What is arc striking? What are the methods of arc striking? What should be paid attention to when striking an arc? The process of igniting and generating a stable arc is called arc striking. There are two arc striking methods: scratching arc striking and direct arc striking.
Scratch arc striking: Align the end of the electrode with the arc striking point, and then use the wrist force to gently scratch the electrode on the surface of the weldment like striking a match. The scratching distance is 10-20 mm, and the electrode is lifted 3-3 mm.
Direct arc striking: First, aim the electrode at the weldment to be welded and gently touch it, and then lift the electrode 2-3 mm at the right time to ignite. When striking an arc, the electrode should be lifted quickly, otherwise it is easy to stick to the workpiece. If sticking occurs, shake the electrode left and right and pull it apart. If it cannot be connected, loosen the welding clamp, cut off the welding circuit, and wait for the weldment to cool slightly before processing.


1. Pay attention to the following points when striking an arc:
(1) After striking an arc, ensure that the arc length does not exceed the diameter of the electrode. Do not use too much force when striking the arc to prevent the arc-strike end coating from cracking or even falling off, which will affect arc striking and welding.
(2) Do not randomly strike the weldment when striking the arc, especially high-strength steel, low-temperature steel, and stainless steel. This is because the arc scratches the part and easily causes hardening or micro-cracks, and stainless steel will reduce corrosion resistance.
(3) When striking the arc, if the welding rod and the weldment are connected together and cannot be removed by shaking, the welding clamp should be removed immediately. After the welding rod cools down, it will be easy to remove.


2. What is spot-fixing before welding?
In order to fix the relative position of the two weldments, positioning welding (usually called spot fixing) should be performed at both ends of the workpiece before welding.
After spot-fixing, the slag should be cleaned up. If the weldment is long, a welding point can be spot-fixed every 200-300mm.


3. What is the positive connection? What is the reverse connection?
During the welding process, the two poles of the arc welding machine are connected to the welding rod and the weldment respectively to form a complete welding circuit. For DC welding machines, one pole is positive and the other pole is negative. When the weldment is connected to the positive pole and the welding clamp (electrode) is connected to the negative pole, it is called the positive connection method (abbreviated as positive connection), and vice versa, it is called the reverse connection method (abbreviated as reverse connection). For AC welding machines, since the polarity of the power supply is alternating, there is no problem with positive connection and reverse connection.


4. How to start welding with arc welding rods?
The start of the weld is the beginning of the weld. Due to the low temperature of the weldment, the temperature of the weldment cannot be quickly raised after the arc is struck. Under normal circumstances, this part of the weld has a slightly higher residual height and a shallower penetration, and even poor fusion and slag inclusions may occur. Therefore, after the arc is struck, the arc should be slightly lengthened to preheat the workpiece, and then the pressure arc is used for normal welding. Flat welding and alkaline electrodes mostly use the reflow method, starting the arc 10 mm from the starting point, reflowing to the starting point, gradually lowering the arc, and at the same time, the electrode is slightly swung to achieve the required weld width, and then normal welding is performed.


5. What is welding arc voltage? What is its function?
The voltage drop between the two ends of the arc (two electrodes) is called arc voltage. It includes cathode voltage drop, anode voltage drop, and arc column voltage drop, which is numerically equal to the sum of the three voltage drops. Arc voltage mainly depends on arc length. Long arc has high arc voltage and short arc has low arc voltage. The larger the arc voltage, the larger the weld width, and the smaller the arc voltage, the smaller the weld width. In the welding process, in order to obtain the appropriate weld width and depth, the arc voltage should be increased accordingly as the welding current increases.


6. What is welding speed? What is its function?
The length of a single weld completed per unit of time is called welding speed. The welding speed depends on the size of the welding current. A large welding current can increase the welding speed, but do not mistakenly believe that the level of welding production efficiency depends on the welding speed; it is not that the higher the welding speed, the higher the welding production efficiency, but the size of the welding current determines the level of welding production efficiency. The larger the welding current, the more metal is melted per unit of time, and the higher the welding production efficiency. Under a certain welding current, the size of the welding speed affects the thickness of the weld. The faster the welding speed, the thinner the weld, and the slower the welding speed, the thicker the weld.


7. Why does the welding arc blow? What are the hazards?
There are three factors that cause arc blow:
(1) The arc blow is caused by excessive eccentricity of the electrode coating: During arc welding, the melting rate of the electrode coating is slower than that of the welding core. During the welding process, the electrode coating forms a sleeve, which is beneficial for protecting the arc. When the thickness of the electrode coating is uneven, the melting rate of the thick coating is slower than that of the thin coating. During the welding process, the length direction of the coating sleeve will be skewed, which will inevitably cause the arc to deviate from the direction of the thin coating.


(2) Arc blow caused by uneven magnetic field strength in the arc area: Due to the asymmetric magnetic field force on both sides of the arc axis, the arc deviates in the direction of weak magnetic field strength. When there is a good conductor (such as a rib plate) on one side of the arc area, the arc deviates to the conductor side. If the cable connecting the workpiece is connected to the left side of the arc axis, the arc blows to the right side.
(3) Arc blow is caused by airflow interference around the arc: the arc blows in the direction of the airflow, that is, from strong airflow to weak airflow.
The result of an arc blow destroys the stability of the arc, makes the arc area poorly protected, and causes serious spatter, which can cause welding defects such as porosity and incomplete penetration.


8. What are the measures to prevent arc blow?
The measures to prevent arc blow are:
(1) Select welding rods with non-eccentric coatings and dry them according to regulations.
(2) Use short arc operation to enhance the "rigidity" of the arc to offset the influence of arc blow.
(3) Adjust the welding angle of the welding rod so that the inclination angle of the welding rod is biased in the opposite direction of the arc blow. The arc blow phenomenon can be improved by changing the welding inclination angle.
(4) When welding the first layer of a butt weld with a gap, a pad or filler wire can be added to the back of the gap to prevent arc blow caused by thermal convection. The reverse segmentation method or the addition of arc-starting plates and arc-extinguishing plates at both ends of the weld can also be used to improve the arc blow caused by heat convection.
(5) When welding steel structures (such as beam-column structures with ribs), AC power should be used as much as possible.
(6) The magnetic field force around the arc should be distributed as evenly as possible by changing the position of the cable (ground wire) connecting the weldment.
9. What are the requirements for welding rods during arc welding?
The following principles should be considered when selecting welding rods:
(1) Select the corresponding type of welding rod (major category) according to the type of metal material to be welded. For example, when welding carbon steel or ordinary low-alloy steel, structural steel welding rods should be used.
(2) The performance of the weld should be the same as that of the parent material, or the chemical composition type of the weld should be the same as that of the parent material to ensure the same performance.
When selecting structural steel welding rods, first select structural steel welding rods with the same strength level according to the "equal strength" principle based on the tensile strength of the parent material. Secondly, for important structures with high requirements for weld performance (ductility, toughness), or steel and structures that are prone to cracking (large thickness, high rigidity, low welding environment temperature, etc.), alkaline electrodes, even ultra-low hydrogen electrodes, and high-toughness electrodes should be used.
When stainless steel electrodes and molybdenum and chromium-molybdenum heat-resistant steel electrodes are selected, electrodes with the same chemical composition type should be selected according to the chemical composition type of the parent material.
(3) The process performance of the electrode must meet the welding requirements. For example, when welding in a non-horizontal position, electrodes suitable for welding in various positions should be selected. For example, when welding vertically downward, welding pipes, bottom layer welding, cover welding, and gravity welding, corresponding special electrodes can be selected. In addition, under the premise of ensuring performance requirements, electrodes with low prices and high deposition efficiency should be given priority.


10. What is tack welding? What should be paid attention to in tack welding?
Before welding, welding is performed to assemble and fix the joint position on the weldment. Assembly and welding are the most important production links in the manufacturing process of welded structures. Tack welding is a method often used in the assembly link. Because the tack weld is left in the welded structure as a formal weld, the welding rod, welding process, and operating skills used for tack welding should be completely consistent with those for formal welds.
Tack welding should comply with the following requirements:
(1) The welding rod for tack welding is the same as the product weld in principle. When a tension plate is used to tack weld alloy structural steel, it is allowed to use a low-hydrogen coated welding rod with a lower strength than the product weld.
(2) The welding rod for tack welding should be dried as required.
(3) The preheating temperature of the tack weld should be the upper limit of the preheating temperature of the product weld. The preheating range of the tack weld is about 150mm around the tack weld.
(4) Slightly larger line energy can be used for tack welding. It is necessary to weld firmly and prevent undercutting. Continuous multi-layer and multi-pass welding can be used.
(5) Avoid tack welding at the weld or structural section where the cross section changes suddenly.