Welding Characteristics Of Austenitic Stainless Steel And Selection Of Welding Rod

Austenitic stainless steel has good weldability and is currently the most widely used in industry. Generally, special technological measures are not required during welding. This paper analyzes in detail the occurrence of hot cracks, intergranular corrosion, Stress corrosion cracking, embrittlement of welded joints (low temperature embrittlement, sigma phase embrittlement, fusion line embrittlement) causes and preventive measures,

Through the theoretical and practical analysis of welding characteristics, the selection principles and methods of electrode selection for austenitic stainless steel in welding different materials and in different working environment conditions are introduced.

Stainless steel is increasingly widely used in aviation, petroleum, chemical and atomic energy industries. Stainless steel is divided into chromium stainless steel, chromium-nickel stainless steel according to chemical composition, and ferritic stainless steel, martensitic stainless steel, austenitic stainless steel and Austenitic-ferritic duplex stainless steel.

Among stainless steels, austenitic stainless steel (18-8 type stainless steel) has better corrosion resistance than other stainless steels; its strength is lower, but its plasticity and toughness are excellent; its welding performance is good, and it is mainly used for chemical containers, equipment and It is the most widely used stainless steel in industry at present.

Although austenitic stainless steel has many advantages, if the welding process is incorrect or the welding material is improperly selected, many defects will occur, which will ultimately affect the performance.

Welding characteristics of austenitic stainless steel

• Prone to thermal cracks

Hot cracks of austenitic stainless steel are relatively easy to produce defects during welding, including longitudinal and transverse cracks of welds, burr cracks, root cracks of backing welding and interlayer cracks of multi-layer welding, etc., especially when the nickel content is relatively high. High austenitic stainless steels are easier to produce.

1. Cause

(1) The liquid and solid phase lines of austenitic stainless steel have a large interval, a long crystallization time, and a single-phase austenite crystallographic orientation is strong, so the impurity segregation is relatively serious.

(2) The thermal conductivity is small and the linear expansion coefficient is large, which will generate large welding internal stress (usually the tensile stress of the weld and heat-affected zone) during welding.

(3) The components in austenitic stainless steel, such as C, S, P, Ni, etc., will form a low melting point eutectic in the molten pool. For example, the melting point of Ni3S2 formed by S and Ni is 645℃, while the melting point of Ni-Ni3S2 eutectic is only 625℃.

2. Preventive measures

(1) Use the dual-phase structure weld to make the weld metal as austenite and ferrite dual-phase structure as much as possible, and control the ferrite content below 3 to 5%, which can disturb the direction of austenite columnar crystals. grain refinement. And ferrite can dissolve more impurities than austenite, thereby reducing the segregation of low-melting eutectic in austenite grain boundaries.

(2) Welding process measures In the welding process, try to use high-quality electrodes with alkaline coating, use small line energy, small current, fast non-swing welding, try to fill the arc pit at the end and use argon arc welding for bottoming, etc. Reduce welding stress and crater cracks.

(3) Control the chemical composition Strictly limit the content of impurities such as S and P in the weld to reduce the low melting point eutectic.

• Intergranular corrosion

Corrosion occurs between grains, which results in a loss of inter-grain bonding, almost complete loss of strength, and fracture along grain boundaries when stressed.

1. cause

According to the theory of depletion of chromium, when the weld and heat affected zone are heated to the sensitization temperature of 450 to 850 °C (dangerous temperature zone), due to the large atomic radius of Cr, the diffusion rate is small, and the supersaturated carbon tends to austenite grains. The boundary diffuses and forms Cr23C6 at the grain boundary with the chromium compound at the grain boundary, resulting in a grain boundary with poor chromium, which is not enough to resist corrosion.

2. Preventive measures

(1) Controlling carbon content

Use low carbon or ultra low carbon (W(C)≤0.03%) stainless steel welding consumables. Such as A002 and so on.

(2) Add stabilizer

Adding Ti, Nb and other elements that have a stronger affinity with C than Cr in steel and welding materials can combine with C to form stable carbides, thereby avoiding chromium depletion at austenite grain boundaries. Commonly used stainless steel and welding materials contain Ti, Nb, such as 1Cr18Ni9Ti, 1Cr18Ni12MO2Ti steel, E347-15 electrode, H0Cr19Ni9Ti welding wire, etc.

(3) Adopt two-way organization

A certain amount of ferrite-forming elements, such as Cr, Si, AL, MO, etc., are melted into the weld by the welding wire or electrode, so that the weld is formed into a dual-phase structure of austenite + ferrite, because Cr is in the The diffusion rate in ferrite is faster than that in austenite, so Cr diffuses to grain boundaries faster in ferrite, which alleviates the phenomenon of depletion of chromium in austenite grain boundaries. Generally, the content of ferrite in the weld metal is controlled to be 5% to 10%. If there is too much ferrite, the weld will become brittle.

(4) Rapid cooling

 Because austenitic stainless steel does not cause hardening, during the welding process, you can try to increase the cooling rate of the welded joint, such as cooling with a copper backing plate or direct watering under the weldment.

In the welding process, measures such as low current, high welding speed, short arc, and multi-pass welding can be used to shorten the time that the welded joint stays in the dangerous temperature area, so as to avoid the formation of a chromium-depleted area.

(5) Carry out solution treatment or homogenization heat treatment. After welding, heat the welded joint to 1050-1100 °C, so that the carbides are redissolved into austenite, and then rapidly cooled to form a stable single-phase austenite structure.

In addition, a homogenization heat treatment at 850-900 °C for 2 hours can also be performed. At this time, the Cr in the austenite grains diffuses to the grain boundaries, and the Cr content at the grain boundaries reaches more than 12% again, so that no grains will be formed. corroded.

• Stress corrosion cracking

Corrosion damage of metal under the combined action of stress and corrosive medium. According to the stress corrosion cracking cases and experimental studies of stainless steel equipment and parts, it can be considered that under the combined action of a certain static tensile stress and a specific electrochemical medium at a certain temperature, the existing stainless steel has the possibility of producing stress corrosion .

One of the biggest features of stress corrosion is the selectivity in the combination of corrosive media and materials. It is easy to cause stress corrosion of austenitic stainless steel, mainly hydrochloric acid and chloride containing chloride ions, as well as sulfuric acid, nitric acid, hydroxide (alkali), seawater, water vapor, H2S aqueous solution, concentrated NaHCO3+NH3+NaCl aqueous solution and other media Wait.

1. Cause

 Stress corrosion cracking is a delayed cracking phenomenon that occurs when welded joints are subjected to 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 manifests as brittle failure without plastic deformation.

2. Preventive measures

(1) Reasonably formulate the forming process and assembly process to minimize the degree of cold work deformation, avoid forced assembly, and prevent all kinds of scars during the assembly process (all kinds of assembly scars and arc burns will become the crack source of SCC, which is easy to cause corrosion. pit.

(2) Reasonable selection of welding consumables The welding seam and the base metal should have a good match, without any bad structure, such as grain coarsening and hard and brittle martensite.

(3) Adopt appropriate welding process to ensure that the welding seam is well formed and does not produce any stress concentration or pitting defects, such as undercut, etc., adopt a reasonable welding sequence to reduce the level of welding residual stress. For example, avoid crisscross welds, change the Y-shaped groove to an X-shaped groove, appropriately reduce the groove angle, use a short weld bead, and use a small line energy.

(4) Post-weld heat treatment for stress relief treatment, such as complete annealing or annealing after welding; post-weld hammering or shot peening is used when heat treatment is difficult to implement.

(5) Production management measures to control 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 layer, lining or cathodic protection, etc., add corrosion inhibitor.

• Embrittlement of welded joints

After the weld of austenitic stainless steel is heated at high temperature for a period of time, the phenomenon of impact toughness will decrease, which is called embrittlement.

1. Low temperature embrittlement of weld metal (475 ℃ embrittlement)

(1) Cause

The dual-phase weld structure containing more ferrite phases (more than 15% to 20%), after heating at 350 to 500 °C, the plasticity and toughness will decrease significantly. Since the embrittlement speed is the fastest at 475 °C, it is called 475 ℃ embrittlement.

For austenitic stainless steel welded joints, corrosion resistance or oxidation resistance is not always the most critical property, but when used at low temperatures, the plastic toughness of the weld metal becomes the critical property.

In order to meet the requirements of low temperature toughness, the weld structure usually hopes to obtain a single austenite structure to avoid the existence of delta ferrite. The presence of delta ferrite always deteriorates the low temperature toughness, and the more the content, the more serious this embrittlement is.

(2) Preventive measures

①On the premise of ensuring the crack resistance and corrosion resistance of the weld metal, the ferrite phase should be controlled at a low level, about 5%.

②Welds that have been embrittled at 475°C can be eliminated by quenching at 900°C.

2. Sigma-phase embrittlement of welded joints

(1) Causes

The long-term use of austenitic stainless steel welded joints in the temperature range of 375 to 875 ° C will produce an inter-FeCr compound called σ phase. The σ phase is hard and brittle (HRC>68).

As a result of the precipitation of σ phase, the impact toughness of the weld drops sharply, which is called σ phase embrittlement. The σ phase generally only appears in the dual-phase structure weld; when the service temperature exceeds 800 ~ 850 ℃, the σ phase will also precipitate in the single-phase austenite weld.

(2) Preventive measures

①Limit the ferrite content in the weld metal (less than 15%); use superalloyed welding materials, that is, high-nickel welding materials, and strictly control the content of Cr, Mo, Ti, Nb and other elements.

② Small specification is adopted to reduce the residence time of weld metal at high temperature

③ The σ phase that has been precipitated is subjected to solid solution treatment when conditions permit, so that the σ phase is dissolved into austenite.

④Heat the welded joint to 1000～1050℃, then cool it quickly. σ phase is generally not produced in 1Cr18Ni9Ti steel.

 3. The fusion line is brittle

(1) Causes

When austenitic stainless steel is used for a long time at high temperature, brittle fracture will occur along a few grains outside the fusion line.

(2) Prevention and control measures

Adding Mo to steel can improve the ability of steel to resist high temperature brittle fracture.

Through the above analysis, only reasonable selection of the above welding process measures or welding materials can avoid the above welding defects. Austenitic stainless steel has excellent weldability, and almost all welding methods can be used for the welding of austenitic stainless steel.

Among various welding methods, electrode arc welding has the advantages of adapting to various positions and different plate thicknesses, and is widely used. The following focuses on analyzing the selection principles and methods of austenitic stainless steel electrodes under different uses.

Key points for selection of electrodes for austenitic stainless steel

Stainless steel is mainly used for corrosion resistance, but is also used as heat resistant steel and low temperature steel. Therefore, when welding stainless steel, the performance of the electrode must match the purpose of the stainless steel. Stainless steel electrodes must be selected according to the base metal and working conditions (including working temperature and contact medium, etc.).

(1) Point 1

Generally speaking, the selection of the electrode can refer to the material of the base metal, and select the electrode with the same or similar composition as the base metal. Such as: A102 corresponds to 0Cr18Ni9, A137 corresponds to 1Cr18Ni9Ti.

(2) Point 2

Since the carbon content has a great influence on the corrosion resistance of stainless steel, the stainless steel electrode whose carbon content of the deposited metal is not higher than that of the base metal is generally selected. Such as 316L must use A022 electrode.

(3) Point 3

The weld metal of austenitic stainless steel shall ensure mechanical properties. This can be verified by welding procedure qualification.

(4) Point 4 (Austenitic heat-resistant steel)

For heat-resistant stainless steel (austenitic heat-resistant steel) working at high temperature, the selected electrode should mainly meet the hot crack resistance of the weld metal and the high temperature performance of the welded joint.

1. For austenitic heat-resistant steels with Cr/Ni≥1, such as 1Cr18Ni9Ti, etc., austenitic-ferritic stainless steel electrodes are generally used, and it is advisable that the weld metal contains 2-5% ferrite. When the ferrite content is too low, the crack resistance of the weld metal is poor; if it is too high, it is easy to form a sigma embrittlement phase during long-term use at high temperature or heat treatment, resulting in cracks.

Such as A002, A102, A137. In some special applications, when all austenitic weld metal may be required, such as A402, A407 electrodes, etc. can be used.

2. For stable austenitic heat-resistant steels with Cr/Ni<1, such as Cr16Ni25Mo6, etc., it is generally necessary to increase the Mo, W, Mn in the weld metal while ensuring that the chemical composition of the weld metal is approximately similar to that of the base metal. The content of such elements can improve the crack resistance of the weld while ensuring the thermal strength of the weld metal. Such as using A502, A507.

(5) Point 5 (corrosion-resistant stainless steel)

For corrosion-resistant stainless steel working in various corrosive media, the electrode should be selected according to the medium and working temperature, and its corrosion resistance should be ensured (do the corrosion performance test of welded joints).

 1. For the medium with the working temperature above 300℃ and strong corrosiveness, the electrode containing Ti or Nb stabilization element or ultra-low carbon stainless steel must be used. Such as A137 or A002 and so on.

 2. For the medium containing dilute sulfuric acid or hydrochloric acid, stainless steel electrodes containing Mo or Mo and Cu are often used, such as: A032, A052, etc.

 3. For equipment with weak corrosion or only to avoid rust pollution, stainless steel electrodes without Ti or Nb can be used. In order to ensure the stress corrosion resistance of the weld metal, superalloyed welding consumables are used, that is, the content of corrosion-resistant alloying elements (Cr, Ni, etc.) in the weld metal is higher than that of the base metal. For example, use 00Cr18Ni12Mo2 type welding materials (such as A022) to weld 00Cr19Ni10 weldments.

(6) Point 6

For austenitic stainless steel working under low temperature conditions, the low temperature impact toughness of the welded joint at the service temperature should be guaranteed, so pure austenitic electrodes are used. Such as A402, A407.

(7) Point 7

Nickel-based alloy electrodes are also available. For example, Mo6 type super austenitic stainless steel is welded with nickel-based welding consumables with Mo up to 9%.

(8) Point 8: Selection of electrode coating type

1. Since the dual-phase austenitic steel weld metal itself contains a certain amount of ferrite, it has good plasticity and toughness. From the perspective of weld metal crack resistance, the basic coating and the titanium calcium type coating electrode are compared. The difference is not as significant as for carbon steel electrodes. Therefore, in practical applications, more attention is paid to the welding process performance, and most of the electrodes with coating type code 17 or 16 (such as A102A, A102, A132, etc.) are used.

2. Only when the structural rigidity is very high or the crack resistance of the weld metal is poor (such as some martensitic chromium stainless steel, pure austenitic chromium-nickel stainless steel, etc.), the choice of coating code 15 can be considered. Basic coated stainless steel electrodes (such as A107, A407, etc.).

In conclusion

To sum up, the welding of austenitic stainless steel has its unique characteristics, and the selection of welding electrodes for austenitic stainless steel is particularly noteworthy. It has been proved by long-term practice that the above measures can be used to achieve different welding for different materials. Methods and electrodes of different materials, stainless steel electrodes must be selected according to the base metal and working conditions (including working temperature and contact medium, etc.). It has a good guiding significance for us, so that it is possible to achieve the expected welding quality.