefore, in practical application, welding electrodes with coating type code of 17 or 16 (such as a102a, A102, a132, etc.) are mostly used from the aspect of welding process performance.
Only when the structural rigidity is large or the crack resistance of weld metal is poor (such as some martensitic chromium stainless steel, chromium nickel stainless steel with pure austenite structure, etc.), the alkaline coated stainless steel electrode with coating code of 15 (such as A107, A407, etc.) shall be considered.
Welding of Ferritic Austenitic duplex stainless steel
(1) Understanding of duplex stainless steel at home and abroad:
Application: mainly used in petrochemical industry, equipment and pipelines for seawater and wastewater treatment with strong corrosive medium (such as high chloride ion content).
Organization and performance characteristics:
It contains high chromium (18-28%) and low nickel (4-10%). The structure at room temperature is austenite+ ferrite, and the ferrite content is usually not less than 50%.
The yield strength can reach 400-550Mpa, which is twice that of ordinary stainless steel
At the same time, it has some characteristics of austenitic stainless steel and ferritic stainless steel, good toughness, high strength, chloride stress corrosion resistance and better than ordinary stainless steel.
It has good welding performance: coMpared with austenitic stainless steel, it has low hot cracking tendency; CoMpared with ferritic stainless steel, it has a lower tendency of heating embrittlement.
There are many varieties of duplex stainless steel, and there are three most commonly used, namely:
Ultra low carbon 18Cr-5Ni-3m O type (also known as Cr18 type and 1805 type):
For example, 00Cr18Ni5Mo3Si2 (1805) listed in the national standard GB4237 hot rolled steel plates of duplex stainless steel; American ASTM S31500 (18Cr-5Ni-3Mo-N, lower limit of tensile strength 630Mpa)
23Cr-4ni Mo type (also known as Cr23 Mo free type and 2305 type):
For example, ASTM S32304 (23Cr-4Ni-mo-cu-N, lower limit of tensile strength 600MPa)
22Cr-5Ni-3m O type (also known as Cr22 type and 2205 type):
For example, ASTM S31803 (22Cr-5Ni-3Mo-N, lower limit of tensile strength 620MPa)
25Cr-7Ni-4mo (also known as Cr25 and 2507): it can be divided into ordinary and super types.
For example, the common duplex stainless steel listed in the national standard GB4237-92 hot rolled steel plates of duplex stainless steel and gb13296-91 has 0Cr26ni5Mo2; American ASTM S31260 (ordinary type, 25Cr-6Ni-3mo-cu-N-w, lower limit of tensile strength 690Mpa), ASTM S32750 (super type, 25Cr-7Ni-4Mo-N, lower limit of tensile strength 800MPa).
(2) Welding characteristics of duplex stainless steel:
It has good weldability and low sensitivity to hot crack. When the proportion of duplex structure is appropriate, the sensitivity to cold crack is also low. However, when the degree of restraint is large and the hydrogen content of weld metal is high, there is still the risk of hydrogen induced crack due to the action of ferrite in duplex structure.
“475 ℃ brittleness” occurs when duplex steel exists for a long time in the range of 300-500 ℃, so the service temperature of duplex steel is often lower than 250 ℃.
Specific electrode selection of various types of duplex steel:
Cr18 duplex steel: Cr22-Ni9-Mo3 ultra low carbon welding materials (such as AWS A5.4 E2209 and er2209 grades) shall be selected; Austenitic stainless steel welding materials containing Mo can also be selected, such as A022Si (E316l-16) and A042 (E309mol-16).
Cr23 Mo free duplex steel: Cr22-Ni9-Mo3 ultra low carbon welding materials (such as AWS A5.4 E2209 and er2209 grades) shall be selected; Austenitic stainless steel welding materials can also be selected, such as A062 (E309l-16).
Cr22: Cr22-Ni9-Mo3 ultra low carbon welding materials (such as AWS A5.4 E2209 and er2209); Austenitic stainless steel welding materials containing Mo can also be selected, such as A042 (E309mol-16).
Cr25 type: select Cr25-Ni5-Mo3 or Cr25-Ni5-Mo4 ultra low carbon welding materials (such as AWS A5.4 e2553 and er2553); High Mo nickel base welding materials without NB can also be selected, such as nicrmo-3 welding materials without NB.
Preheating: not required. The interlayer temperature shall not be higher than 100 ℃.
Welding line energy: Cr18 duplex steel is not more than 15kj/cm; 10-25kj/cm for Cr23 Mo free and Cr22 duplex steel; Cr25 duplex steel is 10-15kj/cm.
Post weld heat treatment: not required.
(3) Key points of welding process for duplex stainless steel:
TIG welding, manual arc welding, MIG welding and submerged arc welding are all acceptable.
To prevent σ Phase and the normal austenite/ferrite phase ratio of weld metal, the “super alloyed” welding material, i.e. high nickel welding material, shall also be used for welding dual-phase steel.
In order to prevent carbide precipitation, the carbon content of weld metal shall be controlled at the level of ultra-low carbon (0.03%).
When there are special requirements for the corrosion resistance of weld metal, alkaline electrode with super duplex steel composition shall also be used.
Welding of super austenitic stainless steel
At present, the new steel widely developed and applied abroad is used in the environment of strong corrosion resistance such as chloride ion. Our coMpany will be in contact with Yantai Wanhua MDI project and Ningbo Formosa Plastics NAE project.
The most typical super austenitic stainless steel abroad is ASTM S31254 (254SMO), with nominal composition of 20Cr-18Ni-6Mo and strength about 50% higher than 316L.
(1) Organization and performance characteristics:
The chemical composition is between ordinary austenitic stainless steel and nickel base alloy, and contains high alloying elements such as Mo, N and Cu, so as to improve the stability and corrosion resistance of austenitic structure, especially the stress corrosion cracking resistance of Cl-.
The structure of the steel is pure austenite.
Cold work hardening tends to be large, and there is a large rebound after cold forming. Therefore, there must be a certain forming allowance during processing. No heat treatment is required after cold forming.
Long time hot forming heating is easy to lead to serious peeling. Solution treatment is generally required after hot forming.
(2) Welding features: the same as other austenitic stainless steels.
(3) Key points of welding process:
Welding method: tungsten argon arc welding is preferred, followed by manual arc welding.
Welding materials: nickel based welding materials, such as welding wire ASTM A5.14 ERNiCrMo-3; Electrode ASTM A5.11 ENiCrMo-12.
Preheating: not required, and the interlayer temperature shall not be higher than 100 ℃.
Welding line energy: control.
Post weld heat treatment: generally not required. If self fusion welding (i.e. TIG welding without filler wire) is adopted, solid solution treatment and quenching shall be carried out to ensure corrosion resistance. It is recommended not to use self fusion welding process as far as possible.
Source: Network Arrangement – China Welding Wire Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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