Problems Existing in Welding Process and Countermeasures

1. The development status of welding technology at home and abroad

With the rapid development of the world’s manufacturing industry, the application of welding technology is becoming more and more extensive, and the level of welding technology is getting higher and higher. New welding techniques and methods are constantly emerging, and professional welding equipment is changing with each passing day. At the same time, domestic and foreign welding equipment manufacturers have also demonstrated their strengths in various ways, especially through the exhibition to display a wide variety of products and advanced technologies.

2. The main defects and solutions in the welding process

(1). Hot crack

Hot cracks can occur in the weld zone or heat-affected zone, distributed along the length of the weld. The microscopic characteristic of a hot crack is cracking along the grain boundary, so it is also called an intergranular crack. Because hot cracks are formed at high temperatures, they have an oxidized color and can be seen immediately after welding. The main cause of hot cracks is the presence of low-melting eutectics (containing impurities such as sulfur, phosphorus, and copper) on the grain boundaries of the weld metal, and the presence of tensile stress in the joints.

The main solution is to select suitable welding materials and strictly control the content of harmful impurities carbon, sulfur, and phosphorus. Fe and FeS are easy to form low melting point eutectic, their melting point is 988 ℃, so it is easy to produce hot cracks. Strictly control the cross-sectional shape of the weld to avoid sudden height and flat arc transition. Reduce the crystallization temperature range, improve the weld structure, refine the weld grain, improve plasticity, and reduce segregation. Determine reasonable welding process parameters and slow down the cooling rate of the weld to reduce welding stress. Such as the use of small line energy, preheating before welding, reasonable welding seam layout, etc.

(2). Cold crack

Cold cracking occurs in carbon steel or alloy steel with high carbon and alloy content. Cold cracks have delayed properties, mainly delayed cracks.

The main cause of cold cracks is that the quenching tendency of welded joints (weld seam and heat-affected zone and fusion zone) is serious, resulting in quenched structure, which leads to embrittlement of joint performance. The hydrogen content of welded joints is relatively high, and a large number of hydrogen molecules are gathered at welding defects, resulting in very large local pressure and embrittlement of joints; too high phosphorus content also produces cold cracks. There is a large tensile stress. Because the diffusion of hydrogen takes time, cold cracks need to be delayed for a period of time after welding. Because it is induced by hydrogen, it is also called hydrogen-induced cracking.

The measures to prevent cold cracks are to use basic electrodes or fluxes to reduce the hydrogen content in the weld metal and improve the plasticity of the weld metal. The electrode flux should be dried, and the weld bevel and nearby base metal should be deoiled, watered, and de-rusted to reduce the source of hydrogen. The workpiece is preheated before welding and cooled slowly after welding (the temperature of most materials can be checked in the table), which can reduce the cooling rate after welding, avoid a hardened structure, and reduce welding residual stress. Take technological measures to reduce welding stress, such as symmetrical welding, multi-layer multi-pass welding with small input energy, etc., and perform stress-removing annealing treatment after welding. Immediately after welding, carry out dehydrogenation (post-heating) treatment, heat to 250 ° C, and keep warm for 2 to 6 hours to make the diffused hydrogen in the weld metal escape from the metal surface.

(3). Slag inclusion and inclusion

The reason for slag inclusion and inclusion is that the groove angle or welding current is too small. There is oxygen cutting or carbon arc gouging slag on the edge of the weldment, the edge is not cleaned, and there are residual oxide iron sheets and carbides. When using acid electrodes, paste slag is formed due to small current or improper transportation. When using basic electrodes, slag inclusions can also occur due to excessive arc length or incorrect polarity.

Preventive measures for slag inclusion and inclusion: remove impurities and dirt on the weld bead, especially keep the welding groove clean and dry, and control the separation of molten iron and slag. According to the requirements of the welding process data, select the appropriate welding current and welding speed, and the swing of the transport bar should be appropriate. When multi-layer welding, strengthen the layer cleaning during the welding process, carefully observe the melting conditions on both sides of the groove, and carefully clean the welding slag on each layer. Use welding rods of appropriate specifications, and select appropriate groove forms and sizes. Improve the operational skill level of welders.

3. Quality Control Measures

(1). Improve the quality level of welding materials

Welding materials have a decisive impact on the welding quality. In addition, the correct use of welding materials has a direct impact on welding costs. Therefore, welding materials must be strictly controlled from the procurement of welding materials to their use. The specific methods include the following: Aspects: First, valid quality certificates should be provided when welding materials enter the site; second, the mechanical properties and chemical composition of welding materials must comply with the requirements of design documents and relevant standards; third, the alloy materials used in toxic and harmful media and high temperature and high-pressure Spectral analysis must be carried out to confirm, and the use of unqualified materials is strictly prohibited; fourth, it is best to set up a first-level warehouse and a second-level warehouse for welding materials on the construction site and pay attention to the humidity and temperature in the warehouse must be controlled; fifth, the quality of welding materials can only be used after passing the inspection. Storage, and at the same time classified and stacked according to specifications and varieties. In addition, welding materials should be accounted for and have a material identification number; Sixth, welding rods must be dried before use, and relevant information about welding rod drying must be recorded

(2).  Improve the welding environment

The welding environment has a direct impact on the welding quality, among which the welding environment must have suitable relative humidity, ambient temperature, and wind speed, thereby improving the internal quality and appearance of the weld structure, while ensuring that the metallographic structure and mechanical properties meet the design requirements match. If the following situations occur in the petroleum welding environment, and no effective protective measures are taken, welding is strictly prohibited: one is rainy and snowy weather; the other is low-hydrogen electrode arc welding with wind speed > 5m/s; Semi-automatic welding, wind speed > 8m/s; fourth, acid electrode arc welding, wind speed > 8m/s; fifth, gas self-shielded welding, wind speed > 2m/s; sixth, relative humidity of the atmosphere > 90%; The temperature required by the process regulation is low.

(3). Strengthen welding inspection and detection

Welding inspection and testing is an important ways to ensure the welding quality of pipelines. It usually requires special attention to the following items: First, the welding quality inspector must check the surface condition of the weld groove, the number of blunt edges, the number of misaligned edges, the groove angle, and composition before welding. Carefully check the gaps, etc. to ensure that the above technical indicators are consistent with the process documents. Second, special personnel must be assigned to supervise and control the whole process of welding, specifically to do the following: conduct a comprehensive inspection of the implementation of the welding process, deal with the problems discovered during the supervision process in a timely manner, or report the problems discovered to the relevant departments in a timely manner. The third is that after each weld is welded, the welder must clean up the slag, spatter, and related defects in time. After the self-inspection is qualified, the weld mark must be made and recorded, and finally, it must be handed over to the welding quality inspector. Secondary confirmation and special inspection, in which welds that pass the self-inspection of welders must be handed over to welding quality inspectors for visual inspection. Fourth, welding quality inspectors must entrust non-destructive testing of welds that require non-destructive testing according to the specified welding appearance quality inspection, flaw detection ratio, and welding operation instructions, and pay attention to prohibiting the establishment of welds to be detected before welding. Fifth, we must pay attention to the problem of welding undercut. Welding undercut will inevitably lead to stress concentration, which will cause fatigue crack source and early fatigue fracture failure of the pipeline. The transition between the material and the weld is smooth, thereby eliminating the adverse effects of the undercut of the weld on the welded joint.

(4). Improve the technical level of welding workers

As an important factor in controlling welding quality, people play a key role in the welding process. It is of great significance to strengthen the control of the professional level and professional quality of welding workers. First of all, welders who participate in the welding of pressure components of pressure pipelines, after passing the basic operation skills test, take up the post with a welder certificate; engage in welding work within the validity period of the certificate to avoid the phenomenon of unlicensed posts; secondly, the construction enterprise should sign a contract with the welder In a legal labor contract, the foreman is responsible for the quality of the nozzle group, guarantees the surface quality of the nozzle, and reasonably controls the gap between the nozzles, the size of the groove, and the amount of misalignment. In combination with the relevant standards of “Nondestructive Testing of Oil and Gas Steel Pipelines”, strengthen the control of welding defects such as misalignment and incomplete penetration, and adopt separate quality rating standards; in addition, the repair process of misalignment and incomplete penetration defects is more complicated. Quality inspectors are required to raise warnings, strictly control quality, find problems and correct them in time, avoid a large number of unqualified products, and improve overall responsibility awareness and quality awareness.

Carry out high-level welder training work. The welder training work should be based on the needs of the first-line construction, proceeding from the actual situation of the enterprise, and adopting different training methods for different engineering projects. Different teaching programs are adopted for different personnel, and more efforts are made in teaching management. Doing a good job in welder training and improving the technical level of welders is an important measure to ensure the quality of construction and strengthen the competitiveness of enterprises. At the same time, select outstanding talents to participate in advanced welding technology training and study, and participate in international welding technology training courses, so that they can become experts and academic leaders in the welding field of enterprises, and can also improve the technical level of welding.

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