Take a close look at that piece of stainless steel. Notice any imperfections? To the naked eye, your material may look smooth and consistent. But a closer exam could reveal a different story.
From mill finishes to applied finishes, the polish that is applied to the surface of your stainless steel can play a role in your product. While some products may only require standard finishes, others may need something extra in order to enhance surface appearances, improve environmental performance or even reduce safety concerns.
But what level of finish means that your metal is truly finished? That answer is ultimately up to you and the specifications you define for your project. Here is a closer look at the various stages of finish for your metal.
Let’s start with basic protection—which applies itself organically. All stainless steels are iron-based alloys containing at least 10.5% chromium. A reaction between that chromium and the oxygen in the atmosphere produces a chromium oxide film at the surface. This allows the film to immediately self-repair in the presence of oxygen in the event of any surface damage.
The next layer comes at the mill. Whether we are talking hot rolled or cold rolled, all stainless steel flat products are treated with a standard mill finish. Let’s review two of the more common mill finishes:
• 2B: This is achieved by cold rolling, heat treating and pickling, along with the application of a light rolling at the end in order to achieve a smooth and reflective sheen. Considered the most widely used surface finish, 2B is the basis for most polished and brushed finishes. Such common stainless steel grades as 201, 304, 304L and 316L come in a standard 2B finish.
• 2D This finish is also achieved by cold rolling, heat treating and pickling, but, unlike 2B, does not receive the light rolling. This achieves a low reflective matte surface appearance that is ideal for industrial and engineering applications yet has different aesthetics than 2B.
Applied finishes may further enhance the surface appearance and environmental performance of your product. This requires physical alteration of the metal, most notably through the use of abrasive material that removes portions from the surface.
The higher the abrasive grit used, the finer and more reflective the look. For example, one common applied finish is #4, which you will find on your average restaurant or kitchen equipment, food processing or dairy equipment, and elsewhere. This finish uses between 120 and 320 abrasive grit, depending on the application. For comparison, applied finish #3 uses between 100-120. The level of the grit impacts the surface roughness, which is typically measured in terms of micro-inches or micro meters, represented as Ra (roughness average). The higher the grit numbers, the finer the polishing lines and more reflective the finish.
Surface consistency is key to this type of finish. For example, some polishes can create a discoloration on the surface—think of a ‘checkerboard’ look to your metal.
Still Not Finished?
In some cases, even with an applied finish your material could be left with minor imperfections—often but not always invisible to the naked eye. For most users, this may not ultimately make a difference for applications of the metal, but, for others, specifically those with food, dairy, or pharmaceutical applications, the presence of microscopic scratches or pits that exist beneath the surface may have the potential to trap bacteria or other contaminants that could create health hazards for the products that are being produced.
One option is to explore finishes that are designed to be ‘pit free’. These types of finishes use an intense polishing method to eliminate any unseen pits, scratches or imperfections. Eliminating these imperfections early could also make the finished product is easier to clean, without the need to use harsh chemicals to eliminate that bacteria that develops. Check out more about what it means to create a pit-free finish from Guy Metals, part of the Ryerson Family of Companies.
In the end, determining your finish and the specific needs of your application and properly conveying those to your producer could be the final step to ensuring your stainless steel is truly complete.