Stainless steel is an alloy made by melting together various metals in an electric arc furnace. However, there is more to it, so let’s take a closer look at the fabrication process.
The Manufacturing Process
Stainless steel is manufactured via:
- Melting
- Casting
- Forming
- Annealing
- Descaling
- Cutting
- Packaging
Below we will discuss some of these steps, as well as some steps in between.
Melting
All the raw elements are fused together at 2800°F for 8-12 hours in an electric arc furnace to create stainless steel. The main additive that makes steel stainless is chromium. Besides it, other elements including silicon, molybdenum, nickel, iron, cobalt, manganese, tungsten, vanadium, and titanium are used in the alloy.
Some additives act as stabilizers. Ferrite stabilizers must be employed in austenitic alloys in a proportional relationship with austenite stabilizers to maintain the austenite composition. Austenite and ferrite stabilizers change the temperature at which different elements can coexist, thus making the alloying process easier.
Depending on the class and grade of stainless steel being manufactured, the fabrication process is slightly different. Specifically, the temperature of melting and heat treatment varies.
Melting stainless steel
Removing Carbon
Carbon content must be calibrated and reduced to the appropriate level before casting molten stainless steel.
Either argon oxygen decarburization (AOD) or vacuum oxygen decarburization (VOD) are employed at this stage—sometimes both are done. The first works by injecting an argon gas combination into the liquid steel, while the second involves injecting oxygen into steel in a heated vacuum chamber.
As the carbon is reduced, the temperature and composition are homogenized, and the mixture is tested for quality.
VOD and AOD
Casting
The liquid molten stainless steel is poured into slab, rod, billet, and other forms to then be put through forming processes with various equipment. Starting with hot rolling, which involves heating the steel and putting it through a lot of pressure rolls, slabs are made into plates, strips, and sheets, while blooms and billets are formed into wires and bars.
Casting stainless steel
Hot-Rolling ＆ Cold-Rolling
Slabs of stainless steel are heated in a furnace during the hot-rolling process. These heated slabs are then passed through pressure rollers. This procedure allows to change some metal properties.
Cold-rolling, on the other hand, does not involve heating the metal; it is passed through rollers and thinned by pressure alone.
The surface of hot-rolled steel is non-oily, scaly, and has gently rounded corners and edges. The surface and edges of cold-rolled steel are extremely smooth, with an oily texture.
Hot-rolling and cold-rolling steel
Annealing
Most forms of stainless steel require an annealing phase after the rolling step. Annealing reduces internal tensions and makes the steel more ductile.
Descaling
During annealing, the steel develops a buildup of material that is usually descaled using acid baths of acids, and chemical treatments. The stainless steel is either immersed in a solution of nitric-hydrofluoric acid or a current is passed over the stainless steel surface during electro-cleaning in a bath of phosphoric acid. A high-pressure water rinse is the final step of this process.
Descaling stainless steel
Hardening
To enhance the hardness of the steel, it is repeatedly reheated and chilled—a process called heat treatment or precipitation hardening. The resulting stainless steel is common in all sorts of industrial applications.
Stainless steel can also be hardened through work and hammering.
Cutting
Using shearing and blanking, stainless steel is cut into the required shapes and sizes. The sheets or rods or stainless steel will be cut according to the manufacturer’s needs or the client’s requirements.
Stainless steel is cut using circular knives, high-speed blades, punches, flames, plasma, or waterjet.
Cutting stainless steel
Finishing
Surface finish is a significant aspect of stainless steel production whenever aesthetics are considered. There are 13 standard finishes, which are the following:
| N0 | Hot-rolled, annealed |
| N1 | Hot-rolled, passivated |
| N2D | Cold-rolled, passivated |
| N2B | N2D, but with additional polish |
| N2BA | Specially annealed to look brighter |
| N3 | Coarse |
| N4 | Brushed |
| N5 | Satin |
| N6 | Matte |
| N7 | Reflective |
| N8 | Mirror |
| N9 | Bead-blasted |
| N10 | Heat-coloured |
Different finishes
Different methods are used for finishing. The finishes produced by heat rolling, annealing, and descaling are dull. On polished rolls, hot rolling is followed by cold rolling to create a brilliant finish. A mirror finish is achieved by polishing with progressively finer abrasives and then thoroughly buffing.
Using nitric or citric acid to clear some iron from the surface of stainless steel is called passivation. As a result, the surface becomes less reactive and less prone to corrosion.
An electrochemical solution is used in the electropolishing process to eat away the top layer of steel so that the metal gains a reflective and smooth surface.
Many finishes are also achieved through the use of abrasive discs that polish the surface of the metal. The finer the abrasives, the smoother will the finish be. For instance, a brushed finish needs very coarse abrasives, while a reflective finish requires extremely small beads and fibers as abrasives.
Finishing stainless steel
Testing & Packaging
To ensure proper quality and adherence to grade composition, mechanical and chemical testing procedures are carried out. Ability to resist impacts, pressures, and strains, as well as the toughness, tensile, and salt-spray corrosion tests are done.
The stainless steel is ready to be packed and supplied to manufacturers. They bend and weld it to make all the products that their clients request.