Stainless steel is well known for its ubiquitous practical uses — making appearances in a range of commercial and industrial applications.
Since stainless steel contains chromium, it’s highly resistant to rust and corrosion.
Its ability to withstand high temperatures, high pressure and still be ductile and malleable makes it ideal for a wide range of oxidation resistant applications.
Stainless steels, by nature, are alloys of iron with a chromium content of at least 10.5 percent.
Other alloying elements in stainless steel include copper, titanium, nickel, molybdenum, nitrogen, and carbon. These elements are added to enhance its properties such as toughness and formability.
Due to its versatility, there is a wide range of stainless steel fabrication techniques.
The Importance of Chromium in Stainless Steel Fabrication
Chromium is a hard, corrosion resistant element that gives stainless steel its corrosion resistant properties. It’s an essential element in the manufacture of stainless steel and has no viable alternative.
In the presence of oxidizing agents, the stainless steel forms a passive film (chromium oxide) invisible to the naked eye on the surface.
As the film adheres strongly to the surface of the steel, it protects it from interacting with the environment, stopping the electrochemical reactions that cause corrosion.
How Stainless Steel is Made
The Manufacture of Stainless Steel Involves a Series of Processes.
1. Melting and Casting
The alloy materials are melted in an electric furnace for around 8 to 12 hours until the metals become molten.
The molten mixture is then decarburized in an Argon-Oxygen Decabuliser (AOD) to remove excess carbon. This involves blowing argon, oxygen, and nitrogen into the molten steel.
At this stage, other alloying elements such as nickel and molybdenum can be added enhance the properties of the stainless steel.
2. Tuning and Forming
Tuning involves stirring the steel slowly to remove unwanted elements and maintain the right chemical composition.
After tuning is complete, the molten steel is cast into forms. This is done by passing the refined stainless steel through a casting machine to form slabs, billets (round or square shapes), blooms (rectangular shapes) and tubes.
3. Hot Rolling
Hot rolling involves heat treatment of the final products after the forming process. It involves heating the slabs to between 1,100 to 1300 degrees Celsius, depending on the type of steel.
Through the heating, billets and blooms are further formed into bar and wire while slabs are formed into strips, sheets, and plates.
The plate thickness can range from 3mm to 60mm.
4. Cold Rolling
Cold rolling involves subjecting the rolls to a relatively low temperature. The process not only reduces the thickness of the rolls but also produces smooth, shiny finished, and cold rolled stainless steel.
5. Annealing and Pickling
Annealing is the process used to soften the stainless steel and improve its ductility. The annealing process results in a scale build up on the steel, which is often cleaned through a process called pickling.
Pickling involves bathing the steel in nitric hydrofluoric acid. Alternatively, the scale can be removed through electrocleaning — a process that involves the use of electric current to get rid of the scale.
Cutting operations are necessary to give the final product the desired shape or size.
Mechanical cutting involves the use of guillotine knives for straight shearing, circular knives for circle shearing, or steel blades. Other methods of cutting include nibbling and blanking.
In applications where the appearance of the stainless steel is important, applying a surface finish can give the steel an attractive look.
Some surface finishes also make the stainless steel easier to clean, which is important for kitchen appliances.
Surface finishes are in most cases the product of the stainless steel fabrication process. For example, annealing, hot scaling, and descaling result in a dull finish.
A combination of annealing and cold rolling in controlled temperatures produces a reflective finish. Abrasive belts and grinding wheels can also be used to polish steel.
To Sum Up
Currently, there are over 60 grades of stainless steel, all categorized by the alloy elements of their microstructure. Within these grades are the three most popular types of stainless steel, namely, austenitic, ferritic, and duplex.
Each type of stainless steel differs depending on the percentage of chromium and the proportion of other elements.
Contact us today for all your iron and steel supplies. We’ve been a successful supplier to the iron and steel industry for over 30 years, and have successfully designed hundreds of overhead crane industrial braking systems for the steel industry.