What we call stainless steel is actually a blanket term for a family of metals selected for engineering because of their resistance to heat and corrosion. Stainless steels are all iron-based alloys, or mixtures of iron and other metals, made up of at least 10.5% chromium. This chromium content reacts with oxygen and moisture in the air to form a very thin, yet very effective protective film that adheres to the surface of the metal. Though this boundary layer is only 2-3 nanometres thick, it covers the entire surface of the material. When scratched or abraded in any way, it self repairs as the chromium reacts with oxygen and moisture to reform the oxide layer. Increasing the chromium content beyond this 10.5% minimum gives the material even greater corrosion resistance properties.
Other metals can be added to the alloy as well, each changing the properties of the resulting stainless steel composition in particular ways. Nickel is a common stainless steel alloy, and the addition of 8% or more can improve corrosion resistance as well as provide a range of other properties. Molybdenum provides further corrosion resistance as well, and nitrogen increases strength and resistance. The addition of other elements in different ratios can add or subtract from corrosion resistance, hardness, strength, or toughness.
Different Kinds of Steel
The different kinds of stainless steels can be classified based on the other metals they are alloyed with. The four basic stainless steel families are ferritic, martensitic, austenitic, and duplex. Within these family groups, there is a range of grades based on the range of their composition and defined by standards that vary by country.
Ferritic steels, for example, have a body-centered-cubic structure, like iron at room temperature, and are mainly alloyed with chromium. Not only is ferritic steel not hardenable by heat treatment, there is also a very low carbon content, together resulting in steel with limited strength. This strength limitation renders it a poor weld steel when welded in thick sections compared to other stainless families. It is also unacceptable for sub-zero temperatures, as it becomes brittle. Despite its comparatively low strength, ferritic steel’s ferromagnetic properties make it useful for other applications. It also has superior corrosion resisance to martensitic stainless steels.
Martensitic steels have a body-centered tetragonal lattice similar to ferritic steels, and their principal alloyed element is also chromium, at a similar rate. Unlike ferritic steels, martensitic stainless steels contain carbon, and can therefore be hardened and strengthened with heat treatment. Increasing the content of carbon in martensitic steels increases its resistance to scratching but decreases its toughness, or ability to absorb impact energy. With added nitrogen and nickel, and lowered carbon, martensitic steels have improved toughness, weldability, and corrosion resistance.
Austenitic Stainless Steels
Austenitic stainless steels are perhaps the most common kind of steel, as they are even more corrosion-resistant than ferritic and martensitic stainless steels. Austenitic stainless steel, like ferritic and martensitic steels, consists of chromium and iron, as well as nickel, and sometimes other alloying elemnets like molybdenum. Surgical steel, for example, or “SAE 304 stainless steel,” is the most familiar of this family, containing 18–20% chromium and 8–10% nickel. Austenitic grade stainless steels have superior corrosion resistance to both ferric and martensitic stainless steels. They do not exhibit a yield point, are extremely formable, and tough in extemely cold temperatures. They do not conduct heat well, and are nonmagnetic.
Duplex Stainless Steels
Duplex stainless steels are alloys of chromium, nickel, molybdenum, copper, and iron. Their microstructure consists of austenite and ferrite, matching the corrosion resistance of austenitic steels with the strength of ferritic steels. It is possible to weld duplex steels, but a balance of austenite and ferrite must be maintained, and forming duplex steels is possible but requires greater force than for austenitic stainless steels.
How Is it Made?
To make stainless steels, the raw materials must be melted together in extremely intense heat in an electric furnace, usually for 8 to 12 hours. After the materials are melted, the molten steel is cast into various forms, depending on what the steel will ultimately be used for. These forms might be rectangular-shaped “blooms”, small round or square “billets”, slabs, rods, or tube rounds.
Casting and Forming
Once the steel is melted and cast, it must be formed into its final shape. In order to do so, it goes through a process called being “hot rolled,” or heated again and rolled through huge rollers. If the molten steel has been cast into blooms or into billets, it will then be formed into 0.5 inch steel wire, or 0.25 inch (.63 cm) steel bar, available in a variety of shapes like rounds, squares, octagons, or hexagons. 3 Slabs of molten steel will be formed into plate, strip and sheet steel, all of which are classified by size. Plate steel is .47cm thick, and over 25cm wide. Strip is less than .47 cm thick, and less than 61cm wide. Sheet is less than .47cm thick, and more than 61cm wide.
Once the stainless steel has been formed into its final shape, it usually must be annealed. Annealing steel involves heating it and subsequently cooling it, which relieves internal stresses and softens the steel. It is possible to age harden, or heat treat, steel in order to give it greater strength, but this can be quite difficult to do, as any small changes can dramatically affect the properties of the finished product. Lower temperatures can cause higher strength and lower fracture toughness (i.e. cause the steel to be more brittle than ductile), and higher temperatures do the opposite, causing the steel to be tougher but of lower strength (i.e. more ductile than brittle).
The steel can be heated at any rate to reach the temperature required to age harden it, which is 900 to 1000 degrees Fahrenheit or 482 to 537 degrees Celsius,3 but the cooling rate must be carefully controlled. If the steel is rapidly cooled by quenching it in cold water immediately, the toughness can increase without losing strength. One example of this quenching process involves dipping the hot steel in a near-freezing ice-water bath for at least two hours. Different types of steels need to be treated in different ways. Austenitic steels need to be heated to a temperature greater than 1900 degrees Farenheit, and then quenched in water for thick sections or in air for thin sections. The steel has to be cooled relatively quickly to keep it clean.
Annealing, or the heat treatment, can cause a precipitate known as scale to build up on the steel. This scale can be removed in several ways, most commonly with a method known as pickling: a nitric-hydrofluoric acid bath. Another method of descaling is known as electrocleaning, which involves the application of an electric current to the surface of the steel using a cathode and phosphoric acid.3 Either of these methods can remove the scale from the steel. Descaling and annealing can be introduced to the steel at different stages in the process, depending on the type of steel being created. Sheet and strip steel, for example, are annealed and descaled immediately after they are hot rolled. They are then cold rolled, or passed through rolls at a much lower temperature to reduce thickness, before being annealed and descaled again. Bar and wire forms have to go through extra steps known as forging and extruding.
After the steel is melted, cast, formed, heat treated, annealed, and then pickled or electrocleaned, it must be cut into its desired shape and size. It can be cut mechanically, using guillotine knives for straight shearing or circular knives for circle shearing. Nibbling involves cuting a series of overlapping holes, and blanking involves metal punches and dies to punch the shape out by shearing it.4 Stainless steel can also be flame cut, involving an oxygen, propane, and iron powder flame, or cut using the plasma jet cutting method, where an ionized gas column melts and cuts the metal.
The final step in the manufacturing of stainless steel is the surface finish, which is necessary to give stainless steel the smooth, reflective surface that it is associated with. Surface finishing also provides further corrosion resistance, and makes the stainless steel easier to clean. Different finishes are applied based on the intended application of the stainless steel. Dull finishes are produced by the hot rolling, annealing, and descaling process. A bright, reflective finish can be produced using cold rolling and annealing, or by grinding or buffing using abrasives. To achieve a mirror finish, the steel must be polished with extremely fine abrasives using grinding wheels or an abrasive belt, and then buffed extensively with cloth wheels and cutting compounds made of very fine abrasive particles.
The Many Uses of Stainless
There is an extremely wide variety of applications for every variety and grade of stainless steel that is manufactured. We use stainless steel in the restaurant and hospitality industry, in the chemical industry, in the hospital and health care industries, and in manufacturing. The great number of uses that stainless steel has will only continue to expand, as its corrosion resistance remains important and new mechanical characteristics are recognized.
Kitchenware and Cutlery
Stainless steels are perhaps best known for their use in kitchenware and cutlery. Fine cutlery like knives use grade 410 and 420, or martensitic steel, which can be hardened and tempered to take and hold a sharp blade. This grade of steel is magnetic, which allows for the use of magnetic knife racks that are seen in some kitchens. Cutlery like forks and spoons are typically grade 304 stainless steel, which is a basic austenitic steel alloy of 18% chromium and 8% nickel. The 18/8 steel can tarnish, but it is ductile and resistant to corrosion, rusting and oxidizing acids. Type 304 stainless steel is perhaps the most common stainless steel and has uses in nearly every industry.
Type 304 is used extensively, along with type 316, in the food production and service industries. Stainless steel delivers no taste to food, making it of great use in kitchens everywhere. And though its anti-corrosive properties are useful, the popularity of stainless steel in the food industry is rather because stainless steel—especially type 316—is so quickly and efficiently cleaned. Type 316 is designed to hold up under tough anti-bacterial treatments.5 The medical industry uses stainless steel for similar reasons, employing stainless steel as standard for many kinds of hospital equipments because it is so easy to clean.
Stainless steel is used extensively in pipes, tanks, pumps, and valves in the chemical, processing, and oil and gas industries because of its non reactivity. One of the first uses for type 304 stainless steel was in the storage of dilute nitric acid, because even in thinner sections it did not corrode.5 Since then, different grades and types of stainless steel have been developed to be resistant to corrosion at very hot and very cold temperatures, making stainless steel an important metal for desalination and sewage plants as well as offshore oil rigs, harbor supports, and ship propellers.
The power generation industry has great use for stainless steel and other corrosion resistant alloys. Nickel alloys are particularly useful in the power industry for its high temperature strength and resistance to oxidation by fossil fuels. Even the nuclear power industry uses stainless steel extensively. Low-cobalt stainless steel is especially useful here.
Engineering and Automotive Industry
Stainless steel is used increasingly in building and construction as well as plumbing, as it is low-maintenance to upkeep and difficult to vandalize. And because it will not rust, even expensive stainless steel reinforcing bar can be used with concrete, and costs less over a life cycle because it does not have to be replaced or repaired.5 Stainless steel is used in the automotive industry for similar reasons, for exhaust systems and catalytic converters mostly, but also for general structure.
Stainless steel is an incredible material whose uses we continue to uncover. As we grow more concerned with the environment, and as our manufacturing industries expand, our uses for stainless steel continue to grow. Our ability to manufacture steel with properties that match the need lends itself to every industry, and our uses for stainless steel will likely only grow as we discover more of its properties.