The welding repair process is suitable for repairingdefects such as pores, sand holes, slag inclusions, cracks (cold and hot cracks) and looseness of various large, medium and small cast steel gears.
1 Defect type of steel casting
There are two kinds of defects in steel castings: the first is the obvious defect, and the arc can directly affect the surface of the whole defect during welding; the other is the dark defect, which can only be repaired by multi-layer and multi-layer overlay welding on the local defect during welding.
2 Requirements before welding repair
1) The casting defects shall be removed until good metal is formed. Meanwhile, the groove shall not have sharp shape, smooth transition, and shall be free of rust, oil, moisture, etc., so as to avoid air holes. After the removal of important casting defects, magnetic particle or dye detection shall be carried out to confirm whether they are removed completely.
2) For cracks caused by shrinkage stress, a crack stopping hole shall be drilled at both ends of the crack before welding to prevent crack propagation, and a groove of 70 ° ~ 80 ° shall be opened on the crack surface.
3) For important steel castings with strength requirements, it shall be ensured that the mechanical properties at the weld joint are not lower than the casting matrix.
3 Welding repair process requirements for steel casting defects
1) Hot welding
Preheating the defects of steel castings can eliminate the hardening layer and internal stress produced during welding repair. Because the weld quality is related to the quality of the transition layer, and the quality of the transition layer is determined by the carbon content of steel castings and the speed of cooling. At the same time, in order to prevent cracks, when welding large and medium-sized steel castings with carbon content more than 0.3%, local or overall preheating (150-250 ℃) must be carried out. The matching welding rod (the welding rod shall be dried according to the regulations of the factory) shall be used to fill the defects according to the method of surfacing process.
2) Cold welding method
When the ambient temperature is higher than 5 ℃, the steel castings with carbon content of 0.2% – 0.3%, wall thickness of less than 20 mm, small shape and simple shape can be welded without preheating.
(3) Chemical composition and impact test
The results of chemical composition analysis showed that only the trace of phosphorus exceeded the standard. The average value of normal temperature impact test is AK = 23.5j, within the normal range.
4 Welding repair of cracks
Before welding repair, the crack area shall be inspected thoroughly, and then the crack shall be repaired to a certain groove form, and the groove bottom shall not be in sharp angle shape. In order to avoid the crack spreading by vibration in the process of chipping, a crack stop hole with a diameter of 8-10 mm should be drilled at both ends of the crack before chipping. Magnetic particle or dye flaw detection shall be carried out to confirm whether the cracks of important castings are removed. J507 welding rod is generally used to repair the cracks, so that the strength and plasticity of the weld after welding can meet the requirements. In the process of welding, attention should be paid to the welding sequence. The preheating area should be 2-3 times larger than the defect area. At the same time, strike the weld properly to eliminate local stress and prevent new cracks.
5 Inspection and acceptance during and after repair welding of steel casting defects
1) Preparation of groove at defect repair;
2) Confirmation of steel casting material and electrode model;
3) The welding repair process shall comply with the process specification;
4) After welding, the appearance quality shall be inspected with a 5-power magnifying glass, and the weld beads shall be arranged in order, and there shall be no welding repair defects such as cracks, pores, slag inclusions, undercuts, etc;
5) For steel castings with hardness requirements, the weld joint must be hardened;
6) For steel castings of special importance or complex shape, the relevant departments shall determine after welding, and magnetic particle or ultrasonic flaw detection shall be carried out 24 hours later.