The diverse surface treatments of stainless steel have expanded its range of applications.
Each type of surface treatment produces a distinct surface, rendering the stainless steel suitable for unique applications.
In the realm of architectural applications, the significance of surface treatment for stainless steel is manifold.
Basic types of stainless steel surface processing
There are approximately five different types of surface treatments that can be utilized for stainless steel. These treatments can also be combined together to produce a greater variety of finished products.
The five categories of surface processing for stainless steel are:
- Rolled surface processing
- Mechanical surface processing
- Chemical surface processing
- Textured surface processing
- Color surface processing
There are also some specialized surface processing techniques available. However, regardless of the chosen method, the following steps should be followed:
- Negotiate the required surface processing with the manufacturer. It is highly recommended to prepare a sample as a standard for future mass production.
- In the case of large-scale usage (such as composite plates), ensure that the base plate or roll material used is from the same batch.
- The manufacturing process should be considered when selecting the surface processing. For example, in order to remove welding beads, the weld seam may need to be ground and the original surface processing must be restored. Patterned plates may be difficult or even unable to meet this requirement.
- For certain surface processing techniques such as grinding or polishing, the texture is directional and is called unidirectional. Using this texture vertically instead of horizontally will make it difficult for dirt to stick to it, and make it easier to clean.
Rolling surface processing
There are three basic rolling surface processing techniques for plates and strips, which are represented by the production process of plates and strips:
- No.1: After hot rolling, annealing, pickling, and descaling, the surface of the treated steel plate is dull and slightly rough.
- 2D: This technique produces a slightly better surface than No.1 processing, but it is still dull. After cold rolling, annealing, descaling, and a final light rolling with a mat roll, the surface becomes smoother.
- 2B: This is the most commonly used technique in architectural applications. Except for the final mild cold rolling with a polishing roller after annealing and descaling, the other processes are the same as 2D. The surface is slightly shiny and can be polished.
In addition to these techniques, there is a special surface processing technique called 2B bright annealing. This technique produces a highly reflective surface by rolling the steel with a polishing roller and then annealing it in a controlled atmosphere. Bright annealing retains its reflective surface and does not produce oxide scale. Since no oxidation reaction occurs during bright annealing, there is no need for further acid pickling and passivation treatment.
Polished surface processing
No.3 surface processing is represented by 3A and 3B.
3A: The surface is uniformly ground, and the abrasive particle size is 80~.
3B: This technique involves rough polishing the surface with uniform straight lines. It is typically achieved by polishing an abrasive belt with a particle size of 180 to 200 on a 2A or 2B plate.
No.4: This surface processing creates a unidirectional surface with poor reflectivity, and it is widely used in architectural applications. The process involves coarse abrasive polishing followed by grinding with an abrasive of 180 particle size.
No.5: This is an improvement over No.4 and involves polishing the No.4 surface with a Tampico polishing brush in an abrasive and oil medium. This surface processing is not included in “British Standard 1449” but can be found in American standards.
No.6: This is called bright polishing and is used to polish a surface that has been finely ground but still has wear scars. Usually, a 2A or 2B board is used along with a fiber or cloth polishing wheel and the corresponding polishing paste.
No.7: This surface processing results in a highly reflective mirror surface, which is often called a mirror surface. Stainless steel is polished continuously with a fine abrasive and then polished with a very fine polishing paste.
It should be noted in architectural applications that this type of surface is prone to leaving fingerprints if used in areas with high human traffic or where people touch frequently. While fingerprints can be wiped off, they can affect the aesthetics of the surface.
The classification of surface finishing can be divided into two types: rolled surface processing and polished surface processing. These types indicate the level of smoothness that can be achieved.
Another effective way to express surface smoothness is by measuring surface roughness.
The standard method for measuring surface roughness is called CLA (Central Line Average). A measuring instrument is used to move laterally on the surface of the steel plate and record changes in peak and valley. The smaller the CLA number, the smoother the surface. The table below shows the final results of different grades based on the surface finishing and CLA numbers.
It is important to remember that during the grinding process, the use of abrasive paper or belt essentially performs a polishing and cutting operation, leaving very fine lines on the surface of the steel plate.
We have experienced difficulties when using alumina as an abrasive due to pressure issues. Any abrasive parts of the equipment, including abrasive belts and wheels, should not be used on non-stainless steel materials before being used on stainless steel. This is because it can lead to contamination of the stainless steel surface.
Electrolytic polishing is a metal removal process in which stainless steel is used as the anode in an electrolyte solution, and the metal is removed from the surface after an electric current is applied. This process is commonly used for machining parts with complex shapes that are difficult to polish using conventional methods.
The process is often used on the surface of cold-rolled steel sheet because it has a smoother surface than hot-rolled steel sheet. However, electrolytic polishing can make surface impurities more visible, especially in stabilized materials like Titanium and Niobium, which can cause differences in the welding zone due to granular impurities.
This technique is effective for removing small welding scars and sharp edges, as it preferentially dissolves surface protrusions. The electrolytic polishing process involves immersing stainless steel in a heated liquid, and the liquid’s proportions involve many proprietary and patented technologies. Electrolytic polishing of austenitic stainless steel is particularly successful.
Textured surface processing
There are numerous patterns that can be applied to stainless steel, and the advantages of creating steel plates with textured or patterned surfaces are as follows:
- Reduced “oil canning”: This term is used to describe the surface of bright materials, such as large-area decorative boards, that do not appear flat from an optical perspective. Even after stretching and straightening, it is difficult to completely flatten the surface, resulting in oil canning.
- Reduced glare in sunlight due to the textured pattern.
- Slight scratches and small area indentations are less visible on patterned plates.
- Increased strength of the steel sheet.
- Provides architects with a range of design options. Patented patterns include linen (used in the Ed Building in London), mosaic patterns, pearl and leather patterns, as well as wave and line patterns. Patterned surfaces are particularly suited for interior decoration, such as elevator panels, counters, wall panels, and entrances. For external applications, it is essential to enable rain and manual flushing to clean the stainless steel and avoid dead corners that collect dirt and airborne impurities, which can cause corrosion and affect aesthetics.
Rough surface processing
Rough surface processing is one of the most commonly used surface treatments. It involves polishing the surface of a steel plate with a nylon abrasive belt or brush after polishing or bright annealing.