Stainless steel
Stainless steel, also known as inox, corrosion-resistant steel (CRES) and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains iron with chromium and other elements such as molybdenum, carbon, nickel and nitrogen depending on its specific use and cost. Stainless steel's resistance to corrosion results from the 10.5%, or more, chromium content which forms a passive film that can protect the material and self-heal in the presence of oxygen.[1]: 3
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.
The biological cleanability of stainless steel is superior to both aluminium and copper, and comparable to glass.[2] Its cleanability, strength, and corrosion resistance have prompted the use of stainless steel in pharmaceutical and food processing plants.[3]
Different types of stainless steel are labeled with an AISI three-digit number.[4] 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.[5]
Properties[edit]
Corrosion resistance[edit]
Although stainless steel does rust, this only affects the outer few layers of atoms, its chromium content shielding deeper layers from oxidation.
The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength.[6] Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure.[7] Corrosion resistance can be increased further by the following means:
Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final material. Any oxidation that forms on the surface (mill scale) is removed by pickling, and a passivation layer is created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.[83][84]
The following designations are used in the U.S. to describe stainless steel finishes by ASTM A480/A480M-18 (DIN):[85]
A wide range of joining processes are available for stainless steels, though welding is by far the most common.[86][52]
The ease of welding largely depends on the type of stainless steel used. Austenitic stainless steels are the easiest to weld by electric arc, with weld properties similar to those of the base metal (not cold-worked). Martensitic stainless steels can also be welded by electric-arc but, as the heat-affected zone (HAZ) and the fusion zone (FZ) form martensite upon cooling, precautions must be taken to avoid cracking of the weld. Improper welding practices can additionally cause sugaring (oxide scaling) and/or heat tint on the backside of the weld. This can be prevented with the use of back-purging gases, backing plates, and fluxes.[87] Post-weld heat treatment is almost always required while preheating before welding is also necessary in some cases.[52] Electric arc welding of type 430 ferritic stainless steel results in grain growth in the HAZ, which leads to brittleness. This has largely been overcome with stabilized ferritic grades, where niobium, titanium, and zirconium form precipitates that prevent grain growth.[88][89] Duplex stainless steel welding by electric arc is a common practice but requires careful control of the process parameters. Otherwise, the precipitation of unwanted intermetallic phases occurs, which reduces the toughness of the welds.[90]
Electric arc welding processes include:[86]
MIG, MAG and TIG welding are the most common methods.
Other welding processes include:
Stainless steel may be bonded with adhesives such as silicone, silyl modified polymers, and epoxies. Acrylic and polyurethane adhesives are also used in some situations.[91]
Most of the world's stainless steel production is produced by the following processes:
World stainless steel production figures are published yearly by the International Stainless Steel Forum. Of the EU production figures, Italy, Belgium and Spain were notable, while Canada and Mexico produced none. China, Japan, South Korea, Taiwan, India the US and Indonesia were large producers while Russia reported little production.[46]
Breakdown of production by stainless steels families in 2017:
Life cycle cost (LCC) calculations are used to select the design and the materials that will lead to the lowest cost over the whole life of a project, such as a building or a bridge.[92][93]
The formula, in a simple form, is the following:[94][95]
where LCC is the overall life cycle cost, AC is the acquisition cost, IC the installation cost, OC the operating and maintenance costs, LP the cost of lost production due to downtime, and RC the replacement materials cost.
In addition, N is the planned life of the project, i the interest rate, and n the year in which a particular OC or LP or RC is taking place. The interest rate (i) is used to convert expenses from different years to their present value (a method widely used by banks and insurance companies) so they can be added and compared fairly. The usage of the sum formula () captures the fact that expenses over the lifetime of a project must be cumulated after they are corrected for interest rate.
Application of LCC in materials selection
Stainless steel used in projects often results in lower LCC values compared to other materials. The higher acquisition cost (AC) of stainless steel components are often offset by improvements in operating and maintenance costs, reduced loss of production (LP) costs, and the higher resale value of stainless steel components.
LCC calculations are usually limited to the project itself. However, there may be other costs that a project stakeholder may wish to consider:
The average carbon footprint of stainless steel (all grades, all countries) is estimated to be 2.90 kg of CO2 per kg of stainless steel produced,[96] of which 1.92 kg are emissions from raw materials (Cr, Ni, Mo); 0.54 kg from electricity and steam, and 0.44 kg are direct emissions (i.e., by the stainless steel plant). Note that stainless steel produced in countries that use cleaner sources of electricity (such as France, which uses nuclear energy) will have a lower carbon footprint. Ferritics without Ni will have a lower CO2 footprint than austenitics with 8% Ni or more. Carbon footprint must not be the only sustainability-related factor for deciding the choice of materials:
Stainless steel is 100% recyclable.[97][98] An average stainless steel object is composed of about 60% recycled material of which approximately 40% originates from end-of-life products, while the remaining 60% comes from manufacturing processes.[99] What prevents a higher recycling content is the availability of stainless steel scrap, in spite of a very high recycling rate. According to the International Resource Panel's Metal Stocks in Society report, the per capita stock of stainless steel in use in society is 80 to 180 kg (180 to 400 lb) in more developed countries and 15 kg (33 lb) in less-developed countries. There is a secondary market that recycles usable scrap for many stainless steel markets. The product is mostly coil, sheet, and blanks. This material is purchased at a less-than-prime price and sold to commercial quality stampers and sheet metal houses. The material may have scratches, pits, and dents but is made to the current specifications.
The stainless steel cycle starts with carbon steel scrap, primary metals, and slag. The next step is the production of hot-rolled and cold-finished steel products in steel mills. Some scrap is produced, which is directly reused in the melting shop. The manufacturing of components is the third step. Some scrap is produced and enters the recycling loop. Assembly of final goods and their use does not generate any material loss. The fourth step is the collection of stainless steel for recycling at the end of life of the goods (such as kitchenware, pulp and paper plants, or automotive parts). This is where it is most difficult to get stainless steel to enter the recycling loop, as shown in the table below:
Nanoscale stainless steel[edit]
Stainless steel nanoparticles have been produced in the laboratory.[101][102] These may have applications as additives for high-performance applications. For example, sulfurization, phosphorization, and nitridation treatments to produce nanoscale stainless steel based catalysts could enhance the electrocatalytic performance of stainless steel for water splitting.[103]
Health effects[edit]
There is extensive research indicating some probable increased risk of cancer (particularly lung cancer) from inhaling fumes while welding stainless steel.[104][105][106][107][108][109] Stainless steel welding is suspected of producing carcinogenic fumes from cadmium oxides, nickel, and chromium.[110] According to Cancer Council Australia, "In 2017, all types of welding fumes were classified as a Group 1 carcinogen."[110]
Stainless steel is generally considered to be biologically inert. However, during cooking, small amounts of nickel and chromium leach out of new stainless steel cookware into highly acidic food.[111] Nickel can contribute to cancer risks—particularly lung cancer and nasal cancer.[112][113] However, no connection between stainless steel cookware and cancer has been established.[114]
