With the rapid increase of railway transportation in China, the requirements for the design and production level of railway locomotives are getting higher and higher. Product upgrades have become a top priority for all railway rolling stock manufacturers. The design of steel structures for rolling stocks is increasingly using high-quality high-performance steel sheets. The surface treatment and coating processes of traditional steel structures have not been able to meet the requirements of production development. It is urgent to find a surface treatment that meets the design and process requirements. New Technology.
2 The main points of the surface treatment and coating process Before the steel structure is coated, in order to make the coating and the substrate have good bonding properties, it is necessary to pretreat the surface of the steel to be coated and the vicinity. First, the surface must be clean and free from dust, oil, water or other dirt and loose oxides. Secondly, when spraying, the combination of the coating and the substrate is generally mechanically combined. Therefore, the coating and the substrate should increase the contact area as much as possible without affecting the quality of the coating. Appropriate roughness helps to improve. Adhesion of the coating to extend the life of the coating. Relevant information: Alkyd paint is used for manual surface descaling. The service life of the coating is 2~3a, and the service life of sandblasting to Sa2.5 can be 10a. The quality of steel surface pretreatment directly affects the apparent quality of the coating engineering and the service life of the coating. The cleanliness and roughness of the surface treatment of the steel structure directly affect the service life of the coating. In order to increase the contact area between the coating and the substrate as much as possible, mechanical roughening and sandblasting coarsening are generally employed. Mechanical roughening includes shot peening, manual grinding, and polishing. Sandblasting roughening is to use dry and angular yellow sand or quartz sand with a diameter of 1~2mm. The compressed air is used to spray the sand from the nozzle of the sandblasting gun to the surface of the steel. The surface of the steel is affected by the impact and friction of the sand. The scale, the rust, the paint film, the grease, and the like are removed, and a process having a certain roughness surface is obtained. In general, the higher the grade of steel, the higher the surface treatment requirements. The surface of the sandblasted steel structure has a surface area of ​​about 90 times that of the surface of the steel which is not subjected to sandblasting, which greatly increases the contact area between the coating and the substrate. This has a great influence on the adhesion of some coating films, especially inorganic zinc-rich coatings.
3 Problems in the original process Previously, the method of surface treatment of steel (including plates and profiles) for steel structures of rolling stocks was to perform simple descaling, after cold and hot processing, to form a steel structure. The primer is then sprayed on the surface of the steel structure. This method has proven to be poor in corrosion resistance through field application of various models. Another method is to chemically treat the surface of the steel (usually phosphating), then directly spray the primer and then weld it into a steel structure. The disadvantage is that the problem that the primer and the phosphating film are not well matched is often caused, and the adhesion of the primer is lowered. In addition, in recent years, a process has been adopted. The steel is first sandblasted and then directly sprayed with a primer, which also has a problem of poor corrosion resistance and even affects the next process.
4 The determination of the new process plan is based on the problems of the original process, in order to meet the requirements of the process and design, and at the same time take into account the inheritance and continuity of the process. Based on the above theory, it is decided to adopt the sandblasting and pre-coating primer phase through experiments. Combined surface treatment and coating process.
4.1 Test materials
(1) Substrate model: Select the high-quality chromium-rich steel plate (N4003) commonly used in recent years, the specification is 150mm × 80mm × 1.5mm, a total of 24 pieces.
(2) Paint:
1 pre-primed primer: use epoxy ester anti-rust paint. Since the pretreated steel is welded into a steel structure, it needs to be welded, cut, polished, etc., and the surface of the paint layer inevitably has a lot of scratches and shedding defects, so the current steel structure of the rolling stock is commonly used. A magnet epoxy ester anticorrosive paint with high temperature resistance, low heat resistance and strong anticorrosive properties is used as a precoat primer for steel surfaces.
2 Primer: Phosphoric acid polyurethane primer is used. After the steel structure is welded and formed, in order to repair the surface defects, and also prepare for the next step of putty or topcoat, it is generally necessary to spray the steel structure once again. The primer is required to be compatible with the putty and topcoat used in the next process. After production verification, zinc phosphate polyurethane primer can fully meet this requirement.
4.2 Test process
4.2.1 Surface treatment of the sample The sample was divided into two groups, A and B, each with 12 pieces, of which group A was sandblasted, group B was not sandblasted, and only polished with 240# sand cloth. The surface of the two sets of samples was cleaned with an organic solvent, dried by a cold air, and placed in a desiccator.
2 The main points of the surface treatment and coating process Before the steel structure is coated, in order to make the coating and the substrate have good bonding properties, it is necessary to pretreat the surface of the steel to be coated and the vicinity. First, the surface must be clean and free from dust, oil, water or other dirt and loose oxides. Secondly, when spraying, the combination of the coating and the substrate is generally mechanically combined. Therefore, the coating and the substrate should increase the contact area as much as possible without affecting the quality of the coating. Appropriate roughness helps to improve. Adhesion of the coating to extend the life of the coating. Relevant information: Alkyd paint is used for manual surface descaling. The service life of the coating is 2~3a, and the service life of sandblasting to Sa2.5 can be 10a. The quality of steel surface pretreatment directly affects the apparent quality of the coating engineering and the service life of the coating. The cleanliness and roughness of the surface treatment of the steel structure directly affect the service life of the coating. In order to increase the contact area between the coating and the substrate as much as possible, mechanical roughening and sandblasting coarsening are generally employed. Mechanical roughening includes shot peening, manual grinding, and polishing. Sandblasting roughening is to use dry and angular yellow sand or quartz sand with a diameter of 1~2mm. The compressed air is used to spray the sand from the nozzle of the sandblasting gun to the surface of the steel. The surface of the steel is affected by the impact and friction of the sand. The scale, the rust, the paint film, the grease, and the like are removed, and a process having a certain roughness surface is obtained. In general, the higher the grade of steel, the higher the surface treatment requirements. The surface of the sandblasted steel structure has a surface area of ​​about 90 times that of the surface of the steel which is not subjected to sandblasting, which greatly increases the contact area between the coating and the substrate. This has a great influence on the adhesion of some coating films, especially inorganic zinc-rich coatings.
3 Problems in the original process Previously, the method of surface treatment of steel (including plates and profiles) for steel structures of rolling stocks was to perform simple descaling, after cold and hot processing, to form a steel structure. The primer is then sprayed on the surface of the steel structure. This method has proven to be poor in corrosion resistance through field application of various models. Another method is to chemically treat the surface of the steel (usually phosphating), then directly spray the primer and then weld it into a steel structure. The disadvantage is that the problem that the primer and the phosphating film are not well matched is often caused, and the adhesion of the primer is lowered. In addition, in recent years, a process has been adopted. The steel is first sandblasted and then directly sprayed with a primer, which also has a problem of poor corrosion resistance and even affects the next process.
4 The determination of the new process plan is based on the problems of the original process, in order to meet the requirements of the process and design, and at the same time take into account the inheritance and continuity of the process. Based on the above theory, it is decided to adopt the sandblasting and pre-coating primer phase through experiments. Combined surface treatment and coating process.
4.1 Test materials
(1) Substrate model: Select the high-quality chromium-rich steel plate (N4003) commonly used in recent years, the specification is 150mm × 80mm × 1.5mm, a total of 24 pieces.
(2) Paint:
1 pre-primed primer: use epoxy ester anti-rust paint. Since the pretreated steel is welded into a steel structure, it needs to be welded, cut, polished, etc., and the surface of the paint layer inevitably has a lot of scratches and shedding defects, so the current steel structure of the rolling stock is commonly used. A magnet epoxy ester anticorrosive paint with high temperature resistance, low heat resistance and strong anticorrosive properties is used as a precoat primer for steel surfaces.
2 Primer: Phosphoric acid polyurethane primer is used. After the steel structure is welded and formed, in order to repair the surface defects, and also prepare for the next step of putty or topcoat, it is generally necessary to spray the steel structure once again. The primer is required to be compatible with the putty and topcoat used in the next process. After production verification, zinc phosphate polyurethane primer can fully meet this requirement.
4.2 Test process
4.2.1 Surface treatment of the sample The sample was divided into two groups, A and B, each with 12 pieces, of which group A was sandblasted, group B was not sandblasted, and only polished with 240# sand cloth. The surface of the two sets of samples was cleaned with an organic solvent, dried by a cold air, and placed in a desiccator.
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