The process of repairing and strengthening the screw is as follows:

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

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

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

4. Trimming and straightening
Ni60 self-fluxing alloy spray welding has good 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 to the screw axis, etc., the screw must be trimmed after welding. The dressing method is to process a mandrel with a center hole and press it into the bearing hole of the screw head. One end has three claws for tightening, one end is centered, and it is processed by turning or grinding, and at the same time, it can eliminate the bending of the screw caused by spray welding. YMO53 is used for cutting tools. The hardness of the welded layer of the screw after spray welding repaired with Ni60 self-fluxing alloy powder can reach HRC60, and the thickness of the welded layer is 2-3mm. Not only the worn out screw can be repaired, but also the screw can be strengthened, and the service life of the screw can be prolonged. There may be slight diffusion between the coating and the roll neck to enhance the bonding strength between the coating and the roll neck. The bonding between sprayed particles is mainly mechanical, and the diffusion bonding, metallurgical bonding, crystal epitaxy, and physical bonding between coatings are supplemented.

5. Workpiece preheating before spraying
The preheating of the workpiece before spraying has great practical significance. It can remove the surface moisture 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. Spray aluminum-clad nickel composite powder F505 as the bonding layer on the surface of the processed journal. The thickness of the bonding layer is 0.10-0.15mm. This layer is only used for bonding, so there is no need 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℃ before use to ensure the fluidity of the powder spray and avoid agglomeration and block the spray channel. In 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. Spray distance 180-200mm; spray angle 90°; spray gun moving speed 6-12mm/min; roller linear speed 8-12m/min.
A neutral flame is used when spraying the working layer, and the powder can be observed from the flame, and the powder is white and bright at the end of the flame, which means that the temperature is higher. If it is dark red or red, the temperature is not enough. At this time, the powder feeding airflow size must be adjusted. And the flame energy rate, so that the powder is heated 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 with multiple layers to make it cool slowly to avoid cracks. When it is cooled to room temperature, rough turning can be carried out, leaving a grinding allowance (about 0.3-0.5mm) for grinding. Grind according to the size required by the drawing and the specified roughness. This completes the repair of the roll. The roll repair cost is about 10,000 yuan, which can save more than 100,000 yuan in the factory (the finished roll is 30,000 yuan per ton, and each roll weighs 3 tons).