AL-6XN® is a registered trademark held by Allegheny Ludlum.
Austenitic is the name given to the face-centered cubic crystal structure (FCC) of ferrous metals. Ordinary iron and steel have this structure at elevated temperatures; certain stainless steels (300 series) have this structure at room temperature. The austenitic grades are not magnetic. They cannot be hardened by heat treatment, but can be hardened significantly by cold-working.
Carbon (C) occurs in nature as the sixth most abundant element in the universe and the 19th element in order of mass in the Earth's crust. Carbon reduces many metals from their oxides when heated with the latter, and small amounts of it greatly affect the properties of iron. Carbides, which are carbon compounds with one or more metallic elements, can form during welding when carbon is present. By limiting the concentration of carbon, carbide formation is avoided, preventing intergranular corrosion in the weld heat affected zones.
Chi phase is a compound with a complex cubic structure, occurring as an intermetallic precipitate. The nominal composition is Fe36Cr12Mo10. The presence of chi phase stainless steel can lead to intergranular corrosion, due to depletion of chromium from the surrounding area.
Chloride ions in aqueous solution can penetrate and destabilize the passive chromium oxide film that protects stainless steel from corrosion. The iron chloride complexes formed are more soluble than the other reaction products, which results in further acceleration of the corrosive process. The corrosion rate increases when either chloride ion concentration or temperature are raised. (See Corrosion.)
Chromium (Cr) is a steel-gray, lustrous, hard, metallic element, and takes a high polish. The addition of a minimum of 12% chromium to steel makes stainless steel 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 film self-repairs in the presence of oxygen if the steel is damaged mechanically or chemically, and thus prevents corrosion from occurring.
Chromium carbide is a compound of chromium and carbon which can form during cooling of welding and other forming processes. Low carbon content steels are chosen to avoid the formation of chromium carbides, which reduce the concentration of chromium in the ajacent areas and thereby lower the corrosion resistance of the weld.
Coefficient of Expansion
The coefficient of expansion is the ratio of the change in length or volume of a body to the original length or volume for a unit change in temperature.
Crevice corrosion is the localized corrosive attack that occurs as a result of the occluded cell that forms at, or immediately adjacent to, an area that is shielded from full exposure to the environment because of close proximity between the metal and the surface of another material. Crevice corrosion may occur in small areas of stagnant solution in crevices, joints and under corrosion deposits. (See Corrosion.)
The elastic modulus (E) of a material represents the relative stiffness of the material within the elastic range and can be determined from a stress-strain curve by calculating the ratio of stress to strain.
Face-Centered Cubic Crystal Structure
The face-centered cubic crystal (fcc) structure is a dense arrangement of particles. Every atom in a face-centered cubic crystal structure forms bonds with twelve adjacent atoms. Austenitic stainless steels maintain the face-centered cubic (fcc) structure due to balancing alloying additions, primarily nickel, that stabilize the austenite phase from elevated to cryogenic temperatures.
A grain is a a portion of a solid metal (usually a fraction of an inch in size), in which the atoms are arranged in an orderly pattern. The irregular junction of two adjacent grains is known as a grain boundary. This narrow zone corresponds to the transition from one crystallographic orientation to another, thus separating one grain from another.
Molybdenum (Mo) metal is silvery white, very hard, but softer and more ductile than tungsten. It has a high elastic modulus, and only tungsten and tantalum, of the more readily available metals, have higher melting points. Molybdenum is added to stainless steel to increase resistance to pitting and crevice corrosion resistance, particularly in marine and acidic environments. 304 stainless steel contains no molybdenum while 316 stainless steel requires a molybdenum content of 2-3% which improves its resistance to chloride corrosion.
Nickel (Ni) is a silvery white metal that takes on a high polish. It is hard, malleable, ductile, somewhat ferromagnetic, and a fair conductor of heat and electricity. Nickel provides high degrees of ductility (ability to change shape without fracture) as well as resistance to corrosion. Approximately 65% of all nickel is used in the making of stainless steel. Nickel alloys are used for the more extreme conditions where general corrosion, pitting or SCC cause failure of the traditional stainless steels.
Elemental nitrogen (N) is a colorless gas making up about 78% of the Earth's atmosphere by volume. In metals, nitrogen is present in solid compounds with other elements, such as iron and chromium. Nitrogen is added to the AL-6XN alloy to minimize chi phase formation, to improve its corrosion resistance, increase its strength over a broad temperature range, and to retain the good formability of austenitic stainless steel.
Oxidizing chlorides are compounds such as ferric chloride, cupric chloride, hypochlorites, etc. which can oxidize metal surfaces, creating corrosion products.
Pitting corrosion is highly localized corrosion of a metal surface, confined to a point or small area, that takes the form of cavities or pits. Pitting typically occurs as a process of local anodic dissolution where metal loss is exacerbated by the presence of a small anode and a large cathode. (See Corrosion.)
Reducing agents readily give up electrons, thereby causing other species to be reduced. The reducing agent is oxidized in the process. Iron(II) in the presence of copper(II) is a reducing agent, giving up an electron and becoming iron(III). Potassium ferricyanide in water is a reducing solution.
Unwanted secondary phases impair corrosion performance and/or embrittle the metal. Examples of secondary phases are carbides, nitrides, and the intermetallic phases formed in metals with higher levels of Cr or Mo: sigma, chi, laves, alpha prime, R and tau.
Sigma phase is an intermetallic compound which can precipitate on grain boundaries as a secondary phase. The general composition of sigma phase is (Fe,Ni)3(Cr,Mo)2.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a cracking process that requires the simultaneous action of a corrosive agent and sustained tensile stress. The stresses may be significantly below the yield strength of the material, and can be residual or applied. Stress-corrosion cracking may occur in combination with hydrogen embrittlement. (See Corrosion.)
Super (austenitic) stainless steels do not fit the definition of a stainless steel because they contain over 50% non-ferrous elements. Compared to conventional austenitic stainless steels, superaustenitic materials have a superior resistance to pitting and crevice corrosion in environments containing halides.
The thermal conductivity of a substance is the quantity of heat in cal/sec passing through a body 1 cm thick with a cross section of 1 sq. cm when the temperature difference between the hot and cold sides of the body is 1 deg. C.
Unified Numbering System
The Unified Numbering System for Metals and Alloys is a systematic scheme in which each metal is designated by a letter followed by five numbers. It is a composition-based system of commercial materials and does not guarantee any performance specifications or exact composition with impurity limits. The UNS is managed jointly by the American Society for Testing and Materials and the Society of Automotive Engineers.