austenitic stainless steel

Selection of welding rod for austenitic stainless steel (marketing department of Shanghai HY Industry Co., Ltd)

1, the understanding of stainless steel.
Stainless steel is the main element Cr higher than 12%, can make steel in a passivated state, but also has the characteristics of stainless steel. Stainless steel is actually stainless steel (resistant to atmospheric, water and other weak corrosive media) and acid-resistant steel (resistant to acids, alkalis, salts and other strong corrosive media) under the general name. Stainless steel is not necessarily acid-resistant, while acid-resistant steel generally has good stainless properties.
Classification of stainless steel
Stainless steel is divided into ferritic, martensitic, austenitic, austenitic + ferritic and precipitation hardening stainless steel according to its microstructure.
Stainless steel is roughly divided into high chromium type stainless steel and high chromium-nickel type stainless steel according to the main alloy elements.
High chromium stainless steel includes two major types of stainless steel: martensitic stainless steel and ferritic stainless steel.
◆High chromium-nickel stainless steels include austenitic, austenitic + ferritic and precipitation-hardening stainless steels.
▼Martensitic stainless steels: commonly represented by Cr13 stainless steels with martensitic organisation at room temperature, such as 1Cr13, 2Cr13, 3Cr13, 1Cr17Ni2, 9Cr18MoV.
▼ Ferritic stainless steels: commonly represented by Cr13 and Cr17, with ferritic organization at room temperature, such as 0Cr13, 0Cr13Al, 1Cr17, 1Cr17Ti, 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 (cast stainless steel)
◇Super austenitic type, such as 254SMo (20Cr-18Ni-6Mo).
☆Cr18Ni9 stainless steel is the basic chromium-nickel austenitic stainless steel.

Stainless steel stainless steel and corrosion resistance is relative and conditional, there is no stainless steel with corrosion resistance to any corrosive environment, so the choice of stainless steel should be based on the specific conditions of use to be reasonably considered. Technicians should pay attention when making material substitutions.
● Organizational characteristics of austenitic stainless steels.
◆ Usually the organization at room temperature is pure austenite, there are some for austenite + a small amount of ferrite, this small amount of ferrite helps to prevent thermal cracking.
◆Can not be strengthened by heat treatment. However, it has significant cold work hardening and can be strengthened by cold deformation.
◆The process hardening produced by cold deformation can be softened by solid solution treatment.
2, austenitic stainless steel welding characteristics.
(1) Thermal cracking.
★Prevention measures.
◇Try to make the weld metal is duplex organization, the content of ferrite is controlled at 3-5% or less. Because ferrite can dissolve a lot of harmful S, P impurities.
◇Try to use alkaline flux coating of high quality welding rod to limit the content of S, P, C, etc. in the weld metal.
(2) intergranular corrosion: According to the theory of poor chromium, the weld and heat-affected zone in the heating to 450-850 ℃ sensitization temperature zone in the grain boundary precipitation of chromium carbide, resulting in poor chromium grain boundary, not enough to resist the degree of corrosion.
★Prevention measures.
◇Use low-carbon or ultra-low-carbon welding consumables, such as A002, etc.; use welding rods containing titanium, niobium and other stabilizing elements, such as A137, A132, etc.
◇Melt a certain amount of ferrite forming elements into the weld by the welding wire or electrode, so that the weld metal becomes austenite + ferrite duplex, (ferrite is generally controlled at 4-12%).

◇Reduce the overheating of the welding pool, choose a smaller welding current and faster welding speed, and accelerate the cooling speed.
◇Stabilized annealing treatment after welding (for weldments with high requirements for intergranular corrosion resistance): 850 ℃/2-3h, air cooling.
(3) Stress corrosion cracking.
● Stress corrosion cracking – the delayed cracking of welded joints subjected to tensile stress in a specific corrosive environment.
Stress corrosion cracking of welded joints of austenitic stainless steel is a more serious form of failure of welded joints, manifested as brittle damage without plastic deformation.
The macroscopic characteristics of stress corrosion cracking: cracks begin to expand from the surface to the interior, and pitting is often the root cause of the cracking. The fracture is often accompanied by various corrosion products and oxidation phenomena.
The three elements that affect stress corrosion cracking: chemical composition, tensile stress, and working medium.

▲ Chemical composition: different materials themselves differ in their sensitivity to stress corrosion.
▲Working medium: mainly the concentration of the medium and the influence of temperature.
① For stress corrosion cracking of carbon steel and low alloy steel.
◇The presence of H2S medium: the concentration of H2S reaches saturation; the temperature of the aqueous H2S solution has the greatest tendency to crack near room temperature.
Presence of NaOH media: Alkali embrittlement can occur in almost all concentration ranges above 5% NaOH, and is most dangerous near 30% NaOH. The critical temperature of alkali embrittlement is about the boiling point, and the minimum temperature of alkali embrittlement is about 60°C.
② For stress corrosion cracking of austenitic stainless steel.
◇The presence of chloride media: almost as long as there is the presence of Cl, stress corrosion cracking can occur; temperature increases, stress corrosion cracking accelerates in dilute solutions with little Cl concentration, there is a SCC sensitive temperature range, generally at 150-300 ℃.
◇The presence of NaOH medium: the concentration of OH- is higher than 0.1% that SCC occurs, 40-42% NaOH is the most dangerous concentration; the lowest temperature at which stress corrosion cracking occurs is about 115°C (for 40-42% NaOH).

◇The presence of high-temperature, high-pressure water media: where the concentration of dissolved oxygen (O) has a great impact on the tendency of stainless steel stress corrosion cracking, and the presence of a sensitive temperature range of 150-300°C, the most likely to produce stress corrosion cracking near 300°C.
▲Joint tensile stress: the presence of tensile stress is a prerequisite for SCC. According to the survey, the stress causing SCC is mainly residual stress, accounting for about 80%, of which residual stress caused by welding accounts for about 30%.
★Stress corrosion cracking prevention measures.
◇Reasonable design: ①Reasonable choice of corrosion-resistant materials, practical use of high Cr, Ni and high Mo austenitic stainless steel is reasonable, duplex stainless steel has the best SCC resistance, super austenitic stainless steel shows obvious stress corrosion resistance; ②Minimize stress concentration and reduce high stress areas.
◇Reasonable formulation of forming processing and assembly process, minimize the degree of cold work deformation, avoid forced assembly, and prevent various scars caused during assembly (various assembly scars and arc burns can become a source of SCC cracks and easily cause corrosion pits).

◇Reasonable choice of welding material: the weld seam should be well matched with the base material and not produce any bad tissue, such as grain coarsening and hard and brittle martensite, etc.
◇Adopt suitable welding technology: ensure that the weld seam is well formed and does not produce any defects of stress concentration or pitting, such as biting edges, etc.; adopt a reasonable welding sequence to reduce the level of residual stress in the weld.
◇Stress relief treatment: post-weld heat treatment, such as complete annealing or annealing after welding; adopting post-weld hammering or shot blasting when it is difficult to implement heat treatment, etc.
◇Production management measures: control of impurities in the medium, such as O2, N2, H2O, etc. in liquid ammonia medium; H2S in liquefied petroleum gas; O2, Fe3+, Cr6+, etc. in chloride solution; anti-corrosion treatment: such as coating, lining or cathodic protection, etc.; addition of corrosion inhibitors.
(4) Low-temperature embrittlement of weld metal.
For austenitic stainless steel welded joints, in low-temperature use, the plastic toughness of the weld metal is the key issue. At this time, the presence of ferrite in the weld tissue always worsens the low-temperature toughness.
★ It follows that, in addition to single-phase austenitic steel, all other types of stainless steel are not suitable for low-temperature conditions.
★ Preventive measures: through the choice of pure austenitic welding material and adjust the welding process to obtain a single austenitic weld.
(5) σ-phase embrittlement of welded joints.
The weld will precipitate a brittle σ-phase in the weld after being heated at high temperature for a certain period of time, resulting in embrittlement of the entire joint and a significant decrease in plasticity and toughness.
The precipitation temperature of the σ phase ranges from 650-850°C. During the high temperature heating process, the σ phase is mainly transformed from ferrite. The longer the heating time, the more σ phase precipitates.
★Preventive measures.
◇Limit the ferrite content in the weld metal (less than 15%); use super-alloyed welding materials, i.e. high nickel welding materials.
◇Adopt small specifications to reduce the residence time of the weld metal at high temperatures.
◇The σ phase that has been precipitated is solid solution treated when conditions permit, so that the σ phase is dissolved into the austenite.
3, austenitic stainless steel welding rod selection points.
Stainless steel is mainly used for corrosion resistance, but 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 use of stainless steel. Stainless steel electrodes must be selected according to the base material and working conditions (including working temperature and contact medium, etc.).
(1) Generally speaking, the choice of welding rod can refer to the material of the parent material, and the choice of the same or similar composition of the parent material welding rod. For example: A102 corresponds to 0Cr19Ni9; A137 corresponds to 1Cr18Ni9Ti.
(2) due to the carbon content of stainless steel corrosion resistance has a great impact, therefore, the general choice of molten metal carbon content is not higher than the parent material of stainless steel welding rod. Such as 316L must choose A022 welding rod.

(3) austenitic stainless steel weld metal should ensure mechanical properties. Can be verified by process assessment
(4) For heat-resistant stainless steel (austenitic heat-resistant steel) working at high temperatures, the electrode selected should mainly meet the weld metal resistance to thermal cracking and high temperature performance of the welded joint.
◆ for Cr / Ni ≥ 1 austenitic heat-resistant steel, such as 1Cr18Ni9Ti, etc., are generally used austenitic – ferritic stainless steel welding rod, to weld metal containing 2-5% ferrite is appropriate. When the ferrite content is too low, the weld metal crack resistance is poor; if too high, it is easy to form σ brittle phase at high temperatures for long-term use or heat treatment, resulting in cracks. Such as A002, A102, A137.
In some special applications, when full austenite weld metal may be required, for example, A402, A407 welding electrodes can be used.
◆ For Cr/Ni <1 stable austenitic heat-resistant steel, such as Cr16Ni25Mo6, etc., should generally ensure that the weld metal has a chemical composition roughly similar to that of the parent material, while increasing the content of Mo, W, Mn and other elements in the weld metal, so as to ensure the thermal strength of the weld metal at the same time, improve the crack resistance of the weld. Such as the use of A502, A507.

(5) For corrosion-resistant stainless steel working in a variety of corrosive media, the welding rod should be selected according to the medium and working temperature, and to ensure its corrosion resistance (to do the corrosion performance test of the welded joint).
◆For the working temperature above 300 ℃, there is a strong corrosive medium, must be used containing Ti or Nb stabilization elements or ultra-low carbon stainless steel welding rod. Such as A137 or A002 etc.
◆For the medium containing dilute sulphuric acid or hydrochloric acid, often use stainless steel electrodes containing Mo or Mo and Cu, such as: A032, A052, etc.
For the work at room temperature, corrosive weak or only to avoid rust pollution of the equipment, can be used without Ti or Nb stainless steel welding rod.
◆In order to ensure the stress corrosion resistance of the weld metal, use super-alloyed welding consumables, i.e. the content of corrosion-resistant alloy elements (Cr, Mo, Ni, etc.) in the weld metal is higher than that of the base material. Such as the use of 00Cr18Ni12Mo2 type of welding materials (such as A022) welding 00Cr19Ni10 weldments.
(6) For austenitic stainless steel working in low-temperature conditions, should ensure that the welded joint in the use of low-temperature impact toughness, so the use of pure austenitic welding rod. Such as A402, A407.
(7) can also be used nickel-based alloy welding rod. Such as the use of Mo up to 9% of the nickel-based welding material welding Mo6 type super austenitic stainless steel.
(8) The choice of electrode flux type.
◆ Due to the duplex austenitic steel weld metal itself contains a certain amount of ferrite, with good plasticity and toughness, from the weld metal crack resistance perspective for comparison, alkaline flux and titanium calcium type flux electrode difference is not as significant as the carbon steel electrode. Therefore, in the actual application, from the welding process performance aspects focus more, most of the use of flux type code 17 or 16 welding rod (such as A102A, A102, A132, etc.).
Only in the structure of rigid or weld metal crack resistance is poor (such as some martensitic chromium stainless steel, pure austenitic organization of chromium-nickel stainless steel, etc.), only when considering the use of alkaline flux-coated stainless steel electrodes (such as A107, A407, etc.).

4, ferritic – austenitic duplex stainless steel welding.
(1) awareness of duplex stainless steel at home and abroad.
● Uses: mainly used for strong corrosive media resistance (such as high chloride ion content) of petrochemical, seawater and wastewater treatment equipment, pipelines and other occasions.
●Organization and performance characteristics.
◇Contains high chromium (18-28%) and low nickel (4-10%). The organization at room temperature is austenite + ferrite, and the ferrite content is usually not less than 50%.
◇Yield strength can reach 400-550MPa, twice that of ordinary stainless steel
◇It has some characteristics of both austenitic stainless steel and ferritic stainless steel, good toughness, high strength and better resistance to chloride stress corrosion than ordinary stainless steel.
◇Good welding properties: lower tendency to thermal cracking compared with austenitic stainless steel; lower tendency to heating embrittlement compared with ferritic stainless steel.

● Types of duplex stainless steels.
There are many varieties of duplex stainless steel, the most commonly used are 3 types, namely.
◇Ultra-low carbon 18Cr-5Ni-3Mo type (also known as Cr18 type, 1805 type).
Such as 00Cr18Ni5Mo3Si2 (1805) which has been included in the national standard GB4237 “Duplex Stainless Steel Hot Rolled Steel Sheet”; ASTM S31500 (18Cr-5Ni-3Mo-N, lower limit of tensile strength 630MPa)
◇23Cr-4Ni-Mo type (also known as Cr23 without Mo, 2305 type).
For example: ASTM S32304 (23Cr-4Ni-Mo-Cu-N, lower limit of tensile strength 600MPa)
◇22Cr-5Ni-3Mo (also known as Cr22, 2205)
For example: ASTM S31803 (22Cr-5Ni-3Mo-N, lower limit of tensile strength 620MPa)
◇25Cr-7Ni-4Mo type (also known as Cr25 type, 2507 type): divided into two types: ordinary and super.

■For example, the ordinary duplex stainless steels that have been included in the national standard GB4237-92 “Duplex Stainless Steel Hot Rolled Steel Sheet” and GB13296-91 are 0Cr26Ni5Mo2; 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.
◇Has good weldability, low hot crack sensitivity, and low cold crack sensitivity when the proportion of duplex tissue is appropriate. However, when the constraint degree is large and the weld metal contains high hydrogen, there is still a risk of hydrogen cracking due to the role of ferrite in the duplex tissue.
◇Duplex steel in the range of 300-500 ℃ when there is a long time, “475 ℃ brittle”, so the use of duplex steel temperature is often lower than 250 ℃.
★Specific welding rod selection for various types of duplex steel.

☆ Cr18 type duplex steel: choose Cr22-Ni9-Mo3 type ultra-low carbon welding material (such as AWS A5.4 E2209 and ER2209 grade); can also choose Mo-containing austenitic stainless steel welding material, such as A022Si (E316L-16), A042 (E309MoL-16).
☆ Cr23 Mo-free duplex steel: choose Cr22-Ni9-Mo3 ultra-low carbon welding consumables (such as AWS A5.4 E2209 and ER2209 grades); can also choose austenitic stainless steel welding consumables, such as A062 (E309L-16).
☆ Cr22 type: choose Cr22-Ni9-Mo3 type ultra-low carbon welding consumables (such as AWS A5.4 E2209 and ER2209 grade); can also choose austenitic stainless steel welding consumables containing Mo, such as A042 (E309MoL-16).
☆Cr25 type: choose Cr25-Ni5-Mo3 or Cr25-Ni5-Mo4 ultra-low carbon welding consumables, (such as AWS A5.4 E2553 and ER2553 grades); also can choose high Mo nickel-based welding consumables without Nb, such as NiCrMo-3 type welding consumables without Nb.
● Preheating: not required. The interlayer temperature should not be higher than 100°C.
●Welding line energy: not more than 15kJ/cm for Cr18 duplex steel; 10-25kJ/cm for Cr23 Mo-free and Cr22 duplex steel; 10-15kJ/cm for Cr25 duplex steel.
● Post-weld heat treatment: not required.
(3) Key points of the welding process for duplex stainless steel.

● Welding methods.
Tungsten argon arc welding, manual arc welding, fusion electrode tungsten gas shielded welding, submerged arc welding are all available.
●Welding materials.
To prevent σ-phase and to ensure the normal austenite/ferrite phase ratio of the weld metal, it is also necessary to use “super-alloyed” welding consumables, i.e. high nickel welding consumables, for welding duplex steels.
To prevent carbide precipitation, the carbon content of the weld metal should be controlled at an ultra-low carbon level (0.03%).
◇When there are special requirements for the corrosion resistance of the weld metal, the alkaline electrode of super duplex steel composition should also be used.
5, super austenitic stainless steel welding.
Currently widely developed and applied new steel grades for resistance to strong corrosive environments such as chloride ions. Our company will be in contact with Yantai Wanhua MDI project and Ningbo Formosa NAE project.
The most typical super austenitic stainless steel is ASTM S31254 (254SMo), with a nominal composition of 20Cr-18Ni-6Mo and a strength approximately 50% higher than 316L.
(1) Organizational and performance characteristics.
◇The chemical composition is between ordinary austenitic stainless steel and nickel-based alloy, containing higher Mo, N, Cu and other alloying elements to improve the stability of the austenitic organization, corrosion resistance, especially to improve the stress corrosion cracking resistance to Cl
◇The organisation of the steel is pure austenitic organisation.
◇There is a greater tendency of cold working hardening and a greater rebound after cold forming. Therefore, a certain amount of forming retention is required during processing. No heat treatment is required after cold forming.
Long-time hot forming heating may lead to serious skinning. Solid solution treatment is generally required after hot forming.
(2) Welding characteristics: The same as other austenitic stainless steels.
(3) Key points of welding process.
Welding method: Preferably tungsten arc welding, followed by manual arc welding.
Welding materials: Nickel-based welding materials, such as welding wire ASTM A5.14 ERNiCrMo-3; welding rod ASTM A5.11 ENiCrMo-12.
●Preheating: not required, interlayer temperature not higher than 100°C.
●Welding line energy: controlled.
● Post-weld heat treatment: Generally not required. If self-fusing welding is used (i.e. no wire filler TIG welding), solid solution treatment and quenching are carried out to ensure corrosion resistance. It is recommended that the self-fusing welding process is not used as far as possible.

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