The pores produced by weld metal can be divided into internal pores, surface stomata and joint stomata.
1. internal stomata: there are two shapes. One is that spherical pores are mostly produced in the middle of the weld. The causes are:
(1) the welding current is too large;
(2) the arc is too long;
(3) the speed of the rod is too fast.
(4) the weld joint is unclean;
(5) the welding rod is dampened and so on.
The above causes the cause of porosity, such as proper adjustment and attention to welding process and operation method, can be solved.
2. sides of stomata: Causes and solutions to surface stomata.
(1) the parent materials contain high contents of C, S and Si. The solution is to replace the base metal or to use the low hydrogen slag system.
(2) welding parts are unclean and easily generate anger. Therefore, the welding part requires the removal of dirt, rust and other dirt before welding. The use of low hydrogen welding electrode is more stringent.
(3) the welding current is too large. Make the rear part of the electrode red, and easily generate anger. Therefore, a suitable welding specification is required. The maximum welding current is not suitable for the tail of the welding rod.
(4) low hydrogen electrode is easy to absorb moisture, so it is necessary to bake 1 hours at 350 degree temperature before use. Otherwise, the air blowholes are easily spread.
3. wave joint blowhole: the use of low hydrogen electrode often easily spread the surface and internal pores at the weld joint. The solution: when welding joint, the arc should start from the arc pit 9 to 10mm in the forward direction of the weld. After the arc burning, the reverse rod is first turned back to the arc pit position, so as to melt and move forward fully, or in the weld seam. This type of blowhole can be avoided by arcing.
1. rigid cracks: often found in welding seam longitudinal cracks, mainly caused by stress generated during welding. The stress of welding is very large in the following cases:
(1) the welded structure is rigid in rigidity.
(2) the welding current is big and the welding speed is fast.
(3) the cooling rate of the weld metal is too fast.
In this case, it is easy to produce longitudinal cracks. The solution is to use proper welding sequence or preheat the workpiece if possible, so as to reduce the rigidity of the structure. Low hydrogen welding rods should be used for extra thick plates and rigid structures with suitable current and welding speed.
2. cracks caused by sulphur: cracks easily occur when the carbon and sulphur content of the welded parent material is high or segregated. Solution: preheating the weldments or using low hydrogen electrodes.
3. gap cracks: the gap cracks occur inside the weld metal and do not develop into the outer capillary cracks. It is considered that the weld metal is subjected to rapid cooling and embrittlement, local stress and hydrogen. The way to prevent this is to slow down the cooling rate of the welding parts, to preheat the welded parts under the possible premise, or to use the low hydrogen electrode to be satisfied.
Three, arc produced partial blow
When the low hydrogen electrode is used to weld the DC welding machine, the phenomenon of partial blow often occurs. It can be solved by the following method.
The 1. line is placed in the direction of the arc blowing.
The 2. lines are divided into more than two.
3. the direction of the arc blowing is welded.
4. take short arc control.