The road to rebirth of mechanical parts - thermal spraying process

       

For a mechanical part that has failed, how to use the thermal spraying process to restore, there are 3 basic steps:

1. Workpiece surface pretreatment



The degree of bonding between the coating and the base material (workpiece material) is the key. If it cannot be bonded or the bonding strength is not enough, the spray coating will peel off from the substrate and fail to meet the requirements of repairing parts. Therefore, in order to better combine the coating and the base material, the target workpiece to be repaired must first undergo surface pretreatment.



Surface preparation. Before spraying, turn or grind the surface of the workpiece, remove the original spray coating and original treatment layer (such as carburized layer, quenching layer) on the workpiece, remove various damages and burrs, correct the uneven wear on the surface, pre- Leave the thickness of the spray coating, etc.

Purification treatment. The purpose is to remove all the dirt on the surface of the workpiece, such as grease, paint, rust and scale and other dirt and so on. Among them, the most critical and representative ones are the grease and rust on the surface of the workpiece, so degreasing and derusting are the most basic pretreatment methods.

Coarse processing. The purpose is to increase the contact surface between the coating and the base material, increase the mechanical bite force of the coating and the base material, and improve the bonding strength of the coating and the base material. The roughening process has different methods for different workpieces, mainly including sandblasting, threading or knurling, electric brushing and so on.

Adhesive base. When spraying some coating materials that are not well bonded to the substrate, it is first necessary to select a material that is well bonded to the body material as a transition layer to strengthen the combination of the coating and the substrate material. Hebei Lingke has obtained a large amount of experimental data after many attempts and applications in this regard.

2. Warm up



The purpose of preheating is to reduce the temperature difference between the coating and the surface of the workpiece, thereby reducing the residual stress between the coating and the substrate due to the difference in thermal expansion, avoiding cracking of the coating, and improving the bonding strength of the coating and the substrate material. The temperature control of preheating depends on the size, shape and material of the workpiece, as well as the thermal expansion coefficient of the substrate and coating materials. This step is a very important process.



3. Spray



The operation of spraying is mainly to select the spraying method and determine the spraying parameters. Different coating functional requirements determine different spraying materials, different spraying materials determine different spraying processes, and different spraying environments determine different spraying methods. For example, plasma spraying is the best choice for spraying ceramic coatings; high-speed flame spraying can be used for spraying carbide cermet coatings; and flame spraying can only be used for spraying plastics. If large-area anti-corrosion projects are sprayed outdoors, flexible and efficient arc spraying or wire flame spraying can be used;  … Therefore, in order to obtain high-quality coatings, it is necessary to select suitable process methods for different purposes and materials. Hebei Lingke has accumulated rich experience in the selection of technology and materials.



4. Treatment after spraying



After the coating is sprayed, most of them cannot be used directly, and various follow-up treatments are required. For example, for the coating of the anti-corrosion layer, it is necessary to carry out sealing treatment, because from a microscopic point of view, the molten particles of the spray material will inevitably form small holes when they adhere to the substrate, and these small holes will allow the corrosive medium to penetrate into the base layer. , so it must be sealed after spraying. According to the processing experience of Hebei Lingke, after adjusting the process and performing multiple multi-layer spraying, the small holes connected to the substrate can basically be eliminated, but in order to ensure the quality to the greatest extent, the hole sealing treatment is still carried out. In addition, for some coatings that are subjected to impact wear or high stress loads, remelting treatment is required to change the bonding with the substrate from mechanical bonding to general metallurgical bonding, thereby improving the bonding strength of the coating. For parts with precision requirements for dimensional repair, machining is required after spraying.