The surface finish of steel affects much more than just its aesthetic. The performance and reliability of stainless steel is also affected by its surface finish. Apart from the performance and reliability, the surface finish affects corrosion resistance and attributes like electrical properties. The finish also influences weldability and manufacturing processes and contributes to different cost-related concerns.
Therefore, choosing the appropriate surface finish of stainless steel is essential.
What is stainless steel surface finish?
Surface finish refers to the appearance of stainless steel, which is created through different manufacturing techniques like polishing, rolling, and blasting. These finishes can range from dull to bright and have unique textures for specialised applications.
Several factors can affect surface finish, including the thickness of the material, the manufacturing process used, the quality of the rolls, and the level of cleanliness during production.
Types of surface finishes on Stainless steel
Stainless steel surface finishes are typically labelled with standard industry codes based on the processing route rather than the visual appearance. Below are some common types of Stainless steel surface finishes-
1. Mill Finish
These finishes represent the initial supply condition for all stainless steel products, whether produced through hot or cold rolling. They are utilised for various applications and serve as a starting point for additional finishing procedures, including mechanical polishing. To enhance corrosion resistance, most of these finishes undergo an acid cleaning process (known as pickling) to remove any scale that develops during hot rolling and annealing.
There are 3 different types of Mill finishes –
No 1 Finish - To create a No. 1 Finish, the steel is hot rolled and then subjected to annealing and pickling in acid, which results in a dull finish. This finish is typically used in applications where a smooth decorative finish is not important, such as in elevated temperature applications.
No 2 Finish – A No. 2D Finish is produced through cold rolling, followed by annealing and pickling in acid. The finish is non-reflective, dull, and smooth, making it ideal for deep drawing applications. This finish can be found in various products, such as auto exhaust components, chemical equipment, and hardware.
No 2B Finish - A No. 2B Finish is achieved through cold rolling, annealing, and pickling, followed by a light temper pass using polished rolls. This finish is smooth, relatively reflective, and bright, making it a general-purpose cold-rolled finish commonly used in producing cookware, small tanks, and pharmaceutical equipment.
2. Bright Annealed Finish
The Bright Annealed Finish is achieved by cold rolling the stainless steel and annealing in a controlled environment to prevent scale formation. Unlike other finishes, this type does not require acid pickling as a thin oxide film is formed during annealing, eliminating the need for pickling. The Bright Annealed Finish is commonly used for trim appliances, surgical instruments, and cookware applications.
3. Mechanically Polished finishes
Mechanically polished finishes are achieved by grinding or polishing the surface with progressively finer abrasives. These finishes are typically designated as "No. 3," "No. 4," "No. 6," and "No. 7" finishes, with a higher number indicating a finer polishing grit.
No. 3 and No. 4 finishes have a dull appearance and are commonly used for architectural applications such as handrails, elevator doors, and storefronts.
No. 6 and No. 7 finishes have a brighter, mirror-like appearance and are used for decorative applications such as automotive trim and decorative hardware.
Mechanically polished finishes offer numerous benefits, including improved corrosion resistance, aesthetics, and enhanced cleanability. However, they can also be more expensive than other finishes and may require additional processing steps to achieve the desired level of polish.
To achieve a mechanically polished finish, the stainless steel is first passed through a series of abrasive belts or wheels, with progressively finer abrasives used at each step. After the final polishing step, the surface may be treated with a passivation solution to enhance its corrosion resistance.
4. Custom finishes
Custom finishes are available for applications which require specific surface characteristics beyond the standard finishes. These finishes can be achieved through various processes, such as electro-polishing, and bead blasting.
Electro-polishing is an electrochemical process that removes a thin layer of the surface material to create a smooth, shiny finish. This process is commonly used for medical and pharmaceutical equipment because it removes surface contaminants and improves corrosion resistance.
Bead blasting, also known as sandblasting, involves shooting small beads or particles at the material's surface to create a matte finish with a uniform texture. This finish is often used when a non-reflective surface is desired, such as outdoor signage or machine parts.
To create a unique texture or pattern, custom finishes can also be achieved by combining different surface treatments, such as brushing and chemical etching. The possibilities for custom finishes are virtually endless, making it possible to achieve the desired surface characteristics for any application.
The surface finishing of stainless steel is highly relevant in its applications as it impacts several important factors beyond just aesthetics. The choice of surface finish can impact corrosion resistance, electrical properties, weldability, and manufacturing operations. It can also contribute to various cost concerns.
In certain applications, such as superconductor components or flexible metal hoses, the surface finish of stainless steel must be specific for the end-use product to function as designed. The wrong finish could cause the product to fail or not meet the required specifications.
Additionally, the surface finish can impact the cleanability and hygiene of stainless steel products used in the food and pharmaceutical industries. The smoother the surface finish, the easier it is to clean and maintain the hygiene of the equipment.
Selecting the appropriate surface finish for a stainless steel application is critical to ensuring the product's performance, reliability, and functionality.
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