At present, low-alloy high-strength steels commonly used in the field of steel structures generally refer to steels having a tensile strength of 500 to 1000 MPa, and those having a tensile strength of more than 1000 MPa are generally referred to as ultra-high-strength steels. The types of low alloy high strength steel can be divided into non-tempered steel and quenched and tempered steel. Commonly used are hot-rolled steel, controlled-rolled steel and normal-fired steel. Generally, non-tempered steel refers to steel with a normal temperature tensile strength of 600 MPa or less.
1. Problems with high-strength steel welding
As the alloying elements in the steel increase, the strength level increases (yield strength ≥ 315 MPa), and the weldability of the steel gradually deteriorates. The main problems that arise are:
1 Hardening tendency of the heat affected zone: rapid cooling during welding leads to martensite structure in the heat affected zone.
2 cold crack: the tendency of cold cracking during welding is increased, and has retardation. For example, the positioning weld is easy to crack, because the weld size is small, the length is short, and the cooling speed is fast.
3 hot crack: hot-rolled, normal-fired steel with yield strength of 315~400 MPa has little tendency to hot crack, but in high-dilution bead of thick-walled sheet (such as root bead or multi-layer submerged arc welding near the edge of the groove) Hot cracks appear in the weld bead.
4 The coarse-grained area is embrittled: the heat-affected zone is heated to a coarse-grained area above 1100 °C. When the welding heat input is too large, the crystal grains will grow rapidly or the Wei's structure will appear, resulting in embrittlement.
2. Main process measures for welding high strength steel
(1) Preheating Preheating is an effective measure to prevent cracking, and it also helps to improve the joint performance, but it will deteriorate the working conditions and complicate the production process. Excessive preheating temperature will also reduce the toughness of the joint. Therefore, whether the preheating and preheating temperature are required before welding should be determined according to the composition (carbon equivalent), plate thickness, structural shape, stiffness and ambient temperature of the base metal.
(2) Selection of welding heat input For steel grades with high carbon equivalent (including V, Nb, Ti), in order to reduce the adverse effects caused by coarse-grain embrittlement in the heat-affected zone, a smaller welding heat input should be selected. Should be controlled below 35kJ / cm.
(3) After heat and post-weld heat treatment, heat refers to the end of welding or after welding a weld, the weldment is immediately heated to 200~250 °C, and kept for a period of time (about 2h), so that the hydrogen in the joint diffuses Out, to prevent delayed cracks. For thick plates and complex areas of stress, post-weld process measures should be taken or covered with a thick enough insulation cotton/felt for slow cooling. When the site conditions permit, high-temperature tempering to eliminate stress should be carried out in time after welding. The purpose is to eliminate welding residual stress and improve the structure without post-heat treatment.
(4) The welding consumables used for welding high-strength steel are selected as the principle of equal strength.
3. Existing welding methods and processes
(1) Welding rod arc welding is applicable to welds of various irregular shapes and various welding positions. When welding arc welding, the welding parameters are mainly selected according to the thickness of the weldment, the shape of the groove, the position of the weld, and the like. When welding the first layer of multi-layer welding and the non-flat welding position, the diameter of the electrode should be smaller. Welding rod arc welding has a wide range of applications, and the welding materials and processes are mature. This welding method can be applied to steel grades within 500~1000MPa. The matching welding rods include CJ607RH, CJ707RH, CJ807RH and CJ107. Generally, under the premise of ensuring the quality of welding, large-diameter welding rods and high-current welding should be used as much as possible to improve production efficiency.
(2) Submerged arc welding has the advantages of high welding efficiency, large penetration depth and mechanical automatic operation, so it is especially suitable for the manufacture of large welded structures, and is mostly used for flat welding and flat fillet welding positions. For steel grades with tensile strength of 500-700 MPa, submerged arc welding can be used for flat welding and flat-angle welding. The welded structure has steel structure, pipelines, bridges, etc. The matching welding wire has CJGNH-1, CJQ-1, CJGX- 1. H10Mn2, CH62CF and H70Q. For steel grades with a tensile strength of 700 MPa or more, there are not many examples of submerged arc welding, mainly because the welding heat input of the submerged arc welding is large, resulting in coarse grain in the weld and heat affected zone, which is reduced due to increased brittleness. Resilience.
(3) CO2 gas shielded welding is currently a highly effective high-efficiency welding method, which includes CO2 gas shielded welding solid wire and flux cored wire. In fact, core wire has a wide application range, and high strength steel of 500~900 MPa uses solid wire. The application field basically includes the existing structure and the effect is better. The narrow gap welding of gas shielded welding wire has the advantages of high productivity, small welding heat input, narrow heat affected zone, etc. The matching welding wire has WH50-6, WH60-G, WH70-G, WH80-G and WH90-G.
(4) Self-protecting flux cored wire is similar to the coating of the electrode in the core, and plays the role of slag formation, gas generation, arc stabilization and deoxidation during welding, so no additional protective gas is needed for welding. Self-protecting flux cored wire has high welding efficiency and strong adaptability, and is especially suitable for field welding, such as pipeline construction. China's West-East Gas Pipeline is self-shielded flux-cored wire welded with X80 pipeline steel.
(5) Both electroslag welding and gas-electric vertical welding have similarities in the form, method and welding efficiency of welded joints. The welding efficiency is very high and the welding heat input is large. In particular, gas-electric vertical welding uses a flux-cored wire, which has high internal quality and beautiful appearance, and is increasingly used in the hull closing and construction of oil storage tanks. Since the electroslag weld and the heat-affected zone are overheated and the grains are coarsened, normalizing is required after welding.
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