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About Stainless Steel



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AUSTENITIC - Refers to 300 series stainless, the most popular of stainless alloys accounting for 85-90 % of stainless fasteners sold. Named for Sir Robert Williams Austen, an English metallurgist, austenitic steel is a crystal structure formed by heating steel, chromium % nickel to high temperature where it forms the characteristics of 300 series stainless steel. An AUSTENITE is a molecular structure where 8 atoms of iron surrounded one atom of carbon, thus limiting the corrosive effects of the carbon. Austenitic fasteners gave high level of corrosion resistance in the stainless family, cannot be hardened by heat treatment, and are almost non magnetic. Sometimes heat & friction in cold forming can cause austenitic stainless to take slight magnetism, but the corrosion resistant properties remains the same. Most commonly used grade is termed as 304. Typical industries using these fasteners include: food, dairy, wine, chemical, pulp & paper, pharmaceutical, boating, swimming pool, pollution control, electronic, medical and hospital equipment, computer, textiles.

 Type 316 stainless has added nickel & added molybdenum (called moly) increase corrosion resistance to chlorides & sulfates, including sulfurous acids in pulp industry. It has superior tensile strength at high temperature compared to 304. Besides pulp & paper, typical industries using 316 are: photographic & other chemicals, ink, textile, bleach, rubber.

CARBIDE PRECIPITATION - Carbon that breaks loose from its bond within the stainless solution when materials are heated between 800 - 1400 degrees. Under severe corrosive conditions, it can result in extra oxidation & surface corrosion.

CARBON - adds strength to stainless steel, but also lowers corrosion resistance. The more carbon there is, the more chromium be added, because carbon offsets 17 times its own weight in chromium to form carbides, thus reducing chromium available for resisting corrosion.

CHROMIUM - A blue white metal, chromium is the most important element providing corrosion resistance in stainless steel. By adding 12 % chromium t o ordinary steel, stainless steel is formed. Chromium offsets the corrosion effects of carbon found in steel & is the primary factor in the ability of stainless steel to form a passive film on its surface providing corrosion resistance.

COLD FORMING (OR) CLOD HEADING (OR) COLD WORKING - When fasteners are produced without heating or small heat below recrystallization temperature (so the raw material bond of stainless remains unchanged) by processing metal wire against various dies at high speed to form a fastener's head or basic shape. Cold working causes an increase in tensile strength & hardness (known as work hardening) and a decrease in ductility.

COPPER - A reddish, metal that is an excellent conductor of heat & electricity. It is malleable, ductile & non magnetic with low to average strength & good corrosion resistance.

CREEP STRENGTH - A measure of resistance of fasteners to stress under elevated temperature. At higher temperature, fasteners can change in dimension under the same load, and is called creep. Creep can cause loosening of fasteners as temperature increases.

DISCONTINUITIES - A  of small or large disfigurations in a fasteners such as pits, tool marks, voids, laps, folds, & seams & inclusions. Minor discontinuities are permissible in both commercial fasteners & those made to various specs.

DRAWING - Where raw material shaped like wire is pulled through die to reduce its diameter to that needed for particular fasteners being manufactured.

DUCTILITY - The ability of fasteners to deform before breaking (for example and elastic would be more ductile than a diamond). Ductility is measurement similar to elongation.

ELONGATION - Stretching a fastener to the point that it breaks. The percentage of elongation at rupture (same as measure of ductility) is determined by dividing the total length after stretching to the original length. Elongation decreases as strength & hardness increases.

MAGNETISM - As related to stainless steel fasteners, 300 series stainless steel is non magnetic in its raw material condition. Cold working can sometimes induces traces of, magnetism in 300 series, depending on the severity of cold working & chemical composition of the stainless. A rise in magnetism is related to increase in tensile strength & work hardening cause by the heat & friction of cold forming & does not reduce corrosion resistance or cause any molecular change in austenitic raw material. A higher portion of nickel can increase stability in stainless steel, thus decreases work hardening & any possibilities of magnetism. Many stainless specs allow 2.0 magnetic permeability which translates to low / medium magnetism.

MANGANESE - A non magnetic metal which improves strength & hardness.

MOLYBDENUM - Nicknamed moly, molybdenum is a metal added to 316 stainless steel, sharply increasing its corrosion resistance to chlorides & sulfates, especially various sulfurous acids in the pulp industry. Molybdenum helps reduce hardness & increase tensile strength at higher temperatures.

NICKEL - A metal added to 300 series stainless to provide corrosion resistance increased strength in both high & low temperature, & increased toughness in low temperatures. Nickel lowers the effects of work hardening, thus reducing traces of magnetism caused by cold forming & making material flow more freely in manufacturing.

PASSIVATING - Technically, passivating is a process of dipping fasteners into 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. The purpose of passivating is to remove both grease left from manufacturing & traces of steel particles which may have rubbed off manufacturing tools onto the fastener.

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.

PROOF LOAD - A test load that a fastener must undergo without showing significant deformation. It is usually 90% of yield strength.

STAINLESS STEEL - With the addition pf 12% chromium to iron, stainless steel is formed. The chromium protects the iron against most corrosion or red colored rust; thus the term stainless steel. The ability of stainless to form a thin layer of protection on its outside surface, called a ' passive film , is its most important characteristic in preventing corrosion.

The over riding purpose of stainless steel is to provide corrosion resistance against : (a) atmospheric condition such as carbon dioxide, moisture, electrical fields, sulfur, salt & chloride compounds (b) Natural & artificially produced chemicals (c) Extreme weather where cold temperature cause brittleness & hot temperature reduce strength & increases corrosion.

TENSILE STRENGTH - A common measure to compare the strength of a fasteners. It is load needed to pull the fasteners apart.

TORQUE OR TORSION STRENGTH - Torque is the force used in twisting, such as tightening a fasteners. Torsion strength is the amount of force needed to twist a fasteners apart. Both measures considered the amount of pressure applied to the fastener & the length of the wrench used in application.

YIELD STRENGTH - The amount of pressure required to cause permanent deformity.


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