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Blasting knowledge of metal and steel structures


In order to make the metal surface have good cleanliness and roughness, it is necessary to perform pretreatment such as rust removal on the surface of the metal structure. There is such a sentence in the anti-corrosion field as "seven points of rust removal, three points of coating", showing the importance of rust removal. In view of the particularity of the hydraulic metal structure, such as large volume, and the construction environment is basically open-air, etc., sandblasting and rust removal methods are still used at this stage. Sandblasting and rust removal control is mainly reflected in the following aspects.

Early control

Before sand blasting, the surface corrosion level of the metal structure substrate was evaluated in accordance with the "Surface Rust Level and Derusting Level of Steel before Painting". Carefully check to remove the attachments such as welding slag and splashes, and clean the surface grease and soluble dirt. Useless welding bodies or joints should also be properly treated.

2. Abrasive control

The sand used for blasting and rust removal requires particles that are hard, angular, dry (water content <2%), free of soil and other impurities; quartz sand is preferred, and coarse river sand is also available. At the same time, sand transportation conditions should be considered , Unit price, recycling capacity and other factors; the particle size of the sand is preferably 0.5 ~ 1.5mm, it must be dried before screening, stored in the shed, indoor, the size of the sieve is: coarse sieve 40 ~ 48 holes / cm2 (particle size 1.2mm), fine sieve 372 ~ 476 holes / cm2 (particle diameter 0.3mm).

After the rust removal plan is determined, the abrasive can be river sand, quartz sand, or a mixture of the two. In the Gangnan Reservoir's danger-removing and strengthening, after pre-spraying comparison, it is best to mix quartz sand and river sand 1: 1 to remove rust.

3. Tool control

During the blasting operation, the air pressure of the air compressor is 6.0 × 105 to 6.5 × 105Pa, and the air pressure variation range is 0.5 × 105 to 1.0 × 105Pa. Generally, the pressure of the sand bucket is 4.5 × 105 ~ 5.5 × 105Pa, and it must not be less than 4.0 × 105Pa. Before blasting, check whether the pressure vessel manufacturer holds the production license issued by the relevant department, whether the protective tools, safety belts (ropes) and oxygen supply devices worn by the blaster are safe and reliable, and the wear of the nozzle. The nozzle should be replaced when the diameter is increased by 25%.

4. Environmental control

In view of the fact that sandblasting and rust removal of hydraulic metal structures is an open-air operation, in addition to dust prevention and environmental protection during construction, the ambient temperature, humidity and metal surface temperature of the construction site must be tested daily, the dew point of the day should be calculated, and construction records should be made. Do not apply when the relative humidity of the air is greater than 85% and the temperature of the metal surface is 3 ° C above the dew point.

5. Process control

The compressed air used for sand blasting must be processed by a cooling device and an oil-water separator to ensure that it is dry and oil-free; the oil-water separator must be cleaned regularly.

The distance from the nozzle to the surface of the base steel is preferably 100-300mm. Before the blasting, the non-blasting parts should be shielded and protected.

The included angle between the spray direction and the surface normal of the base steel is preferably 15 ° to 30 °.

After sandblasting and rust removal, before the next process, if it is raining or other conditions that cause the surface of the base steel to be wet, after the environment has reached the construction conditions, dry the surface with dry compressed air to dry the surface. Sand blasting can not reduce the abrasive requirements, so as not to reduce the roughness.

During the blasting, the nozzle should not stay at a certain place for a long time. The blasting operation should avoid sporadic operations, but the area of the blasting should not be too large at one time. The time interval between the coating or thermal spraying process and the surface pretreatment process should be considered. Manual or power tools should be used to remove rust on the parts that the spray gun cannot spray.

6. Quality control

After the sand blasting is completed, the sand blasting and rust removal parts should be thoroughly inspected first, and then the cleanliness and roughness of the surface of the base steel should be checked. The focus should be on the parts that are not easy to spray, and the rust removal parts of manual or power tools can reduce the requirements appropriately. When checking the cleanliness and roughness of the surface of the base steel, one is forbidden to touch it by hand; the other is to perform it under good scattered sunlight or artificial lighting with comparable illuminance to avoid missed inspections.

After sandblasting and rust removal, the cleanliness of the surface of the metal structure should reach Sa21 / 2, and comparison should be made with the photos provided in "Steel Surface Corrosion Level and Derusting Level before Painting", and there should be at least one comparison point every 2m2.

After sandblasting and rust removal, the surface roughness of the metal structure should reach Ry60 ~ 100μm. According to the technical requirements, the corresponding roughness sample should be taken and compared with a magnifying glass of at least 7 times, and there should be at least one comparison point every 2m2. When measuring directly with a surface roughness meter, there must be at least one evaluation point for every 2m2 surface. The evaluation length is 40mm. Five points are measured within this length range, and the arithmetic average value is used as the surface roughness value of the evaluation point.

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