Welding method is usually selected in accordance with the following principles, the selected welding method must be able to ensure the welding quality, to meet the technical requirements of product design; at the same time can improve welding productivity, reduce manufacturing costs and improve labor conditions.

　　The general method of selection is: for the material properties and structural characteristics of the product, according to the characteristics of various welding methods, combined with the type of production and production conditions and other factors, to do a comprehensive analysis of the selected. Here, the performance of the base material and structural characteristics are often decisive.

　　I. Consideration of base material properties

　　1, the physical properties of the base material must pay attention to the thermal conductivity of the base material, electrical conductivity, melting point and other properties. For high thermal conductivity of metal materials, should be selected with a large heat input, weld penetration ability of welding methods; for heat-sensitive materials, easy to use small heat input welding methods.

　　2, the mechanical properties of the base material mainly refers to the base material strength, plasticity, toughness and hardness. Both the mechanical properties of the base material to see whether it is easy to realize the connection between the metals, but also look at the mechanical properties of the welded joints will not change, will not affect the safety of use after the change.

　　3, the metallurgical properties of the base material The main factor determining the metallurgical properties of the base material is its chemical composition. High-carbon steel or alloy structural steel with high carbon equivalent should be used to slow the cooling rate of the welding method to reduce the tendency of cracking in the heat-affected zone; for metallurgical compatibility of dissimilar metals should be selected solid-phase welding method, such as diffusion welding, brazing and so on.

　　Second, the consideration of the structural characteristics of the product

　　1, the structure of the geometry and size of the main consideration of the product is whether the welding of the required operating space and location. Large metal structures such as ship hulls, there is no operating space difficulties, but its volume is too large, need to choose the method of all-position welding; miniature electronic devices, generally small in size, welded no longer processed, the requirements of precision, it is appropriate to choose a small amount of heat and concentration of welding methods, such as electron beam welding, laser welding and so on.

　　2, the thickness of the weldment Each welding method has a certain thickness and scope of application, beyond which it is difficult to ensure the quality of welding. For fusion welding, is to weld through and not burn through as a prerequisite. The minimum thickness of weldable refers to the stable state of one-sided single-pass welding just weld through and not burn through.

　　3, joint form Welding joint form is usually determined by the product structure shape, use requirements and material thickness and other factors. Butt, lap, T-joint and angle is the most basic form. These joints can adapt to most welding methods, some lap joints are often designed to adapt to certain pressure welding or brazing methods. For the rod, bar, pipe butt, generally appropriate to use flash butt welding or friction welding.

　　4, welding position in the case of welding can not change position on all the weldments, it will be due to the weld in different spatial positions and the use of flat welding, vertical welding, horizontal welding and back welding and other four different positions of the weld. A welding method that can carry out these four positions is called the all-position welding method.

　　Welding material selection principles

　　Should be based on the chemical composition of the base material, mechanical properties, welding performance and combined with the structural characteristics of the pressure vessel, the use of conditions and welding method of comprehensive consideration of the selection of welding materials, if necessary, through the test to determine.

　　Craftsmen, designers should take into account the mechanical properties of welding rod heat treatment specifications, welding parts manufacturing process characteristics (mainly post-weld heat treatment, welding repair) and steel characteristics, the selection of the appropriate welding material. With "G" electrodes plus "specified weld metal Charpy V-notch impact absorption work", chromium-molybdenum steel electrodes, welding of low-temperature steel nickel steel electrodes, to improve the weld metal Charpy V-notch impact acceptance indicators, in order to adapt to the requirements of the steel plate.

　　Usually is the nominal value of the weld metal according to the flux metal to select welding materials, and the actual strength of the flux metal and often exceed the nominal value of a lot of guarantees, such as metallurgical factors or the role of fusion ratio, the actual strength of the weld metal will be far higher than the nominal value of the weld material flux metal. Wish is "low strength" match, the reality may be "such as strong"; wish is "such as strong", the reality may be "super strong! "The actual strength of the weld must be determined by the actual strength level. The matching problem must be analyzed according to the actual strength level of the weld. Welding rods, flux and carbon steel flux-cored wire national standards and product samples do not specify the upper limit of the tensile strength of the molten metal, in the pressure vessel with welding consumables before the introduction of the technical conditions of the order, JB/T4709-2000 provides that "the weld metal should be to ensure the mechanical properties, and should not exceed the parent material standard tensile strength limit value plus 30 MPa ".

　　For heat-resistant low-alloy steel and high-alloy steel weld metal in the mechanical properties under the premise of guaranteeing the chemical composition or corrosion resistance, respectively, "guarantee" the actual meaning of chromium-molybdenum steel is the chemical composition, high-alloy steel is the corrosion resistance "should be higher than or equal to the corresponding base material standard shall be higher than or equal to the lower limit of the value specified in the corresponding base material standard or meet the technical requirements specified in the drawings".