The selection of welding rod must be in order to ensure the safety of the welding structure, reliable use of the premise, according to the chemical composition of the welded material, mechanical properties, plate thickness and joint form, welding structure characteristics, the state of force, the structure of the use of the conditions of the weld performance requirements, welding construction conditions and technical and economic benefits of a comprehensive examination, targeted selection of welding rods, welding test is also required when necessary.

　　The same steel welding electrode selection points

　　(1) Consider the mechanical properties of the weld metal and chemical composition For ordinary structural steel, usually require the weld metal and base material strength, should be used in the molten metal tensile strength equal to or slightly higher than the base material of the electrode. For alloy structural steel, sometimes also require the same or close to the alloy composition and base material. In the welding structure rigidity, high joint stress, welds are prone to cracks in unfavorable circumstances, should be considered than the base material strength of the low-strength welding rod. When the base material of carbon, sulfur, phosphorus and other elements of the content is high, the weld is prone to cracking, should be used to crack-resistant performance of alkaline low-hydrogen type welding rod.

　　(2) Considering the use of welded components and working conditions of dynamic loads and impact loads on the weldment, in addition to meet the strength requirements, the main purpose should be to ensure that the weld metal has a high impact toughness and plasticity, plastic, toughness indicators can be selected from the higher low-hydrogen type electrodes. In the high temperature, low temperature, wear-resistant or other special conditions of work of the weldments, should be selected accordingly heat-resistant steel, low-temperature steel, cladding or other special-purpose electrodes.

　　(3) Considering the characteristics of the welding structure and the force conditions of the complex shape of the structure, the rigidity of the thick and large weldments, due to the welding process produces a large amount of internal stress, easy to make the weld cracks, should be used to crack-resistant alkaline low-hydrogen electrodes with good performance. For the force is not big, the welding part is difficult to clean up the weldment, should be used on the rust, oxide skin, oil insensitive acidic welding rod. For the welding parts that can not be turned over by the conditions, should be used for all-position welding electrodes.

　　(4) Considering the construction conditions and economic benefits in order to meet the requirements of the product performance, should be used in the case of good process of acidic welding rod. In the small or poor ventilation conditions of the occasion, should use acid welding rod or low-dust welding rod. The structure of the welding workload, conditions should be used when the high efficiency welding rod, such as iron powder welding rod, high efficiency gravity welding rod, etc., or the choice of the bottom of the welding rod, standing down to the welding rod and other specialized welding rods, in order to improve welding productivity.

　　Dissimilar steel welding electrode selection points

　　(1) strength levels of different carbon steel + low alloy steel (or low alloy steel + low alloy high-strength steel) generally require that the strength of the weld metal or joints is not less than the minimum or degree of the two welded metal, the choice of the strength of the electrode deposited metal should be able to ensure that the strength of the weld and joints is not less than the intensity of the lower side of the strength of the base material, while the weld metal plasticity and impact toughness should be not less than the strength of the higher plasticity of the side of the base material of the performance of the poor. Therefore, the strength level of the joints can be determined according to the strength level of the two. Therefore, according to both the lower strength level of steel selection of welding rod. However, in order to prevent welding cracks. Should be according to the higher strength level, poorer weldability of the steel to determine the welding process, including welding specifications, preheating temperature and post-weld heat treatment.

　　(2) low alloy steel + austenitic stainless steel should be in accordance with the chemical composition of the molten metal limited to the value of the choice of welding electrodes, generally choose chromium, nickel content, plasticity, crack resistance of better Cr25-Ni13 type austenitic steel electrodes, in order to avoid cracking due to the production of brittle hardened organization. However, the welding process and specifications should be determined according to the less weldable stainless steel.

　　(3) Stainless steel composite steel plate should be considered for the grass-roots level, cladding, transition layer of the welding requirements of the three different properties of the welding electrode. The grass-roots level (carbon steel or low alloy steel) welding, the selection of the corresponding strength level of structural steel welding rod; cladding directly in contact with the corrosive medium, should be selected with the corresponding composition of austenitic stainless steel welding rod. The key is the transition layer (i.e., cladding and grass-roots interface) of the welding, must consider the dilution effect of the base material, should be used chromium, nickel content, plasticity and crack resistance of good Cr25-Ni13 type austenitic steel electrodes.

　　Commonly used metal materials welding electrode selection

　　Carbon steel welding rod selection

　　Carbon steel is a general term for carbon structural and carbon tool steel. China's carbon steel production accounts for about 80% of the total steel production, carbon steel welding is the largest amount of welded metal, the most extensive coverage of a kind. Carbon steel for welding, carbon content of not more than 0.9%. Carbon steel weldability and steel in the steel content is closely related to how much, the higher the carbon content, the worse the steel weldability. Almost all welding methods can be used for carbon steel structure welding, based on the hand arc welding, submerged arc welding and CO2 gas shielded welding is most widely used.

　　Carbon steel welding electrode weld strength is usually less than 540Mpa (55kgf/mm2), in China's national standard GB/T5117-95 carbon steel welding electrode only E43 series and E50 series of two models, that is, the tensile strength of only 420Mpa (43KGF/MM2) and 490Mpa (50kgf/mm2) two strength levels. At present, a large number of welding is used in the welding of 470Mpa level below the electrode. Welding low carbon steel (carbon less than 0.25%) when most of the use of E43XX (J42X) series of electrodes, this series of electrodes have a variety of models, more product grades, according to the specific base material and conditions of use, working conditions, weldments, structural shape and thickness of the steel plate to be selected.

　　Welding of medium carbon steel (C = 0.25% ~ 0.60%) and high carbon steel (C) 0.60%), should be used with lower impurity content, and has a certain desulfurization capacity of alkaline hydrogen-type electrodes. In individual cases, ilmenite-type or titanium-calcium-based electrodes can also be used, but with strict technological measures to match. Medium carbon steel welding, due to the higher carbon content of steel, welding cracks tend to increase, can choose low hydrogen type electrode or weld metal with high plasticity, toughness of the electrode, and most of the cases need to preheat and slow cooling treatment. High-carbon steel welding must take strict preheating, post-heating measures to prevent welding cracks.

　　High-carbon steel welding weld and base material properties are identical to the more difficult, high-carbon steel tensile strength of most of the 675Mpa (69kgf/mm2), the choice of welding consumables should be based on product design requirements. High strength requirements, available J707 or J607 welding rod. Strength requirements are not high, available J506 or J507 and other welding rods; or were selected and the above strength level equivalent to the low alloy steel welding rods. All welding materials should be low hydrogen type.

　　Selection of low alloy high-strength steel welding electrodes

　　Low-alloy high-strength steel according to the strength level and heat treatment status, can be divided into hot-rolled and normalized steel, low carbon tempered steel, medium carbon tempered steel. Low-alloy high-strength steel with the welding rod has a critical impact on the performance of the weld metal.

　　In China's national standard for welding electrodes, the standard for welding electrodes for alloy-containing steels (GB/T 5118-2012) has a wide range of electrodes to choose from, from the E50 series to the E85 series. Generation of alloy steel is generally based on the strength grade of the steel to select the appropriate welding rod, but also need to be based on the parent material weldability, welding structure size, bevel shape and the impact of force, etc., for comprehensive consideration. In the cooling rate is large, so that the strength of the weld increases, the welded joints are prone to cracks in unfavorable circumstances, can be selected than the base material strength of a low level of welding rod. In fact, in the carbon steel welding rod standard also has not and welding rod can also be used for the generation of alloy steel welding.

　　In order to meet the welding requirements of low-alloy steel products, improve the crack resistance of the electrode, the toughness of the weld metal and welding process performance, but also developed a number of special low-alloy steel special electrodes are available for selection, such as ultra-low hydrogen electrodes (JXXXH), high toughness electrodes

　　(JXXXGR), high toughness ultra-low hydrogen electrodes (JXXXRH) and moisture absorption resistant electrodes (JXXXLMA) and so on.

　　Welding hot-rolled and normalized steel, the main basis for the selection of welding materials is to ensure that the strength, plasticity and impact toughness of the weld metal and other mechanical properties match the base material. Do not have to consider the chemical composition of the weld metal and the consistency of the base material. Welding thick components, in order to prevent the emergence of welding cold cracks, can be used "low strength matching" principle, that is, the choice of weld metal strength is lower than the base material strength of welding. Welding strength is too high, will lead to the weld metal plastic, toughness and anti-cracking properties of the reduction.

　　Low-carbon tempered steel has a greater tendency to produce cold cracks, so strict control of hydrogen in the welding material is very important for low-carbon tempered steel electrode should be low-hydrogen or ultra-low hydrogen type electrode. Medium carbon tempered steel welding to ensure that the weld metal plastic, toughness and strength, improve the crack resistance of the weld, should be used in low carbon alloy system, minimize the weld metal sulfur, phosphorus impurity content. For the need for post-weld heat treatment components, should also consider the weld metal alloy composition should be similar to the base material.

　　Heat-resistant steel welding rod selection

　　Low-alloy heat-resistant steel to work at high temperatures for a long time, in order to ensure the high temperature performance of heat-resistant steel, the steel must be added to more alloying elements (such as Cr Mo V Nb, etc.). In the selection of welding materials, first of all, to ensure that the weld performance with the base material to match, with the necessary thermal strength, so the chemical composition of the weld metal should be required to try to be consistent with the base material, if the weld and the base material chemical composition is too related to the high temperature and long-term use of the joint area of certain elements of the diffusion phenomenon (eg, the diffusion of carbon elements in the vicinity of the fusion line), so that the joints under the high temperature performance of the clone.

　　Heat-resistant steel welding rod can generally be selected according to the working temperature of the steel and components temperature. Selection of heat-resistant steel welding consumables is the principle of weld metal alloy composition and properties with the corresponding indicators of the parent material, or should reach the minimum performance indicators proposed by the product technical conditions. In order to improve the weld metal's resistance to thermal cracking, the carbon content in the weld should be slightly lower than the carbon content of the base material, generally should be controlled between 0.07% and 0.15%. Due to the common effect of carbon and alloying elements in steel, heat-resistant steel welding is very easy to form liquid hard tissue, poor weldability. For this reason, heat-resistant steel is generally preheated before welding, tempering treatment after welding.

　　In recent years, in the thin-walled pipe welding commonly used in argon arc welding to play low, acidic welding rod hand arc welding cover process, greatly improving the welding quality. However, this type of electrode crack resistance is inferior to the low-hydrogen type electrodes, in separate use or for thick-walled pipe welding, should choose low-hydrogen type heat-resistant steel electrodes.

　　Selection of low-temperature steel welding rod

　　Cryogenic steel is in the -40 degrees to -196 degrees of low temperature range of low alloy special steel. According to the chemical composition, cryogenic steel is mainly nickel-containing steel and nickel-free steel two categories. Foreign countries generally use nickel-containing cryogenic steel, such as 3.5Ni, 5Ni steel and 9Ni steel; China's use of nickel-free cryogenic steel.

　　Selection of cryogenic steel welding consumables should first consider the joint temperature, toughness requirements and whether the post-weld heat treatment, etc., so that the chemical composition and mechanical properties of the weld metal (especially the impact toughness) and the parent material. After post-weld heat treatment, the weld should still have a high low-temperature toughness. Due to the low temperature toughness of the weld metal puts forward strict requirements, low temperature steel welding rod flux skin are used low hydrogen type. Welding requirements as much as possible using a small welding line energy, to avoid the formation of weld metal and near the seam area of coarse crystal organization and reduce the low temperature toughness. Nickel-containing low-temperature steel in addition to hand arc welding, the main use of argon arc welding welding, the use of the same composition as the base material of the wire, shielding gas for Ar or Ar to add 2% of O2 or 5% ~ 10% of CO2 to improve the formation of the weld.