Stainless steel is everywhere. From cars to kitchens, hospitals to building sites, it’s estimated over half of all manufactured goods contain some quantity of stainless steel.
As for, “how is stainless steel made”, the origins of this question don’t travel very far back in time. It was first invented in the early 20th century, although the actual inventor is a matter of international contention. Whatever the case, this invention revolutionised the manufacturing industry, providing an alternative to traditional carbon steel that was resistant to rust, corrosion and staining.
Let’s take a look at how stainless steel is made.
How is Steel Made Stainless?
There are many types of steel, but the two most common are stainless steel and regular (or carbon) steel. Regular steel is made of iron and carbon. As for “how is steel made stainless” the answer is, by adding chromium. Stainless steel contains a minimum of 10.5% chromium and this is what helps to protect from oxygen and moisture by creating a protective barrier of chromium oxide.
In general, the higher the chromium content, the more corrosion-resistant the steel. It also creates a shiny, reflective surface which is very difficult to tarnish compared to plain steel.
How is Stainless Steel Made? Raw Materials
Overall, stainless steel is an alloy of iron, carbon and chromium, but can contain a number of other elements such as nickel, silicon and molybdenum. Applying differing proportions of each of these elements in how stainless steel is made will affect the characteristics of the resulting product.
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 such as hardness, melting point and shear modulus.
Each unique combination is referred to as a “grade” of stainless steel, such as grades 304 and 430 stainless steel. While grade 304 is noted for its high corrosion resistance, 430 is exceptionally hard. It’ss worth noting that one of the components of more modern stainless steel production is recycled or scrap stainless steel.
How Stainless Steel is Manufactured
The extreme temperatures and toxic nature of “metallurgy” mean that a high level of automation is standard in how stainless steel is made. However, manual input is required throughout. Workers in protective gear are there to feed furnaces, monitor the metal’s chemical composition, perform quality checks, operate machinery and keep track of orders.
How is Stainless Steel Produced? It Starts with Melting
The process of making stainless steel begins with melting down the raw components or materials. These are subjected to intense heat for somewhere between 8 and 12 hours in an electric arc furnace.
How is Steel made Stainless? Carbon Removal
Removing excess carbon from the molten metal, also known as refining it, is a vital part of the answer to “how is stainless steel produced”. For this, it’s placed in a machine called an Argon Oxygen Decarburization (or AOD) converter. A mix of argon and oxygen is piped into the machine, converting carbon and some other impurities to gases or causing them to collect on the liquid’s surface.
Further alloys might be added at this stage to adjust the chemical makeup. Scrap metal is also often added here to bulk up the mix. As a bonus, this scrap can cause a reaction which can be converted to fuel for the factory. For a lower carbon stainless steel, a different machine is used, known as a Vacuum Oxygen Decarburization (VOD) converter.
Fine Tuning the Chemical Composition
The tuning part of how stainless steel is manufactured is just that. Fine tuning the composition of the molten steel. This is done by stirring, and removing any extraneous materials. It also ensures even distribution of the components.
How is Stainless Steel Produced? Forming
With the mix just right, it’s time to cast the stainless steel into temporary forms. Moulds are used to shape the molten metal into slabs, tubes, rods, billets (cubes or cylinders) or blooms (cuboids). These will be further adjusted in the next phases of the manufacturing process.
How Stainless Steel is made into Final Forms: Hot rolling
The hot rolling phase is where the temporary forms are cast into their final, albeit uncut, shapes. For this, the forms are reheated, softening them before they are passed back and forth through hot rollers. The level of heat used will go towards determining the eventual grade of the stainless steel, but it must be above its recrystallization temperature. Slabs might be turned into strips or plates, while blooms might become bars or billets made into wire.
How is Stainless Steel Produced? Cold Rolling
Cold rolling refines and exacts the forms, using a series of smaller rolls to enhance the final finish of the steel. The “cold” part of the process’s name alludes to the fact that this is undertaken at a temperature below that of recrystallization.
How Stainless Steel is made more Ductile: Annealing
Now the steel forms are subjected to gradual heating and cooling. This process, known as annealing, reduces the stresses within the steel, softening it and boosting its ductility.
It’s common for scale to build up on the forms during the annealing process. There are two descaling methods. The first is immersion in hot nitric-hydrofluoric acid, while the second is subjecting it to an electric current or “electrocleaning”.
How is Stainless Steel Produced to Order? Cutting
So, the forms are almost ready, but not yet shaped to the clients’ requirements. So, how is stainless steel produced to meet this criteria? The forms are precision-sliced, usually by mechanical rotating cutting blades, although there are many possible cutting instruments, including guillotine and high-speed saws. Dies, punches and even flame cutting are also options.
The smoother the steel’s finish, the higher its corrosion resistance. There are numerous surface finish types available and various methods to achieve them. Grinding, polishing and buffing can be carried out by wheels and belts with abrasive or smooth surfaces. Etching can be carried out via chemical means, known as wet etching, or by sandblasting, known as dry etching.
And there you have it. The cold, hard facts about how stainless steel is made, including the answers to “how is stainless steel produced” as well as “how is steel made stainless”.