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Principle of Welding Technology

2019-06-20

Welding process and welding method are related to other factors. The operation should be determined according to the material, brand, chemical composition of the welded workpiece, welding structure type and welding performance requirements.


Firstly, welding methods should be determined, such as hand arc welding, submerged arc welding, tungsten argon arc welding, gas shielded arc welding and so on. There are many kinds of welding methods, which can only be selected according to specific conditions. After determining the welding method, the welding process parameters are formulated, and the types of welding process parameters are different. For example, hand arc welding mainly includes: electrode type (or brand), diameter, current, voltage, type of welding power source, polarity connection method, number of welding layers, number of passes, inspection method, etc.


Preheat


Preheating can reduce the cooling rate after welding, reduce the maximum hardness of heat affected zone of medium carbon steel and prevent cold cracking, which is the main technological measure for welding medium carbon steel. Preheating can also improve the plasticity of the joint and reduce the residual stress after welding. Usually, the preheating temperatures of 35 and 45 steels are 150-250 C. The preheating temperature can be raised to 250 - 400 C when the carbon content is higher or the crack tendency is larger due to the thickness and stiffness.


If the weldment is too large and the whole preheating is difficult, local preheating can be carried out. The heating range of local preheating is 150-200 mm on both sides of the weld.


Electrode Conditions


Acidic electrodes are preferred when permitted.


Groove form


Make the weldment as U-shaped groove as far as possible for welding. If it is a casting defect, the groove shape excavated by shovel should be smooth. Its purpose is to reduce the proportion of base metal melting into weld metal, so as to reduce the carbon content in weld and prevent cracks from occurring.


process parameters


Since the proportion of base metal melting to the first layer of weld metal is about 30%, the first layer of weld metal should be welded with low current and slow welding speed as far as possible to reduce the penetration of base metal, which is what we usually call burns (when the current is too large, the base metal is burned).


heat treatment


After welding, the cooling rate should be further slowed down, plasticity and toughness should be increased, hardening tendency should be reduced, and diffusive hydrogen in the joint should be eliminated. Therefore, welding should not be carried out in supercooled environment or in rain. It is better to carry out stress relief heat treatment immediately after welding, especially for weldments with large thickness, high rigidity and severe working conditions (dynamic load or impact load). The tempering temperature for stress relief after welding is 600-650 C, holding for 1-2 h, and then cooling with furnace.


If stress relief heat treatment cannot be carried out after welding, post heat treatment should be carried out immediately.


Basic knowledge of welding process


Welding is a process of metallurgical bonding by means of heating, pressurizing, or combining the two, using or not using welding materials, which causes atoms to diffuse between the two workpieces and forms metallurgical bonding. Welding is widely used in both metal and non-metal applications.


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