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Laser welding is an important part of the application of laser processing technology, and it is the most watched and most promising welding technology in the 21st century.

Laser welding, with its high-energy beam focusing method, can realize deep penetration welding, fast welding and other forms that are difficult to achieve in the welding process, especially the flexible matching of laser welding equipment and mature real-time online detection technology so that it can A high degree of automation is realized in mass production, and a large number of laser welding production lines have been put into industrial production.

Weld Ability of Different Materials

Weldability refers to the difficulty of obtaining high-quality welded joints under certain welding process conditions. The weldability of steel depends on its chemical composition.

Carbon content: the most influential, the lower the carbon content, the less prone to cracks and the better the weldability.

Alloying elements: carbon equivalent Ceq, when the amount of carbon is less than 0.4%, the weldability is excellent, and when the amount of carbon is greater than 0.6%, the weldability is poor.

• Steel with good weldability: low carbon steel (carbon content<0.25); low alloy steel (alloy element content 1~3, carbon content<0.20); stainless steel (alloy element content>3, carbon content<0.18)

• Steel with general weldability: medium carbon steel (alloy element content<1, carbon content 0.25~0.35); low alloy steel (alloy element content<3, carbon content<0.30); stainless steel (alloy element content 13~25, carbon content 0.18)

• Steel with poor weldability: medium carbon steel (alloy element content <1, carbon content 0.35~0.45); low alloy steel (alloy element content 1~3, carbon content 0.30~0.40); stainless steel (alloy element content 13, carbon content 0.20)

• Steels with poor weldability: medium and high carbon steel (alloy element content<1, carbon content>0.45); low alloy steel (alloy element content 1~3, carbon content>0.40); stainless steel (alloy element content 13, carbon content 0.30~0.40)

Weld Ability of Stainless Steel

Classification of Stainless Steel

• Austenitic stainless steel: Cr>18%, containing about 8% Ni and a small amount of Mo, Ti, N, commonly used grades 1Cr18Ni9, 0Cr19Ni9, generally adopt solution treatment (heated to 1050~1150˚C, water cooling or air cooling, to obtain a single-phase austenite structure);

• Ferritin stainless steel: Cr=12-30%, Cr17, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28;

• Duplex stainless steel: Cr=18-28%, Ni=3-10%, duplex structure of ferrite and austenite;

• Martensitic stainless steel: high strength, poor plasticity, and weldability, 1Cr13, 3Cr13, easy to produce cold cracks, need preheating, post heating, high-temperature tempering; F forming elements: Cr, M; A forming elements: Ni, N, Cu, Mn.

Features of Stainless Steel

The thermal expansion coefficient of austenitic stainless steel is 1.5 times that of low carbon steel and ferritin stainless steel → large deformation after welding, high thermal cracking sensitivity;

The thermal conductivity of austenitic stainless steel is 1/3 of that of low-carbon steel, and that of ferritin stainless steel is 1/2 of that of low-carbon steel → heat accumulation, coarse grains, and thermal cracks;

The specific resistance of austenitic stainless steel is more than 4 times that of low carbon steel, and the specific resistance of ferritin stainless steel is 3 times that of low carbon steel → the laser absorption rate is large.

Laser Welding of Austenitic Stainless Steel

Contains more stable austenite elements. Since MS is below room temperature, it has an austenite structure at room temperature.

Base metal: polygonal austenite, a small amount of black banded δ ferrite, and granular M23C6 carbides distributed in dots on the austenite grain boundary.

Weld zone: Austenite is the matrix, and ferrite coexists in the weld in the form of mesh and lath. The microstructure of the weld is determined by the crystallization mode and the solid-state phase transition from ferrite to austenite, and the crystallization mode and solid-state phase transition finally increase with the increase of Creq/Nieq and cooling rate, and the iron in the weld The body shape transitions from skeleton to mesh.

Large heat input coarse grain poor mechanical properties; hot crack: segregation of impurity elements such as P and S; promote hot cracking P>S>Si>Ni, inhibit hot cracking C>Mn>Cr; ferrite phase is 5% -20%, the least tendency to hot cracking.

Laser Welding of Ferritin Stainless Steel

Cr=12%-30 generally does not contain Ni, and the ferrite of the lattice is used as the matrix structure at high temperatures and average temperatures.

Base material: single-phase ferrite structure. Weld zone: ferrite + flaky marten site precipitated at grain boundaries. HAZ: coarse ferrite.

The linear expansion coefficient of ferritin stainless steel is lower than that of austenite, and the tendency of welding hot cracking is not prominent. Generally speaking, it is said that ferritin stainless steel is not easy to weld, which means that the plasticity and toughness of the joint may be reduced and embrittled during the welding process. And its corrosion resistance and embrittlement in long-term service at high temperatures are also problems that cannot be ignored. High-purity F is more weldable than ordinary F (0.05-0.1%C).

FSS grains are easily coarsened above 900°C (minimum heat input and faster cooling rate), embrittled near 475°C (post-weld heat treatment), and stay in the range of 550-820°C to form a σ phase to embrittle the joint (long-term service). Preheating at low temperatures (<150°C) prevents cold cracks (no preheating for high-purity ferritic stainless steel), limits impurities such as C and N, and fills with austenitic welding wire.

Laser Welding Properties of Duplex Stainless Steel

Refers to stainless steel with ferrite and austenite each accounting for about 50%, and the content of fewer phases generally needs to reach at least 30%. In the case of low C content, the Cr content is 18%~28%, and the Ni content is 3%~10%. Some steels contain alloying elements such as Mo, Cu, Nb, Ti, and N.

Base metal: ferrite plus austenite Weld zone: coarse ferrite, low austenite content, precipitated at grain boundaries. HAZ: Ferrite is the matrix, and austenite is interconnected in an acicular shape.

Coarse ferrite is formed in the weld area, the austenite content is low, the grain boundaries are easily corroded, and the weld is prone to embrittlement at low temperatures. The grain size of HAZ is coarse, and the toughness decreases. That is, too high F and rough grain structure seriously affect the joint toughness and corrosion resistance. Add Ni-containing material, nitrogen preheating, and slow cooling (may produce embrittlement phase).

Laser Welding of Martensitic Stainless Steel

Ordinary 1Cr13, 2Cr13, heat-strength martensitic stainless steel such as 2CrWMoV, 2Cr12MoV, ultra-low carbon multi-phase martensitic stainless steel 0.01C-13Cr-7Ni-3Si, 0.03C-12.5Cr-4Ni-0.3Ti, 0.03C-12.5Cr -5.3Ni0.3Mo etc. Except for ultra-low carbon multi-phase M stainless steel, all have a tendency to be brittle and hard.

When the cooling rate is small, the HAZ is prone to produce coarse martensite and carbide; when the cooling rate is high, the HAZ will harden and form coarse martensite; the plasticity and toughness of the HAZ decrease and become brittle

Weld zone: coarse martensite + a certain amount of ferrite; coarse martensite: sensitive to cold cracking, and delayed cracking in the presence of hydrogen; ferrite: distributed between coarse martensite grains, severe It is distributed in a network shape, making the joint more sensitive to cold cracks.

When C<0.1%, it is not necessary to preheat, or it can be preheated to 200; when C=0.1-0.2%, it can be preheated to 200-260; the thin plate can’t be preheated, even if it is preheated, the temperature is 150; when it is austenitic filler, Preheating is not required; after heating (quenching and tempering or high-temperature tempering) 600-750, 1hour.

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