11th of February ,2022

Welding Characteristics and Influencing Factors of Duplex Stainless Steel

Duplex stainless steel has good welding performance. Compared with ferritic stainless steel and austenitic stainless steel, it is neither like the welding heat affected zone of ferritic stainless steel. Unlike austenitic stainless steel, it is more sensitive to welding hot cracks.

Due to its special advantages, duplex stainless steel is widely used in petrochemical equipment, seawater and wastewater treatment equipment, oil and gas pipelines, papermaking machinery and other industrial fields. development prospects.

Welding performance issues often arise with economizer duplex steels. Welding standard duplex steels is not an issue, and there are consumables suitable for these applications regardless of the process. From a metallographic point of view, welding 2101 (1.4162) is not a problem at all, in fact it is even easier to weld than standard grade duplex steels, since this material can actually be welded using the acetylene welding process, whereas for standard duplex materials, This process must always be avoided. The practical problem with welding 2101 is that the viscosity of the weld pool is different, so the wettability is a bit less. This forces the operator to use arc welding more in the process of welding, which is the problem Where it is. Although it can be compensated by choosing a superalloyed welding consumable, we often want to choose a matching welding consumable.

In 2101, there is also a heat-affected zone interaction between the microstructures in the low-temperature heat-affected zone and the high-temperature heat-affected zone, which is more favorable than in 2304, 2205, or 2507. When testing with 2101, it has also been found that due to the lower nickel content, a different type of "temper color" with more nitrogen and manganese is produced, which affects corrosion performance. This loss of composition in the arc and weld pool is due to the evaporation and deposition of nitrogen and manganese, which is a new problem for duplex grades.

The welding characteristics of duplex stainless steel are as follows:

After normal solution treatment of duplex stainless steel (heating at 1020 ℃ ~ 1100 ℃ and water cooling), the steel contains about 50% to 60% austenite and 50% to 40% ferrite. As the heating temperature increases, the ratio of the two phases does not change significantly.

Duplex stainless steel has good low temperature impact toughness. For example, the impact absorption energy of a 20mm thick plate transverse sample can reach more than 100J at -80℃. In most media, its uniform corrosion resistance and pitting corrosion resistance are good, but it should be noted that when this type of steel is heat treated below 950 ℃, its stress corrosion resistance will be significantly deteriorated due to the precipitation of σ phase. Since the ratio of Cr equivalent to Ni equivalent of the steel is appropriate, a large amount of primary austenite is still retained after high temperature heating, and secondary austenite can be formed during the cooling process. As a result, the total amount of austenite phase in the steel is Not less than 30% to 40% so that the steel has good resistance to intergranular corrosion.

In addition, as previously mentioned, the cracking tendency is very low when welding this steel, and preheating and post-weld heat treatment are not required. Due to the high content of N in the base metal, the single-phase ferrite area will not be formed in the welding near seam area, and the austenite content is generally not less than 30%. Applicable welding methods include argon tungsten arc welding and electrode arc welding. Generally, in order to prevent grain coarsening in the near seam area, low line energy welding should be used as much as possible during welding.

duplex stainless steel

The factors affecting the welding quality of duplex stainless steel are mainly reflected in the following aspects:

1. The effect of N content

Gómez de Salazar JM et al. studied the effect of different contents of N2 in the shielding gas on the properties of duplex stainless steel. The results show that with the increase of N2 partial pressure PN2 in the mixed gas, the nitrogen mass fraction ω(N) in the weld begins to increase rapidly, and then changes very little, and the ferrite phase content φ(α) in the weld increases with ω. (N) increases linearly, but the effect of φ(α) on tensile strength and elongation is just the opposite of that of ω(N). With the same ferrite phase content φ(α), the tensile strength and elongation of the base metal are higher than those of the weld. This is due to the difference in microstructure. The increased N content in the weld metal of duplex stainless steel can improve the impact toughness of the joint, which is due to the increase of the γ-phase content in the weld metal and the reduction of Cr2N precipitation.

2. Influence of heat input

Different from the weld zone, the ω(N) of the heat-affected zone does not change during welding, it is the ω(N) of the base metal, so the main factor affecting the structure and performance at this time is the heat input during welding. According to literature, suitable line energy should be selected when welding. If the heat input during welding is too large, the heat-affected zone of the weld will increase, and the metallographic structure will tend to be coarse and disordered, resulting in embrittlement, which is mainly manifested in the decline of the plasticity index of the welded joint. If the welding heat input is too small, it will cause hardened structure and easy to produce cracks, which is also unfavorable to the impact toughness of HAZ. In addition, all factors that affect the cooling rate will affect the impact toughness of HAZ, such as plate thickness, joint form, etc.

3. σ phase embrittlement

Foreign literature has introduced the σ phase embrittlement problem of duplex stainless steel and its weld metal caused by reheating. During the reheating process of the base metal and the weld metal, a fine secondary austenite γ* is formed from the α phase first, and then the σ phase is precipitated. The results show that the brittle cracks all occur at the σ phase and the interface between the matrix and the σ phase. The observation of the base metal fracture shows that there are dimples in the area around the σ phase. Due to the wide area of the α phase, a large number of σ phases will be generated. It reduces the toughness. However, the α-phase region in the weld is small, and the fracture still shows brittle fracture. As long as a small amount of σ-phase is generated, it is enough to cause the reduction of the toughness of the weld metal. Therefore, the σ-phase in the weld metal tends to be brittle. much larger than the parent material.

4. Hydrogen-induced cracking

Hydrogen embrittlement of welded joints of duplex stainless steel usually occurs in the α phase, and the susceptibility to hydrogen embrittlement increases with the increase of peak temperature during welding. The changes of its microstructure are: the peak temperature increases, the content of γ phase decreases, the content of α phase increases, and the amount of Cr2N precipitated from the boundary and interior of α phase increases, so hydrogen embrittlement is very easy to occur.

5. Stress corrosion cracking

Cracks in both the base metal and the weld metal start on the α-phase side of the α/γ interface and propagate within the α-phase. Austenite (γ) acts as a barrier to crack propagation due to its inherently low hydrogen embrittlement susceptibility. Since DSS contains a certain amount of austenite, its tendency to stress corrosion cracking is small.

6. Pitting problems

Pitting corrosion resistance is an important characteristic of duplex stainless steel, which is closely related to its chemical composition and microstructure. Pitting corrosion generally occurs at the α/γ interface and is therefore considered to be generated in the γ* phase between the γ phase and the α phase. This means that the Cr content in the γ* phase is lower than that in the γ phase. The composition of the γ* phase is different from that of the γ phase because the contents of Cr and Mo in the γ* phase are lower than those in the initial γ phase. Further research shows that the pitting corrosion potential of steel with lower N content is more sensitive to the cooling rate. Therefore, when welding duplex stainless steels with lower N content, the control of cooling rate is more stringent. In the welding process of duplex stainless steel, reasonable control of welding line energy is the key to obtaining high-quality duplex stainless steel joints. If the line energy is too small, the cooling rate of the weld metal and the heat-affected zone will be too fast, and the austenite will not have time to precipitate, so that the content of the ferrite phase in the structure will increase; if the line energy is too large, although a sufficient amount of Austenite, but also causes ferrite grain growth in the heat-affected zone and precipitation of σ-equal harmful phases. In general, electrode arc welding (Shieded Metal Arc Welding, SMAW), gas tungsten arc welding (Gas Tungsten Arc Welding, GTAW), flux-cored wire arc welding (Flux-Cored Wire Arc Welding, FCAW) and plasma arc welding (Plasma arc welding) Welding methods such as Arc Welding, PAW) can be used for the welding of duplex stainless steel, and generally do not need to take preheating measures before welding, nor do heat treatment after welding.

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