Stainless steel has found extensive use in industrial, chemical, architectural, and consumer applications for over half a century. Stainless steel is a group of iron-based alloys that contains 10.5% or more of Chromium. The single most important property that distinguishes stainless steel is its corrosion resistance. Corrosion resistance, combined with manufacturing characteristics and good mechanical properties, renders stainless steel as a versatile and valuable metal.
Why is it called ‘Stainless’?
Stainless steel products are an alloy of iron (FE) and Chromium (Cr), consisting of a controlled amount of carbon © content. The Chromium in stainless steel forms a thin layer of chromium oxide on its surface. It is known as the “passive layer” and functions as a reliable protective coating that resists rust. It can even repair itself when damaged. Most stainless steel contains around 18% chromium. It helps harden and toughen the steel and increase its corrosion resistance, especially at high temperatures.
Stainless steel classification and grades
Since stainless steel was invented and developed initially, it has significantly advanced with various moderations in the alloy to yield the desired grades. The various grades of stainless steel are typically categorized under five families, namely,
- Austenitic: The primary microstructure consists of austenite. The austenitic group that contains Chromium and Nickel is identified as a 300 series type. On the other hand, those containing Chromium, nickel, and manganese are known as the 200 series. It shows excellent corrosion resistance, good formability, and strength. One can harden it by cold working, but not by heat treatment. It is non-magnetic in its annealed condition, but cold working may cause it to become slightly magnetic.
- Ferritic: These are plain chromium stainless steel, with chromium content varying between 12% and 18%. It has poor weldability, with lower formability than austenitic grades. It has, however, enhanced engineering abilities. One cannot strengthen or harden it, and it has poor weldability except in thin gauges. Ferritic grades are magnetic.
- Duplex: Duplex stainless steel has an annealed structure that consists of both austenite and ferrite in equal parts. It has relatively higher chromium content anywhere between 18 and 28%). It consists of nickel in moderate amounts (around 4.5 and 8%). It can effectively resist chloride stress corrosion cracking and exhibits twice the yield strength as the parent grades. It finds use in marine equipment, petrochemicals, and desalination plants.
- Martensitic: These are straight-chromium 400 Series types that consist of high carbon (0.1 – 1.2%) content. It shows poor weldability but fairly good ductility. It is hardenable by heat treatment and is magnetic. Martensitic stainless steel can resist corrosion in mild environments.
- Precipitation Hardening: The concerned group of stainless steel is Chromium-nickel alloys, consisting of other elements such as aluminum or copper. It combines austenitic and martensitic properties. After fabrication, you can harden it in a single low-temperature “aging” process. The corrosion resistance ranges from moderate and good and shows high strength. It finds applications in aerospace and other high-technology industries. The selection of appropriate heat-treating parameters can enhance the properties of this stainless-steel family.
These are the five known families of stainless steel. The various families have different metallurgical structures and characteristic properties that make them useful across various industries