The manufacture of wrought stainless steel is a multi-step process: steel scrap is melted, then cast in a workable solid form; The compacted steel is shaped, heat treated, cleaned, and polished to meet desired specifications. Finally, the finished stainless steel is packaged and sent to the manufacturer.
Melt and casting
Steel and alloying metals are loaded into the electric arc furnace. Once in the furnace, the metal is heated to a certain temperature above its melting point, usually over 2800°F. Due to the extreme temperatures, precision, and large volumes required for steel production, the smelting stage generally takes 8 to 12 hours. Throughout this stage, the steel technician regularly checks the bath temperature and chemical composition.
After the steel alloy is completely melted, the mixture is refined. Argon and oxygen gases are pumped into the furnace, where they convert some impurities into gas and cause others to form slag for easy removal.
Fine steel is cast into machined ready-made shapes, including bloom (rectangular), billet (round or square), slabs, rods, and tube rounds.
Form
Hot rolling occurs above the recrystallization temperature of steel. Most cast steel is formed by hot rolling: slabs, blooms, or billets are heated and passed through large rollers, stretching the steel into longer and thinner shapes. Each slab is formed into sheets, plates, or strips, while blooms and billets are formed into wires and rods.
Cold rolling is used when more precise dimensions or superior surface shine are required. It occurs under the recrystallization temperature of steel. Cold rolling uses small-diameter wheels with a series of support wheels to make wide and smooth stainless steel sheets to seal tolerances.
Heat treatment
Heat treatment strengthens the rolled stainless steel by recrystallizing the deformed microstructure. Most stainless steels are heated by annealing – they are heated to a temperature just above their crystallization temperature, and cooled slowly under controlled conditions. This process reduces internal stress and softens stainless steel. Annealing temperature, time, and cooling rate all affect the properties of a complete steel.
Cleaning scale
A rolled piece of stainless steel acquires an oxidized “factory scale” coating, which needs to be cleaned to restore a glossy surface finish. Factory scale is usually removed through chemical means such as electro-cleaning and pickling.
In pickling, stainless steel is immersed in a nitric-hydrofluoric acid bath. Electro-cleaning uses a cathode and phosphoric acid to deliver current to the stainless steel surface. Both processes effectively remove surface scale. The metal whose scale is removed by high-pressure flushing of water, leaving a bright, glossy finish.
Cutting
Stainless steel is cut to a certain shape and size. Most cuts are done mechanically – stainless steel can be shaved with a circular blade, sawed off with a high-speed knife, or punched a punch. Alternative methods such as fire, plasma, and waterjet cutting are sometimes used.
Finish
Stainless steel can be made with a wide variety of surface finishes. The surface finish chosen is not purely aesthetic – certain coatings make stainless steel more resistant to corrosion, easier to clean, or easier to use in manufacturing. The type of final result is determined by the application in question.
The surface finish is the combined result of the fabrication process and finishing method. Hot rolling, annealing, and scale removal result in a dull finish. Hot rolling followed by cold rolling on polishing rolls produces a bright finish, while the combination of cold rolling, annealing, and rubbing with a smooth surface creates a reflective surface.
A series of grinding, polishing, buffing, and sandblasting equipment are used to finish the stainless steel surface.
Work hardening
Work hardening is the process of strengthening a material through deformation. Stainless steel hardens rapidly as a whole, with the exact grade determined by the specific grade. Austenitic steel hardens more easily compared to other grades.
Quality Control and Inspection
There are controls in the process throughout the manufacture and fabrication of stainless steel, but it is not enough to meet international quality standards. Before shipment, each batch of stainless steel must undergo chemical and mechanical testing to ensure it meets the desired specifications.
Mechanical testing measures the physical ability of stainless steel to withstand loads, pressures, and impacts. Mechanical tests include the tensile, Brinell, and toughness tests described above in mechanical properties.
Chemical tests check the exact chemistry of the sample before certifying stainless steel grades. Chemical tests are usually performed by non-destructive spectrochemical analysis.
Corrosion resistance is very important for stainless steels. Steel mills test and measure corrosion resistance by salt spray testing – the longer the steel is not damaged by corrosion after exposure to salt spray, the higher the corrosion resistance.
Manufacturing
Stainless steel is a very useful material used to manufacture a wide variety of products.
Most stainless steel is sent to manufacturers, where it undergoes further forming, heat treatment, machining, and welding to form specific products. Stainless steel is often used to manufacture components for the automobile, ship, aircraft, and food service industries, among many others.
