Intermediate Gas Welder Technology

Author: Chief Editor: Yu Fengying
Publisher:
Publish Date: 1999-06-01
Features:
Fragment:
Principle training requires understanding the influence of gas welding molten pool and gas in the welding area, being familiar with the metal metallurgical process and crystallization process and their characteristics, and mastering the knowledge of welding heat cycle. Gas Welding Metallurgical Process In gas welding metallurgical process, the parent material metal and welding wire are melted under the action of the gas flame, forming a liquid metal part with certain geometric shape on the workpiece, which is called the welding molten pool. In this small molten pool, liquid metal, flux, impurities on the surface of the parent material metal, flame flow, and surrounding air undergo a series of physical and chemical reactions, finally forming weld metal and slag, completing the entire metallurgical process.
I. Morphology of Welding Molten Pool
1. Geometric Shape of Molten Pool
The geometric shape of the molten pool is similar to half an ellipsoid, as shown in Figure 1-1. The geometric shape of the molten pool can be represented by three parameters: the maximum depth Hmax, the maximum width Bmax, and the length L of the molten pool. Under normal conditions, these three parameters increase with the increase of flame energy, and decrease with the increase of welding speed.
2. Temperature Distribution in Molten Pool
The temperature distribution at each point in the molten pool is non-uniform. The longitudinal temperature distribution of the molten pool is shown in Figure 1—2a. From the figure, it can be seen that at the head of the molten pool, since the input heat is greater than the heat loss, the parent material metal is continuously melted as the heat source moves. The temperature at the head of the molten pool is higher, with a larger temperature gradient. The highest temperature in the molten pool is located on the surface of the molten pool under the flame. The temperature at the tail of the molten pool gradually decreases. At the edge of the molten pool, since the input heat is less than the heat loss, metal solidification begins. The temperature gradient at the tail of the molten pool is smaller than that at the head. The transverse temperature distribution in the molten pool is also non-uniform, as shown in Figure 1－2b. Tm is the melting point of the parent material metal. The temperature in the middle of the molten pool is the highest, the temperature at the head of the molten pool is higher, and the temperature at the tail of the molten pool is close to the melting point. In the part below the melting point, the metal has already begun to solidify.
3. Characteristics of Liquid Metal in Molten Pool
The liquid metal in the molten pool is not static but constantly moving. This includes free convection due to the density difference of the liquid metal, forced convection caused by the surface tension difference, and stirring due to the airflow of the gas flame. Due to the convection and stirring caused by the above reasons, the composition of the parent material metal and welding wire metal is well mixed, forming more uniform weld metal. In addition, this movement is beneficial to the escape of harmful gases and non-metallic inclusions, which is advantageous to improving the quality of the weld.