Porosity is the cavity formed when the bubbles in the molten pool fail to escape during solidification during welding. When welding with J507 alkaline electrode, there are mostly nitrogen pores, hydrogen pores and CO pores. The flat welding position has more pores than other positions; there are more base layers than filling and covering surfaces; there are more long arc weldings than short arc weldings; there are more interrupted arc weldings than continuous arc weldings; and there are more arc starting, arc closing and joint locations than welding. There are many other positions to sew. The existence of pores will not only reduce the density of the weld and weaken the effective cross-sectional area of the weld, but also reduce the strength, plasticity and toughness of the weld. According to the characteristics of droplet transfer of J507 welding rod, we select the welding power source, appropriate welding current, reasonable arc starting and closing, short arc operation, linear rod transportation and other aspects to control, and get good quality assurance in welding production.
1. Formation of stomata
Molten metal dissolves a large amount of gas at high temperatures. As the temperature drops, these gases gradually escape from the weld in the form of bubbles. The gas that has no time to escape remains in the weld and forms pores. The gases that form pores mainly include hydrogen and carbon monoxide. From the distribution of stomata, there are single stomata, continuous stomata, and dense stomata; from the location of stomata, they can be divided into external stomata and internal stomata; from the shape, there are pinholes, round stomata, and strip stomata (the stomata are strip-worm-shaped) , which are continuous round pores), chain-like and honeycomb pores, etc. For now, it is more typical for J507 electrodes to produce pore defects during welding. Therefore, taking the welding of low carbon steel with J507 electrode as an example, some discussions are made on the relationship between the causes of pore defects and the welding process.
2.Characteristics of J507 welding rod droplet transfer
J507 welding rod is a low-hydrogen welding rod with high alkalinity. This welding rod can be used normally when the DC welding machine reverses polarity. Therefore, no matter what type of DC welding machine is used, the droplet transition is from the anode area to the cathode area. In general manual arc welding, the temperature of the cathode area is slightly lower than the temperature of the anode area. Therefore, no matter what the transition form is, the temperature will decrease after the droplets reach the cathode area, causing the aggregation of the droplets of this type of electrode and transitioning into the molten pool, that is, the coarse droplet transition form is formed. However, because manual arc welding is a human factor: such as the proficiency of the welder, the size of the current and voltage, etc., the size of the droplets is also uneven, and the size of the molten pool formed is also uneven. Therefore, defects such as pores are formed under the influence of external and internal factors. At the same time, the alkaline electrode coating contains a large amount of fluorite, which decomposes fluorine ions with a high ionization potential under the action of the arc, making the arc stability worse and causing unstable droplet transfer during welding. factor. Therefore, in order to solve the porosity problem of J507 electrode manual arc welding, in addition to drying the electrode and cleaning the groove, we must also start with technological measures to ensure the stability of arc droplet transfer.
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3. Select the welding power source to ensure stable arc
Since the J507 electrode coating contains fluoride with a high ionization potential, which causes instability in the arc gas, it is necessary to choose a suitable welding power source. The DC welding power sources we usually use are divided into two types: rotary DC arc welding machine and silicon rectifier DC welding machine. Although their external characteristic curves are all descending characteristics, because the rotary DC arc welding machine achieves the purpose of rectification by installing an optional commutating pole, its output current waveform swings in a regular shape, which is bound to be a macroscopic phenomenon. Rated current, microscopically, the output current changes with a small amplitude, especially when the droplets transition, causing the swing amplitude to increase. Silicon rectified DC welding machines rely on silicon components for rectification and filtering. Although the output current has peaks and valleys, it is generally smooth, or there is a very small amount of swing in a certain process, so it can be considered continuously. Therefore, it is less affected by the droplet transition, and the current fluctuation caused by the droplet transition is not large. In the welding work, it was concluded that the silicon rectifier welding machine has a lower probability of pores than the rotary DC arc welding machine. After analyzing the test results, it is believed that when using J507 electrodes for welding, a silicon solid welding machine flow welding power source should be selected, which can ensure arc stability and avoid the occurrence of pore defects.
4. Choose the appropriate welding current
Due to the J507 electrode welding, the electrode also contains a large amount of alloy elements in the weld core in addition to the coating to enhance the strength of the weld joint and eliminate the possibility of pore defects. Due to the use of larger welding current, the molten pool becomes deeper, the metallurgical reaction is intense, and the alloy elements are severely burned. Because the current is too large, the resistance heat of the welding core will obviously increase sharply, and the electrode will turn red, causing the organic matter in the electrode coating to decompose prematurely and form pores; while the current is too small. The crystallization speed of the molten pool is too fast, and the gas in the molten pool has no time to escape, causing pores. In addition, the DC reverse polarity is used, and the temperature of the cathode area is relatively low. Even if the hydrogen atoms generated during the violent reaction are dissolved in the molten pool, they cannot be quickly replaced by the alloy elements. Even if the hydrogen gas quickly floats out of the weld, the dissolved The pool is overheated and then cooled rapidly, causing the remaining hydrogen-forming molecules to solidify in the molten pool weld to form pore defects. Therefore, it is necessary to consider the appropriate welding current. Low-hydrogen welding rods generally have a slightly smaller process current of about 10 to 20% than acid welding rods of the same specification. In production practice, for low-hydrogen welding rods, the square of the diameter of the welding rod multiplied by ten can be used as the reference current. For example, the Ф3.2mm electrode can be set at 90~100A, and the Ф4.0mm electrode can be set at 160~170A as the reference current, which can be used as the basis for selecting process parameters through experiments. This can reduce the burning loss of alloy elements and avoid the possibility of pores.
5. Reasonable arc starting and closing
J507 electrode welding joints are more likely to produce pores than other parts. This is because the temperature of the joints is often slightly lower than other parts during welding. Because the replacement of a new welding rod has caused heat dissipation for a period of time at the original arc closing point, there may also be local corrosion at the end of the new welding rod, resulting in dense pores at the joint. To solve the pore defects caused by this, in addition to the initial operation In addition to installing the necessary arc-starting plate at the arc-starting end, at each joint in the middle, lightly rub the end of each new electrode on the arc-starting plate to start the arc to remove the rust on the end. At each joint in the middle, the method of advanced arc striking must be used, that is, after the arc is struck 10 to 20 mm in front of the weld and is stable, it is then pulled back to the arc closing point of the joint so that the original arc closing point can be locally heated until the melt is formed. After pooling, lower the arc and swing it slightly up and down 1-2 times to weld normally. When closing the arc, the arc should be kept as short as possible to protect the molten pool from filling the arc crater. Use arc lighting or swinging back and forth 2-3 times to fill the arc crater to eliminate the pores generated at the closing arc.
6. Short arc operation and linear movement
Generally, J507 welding rods emphasize the use of short arc operation. The purpose of the short arc operation is to protect the solution pool so that the solution pool in the high-temperature boiling state will not be invaded by outside air and produce pores. But in what state the short arc should be maintained, we think it depends on the welding rods of different specifications. Usually short arc refers to the distance where the arc length is controlled to 2/3 of the diameter of the welding rod. Because the distance is too small, not only the solution pool cannot be seen clearly, but it is also difficult to operate and may cause short circuit and arc breakage. Neither too high nor too low can achieve the purpose of protecting the solution pool. It is advisable to transport the strips in a straight line when transporting the strips. Excessive back and forth swing will cause improper protection of the solution pool. For larger thicknesses (referring to ≥16mm), open U-shaped or double U-shaped grooves can be used to solve the problem. During cover welding, multi-pass welding can also be used to minimize the swing range. The above methods are adopted in welding production, which not only ensures the intrinsic quality but also ensures smooth and tidy weld beads.
When operating J507 electrodes for welding, in addition to the above process measures to prevent possible pores, some conventional process requirements cannot be ignored. For example: drying the welding rod to remove water and oil, determining and processing the groove, and proper grounding position to prevent arc deflection from causing pores, etc. Only by controlling the process measures based on the characteristics of the product, we will be able to effectively reduce and avoid pore defects.
Post time: Nov-01-2023
