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1、Causes of terminal cracks
(1) Change of terminal weld temperature field
When the welding heat source is close to the end of the longitudinal weld, the temperature field will change. The closer it is, the greater the change is. The size of the arc striking plate is smaller than that of the cylinder, and the heat capacity is also smaller
The poor heat transfer conditions of the terminal weld lead to local temperature rise, change of the molten pool, increase of penetration depth, longer residence time of the molten pool at high temperature and slower solidification speed. When the size of the arc striking plate is too small, it is obvious that the positioning weld with the cylinder is too short.
(2) Influence of welding heat input
Because the welding heat input of submerged arc welding is greater than that of other welding methods, the penetration is large and covered by flux layer, so the molten pool is large, the solidification speed of molten pool and the cooling speed of weld are slow, resulting in coarse grains and serious segregation, which is easy to lead to thermal cracks
In addition, the transverse contraction of the weld is less than the opening of the gap, so that the transverse tension of the terminal is greater than that of other welding methods. It is obvious for the medium and thick plates with groove and the thin plates without groove.
(3) Other situations
If there is forced assembly, the impurities such as s and P in the base metal are too high and segregated, which will also lead to cracks.
(1) Appropriately increase the size of arc striking plate
Some people think that the arc striking plate only leads the arc pit during arc ending out of the weldment. In order to save steel, it is wrong to make the arc striking plate very small. The arc striking plate has four functions:
Lead the broken part of the weld and the arc pit to the outside of the weldment.
Strengthen the restraint degree of the terminal part of the longitudinal joint and bear large tensile stress.
Improving the temperature field at the terminal is conducive to heat conduction.
Improve the terminal magnetic field distribution and reduce magnetic bias blowing.
The arc striking plate shall have sufficient size and the thickness shall be the same as that of the weldment. The size depends on the size of the weldment and the thickness of the steel plate. For general pressure vessels, it is recommended that its length and width shall not be less than 140 mm.
(2)Attach importance to the assembly and tack welding of arc striking plate
The tack welding between the run on plate and the cylinder shall have sufficient length and thickness, which shall not be less than 80% of the width and thickness of the run on plate. It is required to be continuous welding. On both sides of the longitudinal seam, there shall be sufficient weld thickness for the medium and thick plate.
(3)Pay attention to the tack welding at the end of the cylinder
In order to increase the restraint degree of the longitudinal joint end, the length of the longitudinal joint end shall not be less than 100mm, the thickness shall be sufficient, and there shall be no cracks, incomplete fusion and other defects.
(4) Strictly control the welding heat input
Strictly control the welding heat input, which can not only ensure the mechanical properties of the welded joint, but also play an important role in preventing cracks. The submerged arc welding current has a great impact on the terminal cracks, and the welding current is related to the temperature field and welding heat input.