In manufacturing, welding is a very important process. Because welding involves many subject areas, such as machinery, metallurgy, mechanics, electronics, metal physics, computer and control engineering, etc., welders will encounter a variety of welding in the process of welding production, product manufacturing and engineering construction. Difficulties, which seriously affect or hinder the construction process of welding. These welding problems are collectively referred to as welder’s problems.
Welder difficult problems refer to the difficult problems encountered by welding operators during the welding process, including welding equipment and automatic control, welding material matching, welding methods and processes, welding structure and fracture, welding defects and quality inspection, welding repair Methods, welding safety precautions, etc.
(1) Welding equipment and automatic control
Arc welding is one of the most widely used welding methods. According to different process characteristics, arc welding can be divided into electrode arc welding, submerged arc welding, gas shielded welding and plasma arc welding. Different materials and structures of workpieces require different arc welding processes, and different arc welding processes require different arc welding equipment. For example, the most convenient and widely used electrode arc welding requires an electrode arc welding machine consisting of a power supply device and a welding clamp that supplies power to the arc; the submerged arc welding widely used in industries such as boilers, chemicals, and shipbuilding requires a power supply device Submerged arc welding machine consisting of control box, welding box, etc .; suitable for gas shielded welding of lively metals with chemical properties. It needs a power supply device, control box, welding car (for automatic welding machine) or wire feeding mechanism (semi-automatic). Welding machine), welding torch, gas circuit and water system and other gas shielded welding machine.
The power supply device required by these arc welding methods, that is, the arc welding power source is an important part of the arc welding machine and is a device for supplying electric energy to the welding arc. It should meet the electrical characteristics required for arc welding. Obviously, the electrical performance of the arc welding power supply determines the stability of the welding process to a large extent. Without advanced arc welding power sources, it is difficult to achieve advanced welding processes and welding process automation. Therefore, truly understanding and correctly using the arc welding power source, and then solving the problems in the welding process of the arc welding power source, is the key to further improve the welding quality and production efficiency.
There are three basic types of arc welding current: AC, DC and pulse. The corresponding arc welding power sources are AC arc welding power, DC arc welding power and pulse arc welding power.
①AC arc welding power source
AC arc welding power sources include power frequency AC arc welding power sources (arc welding transformers) and rectangular wave AC arc welding power sources. Arc welding transformer is to change the AC power of the power grid into low-voltage AC power suitable for arc welding. It consists of a transformer, a regulating device and an indicating device. Arc welding transformers have the advantages of simple structure, easy fabrication and repair, low cost, small magnetic bias blowing, small no-load loss, and low noise. However, its output current waveform is a sine wave. Therefore, the arc stability is poor and the power factor is low. It is generally used for electrode arc welding, submerged arc welding and tungsten inert gas shielded welding.
The rectangular wave AC arc welding power source uses semiconductor control technology to obtain rectangular wave AC current. Because the output current zero crossing time is short, the arc stability is good, and the positive and negative half-wave energization time and current ratio can be freely adjusted, so it is particularly suitable for aluminum and aluminum alloy tungsten arc welding.
②DC arc welding power source
DC arc welding power sources include arc welding generators, arc welding rectifiers and inverter arc welding power sources. Arc welding generators are generally composed of special DC generators, regulating devices and indicating devices. Although arc welding generators have played an important role in the history of welding, due to the disadvantages of complicated manufacturing, large noise and no-load loss, large power consumption, low efficiency, and high prices, this type of arc welding power supply has been eliminated.
The arc welding rectifier is composed of a main transformer, a rectifier, and an adjusting device, an indicating device, etc. for obtaining the required external characteristics. It steps down and rectifies the AC power of the grid to obtain DC power. Compared with arc welding generators, it has the advantages of convenient manufacturing, low price, small no-load loss, low noise, etc., and most arc welding rectifiers can adjust welding parameters from a long distance, and can automatically compensate for grid voltage fluctuations to influences. It can be used as a power source for various arc welding methods.
Inverter arc welding power source is a single-phase (or three-phase) AC power that is rectified and converted from an inverter to a mid- to high-frequency AC power of several hundreds to tens of thousands of hertz, and then output AC or DC power after voltage reduction. The whole process is controlled by electronic circuits, so that the power supply can obtain the required external and dynamic characteristics. This type of arc welding power supply has the advantages of high efficiency, energy saving, light weight, small size, high power factor, etc., and can be applied to various arc welding methods. It is a popular arc welding power supply with great development prospects. The inverter arc welding power source can output both AC and DC power, but the latter form is currently commonly used, so it can also be called an inverter arc welding rectifier.
③Pulse arc welding power source
The welding current is transmitted in the form of low-frequency modulated pulses, and is generally composed of an ordinary arc welding power source and a pulse generating circuit. It has the advantages of high efficiency, small heat input, and wide heat input adjustment range. It is mainly used for gas shielded welding and plasma arc welding. It has unique advantages for welding high-alloy materials, thin plates and all-position welding with high thermal sensitivity.
These different types of welding power sources are mainly selected and used according to the material of the metal being welded, the type of electrode and the structural characteristics of the weldment. For example, when welding low-carbon steel with acid electrodes, arc welding transformers with simple structure, reliable and low price should be preferred. When using alkaline electrodes or welding alloy steel and non-ferrous metals, DC arc welding power source must be selected. ; Generally speaking, when there is no special need, AC arc welding power source should be selected as much as possible. However, the arc stability of the AC arc welding power source is poor, and when the DC arc welding power source is used, the arc heat distribution can be adjusted by changing the polarity of the wiring, which can meet higher process requirements.
(2) Matching of welding materials and base materials
The welding materials widely used in welding production mainly include welding rods, welding wires, fluxes, shielding gases and fluxes, and solders. The common characteristics of the above-mentioned several welding process methods are that the welding material is used as the source of the filler metal for the welding seam, and the welding material is used to complete the protection and metallurgical effect of the welding process on the liquid molten pool to obtain high-quality welding metal.
The quality of the welding material plays a decisive role in ensuring the stability of the welding process and obtaining a weld metal that meets the requirements for use. To sum up, the welding materials should have the following functions:
①Ensure stable arc burning and smooth transition of welding droplets.
② Protect the liquid molten pool metal during welding to prevent air intrusion. ③ Carry out metallurgical reactions and transition alloying elements, adjust and control the composition and properties of weld metal.
④Prevent welding defects such as blowholes and cracks.
⑤Improve the welding process performance, and improve the welding efficiency as much as possible on the premise of ensuring the welding quality.
(3) Welding process and method
The welding process is a set of technical regulations in the welding process based on the production properties, drawings and technical requirements of the product, combined with existing conditions, using modern welding technology knowledge and advanced production experience to determine the product processing methods and procedures. Including preparation before welding, welding materials, welding equipment, welding methods, welding sequence, the best choice of welding operations, and post-weld processing. The formulation of welding process is the key link of welding production. Whether it is reasonable or not directly affects product manufacturing quality, labor productivity and manufacturing cost, and is the main basis for managing production, designing welding tooling and welding workshops.
The welding method is the core content of the welding process. According to the characteristics of the welding process, welding is divided into three major categories: fusion welding, pressure welding and brazing, and each category is divided into several different methods according to process characteristics.
Many new welding processes are currently being used in welding production, which has greatly improved welding productivity and welding quality. Submerged arc automatic welding, electroslag welding, CO2 gas shielded welding, TIG welding, MIG welding, resistance are commonly used in heavy machinery, metallurgical and mining machinery, engineering machinery, power station boiler pressure vessels, petrochemicals, rolling stock, and automobiles. Welding and brazing and other welding methods and complete sets of welding process equipment. In particular, automotive production lines have adopted welding robots such as CO2 gas shielded welding, TIG welding, MIG welding, resistance welding robots and automatic production lines, which have greatly improved welding quality and production efficiency. Welding mechanization and automation have reached 35 in total welding workload. % To 45%.
(4) Welding defects and quality inspection
In order to determine whether the quality of the welded structure is consistent, its quality characteristics must be determined. The role of welding inspection is to monitor the formation process of welding product quality, confirm whether the welding products that the company has produced or is producing meet or meet the requirements of conformity quality, and regularly check whether the in-service welding products still have conformity quality. Welding inspection is based on quality standards. Welding quality standards must be formulated based on product performance. The technical documents on which welding inspection is based include the following.
① Relevant technical standards or specifications: The quality rating or acceptance method specified in the relevant technical standards or specifications is a regulatory document that guides welding inspection work.
② Construction drawings and order contracts: Generally, the construction drawings or order contracts of welding products clearly specify or propose specific requirements for the quality of welding (or quality of welds).
③ Inspection technical documents: These documents specify the inspection method and its implementation process, and are the guiding implementation rules for welding inspection work.
Notices of changes in drawings or processes, substitution of materials, and notices of additional or changed inspection requirements shall be properly kept as the basis for welding inspection. The effective use and coordination of various welding inspection methods, as well as the organization and preservation of welding inspection documents, can ensure the effective operation of the quality system of welding products.
Welding standards and specifications generally include welding material approval tests, welding method approval tests, welder skill tests, welding material standards, bevel accuracy standards, welding part appearance standards, and non-destructive inspection standards for welded joints as welding quality standards. These standards are necessary to achieve the manufacture and production of defect-free welded structures and are the procedures that manufacturers should follow.
Welding material approval test, welding method approval test and welding material standards are mainly formulated to prevent material defects. Although it is impossible to verify the performance of a certain part of a specific welded joint, the performance of these welded parts can be guaranteed by using approved welding materials, using approved welding methods, and strictly following the technical procedures.
Welder skill test, groove accuracy standard, welding assembly accuracy standard and welding part appearance standard are formulated to prevent dimensional defects and structural defects. For important welded structural parts, the appearance and non-destructive inspection of the welds shall be performed after the welding is completed. The welds that failed the inspection shall be repaired in accordance with the provisions of the Quality Assurance Manual. 100% appearance and non-destructive testing of welds. The hardness of the weld metal and heat-affected zone at the intersection of the cylinders was tested with a portable hardness tester. The inspection results should meet the technical requirements and be of reliable quality.
