How to avoid weld cracks in steel structure construction

In the construction of steel structure, it is necessary to take measures from many aspects such as material selection, welding process, construction environment and personnel management. Here's how:

1. Material control

Selection of good welding materials: strictly select welding materials that meet national standards and design requirements, including welding rods, wires, fluxes, etc. Ensure that the chemical composition and mechanical properties of the welding material meet the engineering needs, especially to control the content of harmful elements such as sulfur and phosphorus, because these elements will increase the tendency of hot cracks in the weld. Select reputable suppliers to conduct strict quality inspection of the purchased welding materials, including appearance inspection, chemical composition analysis, mechanical properties testing, etc., to ensure reliable material quality.

Matching base material and welding material: According to the characteristics of the base material, strength grade and so on, select the welding material that matches it. Different base materials require welding materials with different compositions and properties to ensure a good combination of the weld and the base material to avoid cracks caused by excessive composition differences. Before welding, it is necessary to carry out welding process assessment, and determine the appropriate welding materials and welding process parameters through tests to ensure the quality and performance of the weld.

Material storage and use: properly keep welding materials to prevent moisture and deterioration. Welding materials should be stored in a dry, well-ventilated warehouse in accordance with the specified temperature and humidity conditions. Before use, the welding material should be dried to remove moisture, especially the welding rod and flux, which must be dried and insulated in strict accordance with the requirements of the product manual. In the process of use, pay attention to the expiration date of the welding material and avoid the use of expired or deteriorated materials.

2. Welding process optimization

Reasonable welding parameters: according to the welding material, thickness, structure and other factors, choose the appropriate welding current, voltage, welding speed and other parameters. Excessive welding current may lead to overheating of weld and increase the probability of hot crack. If the current is too small, it may cause defects such as non-fusion and slag inclusion of the weld, and also affect the quality of the weld. Too fast or too slow welding speed can also adversely affect the quality of the weld, so it is necessary to determine the ideal welding parameters through test and experience, and strictly control them during the welding process.

Adopt appropriate welding sequence: Develop a reasonable welding sequence, follow the principle of symmetry and dispersion, and reduce the concentration of welding stress. For large and complex steel structure members, segmented and layered welding methods should be adopted to avoid concentrated welding in the same part, so that the welding stress is evenly distributed. For example, for the welding of box structures, the internal welds can be welded first, and then the external welds, and in a symmetrical order to reduce the generation of deformation and cracks.

Preheating and post-heating: For some steels with a large hardening tendency, preheating before welding can reduce the welding cooling speed, reduce thermal stress, and prevent the generation of cold cracks. The preheating temperature should be determined according to the material, thickness, welding environment and other factors of the steel, generally between 100-300 ℃. Timely post-heating after welding can promote the hydrogen in the weld to escape and reduce the risk of hydrogen-induced cracks. The post-heating temperature is generally between 200-350 ° C, and the holding time is determined according to the thickness and size of the weldment.

Multi-layer and multi-pass welding: For thicker welds, the use of multi-layer and multi-pass welding can improve the structure and performance of the weld. The thickness of each layer of weld should not be too large, generally controlled between 3-5mm. In the welding process, attention should be paid to cleaning the slag and splash of each layer of weld to ensure a good combination between layers. Multi-layer and multi-pass welding can make the cooling rate of weld more uniform, reduce stress concentration, and reduce the probability of cracking.

3. Construction environment management

Control of ambient temperature and humidity: Perform welding operations at appropriate ambient temperature and humidity. When the ambient temperature is too low, the cooling rate of the weld is too fast, which is easy to crack. In general, when the ambient temperature is below 0 ° C, measures such as preheating should be taken to improve the temperature of the weldment. At the same time, it is necessary to control the environmental humidity and avoid welding in a humid environment, because moisture will increase the hydrogen content in the weld, resulting in hydrogen-induced cracks. If welding in a humid environment cannot be avoided, dehumidification measures should be taken, such as using dehumidification equipment or drying the welding parts.

Windproof measures: When welding in an open air or well-ventilated environment, effective windproof measures should be taken to prevent the welding arc from being disturbed by wind, resulting in unstable welding and affecting the quality of the weld. Facilities such as wind shelter and wind baffle can be set up to control the wind speed within a certain range to ensure the smooth progress of the welding process.

4. Personnel and quality management

Welder training and assessment: strengthen the training and assessment of welders, improve the skill level and quality awareness of welders. Welders must undergo professional training, master the welding process and operating procedures, and be familiar with the performance and use of welding materials. Welders are regularly assessed, including theoretical knowledge and practical operation assessment, and only qualified welders can engage in steel structure welding operations. In the welding process, the welder must strictly abide by the welding process discipline to ensure the welding quality.

Quality inspection and monitoring: Establish a sound quality inspection and monitoring system to track and inspect the entire welding process. Before welding, it is necessary to check the assembly quality of welding parts and the quality of welding materials. In the welding process, it is necessary to monitor the welding parameters, welding sequence, etc., to discover and correct problems in time; After the welding is completed, the appearance inspection and non-destructive testing of the weld should be carried out to ensure that the quality of the weld meets the standards and design requirements. For the defects such as cracks found, it is necessary to deal with them in time, analyze the causes, and take improvement measures to prevent similar problems from happening again.