Joints of steel structures are usually welded, riveted and bolted. Rivet connection due to the cost of steel and labor, now in addition to some heavy and often subjected to dynamic load structure is still used, has been rarely used. Bolted connection, especially the high-strength bolt connection developed in recent years, has a great development prospect, but at present, bolt connection is still not the main way of connection.
Weld connection is the most important connection way in modern steel structure, its advantage is that any shape of the structure can be used weld connection, simple structure, generally do not need splicing material, save labor and steel, and can realize automatic operation, high production efficiency. At present in the industrial and civil building structures, the welded structure has occupied an absolute advantage.
2.Common methods of welding connection
There are three welding methods of steel structure: arc welding, resistance welding and gas welding. Resistance welding is suitable for thin-walled steel welding, gas welding is suitable for thin steel or small structure, arc welding weld quality is more reliable, is the most commonly used welding method. Arc welding includes manual arc welding and automatic or semi-automatic submerged arc welding. In manual arc welding, an arc is produced between the electrode coated with welding flux and the welding parts after electrification, and the welding electrode is melted to form a weld. The welding flux is melted along with the electrode to form the molten slag covering the welding seam. At the same time, a gas is produced to isolate the air from the molten liquid metal and protect the welding seam from the harmful gas in the air. The wire core of the electrode serves both as an electrode and as an added metal in the formation of welds. Therefore, the correct selection of welding rod type is one of the conditions to ensure the quality of welding seam. The manual arc welding electrode shall be suitable for the metal strength of the welding piece. In addition, the weld is divided into vertical welding, vertical welding, horizontal welding and vertical welding, and so on. Lower weld is the most convenient welding, the best quality, should be used as far as possible.
3.Defects and causes of welding connection
Due to various reasons, welds are prone to various defects, such as cracks, edge fusion, root penetration, welding nodus, slag inclusion and porosity. The main reason for the crack is the improper chemical composition of steel, such as high sulfur content of steel will produce hot crack, high phosphorus content will occur cold crack. In addition, improper welding process (referring to the welding method adopted, welding voltage, welding current and welding speed, etc.) and improper welding procedures will also lead to the generation of cracks: cracks can be longitudinal, can also be transverse; It may exist in the weld or in the main metal near the weld. For important structures bearing dynamic loads, low hydrogen electrode should be used, in order to reduce the impact of hydrogen on cracking. Defects such as edge failure to fuse, root failure to penetrate, bite and weld nodules are directly related to the welding process and the welder's technical level of operation.
A.The edge is not fused and the surface of the steel before welding is not completely cleaned, and the welding current is too small and the welding speed is too fast so that the base metal does not reach the melting state;
B.The root is not fully welded and the welding current is not enough and the welding speed is too fast, but also the electrode diameter is too thick and the welder's other improper operation.For butt welds with grooves, attention should be paid to whether the groove shape is appropriate.
C.Bite edge is the defect produced at the base metal near the weld surface. Mainly due to improper selection of welding parameters or due to incorrect operation process, such as the welding current used is too strong and the arc is too long (generally speaking, the arc length should not exceed the diameter of the electrode used);
D.Welding nodule is a metal nodule formed by the molten metal flowing to the undissolved base metal outside the weld seam during welding.
E.Slag inclusion is a kind of defect in the welding seam which can not float to the surface of the metal when the welding seam metal solidifies. If the weld is not carried too fast, slag inclusion can be avoided. When multi-layer welding (the weld completed by the deposition of multi-layer weld metal) before the next weld, the welding slag on the welding surface of the previous layer should be cleaned up, which is also an important measure to avoid slag inclusion;
F.The porosity is formed in the welding process due to the welding electrode coating damp, melting gas invasion of the weld.
Some of these defects can be found and remedied by the appearance inspection after welding, while others require inspection by instruments such as X-ray and ultrasonic inspection.
4.Hazard analysis of defect existence
The harmfulness of defects varies according to the size, nature and location of defects. Generally speaking, cracks are not fused, not weld penetration and bite edge are serious defects. Defects in the tensile zone of components are more serious than those in the compression zone. The existence of defects often leads to stress concentration in the component and crack expansion, and the welded structure is prone to cold and brittle phenomena at low temperatures, which is the reason.
Due to the non-uniform shrinkage during cooling after welding, there will be welding residual stress in the component, which can make part of the section of the component enter into plasticity in advance when it is under load. Reduces the stable critical stress of the component under compression. After welding, due to the non-uniform expansion and contraction of the component welding residual deformation, resulting in the original flat steel plate deformation and so on.
5.Defect handling method
In view of the above defects in addition to the correct design of welding joints, pay attention to minimize stress concentration and welding residual stress, residual deformation generation, manufacturing to choose the correct welding parameters, in strict accordance with the established welding process implementation operation. The welding process evaluation must be carried out for the first steel welding material, welding method, and post-welding heat treatment, and the pre-preparation process should be made according to the evaluation report.