Pores are holes formed when the bubbles in the solution pool fail to escape during solidification during welding. J507 basic welding rod is mostly nitrogen, hydrogen and CO pores. There are more pores in the flat welding position than in other positions. The bottom layer is more than fill and cover; Long arc welding is more than short arc welding. Broken arc welding is more than continuous arc welding. Arc initiation, arc closure and joint are more than other positions of the weld. Due to the existence of pores, it will not only reduce the density of the weld, 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 J507 welding rod droplet transition, selection of welding power supply, appropriate welding current, reasonable arc initiation and closure, short arc operation, linear transport and other aspects of control, the welding production has been a good quality assurance.
1. Formation of stomata
Molten metal dissolves a large amount of gas at high temperature, and with the decrease of temperature, these gases gradually escape from the weld in the form of bubbles, and the gas that is too late to escape remains in the weld to form pores. The main gases that form pores are hydrogen and carbon monoxide. From the distribution of stomata, there are single stomata, continuous stomata and dense stomata. The stomata can be divided into external stomata and internal stomata according to different parts of stomata. From the shape, there are pinholes, round stomata, strip stomata (stomata in the shape of a bar worm, is a continuous round stomata), chain and honeycomb stomata. At present, it is more typical for J507 electrode to produce porosity defects during welding. Therefore, taking J507 electrode welding of low carbon steel as an example, the relationship between the causes of porosity defects and welding technology is discussed
2. Characteristics of droplet transition of J507 electrode
J507 electrode is a high alkalinity low hydrogen type electrode, the electrode can be used normally when the reverse polarity of the DC welding machine. Therefore, no matter what type of DC welder is used, the droplet transfer is from the anode region to the cathode region. In general manual arc welding, the temperature of the cathode region is slightly lower than that of the anode region. Therefore, no matter what kind of transition form is dissolved into the cathode region, the temperature will be reduced, resulting in the polymerization transition of each solution drop of the electrode to the solution pool, that is, the formation of a crude solution drop transition form. However, because manual arc welding is a human factor: such as the proficiency of the welder, the size of the current and voltage are different, the size of the solution drop is also uneven, and the size of the solution pool is also uneven. Therefore, under the influence of external and internal factors, pores and other defects are formed. At the same time, there is a large amount of fluorite in the coating of the basic electrode, which breaks down fluorine ions with high ionization potential under the action of the arc, which makes the stability of the arc worse, and then causes the instability of the droplet transition during welding. Therefore, in order to solve the stoma problem of J507 electrode manual arc welding, in addition to drying the electrode and cleaning the groove, it is necessary to start from the process measures to ensure the stability of the arc dripping transition.
3. Select the welding power supply to ensure stable arc
Because J507 electrode cover contains high ionization potential fluoride, resulting in arc gas instability factors, so it is necessary to choose a suitable welding power source. The DC welding power supply we usually use is divided into two types: rotary DC arc welding machine and silicon rectifier DC welding machine. Although their external characteristic curves are declining characteristics, but because the rotary DC arc welding machine is by selecting the commutator pole to achieve the purpose of rectification, so its output current waveform is a regular shape swing, which is bound to be a rated current on the macro level, and the output current is a small amplitude change on the micro level, especially in the drop transition resulting in an increase in the swing amplitude. For silicon rectifier DC welding machine is filtered by silicon components after rectification, although the output current has peaks and troughs, but on the whole is smooth, or in a process is a very small amount of swing, it can be considered continuous. 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 is concluded that the porosity of silicon rectifier welding machine is lower than that of rotary DC arc welding machine. After analyzing the test results, it is considered that the current welding power supply of silicon integral welding machine should be selected when J507 electrode is used for welding, which can ensure the stability of arc and avoid the occurrence of porosity defects.
4. Select the appropriate welding current
Due to the use of J507 electrode welding, in addition to the coating, the electrode also contains a large number of alloying elements in the welding core to enhance the strength of the weld joint and eliminate the possibility of porosity defects. Due to the use of large welding current, the solution pool becomes deeper, the metallurgical reaction is intense, and the alloying elements are burned seriously. Because the current is too large, the resistance heat of the welding core obviously surges, and the electrode is red, resulting in premature decomposition of organic matter in the electrode coating and formation of pores; And the current is too small. The crystallization rate of the molten pool is too fast, and the gas in the molten pool can not escape quickly enough to produce pores. In addition, DC reverse polarity is adopted, and the temperature of the cathode region is low. Even if the hydrogen atoms generated under the intense reaction are dissolved in the solution pool, they cannot be quickly replaced by alloying elements. Even if the hydrogen quickly emerges outside the weld, and the solution pool cools rapidly after overheating, the residual hydrogen molecules solidify in the solution pool weld, forming porosity defects. Therefore, it is quite necessary to consider the appropriate welding current. The process current of low hydrogen electrode is generally about 10 ~ 20% smaller than that of acid electrode of the same specification. In production practice, the low hydrogen type electrode can be used as the reference current by multiplying the square of the diameter of the electrode by ten. For example, the diameter of 3.2mm electrode can be set to 90 ~ 100A, and the diameter of 4.0mm electrode can be set to 160 ~ 170A as a reference current, and the experiment is used as the basis for selecting the process parameters. This can reduce the burning loss of alloying elements and avoid the possibility of porosity.
5. Reasonable arc initiation and retraction
J507 welding joints have a greater chance of porosity than other parts, because the joint tends to be slightly cooler than other parts during welding. Because the replacement of the new electrode has caused a period of heat dissipation at the original arc recovery, there may also be local corrosion at the end of the new electrode, resulting in dense porosity at the joint. To solve the resulting porosity defects, in addition to installing the necessary arc starter plate at the beginning of the operation, the end of each new electrode is gently wiped on the arc starter plate at the beginning of the arc in the middle of the joint. To remove rust from the ends. In the middle of the joint, it is necessary to use the method of advance arc, that is, after the arc is stabilized at the 10 ~ 20mm before the weld, then pull back to the joint arc retract, so as to locally heat the original arc retract, and then lower the arc after the formation of the solution pool, slightly up and down 1-2 times, that is, normal strip welding. The arc should be kept as short as possible to protect the solution pool from filling the arc pit, and the arc pit should be filled with point arc or swing back and forth 2-3 times to eliminate the porosity at the arc retract.
6. Short arc operation straight line
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 is not invaded by the outside air and produce porosity. However, what state the short arc should be maintained in, we believe that it should vary according to different specifications of the electrode. Usually the short arc means that the arc length is controlled by 2/3 of the diameter of the electrode. Because the distance is too small, not only the solution pool can not be seen clearly, it is not easy to operate, and it will cause short circuit and break arc. Too high or too low can not achieve the purpose of protecting the solution pool. When transporting the rod, it is appropriate to use a straight line, and excessive reciprocating swing will cause improper protection of the solution pool. For large thickness (refers to ≥16mm) can be used to open U-shaped or double U-shaped groove to solve, in the cover welding can also be multi-pass welding to minimize the swing amplitude. The above method is used in the welding production, which not only ensures the internal quality but also the smooth and neat welding pass.
In the operation of J507 electrode welding, in addition to the above process measures to prevent possible porosity, some conventional requirements of the process treatment can not be ignored. For example: drying of welding rod to remove moisture oil, determination and treatment of groove, proper grounding position to prevent arc bias caused by air holes and so on. Only combined with the characteristics of the product from the process measures to control, must be able to effectively reduce and avoid porosity defects.
