1. Importance of surface pretreatment
The purpose of surface pretreatment is to make the surface to be coated reach the rust removal quality and roughness required by the selected coating, and ensure good adhesion between the surface to be coated and the covering layer. The method and index of surface pretreatment are determined by the type of covering layer. The implementation department of surface pretreatment must have relevant equipment and technical operators, and all surface pretreatments should have special technical supervision and inspection. To correctly understand surface pretreatment, we should first have a complete understanding of the factors affecting its process. The thick vertical arrow connects the spraying object with the spraying purpose. The arrows connected to the left and right of the thick arrow indicate the factors that come into play to achieve the expected purpose. The spraying method, abrasive, and carrier for conveying abrasive are selected according to the characteristics, type, and size of the workpiece to be sprayed, as well as the expected purpose after spraying. Since there are many influencing factors involved, it should be very cautious. In the construction of pipeline anti-corrosion layer, there is a saying that “30% material and 70% construction”, which shows the importance of construction. In the construction process, the surface pretreatment of steel pipes (the most basic is “rust removal”) is the top priority, and its quality is directly related to the quality and life of the covering layer. There are such statistics in some literature, indicating that surface treatment is the most important factor among many factors affecting the life of the covering layer.
Through the cost analysis of the covering layer, the cost of surface treatment generally accounts for about 50%. The covering layer of the drag reduction inner coating has a thin film layer, a small number of coatings, and a small amount of paint, so the cost of surface treatment is higher, about 70%. Therefore, in the process design and construction of the drag reduction inner coating, special attention should be paid to the quality of surface pretreatment.
2. Main factors affecting the quality of the covering layer
2.1 The influence of oxide scale: Under the high-temperature conditions of rolling and welding, a layer of oxide scale is naturally generated on the surface of the steel pipe. Its main component is a mixture of iron oxides. From a structural point of view, it is roughly three layers, the outermost layer is Fe3O4 or Fe2O3, the middle layer is FcO and Fe3O4, and the one close to the steel surface is FeO. Under the influence of external environmental conditions, such as temperature, humidity, external force, oxygen, and salt, these oxide scales will crack, peel off and loosen. If they are not removed completely, they will have three major destructive effects on the covering layer: first, the electrode potential of the oxide scale is 0.26V more positive than that of steel, making the steel surface exposed at the place where the oxide scale falls off and cracks become the anode of the galvanic cell and suffer corrosion; second, water vapor is easily condensed in the cracks of the oxide scale. If SO2 is dissolved in it, ferrous sulfate can be generated, which increases the conductivity of the electrolyte and promotes corrosion; second, the oxide scale that has not been removed but has become loose may fall off and bulge completely when the temperature of the pipeline fluctuates greatly, causing the covering layer to crack and peel off.
2.2 The influence of surface dirt: The dirt mentioned here refers to rust products and dust that have not been completely removed from the surface of the steel pipe. It should also include the residual particles that have not been cleaned on the surface of the steel pipe after surface treatment and the new rust that has not been coated within the specified time after surface treatment. Due to their existence, it is difficult to obtain a smooth and uniform coating, weakening its adhesion to the substrate, so that the coating cannot directly contact the steel surface, resulting in reduced adhesion of the coating and affecting the service life of the coating.
2.3 The influence of soluble salts: When there are soluble salts on the steel surface under the coating, due to the different osmotic pressures inside and outside the coating, the moisture in the air will penetrate the coating to reach the surface of the steel, and combine with the soluble salts to cause corrosion on the steel surface and peel off the coating. Among them, chloride is the most important soluble salt. Because of its strongest penetration ability, it is stipulated in the Q/SYXQ11 “Supplementary Technical Conditions for Drag Reduction Coating Layer on the Inner Wall of West-East Gas Pipeline” standard, especially for steel pipes shipped by sea and steel pipes stored in coastal areas for some time, this point should be emphasized.
2.4 Effect of roughness: The adhesion between the coating and the steel pipe surface is determined by the mutual attraction between the polar groups in the coating molecules and the metal surface molecules. In addition to physical effects (dispersion force, induction force, and orientation force), it is mainly mechanical. After the steel pipe surface is treated with abrasive spraying (blasting), the surface roughness increases significantly, and the metal surface area can even increase by 20 times. With the increase of roughness, the surface area increases significantly, and the adhesion between the coating and the steel pipe surface increases accordingly. When the abrasive sprayed (blasted) has edges and corners, the metal surface treated with it not only increases the surface area but also provides a suitable surface geometry for the adhesion of the coating, which is conducive to molecular attraction and mechanical anchoring.
However, unreasonable surface roughness will also hurt the coating. For example, if the roughness is too large, the amount of coating required to fill the “trough” of the anchor pattern will also increase. Too deep troughs are also prone to cause bubbles, which directly affect the quality of the coating. In addition, when the coating is thin, the tip of the crest is easy to expose the surface, destroying the integrity of the coating and causing pitting corrosion.
For the drag-reducing inner coating, the surface roughness of the inner wall of the steel pipe should be required, usually 30 to 50 μm after surface treatment. The surface roughness depends on the process parameters such as the particle size, shape, material, spraying speed, and action time of the abrasive, among which the particle size of the abrasive has the greatest impact on the roughness.
There are many methods for surface treatment. The most reasonable one for pipelines is the commonly used spraying (projectile) method. This is because the violent impact of the abrasive can increase the fatigue strength of the material by about 80%; the surface hardness is also improved to varying degrees; it can also eliminate the internal stress at the weld so that the corrosion resistance of the steel is significantly improved.
3. Basic requirements for plug surface treatment: The surface treatment of steel pipes usually follows technical standards. Industrially developed countries have successively formulated their quality grade standards for rust removal. The most famous of these is the Swedish industrial standard SIS 055900 “Standard for Surface Rust Removal of Steel Materials Before Painting”, which has long been adopted by countries around the world. The International Organization for Standardization has formulated ISO 8501-1 “Pretreatment of Steel Materials Before Painting with Coatings and Related Products – Visual Assessment of Surface Cleanliness – Part 1: Rust Grade and Rust Removal Grade of Uncoated Steel Materials and Steel Materials After Complete Removal of the Original Coating” by the Swedish standard. My country has also formulated GB 8923 “Rust Grade and Rust Removal Grade of Steel Materials Before Painting” concerning ISO standards. The petroleum industry has also formulated SY/T 0407 “Specifications for Surface Pretreatment of Steel Materials Before Painting” to be used in conjunction with GB 8923. Here are some excerpts from the key points in the standard.
3.1 GB 8923 “Rust Grade and Rust Removal Grade of Steel Surfaces Before Painting”: GB 8923 “Rust Grade and Rust Removal Grade of Steel Surfaces Before Painting” focuses on the classification of rust grades and rust removal grades, visual assessment, and the use of color photos of standard samples.
(1) Rust grade Before rust removal, the original rust state of the steel surface is divided into four grades, represented by A, B, C, and D. After rust removal, it should be compared with the original rust grade:
A Steel surface that is completely covered with oxide scale and almost no rust;
B Steel surface that has rusted and part of the oxide scale has peeled off;
C Steel surface that has peeled off the oxide scale due to rust, or can be scraped off, and has a small amount of pitting;
D Steel surface that has completely peeled off the oxide scale due to rust and has widespread pitting. (2) Rust removal level GB 8923 “Rust level and rust removal level of steel surfaces before painting” distinguishes rust removal levels according to different rust removal methods, and then gives different levels according to different methods. “Sa”, “St” and “Fl” represent spray (blast) rust removal, manual and power tool rust removal, and flame rust removal respectively. The Arabic numerals after the letters indicate the degree of rust removal. ① Spray or blast rust removal is represented by “Sa” and is divided into four levels, which are described as follows. Sa1 Mild spray or blast rust removal: There should be no visible grease and dirt on the steel surface, and no loosely attached oxide scale, rust, and coating. Sa2 Thorough spray or blast rust removal: There should be no visible grease and dirt on the steel surface, and the oxide scale, rust, and coating have been basically removed, and the residue should be firmly attached. Sa2.5 Very thorough blasting and blasting rust removal: There should be no visible grease, dirt, oxide scale, rust, or coating on the steel surface, and any remaining traces should be only slight spots or stripes. Sa3 Spraying or blasting rust removal that makes the steel surface clean: There should be no visible grease, dirt, oxide scale, rust, or coating on the steel surface, and the surface should show a uniform metallic color.
② Manual and power tool rust removal Indicated by “St”, GB 8923 gives two levels, namely:
St2 Thorough manual and power tool rust removal: There should be no visible grease and dirt on the steel surface, and no loosely attached oxide scale, rust, and coating.
St3 Thorough manual and power tool rust removal: There should be no visible grease and dirt on the steel surface, and no loosely attached oxide scale, rust, and coating. Rust removal should be more thorough than St2, and the surface of the exposed part of the substrate should have a metallic luster.
③ Flame rust removal Indicated by “F1″, flame rust removal should include the use of a powered wire brush to remove the products attached to the steel surface after the flame heating operation. The standard only gives one grade:
F1 Flame rust removal: The steel surface should be free of oxide scale, rust, coating, and other attachments, and any remaining traces should only be surface discoloration (shadows of different colors).
(3) Evaluation of rust grade and rust removal grade The evaluation method and requirements of visual evaluation and standard photos are given in GB 8923. When evaluating the rust grade, the rust grade indicated in the photo of the corresponding more serious rust grade is used as the evaluation result; when evaluating the rust removal grade, the rust removal grade indicated in the photo that is closest to the steel surface appearance is used as the evaluation result. Many factors affect the visual assessment of the rust removal grade of steel surfaces, including the following: ① Abrasives used for spraying or blasting rust removal, and tools used for manual and power tool rust removal; ② Rust state of steel surfaces that do not belong to the standard rust grade; ③ The color of the steel itself; ④ Differences in the roughness of various parts due to different degrees of corrosion; ⑤ Uneven surface, such as depressions; ⑥ Tool scratches; ⑦ Uneven lighting; ⑧ Shadows caused by different angles of abrasive impacting the surface during spraying or blasting rust removal; ⑨ Abrasives embedded in the surface.
3.2 SY/T 0407 “Specifications for Surface Preparation of Steel Materials Before Painting”: This specification requires use in conjunction with GB 8923, and most of its content is written about the American Steel Structure Painting Committee SSPC standard. Combined with the relevant contents in the pipeline drag reduction process requirements, a brief introduction is as follows:
(1) Surface treatment before and after blasting (blasting) rust removal Before blasting (blasting) rust removal, remove the visible oil, grease, and dirt on the steel surface. After rust removal and before painting, use dry, sleeveless air blowing, vacuum cleaner suction, or brushing to remove the floating rust and dust on the workpiece surface. The steel surface after blasting (blasting) rust removal should be painted before it is contaminated. If the steel surface is contaminated before painting, it should be cleaned again.
(2) Selection of abrasives According to the results of the spray test, zirconium sand, and wire grains are the best abrasives, corundum is the worst, and cast iron crushed grains and two types of fused corundum are in between. The surface rust removal effect of corundum is very slow and poor, and it produces very intense dust flying. Wire grains are particularly suitable for rust removal of delicate cross-sections, and sand also has a good rust removal effect, but both will produce dust. For fused corundum, the abrasive delivery volume is almost only half of that of zircon sand, cast iron crushed grains, and wire grains. For the same rust removal work, the volume of iron abrasives required is 2 to 3 times less than that of mineral materials, that is, heavy particles have a better rust removal effect than light particles. The spraying time required for a certain rust removal effect is related to the selected abrasive. The rust removal effect per unit time decreases in the following order: sand, zircon sand, cast iron crushed grains, 0.65 wire grains, 0.97 wire grains, 0.72 fused corundum, 0.75 fused corundum, and corundum. In actual operation, 0.65 wire grains remove rust faster than 0.97 wire grains. Abrasives should be selected according to the steel grade, type, original rust degree, type of coating used, rust removal method, and surface roughness required for coating. Metal abrasives such as cast steel shot, cast iron shot, cast steel sand, cast iron sand, and steel wire segments can be used for spray (throwing) rust removal. According to the requirements of the coating system for the anchor depth on the steel surface, refer to Table 5-2 to select abrasives. Note that the hardness of the steel shot in the table is HRC 40-50, and the hardness of the steel sand is HRC55-60. The typical anchor depth in the table is the maximum and average surface roughness expected to be achieved under good spray (projectile) conditions (impeller or nozzle). The standard appendix gives the specifications, composition, hardness, and other performance requirements of the steel wire segment. In surface treatment, adding a certain amount of steel wire segments to the abrasive can produce sharp roughness “peaks and valleys”, which is very beneficial to increase the mechanical adhesion between the coating film and the steel surface. The consumption of abrasives is determined by the abrasive life, which is a difficult concept to define. It is usually based on the fragmentation of the abrasive. In engineering, “usable times” is used to indicate its lifespan, which determines the relative cost.
3.3 GB/T13288 “Evaluation of the surface roughness level of steel before painting”: This standard applies to steel surfaces whose rust removal level after metal or non-metal abrasive blasting and blasting is higher than Sa2.5 in GB 8923 “Rust level and rust removal level of steel surface before painting”. The surface roughness formed after abrasive blasting and blasting before painting is divided into three roughness levels.
The influence of roughness parameters on the covering layer depends on the following factors:
①Increase the surface area, improve the adhesion of the coating, and enhance the activation state of the surface;
②Affect the amount of coating;
③Affect the protective effect of the covering layer and the exposure of the peak.
The size of the roughness depends on the following factors:
①The type and specification of the abrasive;
②The spraying speed and angle of the abrasive;
③The flow rate and action time of the sprayed abrasive;
④The type, hardness, and surface structure of the workpiece itself.
3.4 Testing standards for soluble chloride: ISO 8502-2 “Laboratory determination of chloride on clean surfaces” standard specifies a test method for soluble chloride on steel surfaces. This method is to first clean a certain area of the steel surface, and then use the mercuric nitrate titration method with diphenylcarbazone-bromophenol blue as an indicator to analyze and determine the chloride collected in the cleaned steel. In addition, related standards include ISO 8502-5 “Detection of chloride on the surface of the steel to be painted – Chloride ion detection tube method”, ISO 8502-6 “Sampling method for soluble impurities on surfaces to be painted” and ISO 8502-7 “Analysis of soluble impurities on surfaces to be painted – Chloride ion field analysis method”.
4. Pretreatment of the inner surface of steel pipes
To ensure the quality and service life of the inner coating, the coating surface must be thoroughly pretreated before coating. Compared with the anti-corrosion coating, the drag reduction inner coating is thinner, so the surface roughness should belong to the fine (F) grade. According to the requirements of the Q/SY xQ11 standard, the rust removal grade is Sa2.5 and the roughness should be 30-50μm.
Among the several methods of surface treatment, spraying (blasting) on the inner wall of the pipeline is the most suitable. The specific selection should be based on the pipe diameter and equipment conditions. Shot blasting can be used for large-diameter pipes, and sandblasting can be used for small-diameter pipes (such as below 762mm). The Netherlands Metal Research Institute has conducted a special study on spraying rust removal and believes that spraying rust removal can be regarded as a kind of abrasion effect that is expected to be achieved by erosion. The following points are proposed for spraying rust removal technology.
(1) The velocity of the sprayed particles is decisive for the kinetic energy of the particles and is greatly affected by the rebound particles. The particle velocity is a function of the spray distance. (2) The jet angle determines the degree of collision of particles during spraying, which is maximum when the jet angle is 45°. (3) The size of particles is extremely important for the uniformity of rust removal. To achieve the intended purpose, there must be an optimal particle size. The particle size depends largely on the properties of the surface layer (rolling scale, rust, or casting crust) and the surface condition underneath.
4.1 Shot blasting: Shot blasting is the process of using the centrifugal force generated by the high-speed rotation of the blades of the shot blasting machine to shoot abrasives (steel shots, steel wire segments, angular steel sand, etc.) at a very high linear speed onto the inner wall surface of the treated pipe, producing a knocking and grinding effect, removing the surface scale and rust, exposing the surface to the original metal color, and providing a roughness that has anchoring ability for the paint. Shot blasting can not only remove the scale and rust on the surface of the steel pipe but also strengthen the surface of the steel pipe, eliminate residual stress, and improve fatigue resistance and stress corrosion resistance. Shot blasting has high abrasive utilization, fast rust removal speed, and low cost, and is suitable for large-scale operations. Therefore, shot blasting is the first choice for the inner surface treatment of steel pipes. The requirements of the shot blasting process are: preheating of steel pipes, shot blasting rust removal, and surface cleaning.
(1) Preheating of steel pipes: Preheating is to heat the inner surface of the pipe to remove moisture and some oil on the surface. The preheating methods include medium-frequency induction heating, flame heating, and hot water spray heating. When selecting a method, it should be adapted to local conditions, economical and reasonable, and compatible with the assembly line.
① Medium frequency heating has a simple structure. The induction coil is installed on the roller, which does not take up space and consumes less energy. However, medium-frequency heating is not very effective in removing oil and garbage on the surface.
② Flame heating is to burn clean liquefied gas and directly heat the inner surface of the steel pipe with flames, which can burn off the moisture on the surface. The premise of this method is that there must be sufficient liquefied gas supply.
③Hot water spray heating is effective in removing oil and garbage, but the equipment is complex and requires a steam source, a hot water pump, and a ventilation room for hot water evaporation, which occupies a large area.
(2) Shot blasting and rust removal: On the production line, the shot blasting process is carried out in a shot blasting box, which consists of a shot blasting head, an abrasive circulation device, an abrasive cleaning device, and a ventilation and dust removal device. When the steel pipe enters the shot blasting box, the blades of the shot blasting head rotate at high speed driven by the motor, generating a strong centrifugal force. Under the action of the centrifugal force, the abrasive accelerates along the length of the blade until it is thrown out. The thrown abrasive forms a fan-shaped stream and hits the inner surface of the steel pipe to remove oxide scale and rust. After the abrasive is thrown out, the abrasive circulation system will recycle and screen the used abrasive and transfer it to the feed end for reuse.
(3) Surface cleaning: The steel pipes that have been shot blasted contain abrasive dust, rust residue, and other dirt, which need to be cleaned. In some old-fashioned devices, the steel pipes are tilted to pour out the residues. This requires a lot of power and a certain amount of time, so it is rarely used in modern devices. The new cleaning method is to blow with compressed air or vacuum cleaner. With the increase of HSE awareness, ventilation and dust removal devices should be installed in the production line of shot blasting operations to absorb the dust generated during the shot blasting process and separate and recover abrasives.
(4) Abrasives: The abrasives used for shot blasting are mainly iron shots, steel shots, steel wire segments, and angular steel sand. From the perspective of economy and practicality, steel shots are better, while from the perspective of shot blasting effect, steel wire segments are better. The ideal abrasive for shot blasting should be steel shots plus steel wire segments or steel shots plus steel sand, with a ratio of 1:1 to 2:1.
4.2 Sandblasting (shot) treatment: Sandblasting (shot) treatment uses compressed air as the power to spray abrasives (sand or shot) at a certain speed onto the surface of the steel being treated. Under the impact and grinding of the abrasives, the oxide scale, rust products, and other dirt on the metal surface are removed. The sandblasting (shot) treatment device generally includes: a compressed air delivery (processing, storage) system; nozzle, hose, abrasive recovery circulation system; traction lighting electronic control system; dust removal system and air and sand supply system. Many factors affect the rust removal effect of sandblasting (shot), such as air pressure, type and specification of abrasives, spray angle and speed of abrasives, distance from the nozzle to the steel surface, etc. Abrasives should be selected according to the requirements of surface treatment and the original state of the steel surface. Usually, steel shot, steel wire segment, angular steel sand, quartz sand, or their mixture can be used. The requirements for rust removal level and surface roughness of sandblasting (shot blasting) are consistent with the content of quality inspection and the standards mentioned above. From the results, the effects of sandblasting (shot blasting) and shot blasting are the same. The main factors for the selection of methods are economy and conditions. For example, when the pipe diameter is less than 762mm, the distance between the blasting head and the treated surface is not enough, so shot blasting cannot be used, and sandblasting (shot blasting) has to be used. Sandblasting (shot blasting) is a mature technology, and its equipment has also been commercialized. When the inner surface pretreatment of the pipeline is selected, it can be used with a little modification.
4.3 Cleaning operation: The surface treated by blasting (shot blasting) must be cleaned with a brush, compressed air, or a vacuum cleaner. To clean the rust and abrasive fine particles that fall off the surface from the depressions of the “peaks and valleys” of the anchor pattern. For large-mouth steel pipes, the purge method is usually adopted. There are two methods: one is to use a large-displacement vacuum cleaner to suck the main dust and steel shots in the rust removal process while shot blasting. Before the remaining tiny dust is sprayed inside, turn on the air source of the spray gun, and the spray gun will start to purge the inner surface of the steel pipe. The spray gun blows from one end of the steel pipe to the other end, and the dust is sucked away by the vacuum cleaner at the end of the other end. Another method is to use a shot pouring device to lift the steel pipe at a certain angle, so that the steel shots slide down into the recovery device, and then purge the inner wall of the steel pipe, and use a vacuum cleaner to suck away the tiny dust. If it is a wet-treated surface, it must be rinsed with fresh water with sufficient corrosion inhibitor, or rinsed with fresh water first and then passivated. If necessary, a brush should be used for additional treatment to remove all residues.
5. Quality control: There are two main aspects of quality control of the inner surface treatment of steel pipes, namely cleanliness and roughness.
5.1 Cleanliness: According to the requirements of ISO 8501-1 and GB 8923 standards, the inner surface of the steel pipe with drag reduction coating should reach Sa2.5 level after treatment. This level is defined as: the steel surface should be free of visible grease, dirt, scale, rust paint, and other attachments, and any residual traces should only be slight spots or stripes. This cleanliness requirement can be visually inspected. In addition, the ISO 8502 standard also provides a method for detecting surface cleanliness.
ISO8502-1 standard provides a detection method for soluble iron salts remaining on the surface of surface-treated steel. The main method is to clean the steel surface with water, dissolve the soluble iron salt in water, and then use 2,2-bipyridine as an indicator to measure the collected cleaning solution by colorimetry. The reference indicator is that when the content of iron ions on the steel surface is less than 15mg/m2/, it can be considered that there will be no significant impact on the coating.
ISO 8502-2 standard provides a laboratory test method for the content of water-soluble oxides on the surface of steel pipes. This method can be used for the surface of steel pipes before and after surface treatment. The method stipulates that a certain area of the steel surface is first cleaned with a known volume of water, the cleaning water is collected, and then the chloride in the collected cleaning solution is analyzed and determined by mercuric nitrate titration method with diphenylcarbazone-bromophenol blue as an indicator. During the titration process, mercury ions react with free oxygen ions to generate HgCl2. After the chloride ions are consumed, the excess mercury ions appear purple in the mixed indicator, indicating that the titration process is over. Regarding this test, Q/SY XQ11 refers to a 20mg/m2 indicator of relevant foreign standards. However, this indicator refers to whether the steel pipe should be rinsed on the surface before the surface treatment of the steel pipe. According to the requirements of ISO standards, it should be tested again after cleaning. Table 5-5 is the requirement index of foreign standards for salt content on the surface of steel pipes.
ISO 8502-3 standard is a standard for evaluating the degree of dust contamination on the surface of steel to be painted. The standard divides the degree of dust contamination on the steel surface into five levels, defined by standard graphs; the detection method is to stick the steel surface to be tested with pressure-sensitive tape, and then compare the dusty tape with the standard graph to determine the level of dust contamination on the steel surface. ISO 8502-4 standard is a method for evaluating the possibility of condensation on the steel surface before painting. This method measures the dew point under the corresponding environmental conditions by measuring the temperature and relative humidity of the ambient air, then measures the surface temperature of the steel, and evaluates the possibility of surface condensation from the difference between the dew point and the dew point. For solvent-based coatings, the surface temperature of the steel pipe to be painted must be more than 3°C higher than the ambient dew point temperature.
In addition, the International Organization for Standardization ISO/TC35/SCl2 has also formulated other relevant surface cleanliness test method standards, in addition to the ISO 8502-5, ISO 8502-6, and ISO 8502-7 mentioned above, there are: ISO 8502-8 Analysis of soluble impurities on the surface to be painted – sulfate on-site analysis method; ISO 8502-9 Analysis of soluble impurities on the surface to be painted – iron salt on-site analysis method; ISO 8502-10 Analysis of soluble impurities on the surface to be painted – grease on-site analysis method; ISO 8502-11 Analysis of soluble impurities on the surface to be painted – moisture on-site analysis method.
Roughness: GB 13288 standard compiled concerning ISO standard makes corresponding provisions for the roughness assessment after surface treatment. The steps are: remove the dust and debris on the surface, select the appropriate roughness comparison sample (“G” sample and “S” sample) according to the abrasive, and place it close to a certain measuring point on the surface of the steel to be tested for visual comparison. The roughness grade indicated by the sample closest to the appearance of the steel surface is the evaluation grade. If a magnifying glass is used for evaluation, the appearance of the sample and the surface of the steel to be tested should be observed in the magnifying glass at the same time. If the visual evaluation is difficult, you can use the thumbnail or the thumb and index finger to hold the wooden stylus and move it on various parts of the tested surface and the comparison sample and the roughness grade indicated by the closest touch is the evaluation result. The surface roughness reference comparison sample is a flat plate divided into four parts, each with a specified reference surface roughness. The roughness reference value of the surface roughness comparison sample must comply with the provisions of Table 5-6, and its intuitive surface cleanliness should not be lower than Sa2.5. The sample that reflects the surface roughness characteristics obtained by blasting angular sand abrasives (GRIT) is called a “G” sample; the sample that reflects the surface roughness characteristics obtained by blasting shot abrasives (SHOT) is called an “S” sample. There are many methods for testing surface roughness. The roughness comparator method is also commonly used in production. Keane-tator roughness comparator is a commonly used instrument. It consists of a standard template with five sectors converging together. The five sectors are distributed in a five-pin star shape, and there is a hole in the middle of the five-pin star. Each sector represents a standard roughness template. When using it, place the template on the surface to be tested, and use a special magnifying glass placed above the middle hole to compare the surface to be tested with the standard sector to determine the surface roughness value. This method is simple and easy to use, does not require complex tools, and the test results are reliable. The rubbing paper method is another commonly used test method. It uses a special rubbing tape. When using it, peel off the paper backing, place the latex side of the tape on the steel surface, and rub the back of the tape with a smooth tool or other blunt tool in a circle until the surface becomes a uniform gray. Remove the tape and use a spring micrometer to measure the thickness of the rubbing tape. To obtain the roughness height on the film, subtract 50.8μm from the micrometer reading to offset the thickness of the film cushion layer. The instrument should be calibrated during measurement. This method can be seen in ASTM D 4417 Method C. This method is simple and easy to use, and the rubbing impression can be permanently preserved as an archive in the production process.
Post time: Dec-17-2024