The welding characteristics of low alloy steel mainly include the following aspects:
1) For some low-alloy structural steels with low strength grade and low carbon content, the hardening tendency of the weld heat affected zone is not large, but with the increase of the strength grade of low alloy steel, the weld heat affected zone The cold cracking tendency is significantly increased, and the factors affecting the hardening degree of the heat affected zone have two aspects: 1 material and structural form, such as the type, thickness, joint form and weld size of the steel; 2 process factors, such as process, welding Specification, the welding temperature near the weld (equivalent to the surrounding air temperature or preheating temperature). In order to alleviate the hardening tendency of the heat-affected zone, in the process factors, the hardening of the heat-affected zone is often avoided by selecting appropriate process specifications (such as increasing the welding current and slowing the welding speed).
   In addition, as the strength grade of steel increases, the tendency to cold cracking increases. Cold cracks mainly occur in thick plates of high-strength steel, and the main factors affecting the occurrence of cold cracks are: hydrogen content in weld and heat affected zone, degree of hardening in heat affected zone and joint residual stress determined by joint stiffness. .
2) For low-alloy steels with high strength grade and quenching tendency, in addition to the possibility of cold cracks in the joints with extremely high rigidity, delayed cracks which are cold crack properties may be generated. The delayed crack is a cold crack that occurs after the weldment is parked for a period of time (hours, days, or even ten days) without timely heat treatment after welding. The delayed crack develops under the stress and also to the depth of the base metal and the weld metal. Preventing delayed cracks can be controlled from the selection of welding materials and their cleaning, preheating, interlayer insulation, and timely tempering after welding.
   In addition, reheat cracking may occur during post-weld heat treatment of low alloy high strength steel. When welding, the welding material with lower strength should be used as much as possible, so that the deformation occurred during the post-weld heat treatment is concentrated at the weld metal to avoid cracking of the heat affected zone.
3) For the welding of large-thickness rolled ordinary low-alloy steel plates, in the heat-affected zone at the fillet weld of the tee fitting and the T-joint, there may be a crack parallel to the surface of the plate (called lamellar tear). .
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