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Surface processing and application of stainless steel

Stainless steel's wide range of surface finishes broaden its application – different surface finishes make stainless steel surfaces different, making them unique in their applications.

In the field of architectural applications, the surface finish of stainless steel is important for a number of reasons.

A corrosive environment requires a smooth surface because the surface is smooth and not easily fouled. The deposition of dirt can rust or even cause corrosion of stainless steel.

 

In the spacious hall, stainless steel is the most commonly used material for elevator decorative panels. Although the surface handprint can be wiped off, but it affects the appearance, it is best to use a suitable surface to prevent fingerprints.

Hygienic conditions are important in many industries, such as food processing, catering, brewing, and chemicals. In these applications, the surface must be easily cleaned daily, and chemical cleaners are often used.

 

Stainless steel is the best material in this area. In public places, the surface of stainless steel is often ridiculed, but one of its important characteristics is that it can be washed away, which is a remarkable feature of stainless steel over aluminum. The surface of aluminum tends to leave traces and is often difficult to remove. When cleaning the stainless steel surface, it should be cleaned along the lines of stainless steel, because some surface processing lines are unidirectional.

 

Stainless steel is best suited for hospitals or other areas where hygienic conditions are critical, such as food processing, catering, brewing, and chemicals, not only because it facilitates daily cleaning, but sometimes with chemical cleaners, and because it is less prone to bacteria. Tests have shown that stainless steel has the same properties as glass and ceramics in this respect.

 

1. Natural appearance of stainless steel

Stainless steel gives a natural, sturdy and radiant feel, and its natural color gently reflects the color of the surrounding environment.

 

2. Basic types of surface processing

There are roughly five types of surface finishes that can be used for stainless steel, which can be combined to transform more end products.

There are five types: rolling surface processing, mechanical surface processing, chemical surface processing, textured surface processing, and colored surface processing.

There are also some special surface finishes, but regardless of which surface finish is specified, the following steps should be followed:


① Together with the manufacturer, agree on the required surface finish. It is best to prepare a sample as a standard for future mass production.

② In large-area applications (such as composite panels), it must be ensured that the base coil or coil used is in the same batch.

③ In many architectural applications, such as the interior of an elevator, although the fingerprint can be erased, it is not aesthetically pleasing. If you choose a textured surface, it is not so obvious. Mirror stainless steel must not be used in these sensitive places

④ When selecting the surface processing, the manufacturing process should be considered. For example, in order to remove the beads, it is necessary to repair the weld and restore the original surface processing. It is difficult or even impossible for the tread plate to meet this requirement.

⑤ For some surface finishing, grinding or polishing lines are directional, known as one-way. If the texture is made to be vertical rather than horizontal, the dirt is less likely to adhere to it and is easy to clean.

⑥ No matter which finishing process is used, it is necessary to increase the number of process steps, so it is necessary to increase the cost. Therefore, be careful when selecting surface processing.

Therefore, architects, designers, and manufacturers need to understand the surface finish of stainless steel. Through friendly cooperation and mutual communication with each other, we will surely achieve the desired results.

⑦ Based on our experience, we do not recommend the use of alumina as an abrasive unless careful during use. It is best to use a silicon carbide abrasive.

 

3. Standard surface processing

Many types of surface processing have been represented by numbering or other classification methods, and they have been incorporated into relevant standards such as "British Standard BS1449" and "American Iron and Steel Institute Stainless Steel Producers Committee Standards".

 

4. Rolling surface processing

Sheets and strips have three basic rolling surface finishes, which are expressed by the production process of sheets and strips.

No. 1: Hot rolled, annealed, pickled, and descaled. The treated steel sheet surface is a dull surface and is somewhat rough.

No.2D: No. 1 The surface is well processed and is also a dull surface. After cold rolling, annealing, descaling, and finally light rolling with a matte roll.

No.2B: This is the most commonly used in architectural applications. Except for the last light cold rolling with a polishing roller after annealing and descaling, the other processes are the same as 2D, and the surface is slightly illuminated and can be polished.

No. 2B Bright Annealing: This is a reflective surface that is rolled by a polishing roll and subjected to a final annealing in a controlled atmosphere. Bright annealing still maintains its reflective surface and does not produce scale.

Since the oxidation reaction does not occur during the bright annealing process, it is not necessary to perform pickling and passivation treatment.

 

5. Polished surface processing

No. 3: It is represented by 3A and 3B.

3A: The surface is uniformly ground and the abrasive grain size is 80-100.

3B: The matte surface is polished, and the surface has uniform straight lines. It is usually polished with a belt with a particle size of 180~200 on a 2A or 2B board.

No.4: Unidirectional surface processing, low reflectivity, this surface processing may be the most versatile in architectural applications. The process steps are first polished with a coarse abrasive and finally with an abrasive having a particle size of 180.

No. 6: A further improvement to No. 4, polishing the No. 4 surface with a Tampico polishing brush in an abrasive and oil medium. This surface finish is not available in "British Standard 1449" but can be found in the US standard.

No.7: Known as bright polishing, it polishes the surface that has been ground very fine but still has a wear scar.

Typically 2A or 2B plates are used, with fiber or cloth polishing wheels and corresponding polishing pastes.

No.8: Mirror-finished surface with high reflectivity, often referred to as mirror surface finish, because the image it reflects is sharp.

The stainless steel is continuously polished with a fine abrasive and then polished with a very fine polishing paste.

It should be noted in architectural applications that such surfaces can be left handprints if used in places where people are heavily mobile or where people often touch them.

The fingerprint can of course be erased, but sometimes it affects the appearance.

The surface processing described in the “official” standards and in the literature is only a general introduction, and the sample is the most intuitive representation of the type of surface processing. Polishing or metal finishing manufacturers will provide a variety of surface processed samples, the user should discuss with them.

 

6. Surface roughness

The classification of rolling surface processing and polishing surface processing is to illustrate the extent to which it can be achieved. Another effective representation is to measure surface roughness. The standard measurement method is called CLA (centerline average), and the gauge moves laterally on the surface of the steel sheet to record the magnitude of the change in the peaks and valleys. The smaller the CLA number, the smoother the surface. The final results for the different grades can be seen from the surface finishes and CLA numbers in the table below.

Surface processing CLA, micron

2B    0.1-0.5

2A    0.05-0.1

2D   0.4-1.0

3      0.4-1.5

4      0.2-1.5

8      0.2

EP   1/2 of the basic value

EP = electrolytic polishing, which roughly reduces the variation of the peaks and valleys to 1/2 of the original surface.

 

7. Mechanical polishing

Precautions:

We should remember that the grinding with sandpaper or abrasive belt in the grinding operation is basically a polishing cutting operation, leaving a very fine grain on the surface of the steel sheet.

We have had trouble with alumina as an abrasive, in part because of pressure problems.

Any abrasive parts of the equipment, such as abrasive belts and grinding wheels, must not be used for other non-stainless steel materials before use. Because it will contaminate the stainless steel surface.

In order to ensure the consistency of the surface processing, the new grinding wheel or belt should be tested on the same waste to compare with the sample.

 

8. Electrolytic polishing

This is a metal removal process in which stainless steel acts as an anode in the electrolyte and the metal is removed from the surface after energization.

This process is commonly used for the machining of parts because their shape is difficult to polish by conventional methods.

This process is commonly used on the surface of cold rolled steel sheets because its surface is smoother than the surface of hot rolled steel sheets.

However, electrolytic polishing makes the surface impurities more conspicuous, especially titanium and yttrium stabilized materials may cause differences in the weld zone due to particulate impurities.

Small soldering tips and sharp edges can be removed by this process. The process focuses on the protrusions on the surface and preferentially dissolves them.

The electropolishing process involves immersing stainless steel in a heated liquid. The ratio of liquids involves many proprietary technologies and patented technologies.

Austenitic stainless steel has a good electropolishing effect.

 

9. Textured surface processing

There are many types of patterns that stainless steel can use. The advantages of having a steel plate with an added pattern or a textured surface are as follows:

Reducing "oilcan-ning", a term used to describe the surface of a bright material that is not optically angled. For example, a large-area decorative panel, even after stretch straightening or tension tensioning, is difficult to make the surface completely flat, and thus the metal roofing material shrinks.

1 The reticle pattern can support the glare emitted by the sun.

2 If the tread plate has slight scratches and small area indentation, it is not obvious.

3 Increase the strength of the steel plate.

4 Provides the architect with a choice.

Patterns with patent trees include fabric (for the Ed Mansion in London), mosaics, pearls and leather. Ripple and line patterns can also be used.

The textured surface is especially suitable for interior finishes such as elevator panels, counters, siding and entrances.

For external applications, it should be considered that the stainless steel can be cleaned by rain and artificial washing to avoid dead spots that are easy to collect dirt and airborne impurities, so as to avoid corrosion and affecting the appearance.

 

10. Matte surface processing

Mammal surface processing is one of the most commonly used surface finishes. It is polished on a polished or brightly annealed steel surface with a nylon abrasive tape or brush.

 

11. Spray glass ball or shot peening

For internal applications, such as the interior of elevators, mixed surface processing is very popular.

This mixing process is to form a ruthless surface by spraying glass balls, then covering the plastic film by masking, processing into a polished surface, and finally forming a polished and opaque mixed surface.

Stainless steel sand pills can also be used in similar processes.

The glass spheres or pellets to be used must not be used on other materials beforehand, especially on carbon steel. Because the carbon steel particles are embedded in the stainless steel surface, it is easy to cause corrosion.

Ceramic balls can also be used as sprays.

 

12. Color stainless steel

The stainless steel color process was developed by International Nickel Corporation (INCO) in the 1970s, and many companies have licenses to use this process.

As explained earlier, stainless steel does not rust because of its surface chrome oxide film.

The color process uses this film to form a specified color.

Since stainless steel utilizes this layer of film that is always present, it does not fade or maintain as often as paint.

Color stainless steel can also be molded to have no adverse effects on color even in sharp bends.

Regarding the effect on corrosion resistance, tests have shown that the corrosion resistance is enhanced after using this process.

This process is closely related to the operating time, and the color varies with time. The order of color changes is brown, gold, red, purple, and green.

A major feature of the process is that its final appearance reflects the original surface of the material, ie the mirror or polished surface produces a strong metallic luster, while the matte surface finish is matt.

 

Crafting process:

The process is to soak the stainless steel in the solution tank. The solution preferably contains 250 grams of Cr2O3 per liter, and 490 grams of sulfuric acid per liter. The temperature range is 80-85 ° C. The soaking time depends on the color required, and at most More than 25 minutes.

After rinsing the steel plate with clean cold water, it is subjected to cathodic treatment in a liquid having a concentration of 250 g / 1 liter of chloric acid and 2.5 g / 1 liter of phosphoric acid at room temperature for about 10 minutes, and the current density is 0. .2~0.4A/dm2.

In order to prevent damage, the color treatment is immediately followed by a hardening treatment, followed by rinsing in hot water and drying.

 

13. Mixed surface processing

The color stainless steel can be re-patterned, and the proprietary technology developed involves the removal of the "protruding" portion with a corundum belt, so that the end result is a combination of the natural beauty of the steel sheet and the color of the color pattern.

This type of surface is not easy to leave a fingerprint, especially suitable for interior decoration.

Polishing manufacturers can provide surface processed samples.

 

14. Etched surface processing

The pattern is marked on the surface of the steel sheet by a laminating process, and the steel sheet is immersed in a ferric chloride solution (grade 0) to etch away the uncoated portion to form a beautiful pattern on the surface of the stainless steel.

 

Note: Excerpt from <<Stainless Special Issue>>


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