Five Hidden Dangers in Steel Structure Welding

1. Different thicknesses of the plate docking, the transition is not smooth

The use of different thickness and width of the plate docking, do not pay attention to the thickness of the plate is within the standard allowable range. If not within the allowable range and do not make a smooth transition, the weld in the thickness of the plate higher than easy to cause stress concentration and produce weld defects such as non-fusion, affecting the welding quality.

Solution

When exceeding the relevant provisions should be welded into the slope of the weld, the maximum allowable slope should be 1:2.5; or thickness of one or both sides of the slope processed before welding, and the maximum allowable slope value of 1:2.5, when directly subjected to dynamic loads and fatigue calculations need to be carried out on the structure of the slope should not be greater than 1:4. Different widths of the plate butt joints, should be based on factory and site conditions using thermal cutting, machining or grinding wheel grinding method to make a smooth transition, and the maximum allowable slope value of 1:2.5 at the joint.

2. Multi-layer welding discontinuity, resulting in cold cracks

Thick plate multi-layer welding, do not pay attention to the temperature control between layers, such as the interval between layers is too long, do not re-preheat the welding is easy to produce cold cracks in the interlayer; such as the interval is too short, the interlayer temperature is too high (more than 900 ℃), the performance of the weld and the heat-affected zone will have an impact on the performance of the weld, which will cause coarse grain, resulting in a decrease in toughness and plasticity, will leave a joint of the potential hidden problems.

Solution

Thick plate multi-layer welding, should strengthen the control of the temperature between the layers, in the continuous welding process should be tested welded base material temperature, so that the temperature between the layers as far as possible with the preheating temperature to maintain consistency, the highest temperature between the layers should also be controlled. Welding time should not be too long, such as in the case of welding interruption should take appropriate post-heating, heat preservation measures, and again when the welding, re-heating temperature should be appropriately higher than the initial preheating temperature.

3. Welding deformation

Welding does not pay attention to control deformation from the welding sequence, personnel arrangement, bevelling form, welding specifications and operating methods, which leads to deformation after welding, correction is difficult, increase costs, especially thick plates and large workpieces, correction is difficult, mechanical correction is prone to cause cracks or laminar tearing. Correction by flame is costly and easy to cause overheating of workpiece by bad operation. For workpieces with high precision requirements, not taking effective deformation control measures will lead to the installation of the workpiece size can not meet the requirements, or even cause rework or scrap.

Solution

Adopt reasonable welding sequence and choose suitable welding code and operation method, also use anti-deformation and rigid fixing measures.

4. Insufficient size of weld fillet for fusion joints or fillet-to-fillet combination welds.

T-joints, cross joints, corner joints and other requirements for penetration of the butt joints or angle butt joints combination of welds, the foot size is not enough, or the design of the fatigue calculation requirements of the crane beam or similar components of the web and the upper wing edge connection weld foot size is not enough to make the strength and stiffness of the weld can not meet the requirements of the design.

The Solution

T-joints, cross joints, corner joints and other butt or corner butt combination welds that require fusion penetration with insufficient size of the weld foot, or insufficient size of the weld foot of the connection weld between the web and the upper flange edge of the crane girder or similar members with fatigue calculation requirements for the design will make the strength and stiffness of the weld fail to meet the design requirements.

5. Welding in the gap between the joints plug welding rod head or iron block

Due to the welding is difficult to welding rod head or iron and the welded parts into one, will cause unfused, not penetrate and other welding defects, reduce the strength of the connection. If you fill with rusty welding rod tip or iron block, it is difficult to ensure the same material with the base material; if you fill with welding rod tip or iron block with oil, impurities, etc., it will make the weld produce porosity, slag, cracks and other defects. These conditions will make the joint weld quality is greatly reduced, failing to meet the design and specification of the quality requirements of the weld.

Solution

(1) When the assembly gap of the workpiece is large, but does not exceed the permitted range of use, and the assembly gap is more than 2 times the thickness of the thin plate or more than 20mm, the surfacing method should be used to fill in the recessed parts or reduce the assembly gap. It is strictly prohibited to fill in the gap between the joints by filling the welding rod head or iron filler welding method.

(2) Parts processing line, should pay attention to leave enough cutting allowance and welding shrinkage allowance after cutting, control the size of the parts, do not increase the gap to ensure that the external dimensions.