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Top 3 basics for welding of duplex stainless steels

After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?

What is Duplex Stainless Steel?

Duplex stainless steel has the characteristics of both austenitic stainless steel and ferritic stainless steel because it has an austenite + ferrite dual-phase structure, and the content of the two-phase structures is the same. The yield strength can reach 400Mpa ~ 550MPa, which is twice that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has high toughness, low brittle transition temperature, and obvious improvement in intergranular corrosion resistance and welding performance. High conductivity, small coefficient of linear expansion, superplasticity, and magnetic properties. Compared with austenitic stainless steel, the strength of duplex stainless steel is high, especially the yield strength is significantly improved, and the pitting corrosion resistance, stress corrosion resistance, corrosion fatigue resistance, and other properties are also significantly improved.

 Top 3 basics for welding of duplex stainless steels
Top 3 basics for welding duplex stainless steels

What is the classification of duplex stainless steel?

Duplex stainless steels are classified according to their chemical composition and can be divided into four types: Cr18 type, Cr23 (without Mo) type, Cr22 type, and Cr25 type.
For Cr25 type duplex stainless steel, it can be divided into ordinary type and super duplex stainless steel, among which Cr22 type and Cr25 type have been used more in recent years. Most of the duplex stainless steel used in my country is made in Sweden. The specific grades are 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), and SAF2507 (Cr25 type).

2 major welding requirements for duplex stainless steel

Strictly control hydrogen

Duplex stainless steel has good weldability. It is neither easy to embrittle the heat-affected zone when welding ferritic stainless steel, nor prone to welding hot cracks like austenitic stainless steel, but because it has a large amount of ferrite, when When the rigidity is high or the hydrogen content of the weld is high, hydrogen cooling cracks may occur, so it is very important to strictly control the source of hydrogen.

Welding heat input

To ensure the characteristics of dual-phase steel, it is the key to the welding of this type of steel to ensure that the proportion of austenite and ferrite in the microstructure of the welded joint is appropriate. When the cooling rate of the post-weld joint is slow, the second phase change of δ→γ is sufficient, so a duplex structure with a suitable phase ratio can be obtained at room temperature, which requires a suitable large welding heat input during welding. Otherwise, if the cooling rate after welding is fast, the delta ferrite phase will increase, resulting in a serious decrease in the plastic toughness and corrosion resistance of the joint.

Duplex stainless steel welding consumables selection

Welding consumables for duplex stainless steel are characterized in that the weld structure is austenite-dominated duplex structure, and the content of main corrosion-resistant elements (chromium, molybdenum, etc.) sex. To ensure the content of austenite in the weld, the content of nickel and nitrogen is usually increased, that is, the nickel equivalent is increased by about 2% to 4%. In the base metal of duplex stainless steel, there is generally a certain amount of nitrogen content, and a certain amount of nitrogen content is also expected in the welding material, but generally, it should not be too high, otherwise, pores will occur. The higher nickel content thus becomes a major difference between the welding consumable and the base metal.

Corrosion resistance, joint toughness

According to the different requirements of corrosion resistance and joint toughness, the electrode that matches the chemical composition of the base metal should be selected, such as welding Cr22 duplex stainless steel, or Cr22Ni9Mo3 electrode, such as E2209 electrode. When the acid electrode is used, the slag removal is excellent, and the welding seam is beautiful, but the impact toughness is low. When the weld metal is required to have high impact toughness and all-position welding is required, the basic electrode should be used. Alkaline electrodes are usually used when the root is welded. When there are special requirements for the corrosion resistance of the weld metal, a basic electrode with super duplex steel composition should also be used.

Flux cored wire

CNC plate bending machine rolling a stainless plate

For solid gas-shielded welding wire, while ensuring the weld metal has good corrosion resistance and mechanical properties, attention should also be paid to its welding process performance. For flux-cored welding wire, when the weld is required to be beautiful, rutile, or titanium For calcium-type flux-cored welding wire when higher impact toughness is required or welding is performed under conditions of greater restraint, flux-cored welding wire with higher alkalinity should be used.

Alkaline Flux

For submerged arc welding, a wire with a smaller diameter should be used to achieve multi-layer multi-pass welding under small and medium welding specifications to prevent the embrittlement of the welding heat-affected zone and the weld metal, and a matching alkaline flux should be used.

Top 2 welding requirements for duplex stainless steels

Requirements 1#: Control of Welding Thermal Process

Welding line energy, inter pass temperature, preheating and material thickness will affect the cooling rate during welding, thus affecting the structure and properties of the weld and heat-affected zone.

  1. Cooling rates that are too fast or too slow can affect the toughness and corrosion resistance of dual-phase steel welded joints. Excessive α-phase content and increased Cr2N precipitation are caused when the cooling rate is too fast.
  2. Too slow a cooling rate will cause serious coarse grains, and may even precipitate some brittle intermetallic compounds, such as σ phase.
  3. The specific material thickness should also be considered when selecting the line energy.
  4. When welding duplex steels and super stainless steels with a high ω(Cr) content of 25 %, in order to obtain the best weld metal properties, it is recommended that the maximum interpass temperature be controlled at 100 °C. When heat treatment is required after welding, the interpass temperature may not be limited.

Requirements 2#: post weld heat treatment

  • It is best not to carry out heat treatment after welding of duplex stainless steel, but when the content of α phase exceeds the requirements in the welding state or when harmful phases such as σ phase are precipitated, post-weld heat treatment can be used to improve. The heat treatment method used is water quenching.
  • During heat treatment, the heating should be as fast as possible, and the holding time at the heat treatment temperature should be 5 to 30 min, which should be sufficient to restore the equilibrium of the phases.
  • Oxidation of the metal during heat treatment is very serious, and inert gas protection should be considered.
  • For dual-phase steel with ω(Cr) of 22%, heat treatment should be carried out at 1050℃ ~ 1100℃, while dual-phase steel and super duplex steel with ω(Cr) of 25% should be heat treated at 1070℃ ~ 1120℃ heat treatment.