Briefly describe the defects that often occur in welding and their Prevent Methods
Stoma
The pores produced by the weld metal can be divided into internal pores, surface pores, and joint pores.
(1) Internal pores:
There are two shapes. One is that most of the spherical pores are produced in the middle of the weld. Causes: excessive welding current; too long arc; too fast-moving rods; unclean welding parts; damp welding rods, etc.
The above causes of porosity can be resolved if proper adjustments are made and attention to the welding process and operation method is performed.
(2) Surface pores:
Causes and solutions for surface pores:
A) The base material contains high amounts of C, S, and Si and is prone to pores. The solution is either to replace the base material or to use a low-hydrogen slag-based electrode.
B) The welding part is not clean and it is easy to produce pores. Therefore, the welding parts must be cleaned of oil, rust, and other dirt before welding. The requirements are more stringent when using low-hydrogen electrodes for welding.
C) The welding current is too large. The coating on the back half of the electrode turns red, and it is also prone to pores. Therefore, appropriate welding specifications are required. It is advisable to maximize the welding current so that the tail of the electrode is not red.
D) Low-hydrogen electrodes are easy to absorb moisture, so they need to be baked at 350℃ for about 1h before use. Otherwise, pores are prone to appear.
(3) Welding wave joint porosity: The use of low-hydrogen welding rods often tends to appear surface and internal porosity at the weld joint. The solution: when welding wave joints, start to draw at 9-10mm away from the arc crater in the forward direction of the weld. Arc, after the arc burns, first move the rod backward to the crater position, make full melting and then move forward, or start the arc at the welding seam to avoid this type of pores.
The Crack
(1) Rigid Cracks:
Longitudinal cracks throughout the weld are often found during welding, which is mainly caused by the stress generated during welding. Under the following conditions, the welding stress is very high: the welded structure is rigid; the welding current is large, and the welding speed is high; the cooling rate of the weld metal is too fast.
Therefore, it is easy to produce longitudinal long cracks in the above-mentioned situation. Solution: Use a reasonable welding sequence or preheat the workpiece if possible to reduce the rigidity of the structure. Extra-thick plates and highly rigid structures should use low-hydrogen electrodes with appropriate current and welding speed.
(2) Cracks caused by carbon and sulfur elements:
When the carbon and sulfur of the base metal to be welded are high or the segregation is large, it is easy to produce cracks. Solution: preheat the weldment, or use a low-hydrogen electrode.
(3) Fuzzy cracks:
Fuzzy cracks are hairy micro-cracks that occur inside the weld metal and do not develop to the outside. It is considered that the weld metal is embrittled by rapid cooling, and local stress and the influence of hydrogen are generated. The way to prevent this is to slow down the cooling rate of the weldment, preheat the weldment if possible, or use a low-hydrogen electrode to achieve a satisfactory solution.
The arc produces a partial blow
The partial blow phenomenon often occurs when using low-hydrogen electrodes to weld on a DC welding machine. The following methods can be used to solve the problem: the ground wire is placed in the direction of the arc blow; the ground wire is divided into two or more; the welding is carried out along the direction of the arc blow, and the short arc operation is adopted.