Welding processes and methods are related to factors such as the material, grade, chemical composition, structure type of the weldment, and welding performance requirements, and should be determined accordingly during operation.
Firstly, determine the welding method, such as manual arc welding, submerged arc welding, tungsten inert gas (TIG) welding, and gas shielded metal arc welding, among others. There are numerous types of welding methods, which can only be selected based on specific circumstances. Once the welding method is established, then formulate the welding process parameters. The types of welding process parameters vary, with manual arc welding including: electrode type (or brand), diameter, current, voltage, welding power source type, polarity connection, number of welding layers, passes, and inspection methods.
Welding Process Principles
Heating up
Preheating can reduce the cooling rate after welding, which is beneficial for lowering the high hardness in the heat-affected zone of medium carbon steel and preventing the formation of cold cracks, making it a key process measure for welding medium carbon steel. Preheating also improves the plasticity of the joint and reduces residual stresses after welding. Typically, the preheating temperature for 35 and 45 steel grades is between 150 to 250°C. For steels with higher carbon content or those with significant thickness and stiffness, where the tendency for cracks is high, the preheating temperature can be increased to between 250 to 400°C.
If overall preheating is difficult due to the large size of the weldment, local preheating can be performed. The heating range for local preheating is 150 to 200mm on both sides of the weld joint.
Welding Rod Conditions
Prefer acid-core welding rods for licensing.
Bevel form
Weld the welded joints as much as possible into a U-shaped bevel. For casting defects, the bevel shape should be smooth. The purpose is to reduce the amount of base material that melts into the weld metal, thereby lowering the carbon content in the weld and preventing crack formation.
Process Parameters
Due to the high percentage of base material melting into the weld metal, approximately 30%, it is recommended to use a low current and slow welding speed during the first layer welding to minimize the melting depth of the base material, which we commonly refer to as burn-through (when the current is too high, the base material gets burned).
Heat Treatment
After welding, the joint should be kept at a temperature of 200-350℃ for 2-6 hours to further slow down the cooling rate, enhance plasticity and toughness, reduce the tendency to harden, and eliminate diffused hydrogen within the joint. Therefore, welding should not be conducted in sub-cooled conditions or during rain. Post-weld stress relief heat treatment should be immediately applied to the welded parts, especially for thick-walled weldments, high-stiffness structural components, and those operating under severe conditions (dynamic or impact loads). The stress-relief tempering temperature post-weld is 600-650℃, maintaining the temperature for 1-2 hours, followed by furnace cooling.
If stress-relieving heat treatment cannot be performed after welding, post-heat treatment should be carried out immediately.
Basic Welding Processes Knowledge
Welding is a process and method of joining two workpieces by heating, pressure, or both, with or without welding material, to produce interatomic diffusion and form a metallurgical bond. It is widely applied, both to metals and non-metals.
