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Welding technology generally refers to a complete set of technical specifications for the welding process, including welding methods, pre-welding preparations, welding materials, welding equipment, welding sequence, welding operations, process parameters, and post-weld heat treatment. Therefore, different methods entail different welding processes, which gives rise to the concept of welding process parameters. The various physical quantities selected to ensure welding quality are known as welding process parameters. The welding process is an important factor in determining the quality of welds; hence, the importance of establishing a proper welding process is evident. The welding process is primarily determined by the material, grade, and chemical composition of the workpieces to be welded, the type of structure of the weldments, and the required welding properties. First, it is necessary to determine the welding method, such as shielded metal arc welding, submerged arc welding, tungsten inert gas welding, gas metal arc welding, and so on. There are many types of welding methods, and the choice can only be made based on specific circumstances. After determining the welding method, the welding process parameters are established. There are various types of such parameters; for example, in manual arc welding, they mainly include: electrode type (or grade), diameter, current, voltage, type of welding power supply, polarity configuration, number of welding layers, number of passes, inspection methods, and so on. Principle: Preheating helps to reduce the maximum hardness in the heat-affected zone of medium-carbon steel, thereby preventing the formation of cold cracks; this is a key process measure for welding medium-carbon steel. Preheating can also improve the plasticity of the joint and reduce residual stresses after welding. Typically, the preheating temperature for 35 and 45 steel is 150–250°C. If the carbon content is high, or if the thickness and stiffness are great resulting in a higher tendency for cracks, the preheating temperature can be increased to 250–400°C. If the welded piece is too large and overall preheating is difficult, local preheating can be employed; the heating area for local preheating covers 150–200 mm on each side of the weld joint. Welding rod conditions: Acidic welding rods should be given priority when permitted. Groove shape: The welded parts should be prepared with a U-shaped groove as much as possible for welding. In the case of casting defects, the shape of the groove created by chiseling should be smooth; this is done to reduce the proportion of base material that merges into the weld metal, thereby lowering the carbon content in the weld and preventing cracks from forming. Process parameters: Since the proportion of base material that melts into the first layer of weld metal can be as high as around 30%, it is necessary to use a low current and a slow welding speed when welding the first layer of weld, in order to reduce the depth of melting of the base material – what we commonly refer to as burning (the base material being damaged due to excessive current). Heat treatment: After welding, the material should be held at 200–350°C for 2–6 hours to further reduce the cooling rate, increase plasticity and toughness, reduce the tendency to harden, and eliminate diffused hydrogen within the joint. Therefore, welding must not be carried out in an overly cold environment or in the rain. It is advisable to carry out stress-relief heat treatment on the welded parts immediately after welding, especially for thick-walled welded parts, highly rigid structural components, and those operating under severe conditions (such as dynamic or impact loads). The tempering temperature for stress relief after welding is 600–650°C; the part is held at this temperature for 1–2 hours before being cooled in the furnace. If stress-relief heat treatment cannot be carried out after welding, post-weld heat treatment should be performed immediately. Basic knowledge of welding processes: Welding is a processing technique and connection method that involves heating, applying pressure, or using both methods, with or without the use of welding materials, to enable atomic diffusion between two workpieces and thus create a metallurgical bond. Welding has a wide range of applications, and it can be used for both metals and non-metals. Operation methods: There are over 40 metal welding methods, which are mainly classified into three categories: fusion welding, pressure welding, and brazing. Welding by melting: This is a welding method in which the joint between the workpieces is heated to a molten state, and welding is carried out without applying any pressure. During fusion welding, the heat source rapidly heats and melts the joint between the two workpieces to be welded, forming a molten pool. The molten pool moves forward with the heat source; upon cooling, it forms a continuous weld that joins the two workpieces together. During the welding process, if the atmosphere comes into direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize the metal and various alloying elements. Nitrogen, water vapor, and other substances present in the atmosphere that enter the molten pool can also cause defects such as pores, inclusions, and cracks in the weld during the subsequent cooling process, thereby deteriorating the quality and performance of the weld. To improve welding quality, various protection methods have been developed. For example, gas shielded arc welding uses gases such as argon and carbon dioxide to isolate the atmosphere and protect the arc and molten pool during welding ； Similarly, when welding steel, adding ferrotitanium powder with a high affinity for oxygen to the flux of the welding rod enables deoxidation, thereby protecting beneficial elements such as manganese and silicon in the welding rod from oxidation and preventing them from entering the molten pool; this results in high-quality welds after cooling. Pressure welding is a process in which two workpieces are brought together under pressure to achieve atomic bonding in their solid state; it is also known as solid-state welding. The commonly used pressure welding process is resistance spot welding; when current flows through the joint between the two workpieces, the temperature rises due to the high resistance at that location. Once the material reaches a plastic state, it is joined together under axial pressure. The common feature of various pressure welding methods is that pressure is applied during the welding process without the use of filler material. Most pressure welding methods, such as diffusion welding, high-frequency welding, and cold pressure welding, do not involve a melting process; as a result, there is no loss of beneficial alloying elements or intrusion of harmful elements into the weld, just as in fusion welding. This simplifies the welding process and improves the safety and hygiene conditions associated with welding. At the same time, since the heating temperature is lower than that in fusion welding and the heating time is shorter, the heat-affected zone is small. Many materials that are difficult to weld using fusion welding can often be joined by pressure welding to produce high-quality joints with strength equal to that of the base material. Soldering is a welding method that uses a metal material with a lower melting point than that of the workpieces as a solder. The workpieces and the solder are heated to a temperature higher than the melting point of the solder but lower than that of the workpieces. The liquid solder wets the workpieces, fills in the gaps between them, and enables atomic diffusion between the workpieces, thereby achieving welding. The joint formed between two joined objects during welding is called a weld. The areas on either side of the weld are affected by the welding heat during welding, resulting in changes in their structure and properties; this region is known as the heat-affected zone. During welding, due to differences in the workpiece material, welding materials, welding current, etc., overheating, embrittlement, hardening, or softening may occur in the weld and heat-affected zone after welding. This also reduces the properties of the welded part and deteriorates its weldability. This requires adjusting the welding conditions; preheating the joint of the workpieces before welding, maintaining heat during welding, and performing post-weld heat treatment can improve the welding quality of the workpieces. Precautions: Additionally, welding is a process of rapid local heating and cooling. The welded area cannot expand or contract freely due to the constraints imposed by the surrounding workpieces, and as a result, welding stresses and deformations occur in the welded joint after cooling. It is necessary to relieve welding stress and correct welding deformation for all important products after welding. Modern welding techniques are now capable of producing welds free from internal and external defects, with mechanical properties equal to or even superior to those of the materials being joined. The relative positions of the parts to be welded in space are referred to as the weld joint. The strength of the joint is influenced not only by the quality of the weld, but also by its geometric shape, dimensions, loading conditions, and operating environment. The basic forms of joints include butt joint, lap joint, T-joint (direct joint), and corner joint, etc. The cross-sectional shape of the butt joint weld is determined by the thickness of the materials to be welded before welding and the groove shape of the two joint edges. When welding thicker steel plates, various shapes of grooves are cut at the joints to ensure complete penetration, thereby facilitating the insertion of electrodes or welding wires. There are grooves for single-sided welding and grooves for double-sided welding. When selecting a groove type, in addition to ensuring complete penetration, factors such as ease of welding, minimal filler metal consumption, low welding deformation, and low groove machining costs should also be considered. When two steel plates of different thicknesses are butt-jointed, in order to prevent severe stress concentration caused by a sudden change in cross-section, the edge of the thicker plate is gradually tapered down so that both jointed edges have the same thickness. The static and fatigue strength of butt joints are higher than those of other types of joints. For joints operating under alternating or impact loads, or in low-temperature and high-pressure vessels, welding of butt joints is often preferred. The preparatory work prior to welding of lap joints is simple, assembly is convenient, and welding deformation and residual stress are relatively low; therefore, they are frequently used for field installation of joints and in non-critical structures. Generally, lap joints are not suitable for operation under conditions such as alternating loads, corrosive media, high temperatures, or low temperatures. The use of T-joints and corner joints is usually due to structural requirements. The working characteristics of the under-welded fillet welds in T-joints are similar to those of the fillet welds in lap joints. When the weld is perpendicular to the direction of the external force, it becomes a face weld; in this case, the shape of the weld surface causes varying degrees of stress concentration ； The stress conditions in a fully penetrated fillet weld are similar to those in a butt joint. Corner joints have low load-bearing capacity and are generally not used alone; their performance improves only when full penetration is achieved or when there are fillet welds on both the inside and outside. They are often used at the corners of closed structures. Welded products are lighter than riveted, cast, and forged parts, which allows transportation vehicles to reduce their weight and save energy. It has good welding sealing properties, making it suitable for manufacturing various types of containers. Developing combined processing techniques that integrate welding with forging and casting makes it possible to produce large-scale, cost-effective cast-welded and forged-welded structures, resulting in high economic benefits. The welding process enables efficient use of materials; welded structures allow the use of materials with different properties in various sections, thus taking full advantage of the strengths of each material to achieve economic efficiency and high quality. Welding has become an indispensable and increasingly important processing technique in modern industry. In modern metal processing, welding developed later than casting and forging processes, but it has grown at a rapid pace. The weight of welded structures accounts for about 45% of steel production, and the proportion of welded structures made of aluminum and aluminum alloys is also increasing. Future welding processes will require the development of new welding methods, equipment, and materials in order to further improve welding quality and safety reliability, such as improving existing welding sources like arc welding, plasma arc welding, electron beam welding, and laser welding ； By utilizing electronic and control technologies, the process performance of the arc is improved, and reliable and lightweight arc tracking methods are developed. On the other hand, it is necessary to improve the level of mechanization and automation in welding, such as achieving program control and digital control for welding machines ； Develop specialized welding machines that automate the entire process, from preparation and welding to quality control ； On automatic welding production lines, the adoption and expansion of CNC welding manipulators and welding robots can improve the level of welding production and enhance the safety and hygiene conditions in welding processes. Main equipment: 1. Manual shielded metal arc welding machine; 2. Carbon dioxide shielded welding machine; 3. Tungsten inert gas welding machine; 4. Resistance welding machine; 5. Submerged arc welding machine; 6. Welding wire; 7. Flux; 8. Welding auxiliary materials, etc. Temperature control: The temperature of the molten pool has a direct impact on the quality of welding. A higher molten pool temperature results in a larger molten pool and better fluidity of the molten iron, which facilitates welding. However, if the temperature is too high, the molten iron tends to flow downward, leading to burn-through on the back side during single-sided welding with double-sided formation. This can also cause weld defects, make it difficult to control the shape of the weld, and reduce the plasticity of the joint, increasing the risk of cracking when bent. When the pool temperature is low, the molten pool is smaller, the molten iron appears darker, its fluidity is poor, and defects such as lack of penetration, lack of fusion, and slag inclusions are likely to occur. The pool temperature is closely related to factors such as welding current, electrode diameter, electrode angle, and arc combustion time. The following measures are taken to control the pool temperature by addressing these factors. Diameter, welding current, and electrode diameter: The welding current and electrode diameter are selected based on the position of the weld area and the welding layer; when starting welding, larger values for these parameters are used, while smaller values are employed in vertical, horizontal, and overhead welding positions. For the root pass in flat butt welding of 12mm thick plates, a weld rod with a diameter of φ3.2mm is used, with a welding current of 80–85A. For the fill and cover passes, a weld rod with a diameter of φ4.0mm is employed, at a welding current of 165–175A. Choosing the welding current and weld rod diameter appropriately helps to control the temperature of the molten pool, which is essential for achieving proper weld shape. Method: The strip movement method – the temperature of the molten pool with circular strip movement is higher than that with crescent-shaped strip movement, and the temperature with crescent-shaped strip movement is in turn higher than that with zigzag-shaped strip movement. For the root pass in flat welding at 12 mm thickness, zigzag-shaped strip movement is used, along with adjustments to the amplitude of movement and pauses on both sides of the groove. The molten pool temperature was effectively controlled, resulting in a relatively consistent size of the weld pores. The likelihood of weld bulges and burn-through at the root of the groove was reduced, and the problem of incomplete penetration was improved, thereby making one-sided welding with double-sided formation in flat butt joints no longer a challenge. Angle: The angle between the welding rod and the welding direction. When this angle is 90 degrees, the arc is concentrated, resulting in a higher temperature in the molten pool; when the angle is smaller, the arc is dispersed, and the temperature of the molten pool is lower. For example, in flat welding of the root layer using a 12mm welding rod, the angle should be 50–70 degrees, which helps to reduce the temperature of the molten pool and prevents the formation of weld bulges or protrusions on the back side. For example, when changing the welding electrode after welding the root pass in vertical position on 12mm thick plates, a welding electrode angle of 90-95 degrees is used during joint formation; this helps to rapidly increase the temperature of the molten pool, allowing the penetration hole to open smoothly. The back side of the joint achieves a smoother shape, which effectively prevents indentation at the joint area. Time: Arc burning time. In the practical teaching of horizontal and vertical welding of φ57×3.5 pipes, the arc interruption method is used for welding; when welding the root pass, the frequency of arc interruption and the arc burning time have a direct impact on the temperature of the molten pool. Due to the thin wall thickness, the capacity to withstand arc heat is limited; if the arc interruption frequency is reduced to lower the pool temperature, shrinkage cavities are likely to occur. Therefore, the arc combustion time can only be used to control the temperature of the molten pool. If the temperature of the molten pool is too high and the weld hole is large, the arc combustion time can be reduced to lower the temperature of the molten pool; this will result in a smaller weld hole, and an appropriate height of formation inside the tube, thereby preventing the weld seam inside the tube from being too high or causing weld bulges.