Stainless steel, also known as corrosion-resistant steel, is an iron-based alloy, containing a minimum of 11% chromium. Chromium present in it prevents it from getting corroded, by forming a self-healing film of chromium oxide on its surface. Apart from being corrosion resistant, it is also known for its qualities of high ductility, weldability, and cryogenic toughness. It is graded by taking into consideration the environment that it will be assigned to. The Grading system that is most commonly used for grading is the SAE Steel Grades, designated by the Society of Automotive Engineers (SAE), and the UNS grades.
Stainless steel grades are iron alloys that contain more than 10.5% of chromium. Other alloys are added to it to amplify its properties. The grading is based on the metallurgical structure and nature of the alloy. They are categorized into different families on the basis of the properties that they display.
Austenitic Stainless Steel - Series 100-300
The family of Austenitic are the alloys that contain 16% chromium and 6% nickel. Other elements such as copper, titanium, or molybdenum can be added to amplify its properties. The combination of all these elements makes it suitable for use in the applications involving high corrosion resistance and high temperature resistance. It is also effectively resistant to atmospheric corrosion and corrosion by organic, oxidizing, and mineral acids. It displays properties like high ductility, cryogenic toughness, and weldability. This makes it possible to fabricate austenitic steel by punching, spinning, welding, frilling, bending, etc. It is also non-magnetic. The important austenitic grades are the 200 series (chrome-manganese series) and the 300 series (chrome-nickel series). The 300 series is the largest manufactured type in the world. It is also used in the manufacturing of cutlery, kitchen appliances, consumer durables and also for construction purposes.
Ferritic Stainless Steel
The family of Ferritic alloys has chromium content of more than 10.5% and a low carbon content. They are plain chromium grades, which display magnetic properties, and cannot be hardened. They are not suitable for fabrication and are moderately resistant to corrosion. They can be polished and are used for the manufacture of automotive exhausts and sometimes for welding applications.
Duplex Stainless Steel - 2304 and 2205
The family of duplex alloys possesses a high chromium content of 18% to 28% and a moderate nickel content of 1% to 8%. They are a combination of austenitic and ferritic alloy structures and they may also contain molybdenum, copper, manganese, or tungsten. They display high yield strength and resistance to stress corrosion, cracking, and ion attack. These are used in the paper and pulp, petrochemical, and marine application industries, as well as in the desalination plants.
Martensitic Stainless Steel - Series 400
The family of martensitic are alloys having 11.5% to 18% chromium and high carbon content of 0.1% to 2%. It can be easily hardened by subjecting it to heat, and is highly resistant to abrasion, but it displays less resistance to corrosion compared to other alloys. It displays magnetic properties and is used in the manufacturing of surgical instruments, valves, knife blades, etc.
Precipitation Hardening Stainless Steel - Series 600
The family of precipitation hardening alloys contains chromium and nickel. They have the ability to develop high tensile strength. They can be treated with the solutions before being supplied, which makes them easily machinable.
The demand for all the above mentioned stainless steel grades is ever-increasing due to their versatile properties and also because they are totally recyclable.