Raw Materials
Stainless steel metal is formed when the raw materials of nickel, iron ore, chromium, silicon, molybdenum, and others, are melted together. Stainless steel metal contains a variety of basic chemical elements that, when fused together, create a powerful alloy.
Different proportions of stainless steel elements—iron, nickel, chromium, molybdenum, and carbon (among others)—determine the type of stainless steel. The ratio of iron to other materials affects how strong the protective oxide layer is, how resistant the metal is to specific corrosives, and a few other mechanical properties (hardness, melting point, shear modulus, etc.).
These differing ratios of stainless steel components produce the different types of stainless steel alloys. Each unique combination is referred to as a “grade” of stainless steel—such as grade 304 stainless, grade 316 stainless, or grade 420 stainless steel.
Production Process
First, when making stainless steel, the manufacturer has to determine exactly which type of stainless steel they want to make. This is important because the grade of stainless steel they want to make will affect the ratio of stainless steel materials that will be present in the mixture, such as iron, carbon, nickel, etc. These ratios aren’t always exact—sometimes, they’re on a range because of the inevitable risk of variance in the purity of each element in the mix.
Once the raw materials are gathered, the rest of the stainless steel manufacturing process can begin.
- Melting the Raw Materials. The different stainless steel materials are placed in a furnace (typically an electric furnace for modern stainless steel manufacturing applications) and heated to their melting point. This process can take anywhere between 8 and 12 hours Once the metal is molten, stainless steel manufacturing can proceed to the next step.
- Removal of Excess Carbon. The molten material placed into a vacuum oxygen decarburization (VOD) or argon oxygen decarburization (AOD) system to remove excess carbon. Depending on how much carbon is removed, this process could result in a standard or a low-carbon variant of the alloy—for example, 304 versus 304L stainless steel. This can affect the tensile strength and hardness of the final product.
- Tuning or Stirring. To help fine-tune the quality of the final product, the molten steel may be stirred to help distribute and/or remove specific stainless steel components from the mixture. This helps to ensure that the stainless steel is of uniform quality and will meet the specifications required by end users .
- Forming the Metal. As the stainless steel begins to cool, it is put through a variety of forming processes—starting with hot rolling while the steel is still above its crystallization temperature. Hot rolling helps get the steel into a rough shape, and is often used to create billets or blooms of metal. To create metal blooms or billets of precise dimensions, the stainless steel may be cold rolled.
- Heat Treatment/Annealing. To relieve internal stresses and alter the stainless steel’s mechanical properties, it may be annealed (heated and cooled under controlled conditions). If annealed, the steel may need to be descaled so the protective oxide layer isn’t compromised.
- Cutting and Shaping. After the annealing process, stainless steel is put through a variety of cutting and shaping processes to create an ideal final product for the application. The specific operations used to cut the stainless steel will vary depending on the size and shape of the billet/bloom and the desired final product. For example, the steel may be cut mechanically with large metal shears if making thick metal plates. Meanwhile, CNC punch or laser cutting machines may be used to cut shapes out of thinner metal sheets.
- Applying Surface Finishes. The stainless steel manufacturer may apply different surface finishes to their stainless steel billets, blooms, or wires before shipping them to other manufacturers. The specific finish applied will vary depending on the steel’s intended use—but one of the most common surface finishes is simply grinding down the surface to remove impurities and make it smoother.