The various surface treatments of stainless steel have expanded its range of applications.
Each type of surface treatment produces a distinct surface, making stainless steel suitable for unique applications.
In the domain of architectural applications, the importance of stainless steel surface treatment is manifold.
Basic types of stainless steel surface processing
There are approximately five different types of surface treatments that can be used for stainless steel. These treatments can also be combined to produce a greater variety of finished products.
The five categories of surface processing for stainless steel are:
- Laminated surface processing
- Mechanical surface processing
- Surface chemical processing
- Textured surface processing
- Color surface processing
There are also some specialized surface processing techniques available. However, regardless of the method chosen, the following steps must be followed:
- Negotiate the necessary surface processing with the manufacturer. It is highly recommended to prepare a sample as a standard for future mass production.
- In case of large-scale use (such as composite boards), make sure the base plate or roll material used is from the same batch.
- The manufacturing process must be considered when selecting surface processing. For example, to remove weld beads, the weld seam may need to be ground and the original surface processing must be restored. Standardized plates may be difficult or even incapable of meeting this requirement.
- For certain surface processing techniques, such as sanding or polishing, the texture is directional and is called unidirectional. Using this texture vertically instead of horizontally will make it harder for dirt to stick and make cleaning easier.
Rolling surface processing
There are three basic rolling surface processing techniques for sheet and strip, which are represented by the sheet and strip production process:
- No.1: After hot rolling, annealing, pickling and descaling, the surface of the treated steel plate becomes dull and slightly rough.
- 2D: This technique produces a slightly better surface than #1 processing, but is still opaque. After cold rolling, annealing, descaling and a final light rolling with a conveyor roller, the surface becomes smoother.
- 2B: This is the most commonly used technique in architectural applications. Except for final cold smooth rolling with 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 keeps its surface reflective and does not produce oxide scale. As no oxidation reactions occur during bright annealing, there is no need for additional 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 low reflectivity and is widely used in architectural applications. The process involves coarse abrasive polishing followed by grinding with a 180 particle size abrasive.
#5: This is an improvement over #4 and involves polishing the #4 surface with a Tampico polishing brush in an abrasive, oily 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 sanded but still has wear scars. Typically, a 2A or 2B plate is used together with a fiber or fabric polishing wheel and 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 continuously polished 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. Although fingerprints can be erased, they can affect the aesthetics of the surface.
Surface stiffness
The classification of surface finishing can be divided into two types: laminated 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 the peak and valley. The lower the CLA number, the smoother the surface. The table below shows the final results of different grades based on surface finish and CLA numbers.
Mechanical polishing
It is important to remember that during the grinding process, the use of sandpaper or belt essentially performs a polishing and cutting operation, leaving very fine lines on the surface of the steel sheet.
We had difficulties using alumina as an abrasive due to pressure issues. Any abrasive parts of equipment, including abrasive belts and wheels, must not be used on materials other than stainless steel before being used on stainless steel. This is because it can lead to contamination of the stainless steel surface.
Electrolytic polishing
Electrolytic polishing is a metal removal process in which stainless steel is used as an anode in an electrolytic solution and the metal is removed from the surface after applying an electrical current. This process is commonly used to machine complex shaped parts that are difficult to polish using conventional methods.
The process is often used on the surface of cold-rolled steel sheets because it has a smoother surface than hot-rolled steel sheets. However, electrolytic polishing can make surface impurities more visible, especially in stabilized materials such as 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 electropolishing process involves immersing stainless steel in a heated liquid, and the proportions of the liquid 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 preserves”: This term is used to describe the surface of glossy materials, such as large-area decorative plates, that do not appear optically flat. Even after stretching and straightening, it is difficult to completely level the surface, resulting in oil canning.
- Reduced glare in sunlight due to textured pattern.
- Light scratches and indentations in small areas are less visible on patterned plates.
- Greater resistance of the steel sheet.
- It offers architects a variety 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 suitable for interior decoration, such as elevator panels, counters, wall panels and entrances. For exterior applications, it is essential to allow rain and hand washing to clean the stainless steel and avoid dead corners that accumulate 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.
