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1. Surface cleaning
Before the screw spray welding, the remaining plastic attachments on the screw surface should be cleaned up. The cleaning method is to install the screw on the sprayer and rotate it, bake it with a neutral flame of a spray gun, and remove it with a spatula after plasticization. Then use a sandblasting gun to further remove the oxide layer on the welded surface.

2. Warm up
When preheating, use a spraying machine or a vibration surfacing machine to clamp the screw head (transmission part). The speed of the machine tool is adjusted to below the minimum speed of 15r/nin, and the whole screw is preheated to about 270 ℃ with a neutral flame, the muzzle to the screw is about 100mm, so as to reduce the thermal stress of the shaft and ensure the quality of the spray welding layer.

3. Spray welding
A one-step spray welding process is used, using SPH-1/h type spray gun, No. 2 nozzle, oxygen pressure 0.25MPa, acetylene pressure 0.06MPa.
During the rotation, spray welding to repair the worn parts of the screw, and the maximum wear of the screw is about 2-3mm. It is necessary to spray enough size in one operation, and consider the machining allowance. If one spray is not enough, the next repair welding should pay attention to the preheating temperature and preheating uniformity to prevent cracks in the welding layer.

4. Trimming and straightening
Ni60 self-fluxing alloy spray welding has better surface quality. Due to the technical requirements of the screw, that is, the straightness, the runout of the thread surface and the thread working side against the screw axis, etc., the screw should be trimmed after welding. The trimming method is to process a mandrel with a center hole and press it into the bearing hole of the screw head. One end is tightened with three claws, the other end is centered, and it is processed by turning or grinding, and at the same time, it can eliminate the screw bending caused by spray welding. The cutting tool is YMO53. The hardness of the screw weld layer after spray welding repaired with Ni60 self-fluxing alloy powder can reach HRC60, and the thickness of the weld layer is 2-3mm. It not only repairs the worn and failed screw, but also strengthens the screw and prolongs the service life of the screw. There may be a slight diffusion between the coating and the roll neck, which enhances the bonding strength of the coating and the roll neck. The bonding between sprayed particles is mainly mechanical bonding, supplemented by diffusion bonding, metallurgical bonding, crystal epitaxy and physical bonding between coatings.

5. Workpiece preheating before spraying
The preheating of the workpiece before spraying has great practical significance. It can remove the moisture on the surface and reduce the stress accumulation of the coating. Especially for the shaft parts of the size of our factory, the preheating temperature is about 100°C. The aluminum-coated nickel composite powder F505 is sprayed on the surface of the treated journal as a bonding layer. The thickness of the bonding layer is 0.10-0.15mm. This layer is only used for bonding, so it is not necessary to spray too thick. After spraying the bonding layer, spray the working layer of G101 powder. The G101 powder must be preheated and baked at a temperature of 105-250 °C before use to ensure the fluidity of powder spraying and avoid clumping and blocking the spray channel. During the spraying process, the gas pressure, flame temperature, spraying distance, spraying angle, and moving speed are strictly controlled to make the coating uniform and smooth, and the bonding is firm. The specific control parameters are as follows: the coating position is the outer arc surface; the preheating temperature is 100-200°C. The spray distance is 180-200mm; the spray angle is 90°; the moving speed of the spray gun is 6-12mm/min; the linear speed of the roller is 8-12m/min.
When spraying the working layer, a neutral flame is used. The powder can be observed from the flame. At the end of the flame, the powder is white and bright, which indicates that the temperature is high. If it is dark red or red, the temperature is not enough. At this time, the size of the powder feeding airflow should be adjusted. And the flame energy rate, it is advisable to heat the powder to white and bright. At this time, the powder temperature is higher, the speed is faster, and the deposition effect is good.

6. Metal processing after spraying
After spraying, use asbestos to wrap the sprayed part in multiple layers to make it cool slowly to avoid cracks. After cooling to room temperature, rough turning can be carried out, leaving a grinding allowance (about 0.3-0.5mm) before grinding. Grinding shall be done according to the size and roughness required by the drawing. At this point, the repair of the roll is completed. The repair cost of the roll is about 10,000 yuan, which can save more than 100,000 yuan for the factory (the finished roll is 30,000 yuan per ton, and each roll weighs 3 tons).
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