Welding technology and quality control in Urumqi steel structure construction

1. The core application of welding technology in Urumqi steel structure construction

Steel structures are widely used in engineering fields such as super high-rise buildings, long-span bridges, and industrial plants due to their high strength, lightweight, and excellent seismic performance. Welding is the main method of connecting steel structures, and its technical level directly determines the load-bearing capacity and safety of the structure. Current mainstream welding technologies include manual arc welding, gas shielded welding (GMAW/FCAW), submerged arc welding (SAW) and electroslag welding (ESW). The selection of different technologies needs to be comprehensively determined based on engineering needs, component materials and construction environment.

Manual arc welding is highly flexible and suitable for complex nodes or on-site repairs, but the efficiency is low and the quality stability depends on the welder's skills; the editor of Xinjiang Steel Structure said that gas shielded welding (such as carbon dioxide welding) uses inert gas to isolate the air, and the weld shape is beautiful. It is often used for thin-walled components or full melting Penetration welding; submerged arc welding protects the arc through the flux layer, which is suitable for thick plate welding and long welding continuous operations, and the efficiency can reach 5-10 times that of manual welding; electroslag welding is suitable for vertical butt welding of heavy components such as H-shaped steel and box columns, and can complete welding of 30-100mm thickness at one time. In addition, intelligent welding technologies such as robot welding and narrow gap submerged arc welding are gradually becoming more popular, improving welding accuracy and reducing human errors through automated control.

2. Key links in welding quality control

(1) Preparation stage before welding

Materials and Equipment Control

Steel and welding materials (welding rods, welding wires, fluxes) must have factory certificates and third-party testing reports. After entering the site, mechanical property tests (such as tensile and impact) and chemical composition analysis must be conducted according to specifications. The editor of Xinjiang Steel Structure said that special attention should be paid to the carbon equivalent (CEV) when welding low alloy steel. When CEV>0.45%, preheating measures should be taken to prevent cold cracks. The storage of welding materials needs to be moisture-proof and constant temperature. The drying temperature for acidic electrodes is 150-200°C, and the drying temperature for alkaline electrodes is 350-400°C. After 1-2 hours of heat preservation, put it into an 80-100°C insulation cylinder for later use. Welding equipment (such as welding machines, temperature controllers) needs to be calibrated regularly to ensure that parameters such as current, voltage, wire feed speed, etc. are stable.

Joint design and bevel processing

The weld form (butt joint, corner joint, T-shaped joint) and groove size (angle, blunt edge, gap) should meet the design requirements. Full penetration welds usually use V-shaped, X-shaped or U-shaped grooves. The Urumqi steel structure manufacturer said that bevel processing can be processed by mechanical cutting (plasma, flame cutting) or edge milling machine. After cutting, the scale, oil stain and rust need to be removed to reveal the metallic luster. For thick plate welding, if there is no requirement in the design, when the plate thickness is ≥ 25mm and the ambient temperature is lower than 0°C, preheating is required within 100mm on both sides of the groove, and the preheating temperature is controlled at 80-150°C (monitored by a thermometer).

(2) Welding process control

Process parameter optimization

Welding current, voltage, and welding speed constitute the "three elements of welding", and the best combination needs to be determined through the process qualification test (PQR). For example, when Q355B steel is welded with φ1.2mm solid core wire gas shielded welding, the recommended parameters for the flat welding position are current 200-250A, voltage 28-32V, speed 30-40cm/min, and shielding gas flow rate 15-20L/min (Ar+CO₂ mixed gas). The Urumqi steel structure manufacturer said that during multi-layer and multi-pass welding, the thickness of each weld bead should not exceed 1.5 times the diameter of the welding wire, and the inter-layer temperature should be controlled at 150-200°C (inter-layer cooling time <5 minutes) to avoid excessive inter-layer temperature causing coarse grains.

Operating technical specifications

Welders must hold a valid qualification certificate to work and conduct simulation exercises before welding. During vertical welding, use the zigzag bar transport method. During horizontal welding, tilt from bottom to upward by 10-15°. During overhead welding, shorten the arc length (2-3mm) to prevent the molten iron from falling. For complex parts such as beam-column joints, the principle of "welding seams with large shrinkage first, then welding seams with small shrinkage" should be followed. For example, when welding box columns, the longitudinal main welds are welded first, and the transverse partition welds are welded later to reduce welding residual stress.

Environmental factor control

Welding operations must meet environmental requirements: a windproof shed should be installed when the wind speed is >8m/s (gas shielded welding) or >11m/s (other welding); welding is prohibited when the relative humidity is >90% or in rainy days without protective measures; for low-temperature welding (ambient temperature <0℃), the preheating temperature needs to be increased by 20-30℃, and post-heat insulation measures must be taken (250-350℃ insulation for 1-2 hours).

(3) Post-weld inspection and defect treatment

Non-destructive testing (NDT)

The quality levels of welds are divided into first, second and third levels. First-level welds require non-destructive testing, and 20% of second-level welds are randomly inspected (and no less than one). Common detection methods include:

Appearance inspection: Use a magnifying glass (5-10 times) to inspect weld reinforcement (0-3mm), undercut (depth ≤ 0.5mm, length ≤ 10% of weld length), pores, cracks and other surface defects;

Ultrasonic testing (UT): detects internal defects (such as lack of fusion, slag inclusions), with sensitivity up to φ2mm flat-bottomed holes;

Radiographic testing (RT): The Urumqi steel structure manufacturer said that it is suitable for butt welds and can visually display the shape and location of defects. The first-level welding seam is qualified as Class I, and the second-level welding seam is qualified as Class II;

Magnetic particle testing (MT)/penetrant testing (PT): used for surface or near-surface defect detection of ferromagnetic materials, such as cracks and pinholes.

Defect repair

When out-of-standard defects are discovered, the causes need to be analyzed and a repair plan formulated. Crack repair requires first drilling crack relief holes (diameter 8-10mm) at both ends of the crack, and using carbon arc gouging to remove defects until the metallic luster is exposed. The number of repairs should not exceed 2 times (low alloy steel ≤ 3 times), and re-testing is required after repair.

3. Quality control system and management innovation

Whole process quality traceability

Establish a welding quality ledger to record welding material batch numbers, welder numbers, welding parameters, test reports and other information to achieve "traceability of each weld". Use BIM technology to build a three-dimensional model, associate weld information (location, type, inspection results) with the model, and visually manage quality data.

People and training management

Promote the "certificate-based employment + performance evaluation" system for welders, and regularly carry out skill competitions and new process training (such as robot welding operations). Implement qualification certification for welding quality inspectors to ensure the authority of test results.

Intelligent quality monitoring

Urumqi steel structure manufacturers say they apply welding process monitoring systems (such as arc sensors, infrared thermal imaging cameras) to collect current, voltage, temperature and other data in real time, and use AI algorithms to identify abnormal fluctuations and provide early warning. Digital ray detection (DR) is used to replace traditional film rays to realize digital storage of inspection images and remote film evaluation, improving inspection efficiency.

4. Engineering cases and practical experience

In a super high-rise steel structure project (height 368m), the core tube steel column uses Q460GJC-Z35 steel plate (thickness 80mm), and the welding faces the problems of "control of thick plate welding deformation" and "prevention of cold cracks during low-temperature construction". Ensure quality by taking the following measures:

Adopt "narrow gap submerged arc welding + electroslag welding" composite process to reduce heat input (line energy ≤35kJ/cm);

Implement "layered and segmented symmetrical welding", and use hammering to eliminate stress after each layer of welding is completed;

The Urumqi steel structure manufacturer said that during winter construction, a constant-temperature welding workshop (temperature ≥15°C) was built, electric heating sheets were used for preheating (preheating temperature 120°C), and infrared thermometers were used to monitor the interlayer temperature throughout the process. The first pass rate of steel structure welds in this project reached 99.2%, far exceeding the industry average.

5. Conclusion and outlook

The quality control of steel structure welding in Xinjiang needs to focus on "prevention first, process control" as the core, and build an all-round management system of "people, machines, materials, laws and environment" through strict material control, process parameter optimization, intelligent monitoring and other means. In the future, with the development of 5G and digital twin technology, welding quality control will be upgraded in the direction of "real-time perception-intelligent decision-making-closed-loop control", promoting steel structure projects to move towards higher quality and higher efficiency. Construction companies need to continue to innovate technology and management models to provide a solid guarantee for the safety and reliability of steel structure buildings.