The various surface processing of stainless steel broadens its application fields-different surface processing makes the surface of stainless steel different, making it unique in its application.
In the field of construction applications, the surface processing of stainless steel is important for many reasons.
The corrosive environment requires a smooth surface because the surface is smooth and not easy to foul. The deposit of dirt will cause stainless steel to rust and even cause corrosion.
In a spacious hall, stainless steel is the most commonly used material for elevator decorative panels. Although the fingerprints on the surface can be wiped off, it affects the appearance, so it is best to choose a suitable surface to prevent fingerprints from being left.
Sanitary conditions are very important to many industries, such as food processing, catering, brewing and chemical industries. In these applications, the surface must be easy to clean every day, and chemical cleaning agents are often used.
Stainless steel is the best material in this regard. In public places, the surface of stainless steel is often scribbled. However, one of its important features is that they can be washed away. This is a significant feature of stainless steel over aluminum. The surface of aluminum is easy to leave marks, and it is often difficult to remove. When cleaning the surface of stainless steel, you should clean up along the lines of stainless steel, because some of the lines processed on the surface are unidirectional.
Stainless steel is most suitable for hospitals or other areas where sanitary conditions are critical, such as food processing, catering, brewing and chemical engineering, not only because it is easy to clean every day, and sometimes chemical cleaning agents are used, but also because it is not easy to breed bacteria. Tests show that the performance of stainless steel in this respect is the same as that of glass and ceramics.
1. Natural appearance of stainless steel
Stainless steel gives a natural, firm and beautiful feeling, and its natural color softly reflects the color of the surrounding environment.
2. Basic types of surface processing
There are roughly five types of surface processing that can be used for stainless steel, and they can be combined to transform into more final products.
The five categories are: rolling surface processing, mechanical surface processing, chemical surface processing, textured surface processing and color surface processing.
There are also some special surface processing, but no matter which surface processing is specified, the following steps should be followed:
① Negotiate the required surface processing together with the manufacturer, and it is best to prepare a sample as the standard for mass production in the future.
② When using a large area (such as a composite board), it must be ensured that the base coil or coil used is the same batch.
③ In many construction applications, such as the interior of elevators, although the fingerprints can be wiped off, they are very unsightly. If you choose a cloth surface, it will not be so obvious. Mirror stainless steel must not be used in these sensitive places
④ The production process should be considered when selecting the surface processing. For example, in order to remove the weld beads, the weld may need to be ground and the original surface processing must be restored. The tread plate is difficult or even unable to meet this requirement.
⑤ For some surface processing, grinding or polishing lines are directional, which is called unidirectional. If the lines are made vertical rather than horizontal when used, dirt will not easily adhere to it, and it will be easy to clean.
⑥ No matter what kind of finishing is used, it needs to increase the process steps, so the cost is increased. Therefore, be careful when choosing the surface processing.
Therefore, architects, designers, manufacturers and other relevant personnel need to understand the surface processing of stainless steel. Through friendly cooperation and mutual exchanges between each other, the desired effect will surely be obtained.
⑦ According to our experience, we do not recommend using alumina as abrasive, unless you are very careful during use. It is best to use silicon carbide abrasives.
3. Standard surface finish
Many kinds of surface finishes have been represented by numbers or other classification methods, and they have been incorporated into relevant standards, such as "British Standard BS1449" and "American Iron and Steel Association Stainless Steel Producer Committee Standard".
4. Rolled surface processing
There are three basic rolling surface processing for plate and strip, which are represented by the production process of plate and strip.
No.1: After hot rolling, annealing, pickling and descaling. The surface of the treated steel plate is a dull surface, a bit rough.
No. 2D: Better than No. 2D. 1 If the surface is processed, it is also a dull surface. After cold rolling, annealing, descaling, and finally light rolling with a matt roller.
No.2B: This is the most commonly used in construction applications. Except for the last light cold rolling with a polishing roll after annealing and descaling, the other processes are the same as 2D, the surface is slightly shiny and can be polished.
No. 2B Bright Annealing: This is a reflective surface, which is rolled by polishing rolls and final annealed in a controlled atmosphere. Bright annealing still maintains its reflective surface and does not produce oxide scale.
Since no oxidation reaction occurs during the bright annealing process, no pickling and passivation treatments are required.
5. Polished surface processing
No. 3: Represented by 3A and 3B.
3A: The surface is uniformly ground, and the abrasive grain size is 80-100.
3B: The rough surface is polished, and the surface has uniform straight lines. It is usually made by polishing with an abrasive belt with a grain size of 180~200 on the 2A or 2B plate at one time.
No.4: Unidirectional surface processing, low reflectivity, this surface processing may be the most widely used in architectural applications. The process steps are first polishing with coarse abrasives, and finally grinding with abrasives with a particle size of 180.
No.6: It is a further improvement to No.4, which is to polish the surface of No.4 with Tampico polishing brush in abrasive and oil medium. This surface finish is not included in "British Standard 1449", but it can be found in American Standards.
No.7: It is called bright polishing, which is to polish the surface that has been ground very fine but still has abrasion marks.
Usually used is 2A or 2B board, with fiber or cloth polishing wheel and corresponding polishing paste.
No.8: Mirror polished surface, high reflectivity, usually called mirror surface processing, because it reflects a very clear image.
Polish the stainless steel continuously with a fine abrasive, and then polish it with a very fine polishing paste.
In architectural applications, it should be noted that this surface will leave fingerprints if it is used in places where there is a large flow of people or people often touch.
The fingerprints can of course be wiped off, but sometimes it affects the appearance.
The "official" standards and the surface processing described in the literature are only general introductions, and samples can most intuitively indicate the types of surface processing. Polishing or metal finishing manufacturers will provide samples of various surface finishes, and users should discuss with them.
6. Surface roughness
The classification of rolled surface processing and polished surface processing is to indicate the degree that can be achieved, and another effective way to express it is to measure surface roughness. The standard measurement method is called CLA (Centerline Average). The measuring instrument moves laterally on the surface of the steel plate to record the peak and valley changes. The smaller the CLA number, the smoother the surface. The final results of different grades can be seen from the surface finish and CLA number in the table below.
Surface processing CLA, micron
2A 0.05-0. 1
2D 0.4 to 1.0
1/2 of EP basic value
EP=Electropolishing, which can roughly reduce the peak and valley variation to 1/2 of the original surface.
7. Mechanical polishing
We should remember that the grinding with sandpaper or belt in the grinding operation is basically a polishing and cutting operation, leaving very fine lines on the surface of the steel plate.
We have had trouble using alumina as an abrasive, partly because of pressure issues.
Any grinding parts of the equipment, such as abrasive belts and grinding wheels, must not be used for other non-stainless steel materials before use. Because this will contaminate the stainless steel surface.
In order to ensure the consistency of surface processing, new grinding wheels or belts should be tried on waste materials with the same composition first, so that the same products can be compared.
8. Electrolytic polishing
This is a metal removal process in which stainless steel is used as the anode in the electrolyte, and the metal is removed from the surface after power is applied.
This process is usually used for the processing of parts, because their shape is difficult to polish with traditional methods.
This process is often used on the surface of cold-rolled steel sheets because the surface is smoother than that of hot-rolled steel sheets.
But electrolytic polishing will make the surface impurities more obvious, especially the titanium and niobium stabilized materials will cause differences in the weld zone due to particulate impurities.
Small weld scars and sharp edges can be removed by this process. This process focuses on treating the protrusions on the surface and preferentially dissolving them.
The electrolytic polishing process involves immersing stainless steel in a heated liquid. The ratio of the liquid involves many proprietary and patented technologies.
The electrolytic polishing effect of austenitic stainless steel is very good.
9. Textured surface processing
There are many types of patterns that can be used in stainless steel. The advantages of adding patterns or netting surface processing to the steel plate are as follows:
Reduce "metal roofing material shrinkage" (oilcan-ning), which is a term used to describe the surface of bright material, which is not flat from an optical point of view. For example: large-area decorative panels, even after stretching straightening or tension straightening, it is difficult to make the surface completely straight, so the metal roofing material will shrink.
① The net pattern can feed the glare from the sun.
② If the tread plate has slight scratches and small area indentations, it is not obvious.
③ Increase the strength of the steel plate.
④ It provides the architect with a choice.
Patented tree patterns include cloth patterns (used in Ed Building in London), inlay patterns, pearl patterns and leather patterns. Wave and linear patterns can also be used.
The patterned surface is particularly suitable for interior decoration, such as elevator panels, counters, wall panels and entrances.
In external applications, consideration should be given to enabling the stainless steel to be washed and cleaned by rainwater and manual washing to avoid dead spots where dirt and airborne impurities are easy to gather, so as not to cause corrosion and affect appearance.
10. Rough surface processing
The rough surface processing is one of the most commonly used surface processing. It is polished with a nylon abrasive belt or brush on the surface of the polished or bright annealed steel plate.
11. Spray glass ball or shot peening
For interior applications, such as the interior of elevators, hybrid surface finishing is very popular.
This mixing process is to form a matte surface by spraying glass balls, and then through a masking treatment, covering with a plastic film, processing into a polished surface, and finally forming a polished and matte mixed surface.
Stainless steel sand shots can also be used in similar processes.
The glass balls or pellets to be used must not be used on other materials in advance, especially on carbon steel. Because the powder particles of carbon steel will be embedded in the surface of stainless steel, it is easy to cause corrosion.
Ceramic balls can also be used as spray materials.
12. Color stainless steel
The stainless steel color process was successfully developed by the International Nickel Corporation (INCO) in the 1970s, and many companies have licenses to use this process.
As explained earlier, the reason why stainless steel does not rust is because of the chromium oxide film on its surface.
The color process is to use this layer of film to form a specified color.
Because stainless steel uses this film that always exists, it neither fades nor does it require frequent maintenance like paint.
Color stainless steel can also be formed, even in sharp bends, it will not have any adverse effects on the color.
Regarding the impact on corrosion resistance, tests have shown that the corrosion resistance has been enhanced after the process is adopted.
The process is closely related to the operating time, and the color will change with time. The order of color change is brown, gold, red, purple and green.
A major feature of this process is that its final appearance can reflect the original surface of the material, that is, a mirror or polished surface will produce a strong metallic luster, while the color of the matte surface is matte.
The process is to soak stainless steel in a solution tank. The solution should preferably contain 250 grams of Cr2O3 per liter, or 490 grams of sulfuric acid per liter. The temperature range is 80~85℃. The soaking time depends on the color required, at most not More than 25 minutes.
After rinsing the steel plate with clean cold water, it is placed in a liquid with a concentration of 250 g/1 liter of chloric acid and 2.5 g/1 liter of phosphoric acid at room temperature for cathode treatment. The time is about 10 minutes and the current density is 0. ．2~0.4A/dm2.
In order to prevent damage, the hardening treatment is performed immediately after the color treatment, and then rinsed in hot water and dried.
13. Mixed surface processing
Patterns can be added to the colored stainless steel. The proprietary technology developed includes removing the "protruding" parts with corundum belts. In this way, the final result is to combine the natural beauty of the steel plate with the colors of the colored patterns.
This kind of surface is not easy to leave handprints, and is especially suitable for interior decoration.
Polishing manufacturers can provide samples for surface processing.
14. Etched surface processing
Mark the pattern on the surface of the steel plate through the coating process, and then immerse the steel plate in a ferric chloride acid solution (grade 0) to etch away the uncoated part, forming a beautiful pattern on the surface of the stainless steel.
Note: Extracted from <<Stainless Special Issue>>
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