Super duplex stainless steel
is duplex stainless steel with PREN> 40, 25% Cr and high molybdenum (> 3.5%) and high nitrogen (0.22% -0.30%). The main grades are UNS S32550 (UR52N +), S32750 ) And S32760 (Zeron 100) have better resistance to localized media corrosion and better weldability than duplex stainless steels.
Super duplex stainless steel after normal solution treatment (1020 ℃ ~ 1100 ℃ heating and water cooling), the steel contains about 50% ~ 60% austenite and 50% ~ 40% ferrite. As the heating temperature increases, the two-phase ratio change is not obvious.
Super Duplex stainless steel has a good low temperature impact toughness, such as 20mm thick plate horizontal specimen at -80 ℃ impact energy absorbed up to 100J above. In most media, its resistance to uniform corrosion and pitting corrosion are good, but note that this type of steel below 950 ℃ heat treatment, due to the precipitation of σ phase, its stress corrosion resistance will be significantly deteriorated. Due to the appropriate ratio of Cr equivalent and Ni equivalent of the steel, a large amount of primary austenite structure remains after heating at high temperature, and the secondary austenite can also be generated during cooling. As a result, the total amount of austenite phase in the steel Not less than 30% ~ 40% So that the steel has good resistance to intergranular corrosion.
In addition, as previously mentioned, the tendency to crack is very low during the welding of such steels and does not require preheating and post-weld heat treatment. As the base metal contains higher N, the weld near the seam area does not form a single-phase ferrite zone, the austenite content is generally not less than 30%. Suitable welding methods are TIG welding and welding arc welding, etc., generally in order to prevent coarse grain coarsening near the seam area, welding, welding should try to use low line energy.
The factors that affect the welding quality of super duplex stainless steel are mainly reflected in the following aspects:
N content effect
With the increase of partial pressure of N2 in the mixed gas, the mass fraction of nitrogen (ω) starts to increase rapidly and then changes little. 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). The same ferrite phase content of φ (α), the base material tensile strength and elongation are higher than the weld. This is caused by the difference of microstructure. The increase of N content in the duplex stainless steel weld metal can improve the impact toughness of the joint due to the increase of the γ phase content in the weld metal and the decrease of Cr2N precipitation.
Heat input effects
Unlike the weld zone, ω (N) does not change in the HAZ during welding. It is the base material's ω (N), so the main factor affecting the microstructure and properties at this point is the heat input during welding. According to the literature, welding should choose the right line energy. If the welding heat input is too large, heat-affected zone weld area increases, the microstructure tends to coarse grains, disorder, resulting in embrittlement, mainly for the plasticity of welded joints decreased. Such as welding heat input is too small, resulting in hardened tissue and easy to produce cracks, the impact toughness of the HAZ equally unfavorable. In addition, all the factors that affect the cooling rate will affect the impact toughness of the HAZ, such as thickness, joints and other forms.
σ phase embrittlement
In the reheating process of the base metal and the weld metal, a small secondary austenite γ is formed from the α phase and then the σ phase is precipitated. The results show that the brittle fracture occurs both in the σ phase and the interface between the matrix and the σ phase. The observation of the fracture of the base metal shows that all of them are dimples in the surrounding area of the σ phase. Due to the wide α-phase region, However, the α-phase region is small in the weld, and the fracture still shows brittle fracture. As long as a small amount of σ phase is generated, it is enough to cause the decrease of the toughness of the weld metal. Therefore, σ phase in the weld metal tends to embrittle Much bigger than base metal.
Hydrogen embrittlement of super duplex stainless steel welded joints usually occurs in the alpha phase, and the sensitivity of hydrogen embrittlement increases with increasing peak temperature during welding. The changes of microstructure were as follows: the peak temperature increased, the content of γ phase decreased, the content of α phase increased, and the amount of Cr2N precipitated from the boundary and inside of α phase increased.
Stress corrosion cracking
Cracks in the base metal and in the weld metal all start on the α phase side of the α / γ interface and propagate in the α phase. Austenite (γ), due to its inherent low hydrogen embrittlement susceptibility, can act as a barrier to crack propagation. Due to the DSS contains a certain amount of austenite, so its stress corrosion cracking tendency less.