Composition and influencing factors of welded joints
A joint that is joined by welding is called a welded joint. Welded joints include welds, fusion zones and heat affected zones:
1 Weld area: After the joint metal and filler metal are melted, they are cooled and solidified at a faster speed. The weld structure is an as-cast structure crystallized from liquid metal. The crystal grains are coarse, the composition is segregated, and the structure is not dense. However, due to the small weld pool, fast cooling, strict chemical composition control, low carbon, sulfur and phosphorus, the weld chemical composition is adjusted by the alloy to contain certain alloying elements. Therefore, the properties of the weld metal The problem is not big enough to meet the performance requirements, especially the strength is easy to achieve.
2 Fusion zone: The transition between the melt zone and the non-melting zone. The chemical composition of the fusion zone is uneven, and the structure is coarse, which is often a coarse superheated structure or a coarse hardened structure. Its performance is often the worst of the welded joints. The superheat zone (or quenching zone) in the fusion zone and the heat affected zone is the weakest mechanical part of the welded joint, which will seriously affect the quality of the welded joint.
3 Heat affected zone: The area where the microstructure and properties are changed due to the high temperature heating in the weld zone. The heat affected zone of low carbon steel can be divided into a superheat zone, a normalized zone and a partial phase change zone.
(1) The area with the highest heating temperature above 1100 °C in the superheating zone, the crystal grains are coarse, and even the overheated structure is generated, which is called the overheating zone. The plasticity and toughness of the overheated zone are significantly reduced, which is the worst mechanical property in the heat affected zone.
(2) The area with the highest heating temperature in the normalized zone from Ac3 to 1100 °C, air-cooled after welding to obtain a normalized structure with fine grain size, called normalized area. The mechanical properties of the normalized zone are better.
(3) In the phase change zone, the highest heating temperature is from Ac1 to Ac3, and only part of the structure undergoes phase change, which is called partial phase change zone. The grain in this area is uneven and the performance is also poor. In the installation of welding, the fusion welding method is applied more. The welded joint is a non-uniform body formed by local heating of the base metal by a high-temperature heat source and melting and solidification with the molten filler metal. The welded joint can be divided into three parts according to the organization and performance of each part.
The area of ​​the welded joint where the weld melts and solidifies is called the weld, which consists of the molten base metal and the filler metal. The area where the metallurgical structure and mechanical properties of the base metal are affected by heat (but not melted) during welding is called the heat affected zone. The fusion zone is the junction of the weld metal and the heat affected zone in the welded joint. The fusion zone is narrow and has a width of 0.1 to 0.4 mm.
Factors affecting the performance of welded joints:
The mechanical properties of a welded joint are determined by its chemical composition and organization. Therefore, factors affecting the chemical composition of the weld and the microstructure of the welded joint affect the performance of the welded joint.
(1) Welding materials: Welding electrodes for manual arc welding, submerged arc welding and gas shielded welding, which become part of the weld metal after melting, directly affect the chemical composition of the weld metal. Flux also affects the chemical composition of the weld.
(2) Welding method The heat source of different welding methods has different temperature and heat concentration. Therefore, the size of the heat-affected zone and the thickness of the welded joint are different, and the performance of the joint is different. In addition, different welding methods have different mechanical protection effects. Therefore, the purity of the weld metal, that is, the content of harmful impurities, the weld bead performance will also be different.
(3) Welding process The general term for the physical quantities (such as welding current, arc voltage, welding speed, line energy, etc.) selected to ensure the welding quality is called welding process parameters.
Welded joint type:
There are four types of welded joints: butt joints, T-joints, corner joints, and lap joints. Sometimes there are other types of joints in the welded structure, such as cross joints, end joints, crimp joints, sleeve joints, diagonal butt joints, lock bottom butt joints, and the like. It is specified in the national standard GB 985-88.
First, the butt joint
The relatively parallel joint of the two weldments is called the butt joint. This joint is a better joint type from the mechanical point of view. It has better stress condition, less stress concentration, and can withstand large static or dynamic loads. It is welded. The most common type of joint used in the structure.
According to the thickness of the weldment, the welding method and the preparation of the groove, the butt joint can be divided into two types: the non-grooved butt joint and the open groove butt joint.
Second, T-joint
A joint of the end face of one weldment and a surface of the other weldment forming a right angle or a nearly right angle is called a T-joint.
T-joints are used more in steel structural parts and as a contact weld, they can withstand forces and moments in all directions. When selecting, try to avoid single-face fillet welds, because the roots have deep gaps and the carrying capacity is very low. For the higher requirements of the weldment, a K-shaped groove can be used, and whether the root is welded or not according to the force condition is obtained, so that the weld with the large weld foot is economical than the groove without the groove, and the joint fatigue strength is high.
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