Stainless steel, an iron-carbon alloy with a minimum of 10.5% chromium, is renowned for its corrosion resistance, deriving its name from its stain-free, corrosion-resistant, and rust-free properties.
In today’s demanding environments, stainless steel effortlessly performs throughout its lifespan. Accessing various qualities and easily attainable surface characteristics is straightforward.
Demonstrating high resistance to corrosion and staining, selecting the right quality is crucial for specific applications. Chromium forms a protective chromium oxide layer when exposed to oxygen, providing excellent oxidation resistance. This passive layer acts as a barrier, preventing water, gas, or oxygen penetration to the underlying metal.
Stainless steel’s corrosion resistance, low maintenance costs, affordability, and attractive appearance make it indispensable in commercial applications. Despite over 150 grades, 15 are widely recognized.
Stainless steel, like its counterparts, is used in various forms. It doesn’t tarnish, and its slightly lighter density than silver provides flexibility for designers.
Moreover, stainless steel is 100% recyclable, with about 60% derived from recycled materials.
The corrosion resistance of iron-chromium alloys was noted in 1821, and the aluminothermic method in the 1890s paved the way for practical utilization.
The modern era of stainless steel began in the early 1910s when its corrosion-resistant properties were discovered. 18-8 austenitic stainless steel (304) achieved success around the 1920s.
Varieties of Stainless Steel
Stainless steel comes in various types. For instance, adding nickel stabilizes the microstructure, resulting in a non-magnetic, less brittle steel. Increased carbon content enhances hardness for products like razors and cutting edges. Manganese, found in many steels, aids in preserving the austenitic structure.
Austenitic stainless steels, like the 300 series, contribute around 60% to global stainless steel production. Examples include 304 and 316, with 18% chromium and 8% nickel. They offer high ductility, excellent corrosion resistance, and are non-magnetic. “Super-austenitic” steels, such as AL-6XN and 254SMO, show superior corrosion resistance due to high molybdenum and nitrogen. While costlier, comparable performance can be achieved with less expensive ferritic or duplex stainless steels.
Ferritic stainless steels, known for their high chromium content (10.5% to 30%), lack nickel and exhibit excellent corrosion resistance. They are magnetic, cannot undergo heat treatment, and are easily formable. Recent developments have produced cost-effective ferritic grades like 430 and 442, offering corrosion resistance similar to austenitics without the use of expensive nickel.
Martensitic stainless steels, resembling low-alloy or carbon steels, gain increased strength through heat treatment due to extra carbon content. With 12% to 15% chromium, 0.2% to 1.0% molybdenum, and 0.1% to 1.2% carbon, they are magnetic. Heat-treated for hardness up to 60 HRC, tempering ensures optimal corrosion resistance. While slightly lower in corrosion resistance, they offer high machinability. Martensitic steels excel in applications requiring strength, wear resistance, and corrosion resistance, especially in tool steel applications.
Duplex stainless steels, with an even mix of ferrite and austenite, offer corrosion resistance influenced by alloy composition. Stronger than austenitics, especially against pitting and stress corrosion, they feature higher chromium, molybdenum, and lower nickel. However, susceptibility to embrittlement at extreme temperatures requires annealing after brief exposures above 300°C or below -50°C. The widely used 2205 duplex stainless steel showcases this dual-phase microstructure.
Known as “precipitation-hardened stainless steels,” these alloys blend martensitic and austenitic qualities with chromium and nickel. Achieving high strength through heat treatment like martensitic steels, they also maintain corrosion resistance similar to austenitic grades. The hardening process involves adding alloy elements such as copper, aluminum, titanium, niobium, and molybdenum. A prominent grade in this category is 17-4 PH, or 630, named after its 17% chromium and 4% nickel content, with 4% copper and 0.3% niobium.