Issues to be noted in the grid welding process

Spatial structures are widely used in large-span buildings due to their advantages such as reasonable spatial force, light weight and high rigidity.

1. Pre-welding preparation should be meticulous

(1) Material quality control

The quality of materials such as steel, welding rods, welding wires and flux is the basis. The quality certification documents must be strictly reviewed to ensure that their varieties, specifications and performance are consistent with the design and relevant standards. For example, the Q235 and Q355 steels commonly used in grids must ensure that their mechanical properties such as yield strength and tensile strength meet the standards. The type of welding rod must match the parent material. For example, when welding Q235 steel, E43 series welding rods can be used. At the same time, the material should be inspected for appearance. There should be no defects such as cracks and interlayers on the surface of the steel, and the welding rods and welding wires should not have coating shedding or rust.

(2) Welding equipment debugging

Select suitable welding equipment with good performance, and conduct comprehensive debugging and inspection. The current and voltage regulation of the electric welding machine should be accurate and stable, and the gas flow control device of the gas shielded welding machine should be normal. For example, the gas flow rate of a CO2 gas shielded welding machine is generally controlled at 15-25L/min. In addition, the grounding of the equipment should be checked to ensure safety and reliability, and to avoid equipment failure affecting the welding quality.

(3) Welding site and environmental control

The welding site should be dry and clean, and avoid open-air welding in bad weather. When the ambient temperature is lower than the specified value, the weldment should be preheated. For example, the welding temperature of ordinary carbon structural steel should not be lower than -20℃, and that of low-alloy structural steel should not be lower than -10℃. When the humidity is high, moisture-proof measures should be taken, such as building a protective shed. At the same time, the site should be reasonably planned to provide sufficient operating space for welders.

2. Strict control of the welding process

(1) Reasonable selection of process parameters

According to factors such as steel material, thickness, and joint form, the welding current, voltage, welding speed, and number of welding layers should be reasonably determined. For example, when welding thicker steel, the welding current and voltage should be appropriately increased, but not too large, so as to avoid problems such as weld overheating, deformation, or pores. For multi-layer and multi-pass welding, the thickness of each weld is generally controlled at 3-4mm. And the operation must be strictly in accordance with the parameters determined by the welding process assessment, and no changes can be made at will.

(2) Standard and proficient operation techniques

Welders need to have proficient operation skills and master the correct welding techniques. When performing manual arc welding, the electrode angle and the electrode movement method should be appropriate to ensure the weld width, excess height and fusion quality. For example, when performing flat welding, the angle between the electrode and the weld surface is 60°-80°. When performing multi-layer and multi-pass welding, attention should be paid to cleaning between layers, removing impurities such as welding slag and spatter, and ensuring that each layer of weld is well fused.

(3) Scientific arrangement of welding sequence

A reasonable welding sequence can reduce welding deformation and residual stress. Symmetrical welding, segmented welding and other methods can be used for large spatial structures. First weld the weld with large shrinkage, then weld the weld with small shrinkage; first weld the transverse weld, then weld the longitudinal weld. For example, when welding the cross-shaped node of the grid, the transverse weld should be welded first, then the longitudinal weld, so that the stresses offset each other.

3. Do not relax the inspection after welding

(1) Appearance inspection

After welding is completed, first conduct an appearance inspection. The weld surface should be flat and free of defects such as cracks, weld bumps, pores, slag inclusions, and undercuts. The weld excess height, width and other dimensions should meet the design and specification requirements. For example, undercuts are not allowed for first-level welds, and the undercut depth of second-level welds shall not exceed 0.05t (t is the thickness of the weldment), and the continuous length shall not exceed 100mm.

(2) Non-destructive testing

For important grid node welds and welds that require flaw detection in the design, non-destructive testing such as ultrasonic flaw detection and radiographic flaw detection shall be carried out in accordance with regulations. The test results shall meet the qualified grade requirements of relevant standards. If defects exceeding the standard are found, the reasons shall be analyzed, a repair plan shall be formulated, and re-testing shall be carried out after repair.

4. Safety issues need to be taken seriously

Welders shall wear personal protective equipment, such as welding masks, protective gloves, protective clothing, etc., to prevent arc and splash damage. Fire extinguishing equipment shall be equipped in the welding site, and flammable and explosive items shall not be stored around. When welding near flammable and explosive areas, fire approval must be obtained and fire isolation zones must be set up.