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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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