How to control the temperature of the molten pool during the weld of the welded pipe

How to control the temperature of the molten pool during the weld of the welded pipe

Molten pool refers to the base metal part melted into pool shape due to welding arc heat. The liquid metal part with geometric shape formed on the weldment during welding is called molten pool. The temperature of liquid metal in molten pool is much higher than that of general pouring molten steel. The average temperature of transition droplet is about 2300 ℃, the average temperature of molten pool is about 1700 ℃, and the maximum temperature can reach 2900 ℃ The liquid metal is superheated.

In the welding operation, defects such as flash, burn through, incomplete penetration, concave, slag inclusion and poor forming often appear. The main reasons for these defects are that in the welding operation process, it is not good at observing the change of molten pool temperature and controlling the temperature of molten pool, resulting in the above defects. The temperature of molten pool directly affects the welding quality. The temperature of molten pool is high, the molten pool is large and the fluidity of molten iron is poor Good, easy to fuse, but when it is too high, the molten iron is easy to flow down, the back side of single side welding and double side forming is easy to burn through, forming flash, forming is difficult to control, the joint plasticity is decreased, bending is easy to crack, when the molten pool temperature is low, the molten iron is small, the molten iron is dark, the fluidity is poor, and it is easy to have defects such as incomplete penetration, incomplete fusion, slag inclusion, etc.

When welding, the current flowing through the weldment is called welding current. The diameter of the electrode refers to the cross-section size of the filler metal rod. In short, whether the electrode can be properly melted depends on the current passing through. If the current is too small, it is difficult to start the arc. The electrode is easy to stick to the weldment, with thick fish scale and poor fusion on both sides. If the current is too large, the welding spatter and smoke will be large, the electrode will be red and the molten pool will be damaged The surface is very bright, easy to burn through and undercut.

Suitable current, easy to ignite, stable arc, small spatter, even crackle sound can be heard, smooth transition from both sides of the weld to the base metal, small fish scale on the surface, welding slag is easy to knock, and in its application, there is a complex relationship, in vertical, horizontal and upward position, the current is correspondingly smaller than that of flat welding, usually about 10% less than that of flat welding, also in vertical position The diameter of the electrode is usually smaller than that of the flat welding.

Transportation method

The welding rod is sent along the axis to the molten pool direction to make the arc length remain unchanged after melting. Therefore, the speed of the electrode feeding into the molten pool is equal to the melting speed of the electrode. If the speed of the electrode feeding is less than the melting speed of the electrode, the arc length will gradually increase, leading to arc breaking. If the welding rod is sent too fast, the arc length will be shortened rapidly, and the electrode does not contact the welding parts at the end Short circuit will also extinguish the arc.

Crescent shape welding method: the end of the welding rod swings left and right along the welding direction in crescent shape, the middle action should be fast, and the two sides should stay for a while. This method can control the temperature of the molten pool, and the molten pool is shallow, so it is necessary to prevent the undercut on both sides. Crescent shape welding is one of the main welding methods for single side welding and double side forming continuous arc welding.

Zigzag welding method: the end of the welding rod swings forward in zigzag shape, and the two sides stay slightly to prevent undercut. This method is easy to operate, and is suitable for the welding of each layer of weld bead of butt weld in horizontal, vertical and overhead welding positions. The temperature of the circle welding pool is higher than that of the crescent welding pool, and the temperature of the crescent welding pool is higher than that of the zigzag welding pool The range and pause on both sides of the groove control the temperature of the molten pool, make the size of the molten hole basically consistent, reduce the probability of the groove root not forming a weld bead and burning through, and improve the incomplete penetration, so that the single side welding and double side forming of butt flat welding is no longer difficult.

When the angle between the electrode and the welding direction is 90 degrees, the arc is concentrated, the bath temperature is high, the included angle is small, the arc is dispersed, the bath temperature is low, the arc combustion time, and the arc combustion time are used to control the bath temperature. If the bath temperature is too high and the molten hole is large, the arc combustion time and the bath temperature can be reduced. At this time, the molten hole becomes small, and the internal molding height is moderate, Avoid too high welding seam or weld bead inside the pipe.