The Elements of Steel . Streamliners: America's Lost Trains . WGBH American Experience | PBS Iron is the main ingredient in various forms of iron and steel, but the various types of metals contain other elements as well. Sometimes these elements are unwanted; other times they're intentionally added. The Elements Used in Steel Carbon (C): Carbon, a nonmetallic element, forms a number of organic and inorganic compounds and can be found in coal, petroleum and limestone. It is the principle strengthening element in carbon steels and low-alloy steels. Atomic number 6, atomic weight 12.01115. Manganese (Mn): Manganese is a brittle, metallic element that exists in the ore of pyrolusite. When making steel, it reacts with sulfur and helps to increase the metal's resistance to heat. Atomic number 25, atomic weight 54.9380. Phosphorus (P): Phosphorus is a poisonous, nonmetallic element that helps protect metal surfaces from corrosion. Atomic number 15, atomic weight 30.9738.

Sulfur (S): Sulfur is a nonmetallic element found mainly in volcanic and sedimentary deposits. Sulfur, in the form of iron sulfide, can cause steel to be too porous and prone to cracking. Atomic number 16, atomic weight 32.064. Silicon (Si): Silicon is the second most abundant element in the earth's crust and can be found in rocks, sand and clay. It acts as a deoxidizer in steel production. Atomic number 14, atomic weight 28.086. Nickel (Ni): Nickel is a hard, metallic element that found in igneous rocks. Without nickel, stainless steel would be less resistant to heat and corrosion. Atomic number 28, atomic weight 58.71. Chromium (Cr): Chromium, a metallic element, is found in the earth's crust. It is used in the production of stainless steel to make the steel resistant to oxidation and corrosion. Atomic number 24, atomic weight 51.996.

The Elements of Steel Composition (percent by mass) Cast Iron Stainless Steel - General Information - Alloying Elements in Stainless Steel Stainless Steel - General Information - Alloying Elements in Stainless Steel Alloying Elements in Stainless Steel Stainless Steels comprise a number of alloying elements according to the specific grade and composition. Listed below are the alloying additions with the reason for their presence, whilst attched is a summary table. Carbon (C): Iron is alloyed with carbon lo make steel and has the effect of increasing the hardness and strength of iron. Pure iron cannot be hardened or strengthened by heat treatment but the addition of carbon enables a wide range of hardness and strength. In Austenitic and Ferritic stainless steels a high carbon content is undesirable, especially for welding due to the threat of carbide precipitation. Manganese (Mn): Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an Austenite forming element and has been used as a substitute for nickel in the AISI200 Series of Austenitic Stainless Steels, e.g. AISI 202 as a substitute for AISI 304.

Chromium (Cr): Chromium is added to steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steels have a minimum of 10.5% Chromium (traditionally 11 or 12%). This gives a very marked degree of general corrosion resistance when compared to steels with a lower percentage of Chromium. The corrosion resistance is due to the formation of a self-repairing passive layer of Chromium Oxide on the surface of the stainless steel. Nickel (Ni): Nickel is added in large amounts, over about 8%, to high Chromium stainless steels to form the most important class of corrosion and heat resisting steels. These are the Austenitic stainless steels, typified by 18-8 (304/1.4301), where the tendency of Nickel to form Austenite is responsible for a great toughness (impact strength) and high strength at both high and low temperatures. Nickel also greatly improves resistance to oxidation and corrosion. Molybdenum (Mo): Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting and crevice corrosion especially in chlorides and sulphur containing environments.

Nitrogen (N): Nitrogen has the effect of increasing the Austenite stability of stainless steels and is, as in the case of Nickel, an Austenite forming element. Yield strength is greatly improved when nitrogen is added to stainless steels as is resistance to pitting corrrosion. Copper (Cu): Copper is normally present in stainless steel as a residual element. However, it is added to a few alloys to produce precipitation hardening properties or to enhance corrosion resistance particularly in sea water environments and sulphuric acid. Titanium (Ti): Ttitanium is added for carbide stabilization especially when the material is to be welded. It combines with carbon to form titanium carbides, which are quite stable and hard lo dissolve in steel, which tends to minimise the occurrence of inter-granular corrosion. Adding approximately 0.25 / 0.60% titanium causes the carbon to combine with titanium in preference to chromium, preventing a tie-up of corrosion-resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.

However, the use of titanium has gradually decreased over recent years due to the ability of steelmakers to deliver stainless steels with very low carbon contents that are readily weldable without stabilisation. Phosphorus (P): Phosphorus is usually added with sulphur, to improve machinability. The Phosphorus present in Austenitic stainless steels increases strength. However, it has a detrimental effect on corrosion resistance and increases the tendency of the material to crack during welding. Sulphur (S): When added in small amounts Sulphur improves machinability. However, like Phosphorous it has a detrimental effect on corrosion resistance and weldability. Selenium (Se): Selenium was previously used as an addition to improve machinability. Niobium / Colombium (Nb): Niobium is added to steel in order to stabilise carbon, and, as such, performs in the same way as described for Titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service. SiIicon (Si): Silicon is used as a deoxidising (killing) agent in the melting of steel and as a result most steels contain a small percentage of Silicon.

Cobalt (Co): Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and, as a result, any stainless steel that is in nuclear service will have a Cobalt restriction, usually approximately 0.2% maximum. This problem is emphasized because there is normally a residual Cobalt content in the Nickel used in producing Austenitic stainless steels. Calcium (Ca): Small additions are used to improve machiniability, without the detrimental effects on other properties caused by Sulphur, Phosphorus and Selenium. Why Is Stainless Steel Stainless? Why Is Stainless Steel Stainless? Why Is Stainless Steel Stainless? What It Is and How It Works! Cookware is often made of stainless steel because it is durable, inexpensive, and doesn't rust. Coyau, Creative Commons Updated July 18, 2016. What Is Stainless Steel and Why Is it Stainless? In 1913, English metallurgist Harry Brearly, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel gives it stain resistance.

In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel . It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain 'less' than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion .

continue reading below our video 10 Best Universities in the United States (Iron, on the other hand, rusts quickly because atomic iron is much smaller than its oxide, so the oxide forms a loose rather than tightly-packed layer and flakes away.) The passive film requires oxygen to self-repair, so stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment. Types of Stainless Steel The three main types of stainless steels are austenitic, ferritic, and martensitic. These three types of steels are identified by their microstructure or predominant crystal phase. Austenitic: Austenitic steels have austenite as their primary phase (face centered cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not hardenable by heat treatment. The most familiar stainless steel is probably Type 304, sometimes called T304 or simply 304.

Type 304 surgical stainless steel is an austenitic steel containing 18-20% chromium and 8-10% nickel. Ferritic: Ferritic steels have ferrite (body centered cubic crystal) as their main phase. These steels contain iron and chromium, based on the Type 430 composition of 17% chromium. Ferritic steel is less ductile than austenitic steel and is not hardenable by heat treatment . Martensitic: The characteristic orthorhombic martensite microstructure was first observed by German microscopist Adolf Martens around 1890. Martensitic steels are low carbon steels built around the Type 410 composition of iron, 12% chromium, and 0.12% carbon. They may be tempered and hardened. Martensite gives steel great hardness, but it also reduces its toughness and makes it brittle, so few steels are fully hardened. There are also other grades of stainless steels, such as precipitation-hardened, duplex, and cast stainless steels. Stainless steel can be produced in a variety of finishes and textures and can be tinted over a broad spectrum of colors.

Passivation There is some dispute over whether the corrosion resistance of stainless steel can be enhanced by the process of passivation. Essentially, passivation is the removal of free iron from the surface of the steel. This is performed by immersing the steel in an oxidant, such as nitric acid or citric acid solution . Since the top layer of iron is removed, passivation diminishes surface discoloration. While passivation does not affect the thickness or effectiveness of the passive layer, it is useful in producing a clean surface for a further treatment, such as plating or painting. On the other hand, if the oxidant is incompletely removed from the steel, as sometimes happens in pieces with tight joints or corners, then crevice corrosion may result. Most research indicates that diminishing surface particle corrosion does not reduce susceptibility to pitting corrosion. Additional Reading Citric Acid Passivation of Stainless Steel - Lee V. Kremer's article discusses the use of citric acid passivation as an alternative to nitric acid passivation of stainless steel. Passivation of Stainless Steel - Dan Englebert's article defines passivation and describes how it is performed and verified.

Phase Diagram Glossary - These are definitions of several relevant terms, including 'ferritic steel', 'microstructure', and 'plain carbon steel'. The Stainless Steel Information Center - The Specialty Steel Industry of North America sponsors this resource, which includes general information about stainless steel, industry news, publications, workshops and training, and information for students. What Are Martensites? - This site offers a description and pictures of the martensitic crystal transformation in steels. What elements make up stainless steel? | Reference.com What elements make up stainless steel? A: Quick Answer According to U.K. metals stockholder Aalco, stainless steel production includes the alloying of iron with carbon, manganese, chromium, nickel, molybdenum, nitrogen, copper, titanium, phosphorus, sulphur, selenium, niobium, silicon, cobalt and calcium. Alloying additions depend on the steel's grade and composition. Full Answer Aalco explains that the various alloying additions serve varying purposes. Alloying iron with carbon is essential to manufacturing steel. The alloying addition of manganese serves to increase the strength of the stainless steel. The source also discusses the important role the alloying addition of chromium makes.

Chromium typically constitutes 11 or 12 percent of stainless steel, serving as protection against damage from oxidation. SSINA: Stainless Steel: Overview STAINLESS STEEL OVERVIEW: ALLOYING ELEMENTS IN STAINLESS STEEL Metals are rarely used in their pure form. Alloying elements are added to change their properties. âStainless Steelsâ are iron-based alloys that meet the ASTM A941 definition for this alloy family, specifically âa steel that conforms to a specification that requires, by mass percent, a minimum chromium content of 10.5 or more, and a maximum carbon content of less than 1.20.â There are over 150 different stainless steels with unique alloying element combinations. These alloying additions improved corrosion resistance in different service environments and determine strength levels, formability, machinability and other desirable characteristics. ROLE OF ALLOYING ELEMENTS IN STAINLESS STEEL CARBON Carbon is always present in stainless steel. The amount of carbon is the key. In all categories except martensitic, the level is kept quite low. In martensitic grade the level is deliberately increased to obtain high strength and hardness.

Heat treating by heating to a high temperature, quenching and then tempering develops the martensitic phase. Carbon can have an effect on the corrosion resistance. If the carbon is allowed to combine with the chromium (to form chrome carbides), it may have a detrimental effect on the ability of the “passive” layer to form. If, in localized areas, the chrome is reduced to below 10.5%, the layer will not form CHROMIUM Chromium is a highly reactive element and accounts for the “passive” nature of all stainless steels. The resistance to the chemical effects of corrosion and the typical “rusting” (oxidation) that occurs with unprotected carbon steel, is the direct result of the presence of chromium. Once the composition contains at least 10.5% chromium, an adherent and insoluble surface film is instantaneously formed that prevents the further diffusion of oxygen into the surface and prevents the oxidation of the iron in the matrix. The higher the chromium level the greater the protection. NICKEL Nickel is the essential allying element in the 300 series stainless steel grades.

The presence of nickel results in the formation of an “austenitic” structure that gives these grades their strength, ductility and toughness, even at cryogenic temperatures. It also makes the material non-magnetic. While the role of nickel has no direct influence on the development of the “passive” surface layer, it results in significant improvement in resistance to acid attack, particularly with sulfuric acid. MOLYBDENUM The addition of molybdenum to the Cr-Fe-Ni matrix adds resistance to localized pitting attack and better resistance to crevice corrosion (particularly in Cr-Fe ferritic grades). It helps resist the detrimental effects of chlorides (316 with 2% moly is preferred over 304 in coastal and de-icing salt situations). The higher the molybdenum content (there are stainless steels at 6% moly), the better the resistance to higher chloride levels. MANGANESE Generally manganese is added to stainless steels to assist in de-oxidation, during melting, and to prevent the formation of iron sulfide inclusions which can cause hot cracking problems.

It is also a “austenite” stabilizer and when added in higher levels (from 4 to 15%) replaces some of the nickel in the 200 series stainless steel grades. SILICON & COPPER Small amounts of silicon and copper are usually added to the austenitic stainless steels containing molybdenum to improve corrosion resistance to sulfuric acid. Silicon also improves oxidation resistance and is a “ferrite” stabilizer. In “austenitic stainless steels, high silicon contents improves resistance to oxidation and also prevents carburizing at elevated temperatures (309 and 310 are examples) NITROGEN In “austenitic” and “duplex” stainless steels, nitrogen increases the resistance to localized pitting attack and inter-granular corrosion. Low carbon “austenitic” grades (designated with an “L” since they contain less than 0.03% carbon), are suggested for welding operations, since the lower carbon minimizes the risk of sensitization. The low carbon levels, however, tend to reduce the yield strength. The addition of nitrogen helps to raise the yield strength levels back to the same level as standard grades.

NIOBIUM Niobium additions prevents inter-granular corrosion, particularly in the heat effected zone after welding. Niobium helps prevent the formation of chrome carbides, that can rob the microstructure of the required amount of chromium for passivation. In “ferritic” stainless steels the addition of niobium is an effect way to improve thermal fatigue resistance. TITANIUM Titanium is the main element used to stabilize stainless steel before the use of AOD (Argon-Oxygen Decarburization) vessels. When stainless steel is melted in air, it is difficult to reducing the carbon levels. 302, the most common grade before AOD’s, was allowed to have a maximum carbon level of 0.15%). At this high level, something was needed to stabilize the carbon and titanium was the most common way. Titanium will react with the carbon to form titanium carbides and prevent the formation of chrome carbides, that could affect the formation of the “passive” layer. Today all stainless steel are finished in an AOD vessel and the carbons levels are generally low due to the absence of oxygen.

The most common grade today is 304 (with 0.08 max carbon, although in reality the levels are lower). SULFUR Sulfur is generally kept to low levels as it can form sulfide inclusions. It is used to improve machinability (where these inclusion act as “chip breakers). The addition of sulfur, however, does reduce the resistance to pitting corrosion. Stainless steel corrosion Stainless Steel Corrosion The main reason for the existence of the stainless steels is their resistance to corrosion. Chromium is the main alloying element, and the steel should contain at least 11 %. Chromium is a reactive element, but it and its alloys passivate and exhibit excellent resistance to many environments. Higher Cr contents may be necessary for to improve the corrosion resistance of stainless steels in more aggressive media. Among other alloying elements, Nickel is the most important : it is added to control the alloy microstructure and to improve the corrosion resistance in acidic or caustic media. The addition of other elements such as Molybdenum, Tungsten, Copper, Silicon, Titanium, Niobium, Nitrogen , … enables a wide range of properties to be obtained.

All stainless steels contain some carbon. Even though it is difficult to get much less than about 0.03 % and sometimes carbon is deliberately added up to 1.00% or more, modern melting processes allow to reduce the content below 0.015 or even 0.010%, providing stainless steels with superior resistance to intergranular corrosion . The more carbon there is, the more chromium must be used, because carbon can take from the alloy about seventeen times its own weight of chromium to form carbides. Chromium carbide is of little use for resisting corrosion. The carbon, of course, is added for the same purpose as in ordinary steels to make the alloy stronger. Nitrogen is often added in modern stainless steels (especially in duplex grades or in highly alloyed austenitics); this element provides high mechanical properties, better resistance to pitting initiation and more generally improves the corrosion resistance thanks to a better microstructural stability. From a microstructural point of view, alloying elements may be classified as either Austenite formers (Nickel, Nitrogen, Carbon, …) or Ferrite formers (Chromium, Molybdenum, …).

Depending upon the ratio between austenite former elements and ferrite former elements, the resulting microstructure may be Ferritic (Body-Centered-Cubic crystal) or Austenitic (Face-Centered-Cubic crystal). The Martensitic structure (Close-Packed Hexagonal crystal) is the result of the transformation of austenite to martensite during cooling or during strong cold working. The Duplex microstructure is a mixture of Austenite and Ferrite (generally about 50/50%) which is the result of an accurate balance between Austenite and Ferrite former alloying elements. A large number of stainless steels are available. Their corrosion resistance, mechanical properties, and cost vary over a broad range. For this reason, it is important to specify the exact stainless steel desired for a given application. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook. The new interactive software CorrIntel™ covers the topic of the Corrosion Resistance and Metallurgy of Stainless Steels and Nickel base Alloys. There are five main types of stainless steel: ferritic, martensitic, austenitic, precipitation hardening and duplex.

The ferritic and martensitic grades are so named because of their crystal structures. Both are iron-chromium-based alloys and were the type of stainless steel first developed in the early 1900’s. The ferritic and martensitic stainless steels are fully magnetic; duplex stainless steels are partly magnetic. The martensitic stainless steels can be hardened by a heat treatment similar to that used to harden ordinary steel, namely, heating to a high temperature, quenching, then reheating to an intermediate temperature (tempering) to achieve the desired balance of hardness and ductility. Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at atmospheric and elevated temperatures, and are produced to cover a wide range of mechanical and physical properties for particular applications. Elements in Steel - West Yorkshire Steel Co Ltd Copper can be present in stainless steels for precipitation hardening properties. Used in “weathering” steels. Chromium (Cr) Chromium is added to steel to increase corrosion and oxidation resistance. It also increases hardenability and combined with high carbon improves wear and abrasion resistance. Iron (Fe) Iron is the base metal in steel, combining with other alloying elements to form all steel types.

It is relatively soft and weak in its pure form. Lead (Pb) Lead is added to improve machineability. Manganese (Mn) Manganese contributes to strength and hardness with variable carbon content. It is an austenite forming element in some steels and has a significant effect on hardenability. Molybdenum (Mo) Molybdenum is added to nickel chrome alloy steels to improve strength and hardness and also in chromium nickel austenitic steels it improves corrosion resistance. Molybdenum is used in some high speed steel grades. Nickel (Ni) Nickel is an important element which increases hardenability, tensile and impact values of steels. Added to high chromium stainless steels in amounts of over 8% it produces austenitic structures which gives high temperature strengths and resistance to oxidation and corrosion. Niobium (Nb) Niobium stabilises carbon in the same way as titanium and strengthens steels for high temperature service. Nitrogen (N) Nitrogen is added to stainless steel to improve the austenitic stability with increased yield strength. Phosphorous (P) Phosphorous is normally controlled to low levels but higher phosphorous can be used to improve machineability.

Selenium (Se) Selenium is added to steel to improve machineability. Silicon (Si) Silicon is a principal deoxidiser in steel, used in silicon manganese, corrosion and heat resisting steels. Sulphur (S) Sulphur is often added to improve machineability, but does decrease ductility and notch impact toughness. Tantalum (Ta) Tantalum is a refractory metal which is highly corrosion resistant. It is part of the refractory metals group, which are widely used as minor components in alloys and precipitation hardening stainless steels. Titanium (Ti) Titanium is commonly added to steel for carbide stabilization, combining with carbon to form titanium carbides. Tungsten (W) Tungsten is a major element in high speed and some tool steels. In the heat treated condition it retains hardness at elevated temperatures and is particularly useful for cutting tools. Vanadium (V) Vanadium helps improve fatigue stress and wear resistance when used with other alloying elements. Zirconium (Zr) Zirconium can be added to high strength low alloy steels, affecting inclusion improvement, giving toughness and ductility in bending modes.

All About Stainless Steel Home --> Learning Center --> All About Stainless Steel All About Stainless Steel The many unique values provided by stainless steel make it a powerful candidate in materials selection. Engineer, specifiers and designers often underestimate or overlook these values because of what is viewed as the higher initial cost of stainless steel. However, over the total life of a project, stainless is often the best value option. What is Stainless Steel? Stainless steel is essentially a low carbon steel which contains chromium at 10% or more by weight. It is this addition of chromium that gives the steel its unique stainless, corrosion resisting properties. The chromium content of the steel allows the formation of a rough, adherent, invisible, corrosionresisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen. There are more than 60 grades of stainless steel. However, the entire group can be divided into five classes.

Each is identified by the alloying elements which affect their microstructure and for which each is named. Benefits of Stainless Steel CORROSION RESISTANCE Lower alloyed grades resist corrosion in atmospheric and pure water environments, while high-alloyed grades can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments, properties which are utilized in process plants. FIRE AND HEAT RESISTANCE Special high chromium and nickel-alloyed grades resist scaling and retain strength at high temperatures. HYGIENE The easy cleaning ability of stainless makes it the first choice for strict hygiene conditions, such as hospitals, kitchens, abattoirs and other food processing plants. STRENGTH-TO-WEIGHT ADVANTAGE The work-hardening property of austenitic grades, that results in a significant strengthening of the material from cold-working alone, and the high strength duplex grades, allow reduced material thickness over conventional grades, therefor cost savings. Properties Stainless steel is defined as a ferrous alloy with a minimum of 10% chromium content. The name originates from the fact that stainless steel does not stain, corrode or rust as easily as ordinary steel.

This material is also called corrosion resistant steel when it is not detailed exactly to its alloy type and grade, particularly in the aviation industry. Stainless steels have higher resistance to oxidation (rust) and corrosion in many natural and man made environments; however, it is important to select the correct type and grade of stainless steel for the particular application. High oxidation resistance in air at ambient temperature is normally achieved with additions of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments. The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, meaning the metal stays shiny. It is, however, impervious to water and air, protecting the metal beneath. Also, when the surface is scratched this layer quickly reforms. This phenomenon is called passivation by materials scientists, and is seen in other metals, such as aluminium. When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off causing the parts to weld together. When disassembled, the welded material may be torn and pitted, an effect that is known as galling.

Commercial value of stainless steel Stainless steel's resistance to corrosion and staining, low maintenance, relative inexpense, and familiar luster make it an ideal base material for a host of commercial applications. There are over 150 grades of stainless steel, of which fifteen are most common. The alloy is milled into sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, and building material in skyscrapers and large buildings. See "Use in sculpture and building facades", below, for more. Stainless steel is 100% recyclable. In fact, over 50% of new stainless steel is made from remelted scrap metal, rendering it a somewhat eco-friendly material. Types of stainless steel There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels are used as razor blades, cutlery, tools etc. Significant quantities of manganese have been used in many stainless steel compositions.

Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost. Stainless steels are also classified by their crystalline structure: Austenitic stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition is 18% chromium and 10% nickel, commonly known as 18/10 stainless is often used in flatware. Similarly 18/0 and 18/8 is also available. "Superaustenitic" stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high Molybdenum contents (>6%) and nitrogen additions and the higher nickel content ensures better resistance to stress-corrosion cracking over the 300 series.

The higher alloy content of "Superaustenitic" steels means they are fearsomely expensive and similar performance can usually be achieved using duplex steels at much lower cost. Ferritic stainless steels are highly corrosion resistant, but far less durable than austenitic grades and cannot be hardened by heat treatment. They contain between 10.5% and 27% chromium and very little nickel, if any. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely strong and tough as well as highly machineable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), no nickel, and about 0.1-1% carbon (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic.

It is also known as "series-00" steel. Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, uses about 17% chromium and 4% nickel. Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50:50 mix although in commercial alloys the mix may be 40:60 respectively. Duplex steel have improved strength over austenitic stainless steels and also improved resistance to localised corrosion particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised by high chromium (19-28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. History A few corrosion-resistant iron artifacts survive from antiquity. A famous (and very large) example is the Iron Pillar of Delhi, erected by order of Kumara Gupta I around the year AD 400.

However, unlike stainless steel, these artifacts owe their durability not to chromium, but to their high phosphorus content, which together with favorable local weather conditions promotes the formation of a solid protective passivation layer of iron oxides and phosphates, rather than the non-protective, cracked rust layer that develops on most ironwork. The corrosion resistance of iron-chromium alloys was first recognized in 1821 by the French metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. However, the metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be of practical interest. This situation changed in the late 1890s, when Hans Goldschmidt of Germany developed an aluminothermic (thermite) process for producing carbon-free chromium. In the years 1904-1911, several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel.

In 1911, Philip Monnartz of Germany reported on the relationship between the chromium content and corrosion resistance of these alloys. Harry Brearley of the Brown-Firth research laboratory in Sheffield, England is most commonly credited as the "inventor" of stainless steel. In 1913, while seeking an erosion-resistant alloy for gun barrels, he discovered and subsequently industrialized a martensitic stainless steel alloy. However, similar industrial developments were taking place contemporaneously at the Krupp Iron Works in Germany, where Eduard Maurer and Benno Strauss were developing an austenitic alloy (21% chromium, 7% nickel), and in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless. Already in the year 1908 Krupp had built a famous sailing-yacht featuring a chrome-nickel steel hull, or so it seems its wreck being currently investigated by the Bureau of Archaeological Research of the State of Florida. Stainless Steel Grades Type 301-highly ductile, for formed products. Also hardens rapidly during mechanical working.

Type 303-Free machining version of 304 via addition of sulfur Type 304-the most common; the classic 18/8 stainless steel. Type 316-the next most common; for food and surgical stainless steel uses; Alloy addition of molybdenum prevents specific forms of corrosion. Also known as "marine grade" stainless steel due to its increased ability to resist saltwater corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. 400 Series-ferritic and martensitic chromium alloys Type 408-heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel. Type 409-cheapest type; used for automobile exhausts; ferritic (iron/chromium only). Type 410-martensitic (high-strength iron/chromium). Type 416 Type 420-"Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel". Also known as "surgical steel". Type 430-decorative, e.g., for automotive trim; ferritic.

Type 440-a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly. 500 Series-heat resisting chromium alloys 600 Series-martensitic precipitation hardening alloys Type 630-most common PH stainless, better known as 17-4; 17% chromium, 4% nickel Common Steel Alloying Agents | Properties and Effects Common Steel Alloying Agents | Properties and Effects Search the site Common Steel Alloying Agents | Properties and Effects How do alloying agents benefit steel? Updated November 20, 2016 Steel is essentially iron and carbon alloyed with certain additional elements. The process of alloying is used to change the chemical composition of steel and improve its properties over carbon steel or adjust them to meet the requirements of a particular application. Benefits of Steel Alloying Agents: Different alloying elements each have their own effect on the properties of steel. Some of the properties that can be improved through alloying include: Stabilizing austenite: Elements such as nickel , manganese , cobalt , and copper increase the temperatures range in which austenite exists.

Stabilizing ferrite: Chromium , tungsten , molybdenum , vanadium, aluminum , and silicon can have the effect of lowering carbon's solubility in austenite. This results in an increase in the amount of carbides in the steel and decreases the temperature range in which austenite exists. Carbide forming: Many minor metals, including chromium , tungsten, molybdenum, titanium , niobium, tantalum and zirconium, form strong carbides that - in steel - increase hardness and strength. Such steels are often used to make high-speed steel and hot work tool steel. Graphitizing: Silicon, nickel, cobalt and aluminum can decrease the stability of carbides in steel, promoting their breakdown and the formation of free graphite. Decrease of eutectoid concentration: Titanium, molybdenum, tungsten, silicon, chromium, and nickel all lower the eutectoid concentration of carbon. Increase corrosion resistance: Aluminum, silicon and chromium form protective oxide layers on the surface of steel, thereby protecting the metal from further deterioration in certain environments.

Common Steel Alloying Agents: Below is a list of commonly used alloying elements and their affect on steel (standard content in brackets): Aluminum (0.95-1.30%): A deoxidizer. Used to limit the growth of austenite grains. Boron (0.001-0.003%): A hardenability agent that improves deformability and machinability. Boron is added to fully killed steel and only needs to be added in very small quantities to have a hardening affect. Additions of boron are most effective in low carbon steels. Chromium (0.5-18%): A key component of stainless steels . At over 12 percent content, chromium significantly improves corrosion resistance. The metal also improves hardenability, strength, response to heat treatment and wear resistance. Cobalt: Improves strength at high temperatures and magnetic permeability. Copper (0.1-0.4%): Most often found as a residual agent in steels, copper is also added to produce precipitation hardening properties and increase corrosion resistance.

Lead : Although virtually insoluble in liquid or solid steel, lead is sometimes added to carbon steels via mechanical dispersion during pouring in order to improve machinability. Manganese (0.25-13%): Increases strength at high temperatures by eliminating the formation of iron sulfides. Manganese also improves hardenability, ductility and wear resistance. Like nickel, manganese is an austenite forming element and can be used in the AISI 200 Series of Austenitic stainless steels as a substitute for nickel. Molybdenum (0.2-5.0%): Found in small quantities in stainless steels, molybdenum increases hardenability and strength, particular at high temperatures. Often used in chromium-nickel austenitic steels, molybdenum protects against pitting corrosion caused by chlorides and sulfur chemicals. Nickel (2-20%): Another alloying element critical to stainless steels, nickel is added at over 8% content to high chromium stainless steel. Nickel increases strength, impact strength and toughness, while also improving resistance to oxidization and corrosion. It also increases toughness at low temperatures when added in small amounts.

Niobium: Has the benefit of stabilizing carbon by forming hard carbides and, so, is often found in high-temperature steels. In small amounts, niobium can significantly increase the yield strength and, to a lesser degree, tensile strength of steels as well as have a moderate precipitation strengthening affect. Nitrogen: Increases the austenitic stability of stainless steels and improves yield strength in such steels. Phosphorus: Phosphorus is often added with sulfur to improve machinability in low alloy steels. It also adds strength and increases corrosion resistance. Selenium: Increases machinability. Silicon (0.2-2.0%): This metalloid improves strength, elasticity, acid resistance and results in larger grain sizes, thereby, leading to greater magnetic permeability. Because silicon is used in a deoxidizing agent in the production of steel , it is almost always found in some percentage in all grades of steel. Sulfur (0.08-0.15%): Added in small amounts, sulfur improves machinability without resulting in hot shortness.

With the addition of manganese hot shortness is further reduced due to the fact that manganese sulfide has a higher melting point than iron sulfide. Titanium: Improves both strength and corrosion resistance while limiting austenite grain size. At 0.25-0.60 percent titanium content, carbon combines with the titanium, allowing chromium to remain at grain boundaries and resist oxidization. Tungsten: Produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures. Vanadium (0.15%): Like titanium and niobium, vanadium can produce stable carbides that increase strength at high temperatures. By promoting a fine grain structure, ductility can be retained. Zirconium (0.1%): Increases strength and limits grains sizes. Strength can be notably increased at very low temperatures (below freezing). Steel's that include zirconium up to about 0.1% content will have smaller grains sizes and resist fracture. Sources: How stainless steel is made - material, manufacture, used, processing, parts, components, structure, steps Process The manufacture of stainless steel involves a series of processes.

First, the steel is melted, To make stainless steel, the raw materials—iron ore, chromium, silicon, nickel, etc.—are melted together in an electric furnace. This step usually involves 8 to 12 hours of intense heat. Next, the mixture is cast into one of several shapes, including blooms, billets, and slabs. and then it is cast into solid form. After various forming steps, the steel is heat treated and then cleaned and polished to give it the desired finish. Next, it is packaged and sent to manufacturers, who weld and join the steel to produce the desired shapes. Melting and casting 1 The raw materials are first melted together in an electric furnace. This step usually requires 8 to 12 hours of intense heat. When the melting is finished, the molten steel is cast into semi-finished forms. These include blooms (rectangular shapes), billets (round or square shapes 1.5 inches or 3.8 centimeters in thickness), slabs, rods, and tube rounds. Forming 2 Next, the semi-finished steel goes through forming operations, beginning with hot rolling, in which the steel is heated and passed through huge rolls.

Blooms and billets are formed into bar and wire, while slabs are formed into plate, strip, and sheet. Bars are available in all grades and come in rounds, squares, octagons, or hexagons 0.25 inch (.63 centimeter) in size. Wire is usually available up to 0.5 inch (1.27 centimeters) in diameter or size. Plate is more than 0.1875 inch (.47 centimeter) thick and over 10 inches (25.4 centimeters) wide. Strip is less than 0.185 inch (.47 centimeter) thick and less than 24 inches (61 centimeters) wide. Sheet is less than 0.1875 (.47 centimeter) thick and more than 24 (61 centimeters) wide. Heat treatment 3 After the stainless steel is formed, most types must go through an annealing step. Annealing is a heat treatment in which the steel is heated and cooled under controlled conditions to relieve internal stresses and soften the metal. Some steels are heat treated for higher strength.

However, such a heat treatment—also known as age hardening —requires careful control, for even small changes from the recommended temperature, time, or cooling rate can seriously affect the properties. Lower aging temperatures produce high strength with low fracture toughness, while higher-temperature aging produces a lower strength, tougher material. Though the heating rate to reach the aging temperature (900 to 1000 degrees Fahrenheit or 482 to 537 degrees Celsius) does not effect the properties, the cooling rate does. A post-aging quenching (rapid cooling) treatment can increase the toughness without a significant loss in strength. One such process involves water quenching the material in a 35-degree Fahrenheit (1.6-degree Celsius) ice-water bath for a minimum of two hours. The type of heat treatment depends on the type of steel; in other words, whether it is austenitic, ferritic, or martensitic. Austenitic steels are heated to above 1900 degrees Fahrenheit (1037 degrees Celsius) for a time depending on the thickness. Water quenching is used for thick sections, whereas air cooling or air blasting is used for thin sections.

If cooled too slowly, carbide precipitation can occur. This buildup can be eliminated by thermal stabilization. In this method, the steel is held for several hours at 1500 to 1600 degrees Fahrenheit (815 to 871 degrees Celsius). Cleaning part surfaces of contaminants before heat treatment is sometimes also necessary to achieve proper heat treatment. Descaling 4 Annealing causes a scale or build-up to form on the steel. The scale can be removed using several processes. One of the most common methods, pickling, uses a nitric-hydrofluoric acid bath to descale the steel. In another method, electrocleaning, an electric current is applied to the surface using a cathode and phosphoric acid, and the scale is removed. The annealing and descaling steps occur at different stages depending on the type of steel being worked. Bar and wire, for instance, go through further forming steps (more hot rolling, forging, or extruding) after the initial hot rolling before being annealed and descaled. Sheet and strip, on the other hand, go through an initial annealing and descaling step immediately after hot rolling.

After cold rolling (passing through rolls at a relatively low temperature), which produces a further reduction in thickness, sheet and strip are annealed and descaled again. A final cold rolling step then prepares the steel for final processing. Cutting 5 Cutting operations are usually necessary to obtain the desired blank shape or size to trim the part to final size. Mechanical cutting is accomplished by a variety of methods, including straight shearing using guillotine knives, circle shearing using circular knives horizontally and vertically positioned, sawing using high speed steel blades, blanking, and nibbling. Blanking uses metal punches and dies to punch out the shape by shearing. Nibbling is a process of cutting by blanking out a series of overlapping holes and is ideally suited for irregular shapes. Stainless steel can also be cut using flame cutting, which involves a flame-fired torch using oxygen and propane in conjunction with iron powder. This method is clean and fast. Another cutting method is known as plasma jet cutting, in which an ionized gas column in conjunction with an electric arc through a small orifice makes the cut. The gas produces extremely high temperatures to melt the metal.

Finishing 6 Surface finish is an important specification for stainless steel products and is critical in applications where appearance is also important. Certain surface finishes also make stainless steel easier to clean, which is obviously important for sanitary applications. A smooth surface as obtained by polishing also provides better corrosion resistance. On the other hand, rough finishes are often required for lubrication applications, as well as to facilitate further manufacturing steps. Surface finishes are the result of processes used in fabricating the various forms or are the result of further processing. There are a variety of methods used for finishing. A dull finish is produced by hot rolling, annealing, and descaling. A bright finish is obtained by first hot rolling and then cold rolling on polished rolls. A highly reflective finish is produced by cold rolling in combination with annealing in a controlled atmosphere furnace, by grinding with abrasives, or by buffing a finely ground surface. A mirror finish is produced by polishing with progressively finer abrasives, followed by extensive buffing. For grinding or polishing, grinding wheels or abrasive belts are normally used. Buffing uses cloth wheels in combination with cutting compounds containing very fine abrasive particles in bar or stick forms.

Other finishing methods include tumbling, which forces The initial steel shapes—blooms, billets, slabs, etc.—are hot rolled into bar, wire, sheet, strip, and plate. Depending on the form, the steel then undergoes further rolling steps (both hot and cold rolling), heat treatment (annealing), descaling Ito remove buildup), and polishing to produce the finished stainless steel. The steel is then sent the end user. movement of a tumbling material against surfaces of parts, dry etching (sandblasting), wet etching using acid solutions, and surface dulling. The latter uses sandblasting, wire brushing, or pickling techniques. Manufacturing at the fabricator or end user 7 After the stainless steel in its various forms are packed and shipped to the fabricator or end user, a variety of other processes are needed. Further shaping is accomplished using a variety of methods, such as roll forming, press forming, forging, press drawing, and extrusion. Additional heat treating (annealing), machining, and cleaning processes are also often required. There are a variety of methods for joining stainless steel, with welding being the most common. Fusion and resistance welding are the two basic methods generally used with many variations for both.

In fusion welding, heat is provided by an electric arc struck between an electrode and the metal to be welded. In resistance welding, bonding is the result of heat and pressure. Heat is produced by the resistance to the flow of electric current through the parts to be welded, and pressure is applied by the electrodes. After parts are welded together, they must be cleaned around the joined area. Quality Control In addition to in-process control during manufacture and fabrication, stainless steels must meet specifications developed by the American Society for Testing and Materials (ASTM) with regard to mechanical properties such as toughness and corrosion resistance. Metallography can sometimes be correlated to corrosion tests to help monitor quality. The Future Use of stainless and super stainless steels is expanding in a variety of markets. To meet the requirements of the new Clean Air Act, coal-fired power plants are installing stainless steel stack liners. Other new industrial applications include secondary heat exchangers for high-efficiency home furnaces, service-water piping in nuclear power plants, ballast tanks and fire-suppression systems for offshore drilling platforms, flexible pipe for oil and gas distribution systems, and heliostats for solar-energy plants.

Environmental legislation is also forcing the petrochemical and refinery industries to recycle secondary cooling water in closed systems rather than simply discharge it. Reuse results in cooling water with elevated levels of chloride, resulting in pitting-corrosion problems. Duplex stainless steel tubing will play an increasingly important role in solving such industrial corrosion problems, since it costs less than other materials. Manufacturers are developing highly corrosion-resistant steels in respond to this demand. In the automotive industry, one steel manufacturer has estimated that stainless-steel usage per vehicle will increase from 55 to 66 pounds (25 to 30 kilograms) to more than 100 pounds (45 kilograms) by the turn of the century. New applications include metallic substrates for catalytic converters, air bag components, composite bumpers, fuel line and other fuel-system parts compatible with alternate fuels, brake lines, and long-life exhaust systems. With improvements in process technology, superaustenitic stainless steels (with nitrogen contents up to 0.5 percent) are being developed. These steels are used in pulp-mill bleach plants, sea water and phosphoric-acid handling systems, scrubbers, offshore platforms, and other highly corrosive applications.

A number of manufacturers have begun marketing such materials in sheet, plate, and other forms. Other new compositions are being developed: ferritic iron-base alloys containing 8 and 12 percent Cr for magnetic applications, and austenitic stainless with extra low sulfur content for parts used in the manufacture of semiconductors and pharmaceuticals. Research will continue to develop improved and unique materials. For instance, Japanese researchers have recently developed several. One is a corrosion-resistant stainless steel that displays the shape-memory effect. This type of material returns to its original shape upon heating after being plastically deformed. Potential applications include assembly components (pipe fittings, clips, fasteners, clamps), temperature sensing (circuit breakers and fire alarms), and springs. An improved martensitic stainless steel has also been developed for precision miniature and instrument rolling-contact bearings, which has reduced vibration levels, improved life expectancy, and better surface finish compared to conventional materials. Where To Learn More

Question: Which element is added to steel to make it stainless?
Answer: Chromium