Austenitic Stainless Steel Welding and Optimal Welding Rod Selection

Austenitic stainless steel boasts excellent weldability, making it the most widely used in industry today. Typically, it requires no special process measures during welding. This article offers a detailed analysis of austenitic stainless steel welding, covering issues such as hot cracking, intergranular corrosion, stress corrosion cracking, and joint embrittlement (low-temperature brittleness, sigma phase brittleness, fusion line fracture), along with their causes and preventive measures.

Through theoretical and practical analysis of welding characteristics, we focus on the principles and methods of electrode selection when welding different materials and operating under varying working environment conditions. Only with reasonable process measures and electrode selection can a perfect weld seam be achieved.

Stainless steel is increasingly utilized in industries such as aerospace, oil, chemical, and nuclear energy. Classified by chemical composition, stainless steel can be chromium stainless steel or chromium-nickel stainless steel. By structure, it can be ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, or austenitic-ferritic duplex stainless steel.

Among these, austenitic stainless steel (18-8 type stainless steel) offers superior corrosion resistance compared to other stainless steels. While its strength is relatively low, it has excellent plasticity and toughness, and good weldability. Its primary applications include chemical containers, equipment, and parts, making it the most widely used stainless steel in industry today.

However, despite the many advantages of austenitic stainless steel, improper welding techniques or incorrect selection of welding materials can lead to many defects, ultimately affecting performance.

Welding Characteristics of Austenitic Stainless Steel

(I) Prone to Hot Cracking

Austenitic stainless steel is susceptible to hot cracking during welding, including longitudinal and transverse cracks in the weld, arc crater cracks, root cracks in the initial weld, and interlayer cracks in multi-layer welding. This is especially prevalent in austenitic stainless steel with a higher nickel content.

1. Causes

(I) Austenitic stainless steel has a large liquid-solid phase line interval and a long crystallization time. The austenite crystallization is strongly directional, leading to severe impurity segregation.

(II) The low thermal conductivity and high linear expansion coefficient generate significant welding internal stress, generally involving tensile stress in the weld and heat-affected zone.

(III) Components such as C, S, P, and Ni in austenitic stainless steel form low melting point eutectics in the melt pool. For example, Ni₃S₂ formed by S and Ni has a melting point of 645°C, while the eutectic of Ni-Ni₃S₂ melts at only 625°C.

2. Preventive Measures

(I) Employing duplex structure in the weld seam. Attempt to make the welded metal present a dual phase structure of austenite and ferrite, keeping the ferrite content below 3-5%. This can disrupt the direction of austenite columnar crystals and refine the grains. Additionally, ferrite can dissolve more impurities than austenite, thus reducing the segregation of low melting point eutectics at the austenite grain boundaries.

(II) Welding process measures. Preferably opt for high-quality welding rods with basic slag coatings, use low linear energy, small current, and fast non-oscillating welding. Fill up the arc crater at the end, and employ argon arc welding for the base layer if possible. This reduces welding stress and potential for arc pit cracking.

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

(II) Intergranular Corrosion

Corrosion occurring between grains leads to the loss of intergranular bonding strength, nearly eradicating its strength entirely. Upon stress application, fracturing occurs along grain boundaries.

1. Causes

According to the chromium depletion theory, when the weld and heat-affected zone are heated to sensitization temperatures of 450–850°C (danger temperature zone), due to the large atomic radius of Cr and its slow diffusion rate, oversaturated carbon diffuses toward the austenite grain boundaries. It combines with chromium compounds at the grain boundaries to form Cr23C6, resulting in chromium-depleted grain boundaries that are insufficient to resist corrosion.

2. Preventive Measures

(1) Control Carbon Content

Use low or ultra-low carbon (W_(C)≤0.03%) stainless steel welding materials, such as A002, etc.

(2) Add Stabilizers

Adding elements like Ti, Nb etc., which have a stronger affinity with C than Cr, to the steel and welding materials. These elements form stable carbides with C, thus preventing chromium depletion at the austenite grain boundaries. Common stainless steel materials and welding materials contain Ti, Nb, such as 1Cr18Ni9Ti, 1Cr18Ni12MO2Ti steel, E347-15 welding rods, H0Cr19Ni9Ti welding wire, etc.

(3) Adopt Dual Phases

By melting a certain amount of ferrite-forming elements into the weld from the welding wire or rod, such as Cr, Si, Al, Mo, etc., the weld forms a dual-phase structure of austenite + ferrite. Since Cr diffuses faster in ferrite than in austenite, Cr diffuses to the grain boundaries more quickly in ferrite, alleviating the chromium depletion of the austenite grain boundaries. Generally, the ferrite content in the weld metal is controlled to be between 5% and 10%. Excess ferrite can make the weld brittle.

(4) Rapid Cooling

Since austenitic stainless steel does not undergo hardening, during welding, the cooling rate of the welded joint can be increased, such as by using a copper backing plate underneath the weldment or direct water cooling.

In welding technology, measures can be taken such as using low current, high welding speed, short arc, multi-pass welding, etc., to shorten the dwell time of the welded joint in the danger temperature zone, thereby preventing the formation of chromium-depleted zones.

(5) Carry Out Solution Treatment or Homogenizing Heat Treatment

After welding, the welded joint is heated to 1050–1100°C, causing the carbides to dissolve back into the austenite, followed by rapid cooling to form a stable single-phase austenite structure.

Additionally, a homogenizing heat treatment at 850–900°C for 2 hours can also be carried out. At this time, the Cr inside the austenite grains diffuses to the grain boundaries, and the Cr content at the grain boundaries reaches more than 12% again, thus preventing intergranular corrosion.

(III) Stress Corrosion Cracking

This refers to the corrosive damage that metals undergo under the combined action of stress and corrosive media. Based on the examples of stress corrosion fracture in stainless steel equipment and components and experimental research, it can be inferred that under certain conditions of static tensile stress and temperature, all existing stainless steel grades may potentially undergo stress corrosion.

One of the key characteristics of stress corrosion is the selectivity in the combination of the corrosive medium and the material. Media that easily cause austenitic stainless steel stress corrosion primarily include hydrochloric acid and chloride-containing media, as well as sulfuric acid, nitric acid, hydroxides (alkali), seawater, steam, H₂S aqueous solution, concentrated NaHCO₃+NH₃+NaCl aqueous solution, and others.

1. Causes

Stress corrosion cracking is a delayed cracking phenomenon that occurs when a welded joint is subjected to tensile stress in a specific corrosive environment. Stress corrosion cracking in austenitic stainless steel welded joints is a rather severe form of failure, appearing as brittle fracture with no plastic deformation.

2. Preventive Measures

(1) Develop a rational forming, processing, and assembly procedure to minimize cold work deformation, avoid forced assembly, and prevent the formation of various scars during assembly. All assembly scars and arc burn marks can become sources of stress corrosion cracking (SCC) and lead to corrosion pits.

(2) Select appropriate welding materials. The weld seam and base metal should match well, avoiding the formation of any unfavorable structures, such as grain coarsening and hard, brittle martensite.

(3) Adopt suitable welding procedures to ensure a good weld seam formation, without any stress concentration or pitting defects, such as undercutting. Implement a rational welding sequence to reduce the level of welding residual stress. For example, avoid cross welds, change Y-shaped grooves to X-shaped ones, appropriately reduce the groove angle, use short weld paths, and employ low linear energy.

(4) Eliminate stress by post-weld heat treatment, such as complete annealing after welding or normalizing. In cases where heat treatment is impractical, post-weld hammering or shot peening can be employed.

(5) Implement production management measures to control impurities in the medium, such as O₂, N₂, and H₂O in liquid ammonia medium, H₂S in liquefied petroleum gas, and O₂, Fe³⁺, Cr⁶⁺ in a solution of chlorides. Apply corrosion prevention treatments, such as coating, lining, or cathodic protection, and add corrosion inhibitors.

(IV) Brittle Fracture of Weld Joints

After being heated at high temperatures for a period of time, the impact toughness of austenitic stainless steel welds decreases, a phenomenon referred to as embrittlement.

1. Low-Temperature Embrittlement of Weld Metal (475°C Embrittlement)

(1) Causes

The duplex weld structure, containing a higher proportion of ferritic phase (over 15%-20%), will significantly reduce in plasticity and toughness after being heated at 350-500°C. Because the embrittlement speed is fastest at 475°C, this is referred to as 475°C embrittlement.

For austenitic stainless steel weld joints, corrosive resistance or oxidation resistance is not always the most critical property. At low temperatures, the plasticity and toughness of the weld metal become the key performance indicators.

To meet the requirements of low-temperature toughness, the weld structure typically aims to achieve a singular austenitic structure, avoiding the presence of δ delta ferrite. The presence of δ delta ferrite always worsens the low-temperature toughness, and the more it contains, the more severe the embrittlement.

(2) Preventive Measures

① While ensuring the crack resistance and corrosion resistance of the weld metal, the ferritic phase should be kept at a lower level, around 5%.

② For welds that have undergone 475°C embrittlement, it can be eliminated by quenching at 900°C.

2. σ Phase Embrittlement in Weld Joints

(1) Causes

When austenitic stainless steel weld joints are used for extended periods within a temperature range of 375-875℃, a compound known as the σ phase forms between Fe and Cr. The σ phase is hard and brittle (HRC>68).

The precipitation of the σ phase results in a drastic drop in the weld’s impact toughness, a phenomenon referred to as σ phase embrittlement. The σ phase typically only occurs within duplex structure welds; however, when the operating temperature exceeds 800-850℃, the σ phase can precipitate within single-phase austenitic welds.

(2) Preventative Measures

① Restrict the ferrite content in the weld metal (less than 15%), use superalloy welding materials, specifically high-nickel welding materials, and strictly control the content of elements such as Cr, Mo, Ti, Nb.

② Adopt smaller specifications to reduce the dwell time of the weld metal at high temperatures.

③ For already precipitated σ phases, if conditions permit, conduct solid solution treatment to dissolve the σ phase into the austenite.

④ Heat the weld joint to 1000-1050℃ and then rapidly cool. The σ phase typically does not form in 1Cr18Ni9Ti steel.

3. Fusion Line Fracture

(1) Causes

When austenitic stainless steel is used at high temperatures for extended periods, brittle fracture can occur along the fusion line.

(2) Preventative Measures

Adding Mo to the steel can enhance the steel’s high-temperature fracture resistance.

Through the above analysis, choosing the appropriate welding process measures or welding materials can prevent the aforementioned welding defects. Austenitic stainless steel has excellent weldability, and nearly all welding methods can be applied to austenitic stainless steel welding.

Among various welding methods, stick electrode welding has the advantage of adapting to different positions and thicknesses, and it is widely used. The following will focus on the principles and methods for selecting austenitic stainless steel electrodes for different applications.

Key Points in Choosing Welding Rods for Austenitic Stainless Steel

Stainless steel is primarily used for corrosion resistance, but it also serves as heat-resistant steel and low-temperature steel. Hence, when welding stainless steel, the welding rod’s characteristics must align with the steel’s purpose. The selection of stainless steel welding rods must be based on the parent material and the working conditions, including the working temperature and contact medium.

Comparison Chart for Different Stainless Steel Grades and Welding Rod Types and Models

(I) Point One

Generally, the selection of welding rods can refer to the material of the base metal, choosing a rod with a composition similar to or the same as the base metal. For instance, A102 corresponds to 0Cr18Ni9, and A137 corresponds to 1Cr18Ni9Ti.

(II) Point Two

Given that the carbon content significantly affects the corrosion resistance of stainless steel, it is typical to choose stainless steel welding rods with a deposited metal carbon content no higher than that of the base metal. For example, A022 welding rods must be used with 316L.

(III) Point Three

The weld metal of austenitic stainless steel should ensure mechanical performance, which can be verified through welding process evaluation.

(IV) Point Four (Austenitic Heat-Resistant Steel)

For heat-resistant stainless steel (austenitic heat-resistant steel) that operates at high temperatures, the chosen welding rod should mainly be able to meet the heat crack resistance of the weld metal and the high-temperature performance of the welded joint.

1. For austenitic heat-resistant steel with a Cr/Ni ratio greater than or equal to 1, such as 1Cr18Ni9Ti, austenitic ferritic stainless steel welding rods are generally utilized, with 2-5% ferrite in the weld metal being suitable. If the ferrite content is too low, the crack resistance of the weld metal is poor; if it is too high, sigma phase brittleness can easily form during long-term high-temperature use or heat treatment, causing cracks. For example, A002, A102, A137. In some special applications, if a fully austenitic weld metal is required, rods like A402, A407 can be used.

2. For stable austenitic heat-resistant steel with a Cr/Ni ratio less than 1, such as Cr16Ni25Mo6, while ensuring the weld metal has a chemical composition roughly similar to the base metal, the content of elements such as Mo, W, Mn in the weld metal should be increased to ensure the thermal strength of the weld metal and improve its crack resistance. For example, A502, A507 can be used.

V. Point Five (Corrosion-Resistant Stainless Steel)

For corrosion-resistant stainless steel that operates in various corrosive media, the welding rod should be selected according to the medium and working temperature, ensuring its corrosion resistance (corrosion performance testing of the welded joint).

1. For a working temperature above 300℃ and a highly corrosive medium, it’s necessary to use welding rods made from stainless steel with Ti or Nb stabilizing elements or ultra-low carbon, such as A137 or A002.

2. For media containing dilute sulfuric acid or hydrochloric acid, commonly selected are stainless steel welding rods that contain Mo or both Mo and Cu, such as A032 or A052.

3. For operations in less corrosive media or equipment simply to avoid rust contamination, stainless steel welding rods without Ti or Nb can be used. To ensure the welded metal’s resistance to stress corrosion, super alloyed welding materials are used, i.e., the content of corrosion-resistant alloy elements (Cr, Ni, etc.) in the welded metal is higher than the parent material. For instance, 00Cr18Ni12Mo2 type welding material (such as A022) is used to weld 00Cr19Ni10 parts.

(VI) Point Six

For austenitic stainless steel operating under low-temperature conditions, the welded joint’s low-temperature impact toughness at the usage temperature should be ensured, thus pure austenitic welding rods are used, such as A402 or A407.

(VII) Point Seven

Nickel-based alloy welding rods may also be chosen. For instance, a nickel-based welding material with 9% Mo is used to weld Mo6 type super austenitic stainless steel.

VIII. Point Eight (Selection of Welding Rod Coating Types)

1. As the welding metal of duplex austenitic steel inherently contains a certain amount of ferrite, it has good plasticity and toughness. From the perspective of the welding metal’s crack resistance, there isn’t a notable difference between basic and rutile coatings like carbon steel welding rods. Therefore, in practical applications, more emphasis is placed on the welding process performance, and welding rods with coating type codes 17 or 16 (like A102A, A102, A132, etc.) are mostly used.

2. Only when the structure is very rigid or the welding metal’s crack resistance is poor (like some martensitic chromium stainless steel, pure austenitic chromium-nickel stainless steel, etc.), should one consider using stainless steel welding rods with a basic coating with a code of 15 (such as A107, A407).

Conclusion

In summary, the welding of austenitic stainless steel has its unique characteristics. The selection of welding rods during the welding process of austenitic stainless steel is particularly noteworthy. Long-term practice has shown that the measures mentioned above can achieve different welding methods and welding rods for different materials.

The selection of stainless steel welding rods must be based on the parent material and working conditions (including working temperature and contact medium). This has a significant guiding impact on us, and only then is it possible to achieve the desired welding quality.