Various "Problems" and "Prescriptions" in the Process of Stainless Steel Pipe Welding (2)

  • By:Iris Liang
  • Date:2021/12/29

Next, Hangao Tehc (SEKO Machinery) will continue to take you to understand the problems that may occur during the welding process and help you prevent and solve such problems.

03 Welding hot cracks (crystallization cracks in welds, liquefaction cracks in the heat-affected zone)

The sensitivity of thermal cracking mainly depends on the chemical composition, organization and performance of the material. Ni is easy to form low melting point compounds or eutectic with impurities such as S and P. The segregation of boron and silicon will promote thermal cracking.

The weld seam is easy to form a coarse columnar crystal structure with strong directionality, which is conducive to the segregation of harmful impurities and elements. This promotes the formation of a continuous intergranular liquid film and improves the sensitivity of thermal cracking. If the welding is not uniformly heated, it is easy to form a larger tensile stress and promote the generation of welding hot cracks.

Preventive measures:

a. Strictly control the content of harmful impurities S and P.

b. Adjust the organization of the weld metal. The dual-phase structure weld has good crack resistance. The delta phase in the weld can refine the grains, eliminate the directionality of single-phase austenite, reduce the segregation of harmful impurities at the grain boundary, and the delta phase can dissolve more The S and P can reduce the interfacial energy and organize the formation of intergranular liquid film.

c. Adjust the weld metal alloy composition. Appropriately increase the content of Mn, C, and N in the single-phase austenitic steel, and add a small amount of trace elements such as cerium, pickaxe, and tantalum (which can refine the weld structure and purify the grain boundary), which can reduce the sensitivity of thermal cracking.

d. Process measures. Minimize the overheating of the molten pool to prevent the formation of thick columnar crystals. Use small heat input and small cross-section weld beads. An arc stabilizing device can be added during welding to reduce the area of ​​the molten pool, improve the working efficiency of the welding gun, and improve the welding quality.

For example, 25-20 austenitic steel is prone to liquefaction cracks. It is possible to strictly limit the impurity content and grain size of the base material, adopt high-energy density welding methods, small heat input and increase the cooling rate of the joints.

04 Embrittlement of welded joints

Heat-strength steel should ensure the plasticity of welded joints to prevent high-temperature embrittlement; low-temperature steels are required to have good low-temperature toughness to prevent low-temperature brittle fracture of welded joints.

05 Large welding distortion

Due to the low thermal conductivity and large expansion coefficient, the welding deformation is large, and clamps can be used to prevent deformation. The welding method of austenitic stainless steel and the selection of welding materials:

Austenitic stainless steel can be welded by argon tungsten arc welding (TIG), molten argon arc welding (MIG), plasma argon arc welding (PAW) and submerged arc welding (SAW).

Austenitic stainless steel has a low welding current because of its low melting point, low thermal conductivity, and high electrical resistivity. Narrow welds and beads should be used to reduce high-temperature residence time, prevent carbide precipitation, reduce weld shrinkage stress, and reduce thermal crack sensitivity.

The composition of welding material, especially Cr and Ni alloying elements, is higher than that of base material. Use welding materials containing a small amount (4-12%) of ferrite to ensure good crack resistance (cold cracking, hot cracking, stress corrosion cracking) performance of the weld.

When the ferrite phase is not allowed or impossible in the weld, the welding material should be the welding material containing Mo, Mn and other alloy elements.

The C, S, P, Si, and Nb in the welding material should be as low as possible. Nb will cause solidification cracks in the pure austenitic weld, but a small amount of ferrite in the weld can be effectively avoided.

For welding structures that need to be stabilized or stress-relieved after welding, Nb-containing welding materials are usually used. Submerged arc welding is used to weld the middle plate, and the burning loss of Cr and Ni can be supplemented by the transition of the flux and the alloy elements in the welding wire;

Due to the large penetration depth, care should be taken to prevent the occurrence of hot cracks in the center of the weld and the reduction of corrosion resistance in the heat-affected zone. Attention should be paid to choosing a thinner welding wire and a smaller welding heat input. The welding wire needs to be low in Si, S, and P.

The ferrite content in the heat-resistant stainless steel weld should not exceed 5%. For austenitic stainless steel with Cr and Ni content greater than 20%, high Mn (6-8%) welding wire should be used, and alkaline or neutral flux should be used as the flux to prevent the addition of Si to the weld and improve its crack resistance.

The special flux for austenitic stainless steel has very little increase of Si, which can transfer the alloy to the weld and compensate for the burning loss of alloy elements to meet the requirements of weld performance and chemical composition.

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