Stainless steel

Stainless steel is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Alternatively, it is known as inox (an abbreviation of the French term inoxydable, meaning non-oxidizable), corrosion-resistant steel (CRES), Nirosta (an abbreviation of the German term nichtrostender Stahl) or rustless steel. Stainless steel's resistance to corrosion comes from its chromium content of 10.5% or more, which forms a passive film that protects the material and can self-heal when exposed to oxygen. It can be further alloyed with elements like molybdenum, carbon, nickel and nitrogen to enhance specific properties for various applications. The alloy's properties, such as luster and resistance to corrosion, are useful in many applications. Stainless steel can be rolled into sheets, plates, bars, wire, and tubing. These can be used in cookware, cutlery, surgical instruments, major appliances, vehicles, construction material in large buildings, industrial equipment (e.g., in paper mills, chemical plants, water treatment), and storage tanks and tankers for chemicals and food products. Some grades are also suitable for forging and casting. The biological cleanability of stainless steel is superior to both copper and aluminium, and comparable to glass. Its cleanability, strength, and corrosion resistance have prompted the use of stainless steel in pharmaceutical and food processing plants. Different types of stainless steel are labeled with an AISI three-digit number. The ISO 15510 standard lists the chemical compositions of stainless steels of the specifications in existing ISO, ASTM, EN, JIS, and GB standards in a useful interchange table.

Properties

Corrosion resistance Although stainless steel does rust, this only affects the outer few layers of atoms. Its chromium content shields deeper layers from oxidation. The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength. Stainless grades vary chromium and molybdenum contents to suit the environment the alloy must endure. Corrosion resistance can be increased further by the following means:

increasing chromium content to more than 11% adding nickel to at least 8% adding molybdenum (which also improves resistance to pitting corrosion) Multi-layer protection, e.g., including both chromium oxide and manganese-based layers, which can withstand saltwater electrolysis at up to 1700 mV.

Strength The most common type of stainless steel, 304, has a tensile yield strength around 210 MPa (30,000 psi) in the annealed condition. It can be strengthened by cold working to a strength of 1,050 MPa (153,000 psi) in the full-hard condition. The strongest commonly available stainless steels are precipitation hardening alloys such as 17-4 PH and Custom 465. These can be heat treated to have tensile yield strengths up to 1,730 MPa (251,000 psi).

Melting point The melting point of stainless steel ranges from 1,325 to 1,530 °C (2,417 to 2,786 °F), depending on the alloy, which is near that of ordinary steel, and much higher than aluminium or copper.

Conductivity Like steel, stainless steels are relatively poor conductors of electricity, with significantly lower electrical conductivities than copper. In particular, the electrical contact resistance (ECR) of stainless steel arises as a result of the dense protective oxide layer and limits its functionality in applications as electrical connectors. Copper alloys and nickel-coated connectors tend to exhibit lower ECR values and are preferred materials for such applications. Nevertheless, stainless steel connectors are employed in situations where ECR poses a lower design criterion and corrosion resistance is required, for example in high temperatures and oxidizing environments.

Magnetism Martensitic, duplex and ferritic stainless steels are magnetic, while austenitic stainless steel is usually non-magnetic. Ferritic steel owes its magnetism to its body-centered cubic crystal structure, in which iron atoms are arranged in cubes (with one iron atom at each corner) and an additional iron atom in the center. This central iron atom is responsible for ferritic steel's magnetic properties. This arrangement also limits the amount of carbon the steel can absorb to around 0.025%. Grades with low coercive field have been developed for electro-valves used in household appliances and for injection systems in internal combustion engines. Some applications require non-magnetic materials, such as magnetic resonance imaging. Austenitic stainless steels, which are usually non-magnetic, can be made slightly magnetic through work hardening. Sometimes, if austenitic steel is bent or cut, magnetism occurs along the edge of the stainless steel because the crystal structure rearranges itself.

Wear Galling, sometimes called cold welding, is a form of severe adhesive wear, which can occur when two metal surfaces are in relative motion to each other and under heavy pressure. Austenitic stainless steel fasteners are particularly susceptible to thread galling, though other alloys that self-generate a protective oxide surface film, such as aluminum and titanium, are also susceptible. Under high contact-force sliding, this oxide can be deformed, broken, and removed from parts of the component, exposing the bare reactive metal. When the two surfaces are of the same material, these exposed surfaces can easily fuse. Separation of the two surfaces can result in surface tearing and even complete seizure of metal components or fasteners. Galling can be mitigated by the use of dissimilar materials (bronze against stainless steel) or using different stainless steels (martensitic against austenitic). Additionally, threaded joints may be lubricated to provide a film between the two parts and prevent galling. Nitronic 60, made by selective alloying with manganese, silicon, and nitrogen, has demonstrated a reduced tendency to gall.

Density The density of stainless steel ranges from 7.5 to 8.0 g/cm3 (0.27 to 0.29 lb/cu in) depending on the alloy.

History

… excerpt ends here. Continue reading the full article.