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Structure and function of welded joint

1. weld zone:
The weld line of arc welding is formed by solidification of liquid metal in the molten pool. It belongs to the forging structure, and the crystal grain is vertical to the columnar crystal shape of the molten pool wall. The crystallization process makes the chemical composition and impurities easy to form segregation in the middle of the weld seam, causing the mechanical function of the weld metal to decrease, so that the welding is improved by proper swing and alloying.
2. fusion zone:
The fusion zone is the transition zone between the weld metal and the heat affected zone in the welded joint. The area is very narrow, with both sides being completely melted weld zone and completely non melting heat affected zone. The maximum temperature of the fusion zone is between the solid and liquidus of the alloy. The fusion zone has significant chemical unaverage, resulting in unequal tissue, which is characterized by a small amount of cast structure and large superheated tissue, so that the plasticity is poor, the strength is low, and the brittleness is large. It is easy to produce welding cracks and brittle fracture, which is one of the weakest links in the welding joint.
3. heat affected zone:
The welding heat affected zone is the area where the structure and mechanical properties of the base material change due to the thermal influence (but not melting) during the welding process. The welding heat affected zone includes superheated zone, normalizing zone and sectoral transformation zone. The structure and function of welding heat affected zone basically reflect the function and quality of welded joints.
Microstructure change of low carbon steel welded joint
1) overheated zone: in the welding heat affected zone, the area with overheating structure or grain coarsening is called the overheated zone. The maximum temperature range of the heating is from solidus to 1100 degrees, and the width is about 1 ~ 3mm. Because of the coarse grain, the plasticity and toughness of the material decrease, and cracks often occur at overheating.
2) normalizing area: the highest temperature range is 1100 to Ac3, and the width is about 1.2 to 4.0mm. Because the metal recrystallized and then cooled in the air. Therefore, the average fine normalizing microstructure can be obtained with good mechanical properties.
3) the phase transition zone: the highest temperature range in the area is Ac1 to Ac3, and only phase transformation occurs in the sector. Due to recrystallization of the Department metals, the refined ferrite and pearlite can be obtained after cooling, while the non recrystallized sectors get coarse ferrite. Because the grain size is different, the mechanical function is poor. In general, when the weldment is heated to below Ac1, the microstructure of the steel will not change. For cold rolled steel products, recrystallization will also occur at the temperature of 450 Ac1 to soften the steel.
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