STAINLESS STEEL TERMS
The following are some terms and their definitions you will encounter when dealing with stainless steel.
For discussions of 18-8, 304 & 316 stainless steel click here.
AN: Stands for Air Force-Navy.
ANSI: Stands for American National Standards Institute.
ASME: Stands for American Society of Mechanical Engineers.
ASTM: Stands for American Society of Testing and Materials.
AIRCRAFT QUALITY: Fittings made with a particularly high level of attention in manufacture and inspection.
ANNEAL: To heat metal in order to lower its hardness. The term anneal refers to the heat treatment given all 300 series stainless and most 400 series stainless by the steel mill after the raw material has been completed but before fittings are manufactured. Anneal also refers to the heat treatment given 400 series stainless fittings after their manufacture (also called hardening and tempering) to lower hardness and increase toughness. For example, fittings of 410 stainless may score over 200,000 psi after manufacture and be too brittle. By annealing at 1000 degrees F., tensile strength would reduce to 125,000-150,000 psi, while annealing the same material to 500 degrees F. would bring tensile to 160,000-190,000 psi.
BL: Stands for "Break Load", meaning the weight at which a product will break. Usually about 4 to 5 time the WLL (working load).
ELONGATION: Stretching a fitting to the point that it breaks. The percent of elongation at rupture (same as measure of ductility) is determined by dividing to total length after stretching to the original length. Elongation decreases as strength and hardness increase.
GALVANIC CORROSION:An accelerated degree of corrosion occurring when two different metals are in contact with moisture, particularly seawater. All metals have what is termed a specific electric potential, so that low-level electric current flows from one metal to another. A metal with a higher position in the galvanic series (see below) will corrode sacrificially rather than one with a lower position, meaning stainless, for example, will corrode before gold. The further apart the metals on the chart, the more electric current will flow and the more corrosion will occur. No serious galvanic action will occur by combining the same metals only dissimilar ones. To prevent galvanic corrosion, use insulation, paint or coatings when separating dissimilar metals; or put the metal to be protected next to a metal which is not important in the assembly, so it can corrode sacrificially. The metals listed at the top of the table below will corrode first due to galvanic reaction before those at bottom of the table.
MS: Stands for Military Standards. The overriding characteristic of MS fittings compared to commercial products is the extensive inspection and lot traceability for MS, guaranteeing the chemical, physical and dimensional qualities. While commercial fittings may look similar and happen to pass many tests given MS products, the commercial fittings lack the pedigree of guaranteed quality or chemical, physical and dimensional aspects that users who order MS fittings rely on.
MAGNETIC PERMEABILITY: Test simply determines the level of magnetism.
MARINE QUALITY: This term differs depending on the application.Fittings and hardware are usually made of Type 316 stainless to resist corrosion. They are usually highly polished. Nuts bolts, screws, and other fasteners and items where strength and wear are required are usually made of Type 304 (18-8).
MOLYBDENUM: Nicknamed moly, molybdenum is a metal added to 316 stainless steel, sharply increasing its corrosion resistance to chlorides and sulfates especially various sulfurous acids in the pulp industry. Molybdenum helps reduce hardness and increase tensile strength at higher temperatures.
MONEL: Invented by the International Nickel Co., and composed basically to two-thirds nickel, one-third copper. Monel has good strength, excellent corrosion resistance against salt water and in high temperatures, and is very expensive.
PASSIVATING: A very confusing term, since the common usage has taken on a different meaning that the technical definition. In SUNCOR's experience, users (including engineers) of commercial fittings seldom mean the old technical terminology. Technically, passivation is not cleaning but is a process of dipping fittings into a nitric acid solution to rapidly form a chromium oxide on the surface of the material, creating a passive film that protects stainless from further oxidation (see PASSIVE FILM). The purpose of passivation is to remove both grease left from manufacturing and traces of steel particles that may have rubbed off manufacturing tools onto the fitting. In common commercial parlance (meaning non-military and aerospace), passivating means cleaning to users, and the terms "passivating" and "cleaning" are used interchangeably. A wide range of cleaning methods using different mixtures containing nitric, phosphoric and other acids or simply exposing cleaned stainless fittings to air for a period of time will result in a "passivated" condition. For fittings that have been properly cleaned, it is impossible to determine the method of cleaning or passivation that was used. AN/MS/NAS fittings sold by SUNCOR have been cleaned, descaled, and passivated to the applicable engineering specifications.
PASSIVE FILM: The major characteristic of stainless is its ability to form a thin layer of protection called a "passive film" on its outside surface. This film results from a continual process of low-level oxidation, so oxygen from the atmosphere is needed for the passive film to exist. Once formed, it prevents further oxidation or corrosion from occurring. Even if chipped or scratched, a new passive film on stainless will form.
PICKLING: Removing surface impurities by using chemicals.
PITTING CORROSION: Pitting indicates deep corrosion in localized spots on a fastener. Dirt or grease on certain portions of a fastener may block oxygen from that surface, thus impeding the passsive film that protects stainless from corrosion.
PROOF LOAD: A test load that a fitting must undergo without showing significant deformation. It is usually 90% that of yield strength.
SOLUTION ANNEALED (same as CARBIDE SOLUTION ANNEALED) : A process of heating and removing carbide precipitants (carbon that has broken loose from its stainless steel solution) by heating a finished fitting to over 1,850 degrees F. and cooling it quickly, usually in water, so carbon content goes back into the stainless solution.
THREAD: Class 1 threads are a loose tolerance. Class 2 threads comprise 90% of stainless fittings and are normal commercial tolerance. Class 3 threads have a stricter tolerance and tighter fit such as socket cap and set screws. No definite relationship exists between tensile strength and tightness or looseness of fit. The symbol "A" added to threads, such as 2A, means external threads (screws), and "B" means internal (nuts).
WLL: Stands for "Work Load Limit", this is the recommended weight limit for safe use of a product.