When choosing CO2 gas shielded welding wire, must according to the welding pieces of the chemical composition of parent metal, welding method, and the mechanical properties of welded joint, the welding structure, welding pieces of the constraint degree can heat treatment after welding and weld metal of high temperature resistant, good low temperature resistance, corrosion resistance and other use conditions make a comprehensive consideration, and then after welding process evaluation, comply with the technical requirements shall be determined after welding structure .
When welding hot-rolled steel, normalizing steel and low carbon quenched and tempered steel using CO2 gas shielded welding, the first consideration is that the mechanical properties of the weld metal are close to or equal to that of the weld base metal, and whether the chemical composition of the weld metal is the same as that of the weld base metal is the secondary consideration. When welding the structure with large constraint, in order to prevent welding cracks, the principle of low matching can be adopted, that is, the strength of the weld metal is slightly lower than that of the base metal of the weldment. When selecting welding wire according to the requirement of equal strength, the influence of plate thickness, joint form, groove shape, weld distribution and welding heat input on the mechanical properties of welding metal should be fully considered.
When choosing welding wire before carbon quenching and tempering steel in welding, while strictly controlling S, P and other impurities in weld metal, it should also ensure that the main alloy composition of weld metal is similar to that of base metal, so as to ensure that the mechanical properties of weld metal are consistent with base metal when tempering and tempering after welding. When welding the base metal of two different strength grades, the welding wire should be selected according to the base metal of low strength grade, and the plasticity of the weld should not be lower than the base metal of low plasticity. Welding parameters should be formulated for base metal with poor weldability.
The weld metal of alkaline fluxcored wire welding has good plasticity, toughness and crack resistance, and the relative fluidity of alkaline slag is good, which facilitates the gas escape between welding pool and slag and reduces the tendency of welding seam to form porosity. Alkaline slag fluoride in (CaF2, etc.) can also prevent oxygen dissolved in the welding pool, the diffusible hydrogen content in the weld is very low, so the basic flux cored wire to steel plate surface coated with anti-rust agent has strong ability of stomatal resistance and resistance to pit, the coated with iron oxide and sulfide type paint primer of steel also has good welding effect. The disadvantages of alkaline flux-cored wire are: the welding path is convex and the splash is larger; During the welding process, the droplet of welding wire is coarse particle transition. The fluidity of welding slag is too large, it is not easy to achieve all-position welding; In the process of welding, it is easy to cause defects such as lack of fusion. Therefore, in recent years, alkaline flux cored wire is gradually replaced by titanium flux cored wire.
Titanium flux cored wire is also known as rutile type welding wire, this kind of welding wire belongs to acid slag system, slag flow is good, solidification temperature range is very small, suitable for all-position welding. During the welding process, the arc is stable and the welding wire droplet becomes spray transition, which greatly improves the mechanical properties of flux cored wire, especially the low temperature toughness of weld metal. Titanium flux cored wire is dominant in flux cored wire.
Low alloy heat resistant steel with a certain amount of Cr and Mo elements is widely used in the manufacture of boilers, petroleum industry refining equipment, synthetic chemical industry equipment and high temperature and high pressure hydrogen resistant materials. In use, steel and welded joints are required to be chemically stable and have sufficient creep resistance and durable strength under high temperature and pressure. Choose cr-Mo heat resistant steel welding wire, first of all to ensure that the chemical composition and mechanical properties of the weld should be as consistent as possible with the base material, so that the weld has a certain high temperature strength, good oxidation resistance and resistance to gas medium corrosion ability at the working temperature. Secondly, while considering the weldability of welding wire, welding wire with high impurity content or high strength should be avoided. The mass fraction of carbon in Cr-Mo heat resistant steel wire should be controlled between 0.07% and 0.12%, too low carbon content will reduce the high temperature strength of Cr-Mo heat resistant steel weld, too high carbon content is prone to weld crystallization crack. In recent years, in order to reduce the preheating temperature before welding, or even not preheating, improve the crack resistance of Cr-Mo heat-resistant steel weld, welding wire ω(C)≤0.04% ultra low carbon Cr-Mo heat-resistant steel welding wire.
Welding takong degrees, not heat treatment after welding of Cr, Mo alloy steel structure repair welding and poor weldability and welding defects of heat resistant steel (such as 12 cr5mo), should choose good plasticity, low intensity, a large quantity of dissolved hydrogen Cr – Ni austenite size no preheating welding wire, to improve the toughness of welded joint, reduce welding stress, prevent the welding cracks Purpose. N-type welding wire is not suitable for Cr-Mo heat-resistant steel welding structure working under cyclic heating and cooling conditions, because there is a great difference in thermal expansion coefficient between Cr-Ni type welding wire and Cr-Mo heat-resistant steel, the welding structure will crack due to the action of thermal stress during use.
Low temperature steel is a special steel for welding structure at low temperature (-25–253℃). In use process, should ensure that under the condition of low temperature working, have enough strength, plasticity, toughness and good weldability, temper brittleness, and sensitivity to strain aging brittleness, work in less than the minimum temperature, the steel and welding joint area has enough ability to resist rupture, so as to achieve the purpose of safe use in low temperature. Low temperature steel welding wire, whether solid cored wire or flux cored wire, mostly with the base material containing Ni equivalent nickel alloyed low alloy steel welding wire, wire C, S, P and other impurities the lower the better. If trace elements T and B are added to the welding wire, the weld grain can be refined, so as to improve the toughness of the weld.
Low temperature steel flux cored wire is classified into titanium slag, alkaline slag and metal powder slag according to slag system. Titanium type low temperature steel flux-cored wire has good all position welding operation, and the weld appearance, but the diffusion of oxygen content on the high side, the weld metal toughness and crack resistance of alkaline slag system of flux-cored wire is better than alkaline slag series low temperature steel flux-cored wire, though the low temperature toughness and crack resistance of titanium type low temperature steel flux-cored wire is good, but the poor welding manufacturability. In recent years, in titanium type flux-cored wire welding powder to add element of a strong oxidation and hydrogen group yuan, improve the effect of the DNA of weld pool and hydrogen to, make it has both titanium wire type flux-cored wire welding process good advantages, but also has basic flux-cored wire crack resistance high characteristic, the titanium type low temperature steel flux-cored wire, can control the mass fraction of oxygen in weld metal at about 0.06%, the diffusion of oxygen The content is less than 5mL/100g.
Weathering steel has a certain atmospheric corrosion resistance and seawater corrosion performance, because the steel contains a small amount of Elements such as Cu, P, Cr, Ni, its atmospheric corrosion resistance than ordinary carbon steel 1/3 higher than. The welding wire for weathering steel can be divided into Cu-Cr system, Cu-Cr-Ni system and Cu-Ni system according to its deposited metal alloy system. The tensile strength of weld metal can be divided into 490MPa and 590MPa. When selecting the welding wire, the chemical composition of the welding wire should be consistent with the welding weathering steel alloy system. Because P is easy to cause cracks and embritchment in the weld, it is not necessary to add P in the welding wire to improve the atmospheric corrosion resistance of the weathering steel weld.