(1) Selection of carbon steel electrode
Carbon steel is a general term for carbon structural steel and carbon tool steel. China’s carbon steel output accounts for more than 80% of the total steel output. The weldability of carbon steel is closely related to the carbon content of the steel type. The higher the carbon content, the worse the weldability of the steel. Carbon steel welding rods currently use a large number of welding rods below 490MPa. When welding low carbon steel (carbon less than 0.25%), most of the electrodes of the E43 ×× (J42 ×) series are used. This series of electrodes has a variety of models and more product grades. Depending on the specific base material and use conditions, working conditions, Welding structure shape and steel plate thickness are selected. When welding medium carbon steel (C = 0.25% ～ 0.60%) and high carbon steel (C> 0.60%), an alkaline low hydrogen electrode with low impurity content and a certain desulfurization ability should be selected. A comparison of the process performance of various low-carbon steel electrodes is shown in Table 3-6.
Table 3-6
In some cases, ilmenite-type or calcium-titanium-type electrodes can also be used, but strict process measures should be used in conjunction with medium carbon steel welding. Due to the high carbon content of the steel, the tendency of welding cracks to increase, low hydrogen electrodes or Weld metal has higher plastic and tougher electrodes, and in most cases requires preheating and slow cooling. High-carbon steel welding must take strict pre-heating and post-heating measures to prevent welding cracks. It is difficult to weld the same properties of the high-carbon steel and the base metal. The tensile strength of the high-carbon steel is mostly above 675MPa. The selection of the welding material should depend on the product design requirements. When strength requirements are high, J707 or J607 electrodes can be used; when strength requirements are not high, J506 or J507 electrodes can be used; or low-alloy steel electrodes or filler metals equivalent to the above strength grades can be selected respectively. All welding materials should be of low hydrogen type.
(2) Selection of low-alloy high-strength steel electrodes
Low-alloy high-strength steel can be further classified into hot-rolled and normalized steel, low-carbon quenched and tempered steel, and medium-carbon quenched and tempered steel according to the strength level and heat treatment state. Welding materials (including electrodes, wires, fluxes, etc.) for low-alloy high-strength steels have a critical impact on the properties of the weld metal. Among the national standards for welding electrodes in China, there are various types of welding electrodes from the E50 to E85 series. Low alloy steel generally selects the corresponding electrode according to the strength grade of the steel. At the same time, it needs to be comprehensively considered according to the weldability of the base metal, the size and shape of the welding structure and the force. Under the disadvantages that the cooling rate is large, the strength of the weld is increased, and the weld joint is prone to cracks, an electrode with a strength lower than that of the base metal can be selected. Actually, there are many electrodes in the carbon steel electrode standard that can also be used for low alloy steel welding. The selection of common low alloy steel electrodes is shown in Table 3-7.
Table 3-7
When welding hot rolled and normalized steel, the main basis for selecting the welding material is to ensure that the mechanical properties such as the strength, plasticity and impact toughness of the weld metal match the base metal. It is not necessary to consider the consistency of the chemical composition of the weld metal and the base metal. When welding thick and thick components, in order to prevent welding cold cracks, the “low-strength matching” principle can be used, that is, the welding material with the strength of the weld metal lower than that of the base metal is selected. Excessive strength of the weld will lead to a decrease in weld metal plasticity, toughness and crack resistance.
