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Listed below are a
overviews of the following subjects:
-
Fastener Materials
-
Platings and Coatings
-
Characteristics of
Threaded Fasteners
-
Types of Threads
-
Characteristics of Rivets
- Hydrogen
Embrittlement
- Stress
Embrittlement
-
Mechanical Engineering
Schools
-
Society of
Manufacturing Engineers
Fastener Materials:
End users and OEM's
need a solid foundation for decision making and
normally look for guidance from fastener
manufacturers. Today, distributors are serving as a
key link in fastener supply to end user clients.
Bolts can be made from
many different materials, but most bolts are made of
carbon steel, alloy steel, or stainless steel.
Stainless steels include both iron and nickel based
chromium alloys. Titanium and aluminum bolts have
limited usage, primarily in the aerospace industry.
When selecting a grade
(chemistry) of steel, several factors must be
considered. Among these factors are product
diameter, geometry (shape), amount of cold (or hot)
forming required, finished mechanical properties,
heat treat capabilities, application requirements,
steel availability, and cost.
If a decision has to be made without answers to most
of the above concerns, as a minimum, the chemistry
should be selected based on the desired mechanical
properties of the finished part. In general, Grade 5
products are made from plain (non-alloy) medium
carbon steel such as 1038. Grade 8 products are made
from medium carbon alloys such as 4037, 4140, 8637
and 4340.
Carbon steel is the cheapest and most common bolt
material. Many hardware stores sell carbon steel
bolts, which are usually zinc plated to resist
corrosion. The typical ultimate strength of this
bolt material is 55 ksi.
An alloy steel is a high-strength carbon steel that
can be heat treated up to 300 ksi. However, it is
not corrosion resistant and must therefore have some
type of coating to protect it from corrosion.
Aerospace alloy steel fasteners are usually cadmium
plated for corrosion protection.
Bolts of stainless steel (CRES) are available in a
variety of alloys with ultimate strength ranging
from 70 to 220 ksi. The major advantage of using
CRES is that it normally requires no protective
coating and has a wider service temperature range
than plain carbon or alloy steels.
Steel Fasteners:
Over 90 percent of all fasteners are manufactured
using carbon steel. The reason is quite simple.
Steel has excellent workability, offers the broadest
range of attainable comparison with other commonly
used fastener materials, it's inexpensive.
In general, use of carbon steel for fasteners can be
catalogued into 3 broad groupings, low carbon,
medium carbon, and alloy steel. They come in varying
grades with variable strength characteristics.
Stainless Steel
Fasteners: All stainless steel contains some
carbon, however carbon content is usually quite low
and it's carefully controlled. Additionally all
stainless steels have a mix of other alloying
elements. Nickel is the most important. It
dramatically improves corrosion resistance, adds
toughness in low temperature exposures, and helps
retain strength.
Austenitic
Stainless Steel Fasteners: About 80% of all
stainless steel fasteners are produced from the
austenitic grades, they are commercially nicknamed
"18-8 Series" or "300 Series Stainless." They have
better corrosion resistance than other stainless
groups, and are non-magnetic. They are non-heat
treatable, however they have excellent strength
properties and toughness at extremely low
temperatures. (Types 302, 303, 304, 305, 316, 321,
384)
Martensitic
Stainless Steel Fasteners: About 10% of all
stainless steel fasteners are made using martensitic
stainless steel, which are magnetic. The corrosion
resistance offered by martensitic steels is not as
good, or as versatile as other group, however in
their heat-treated condition; they perform
adequately in most atmospheric and marine exposures.
(Types 410, 416, and 431)
Brass Fasteners:
Used for corrosion resistance, and its thermal and
electrical conductivity. Usually recognized for its
yellow or gold coloring, and the most popular of
copper alloys. Brass tends to be soft and not high
strength, it is also non-magnetic.
Bronze Fasteners:
Similar to brass in application but provides higher
strength. Has more of a red/orange color to it.
Aluminum Fasteners:
This dull silver or gray non-magnetic material is
synonymous with lightweight. Aluminum fasteners
weigh about one-third those of steel. The
strength-to-weight ratio of aluminum fasteners is
better than any other commercially used fastener
material, but only the most common fasteners are
available in this material.
Nylon Fasteners:
This non-metallic plastic is lightweight, corrosion
resistant, they have excellent thermal and
electrical insulating properties, and they are
easily colored for appearance, matching and
identification. To their detriment, plastics are low
strength, they can't tolerate evenly moderately
elevated temperatures, and many of them quickly
embrittle in relatively low temperatures.
Plating and
Coatings For Fasteners:
Usually coatings and
platings are less expensive than going to an upgrade
of material like stainless steel from a basic carbon
steel. Coatings or platings may help to improve
appearance, control torque tension, minimize thread
seizure, and may serve as product identifiers in
addition to simply providing corrosion protection.
Most plating processes
are electrolytic and generate hydrogen. Thus, most
plating processes require baking after plating at a
temperature well below the decomposition temperature
of the plating material to prevent hydrogen
embrittlement. However, heating the plating to its
decomposition temperature can generate free hydrogen
again. Thus, exceeding the safe operating
temperature of the plating can cause premature
fastener failure due to hydrogen embrittlement as
well as loss of corrosion protection.
Cadmium Plating of
Fasteners:
The most common aerospace fastener plating material
is cadmium. Plating is done by electrodeposition and
is easy to accomplish. However, cadmium-plated parts
must be baked at 375 F for 23 hours, within 2 hours
after plating, to prevent hydrogen embrittlement.
Since cadmium melts at 600 F, its useful service
temperature limit is 450 F.
Zinc Plating of Fasteners:
Zinc is also a common type of plating. The hot-dip
method of zinc plating is known commercially as
galvanizing. Zinc can also be electrodeposited.
Because zinc plating has a dull finish, it is less
pleasing in appearance than cadmium. However, zinc
is a sacrificial material. It will migrate to
uncoated areas that have had their plating scratched
off, thus continuing to provide corrosion
resistance. Zinc may also be applied cold as a
zinc-rich paint. Zinc melts at 785 F but has a
useful service temperature limit of 250 F. (Its
corrosion-inhibiting qualities degrade above 140 F.)
Phosphate Coatings for Fasteners:
Steel or iron is phosphate coated by treating the
material surface with a diluted solution of
phosphoric acid, usually by submerging the part in a
proprietary bath. The chemical reaction forms a
mildly protective layer of crystalline phosphate.
The three principal types of phosphate coatings are
zinc, iron, and manganese. Phosphate-coated parts
can be readily painted, or they can be dipped in oil
or wax to improve their corrosion resistance.
Fasteners are usually coated with either zinc or
manganese phosphate. Hydrogen embrittlement seldom
is present in such parts. Phosphate coatings start
deteriorating at 225 F (for heavy zinc) to 400 F
(for iron phosphate).
Nickel Plating of Fasteners:
Nickel plating, with or without a copper strike
(thin plating), is one of the oldest methods of
preventing corrosion and improving the appearance of
steel and brass. Nickel plating will tarnish unless
followed by chromium plating. Nickel plating is more
expensive than cadmium or zinc plating, and also
must be baked as cadmium to prevent hydrogen
embrittlement. Nickel plating is good to an
operating temperature of 1100 F, but is still not
frequently used for plating fasteners because of its
cost.
Chromium Plating of
Fasteners:
Chromium plating is commonly used for automotive and
appliance decorative applications, but it is not
common for fasteners. Chromium-plated fasteners cost
approximately as much as stainless steel fasteners.
Good chromium plating requires both copper and
nickel plating prior to chromium plating. Chromium
plating also has hydrogen embrittlement problems.
However, it is acceptable for maximum operating
temperatures of 800 to 1200 F.
Characteristics of
Threaded Fasteners:
Threaded fasteners are
probably one of the most commonly used types of
fasteners. Threaded fasteners enjoy the following
primary advantages:
Strength: Properly assembled, threaded
fasteners provided very high strength connections.
Bolted joints and screws are most often stronger
than a glued or nailed joint. The primary reason
this is true is that, while glues and nails hold two
materials together, a bolt actually squeezes the
joint together with a clamping force (called the
preload), that must be overcome before the joint can
separate. This is also true, albeit to a lesser
extent, with wood screws, although bolted joints are
much stronger, since the limiting factor in a
screwed joint is often the material itself, not the
screw.
Low Cost:
Bolts, washers, and nuts are extremely inexpensive,
and are widely available in most any size desired.
Reversibility: Bolted joints are one of the types of
joints that are reversible i.e., the joint can be
taken apart after it is put together. Glued joints,
press-fit joints, and welds/brazes are obviously
very difficult if not impossible to take apart. This
feature of bolted joints is especially important in
experimental machines, since the design is very
often changed. Holes for bolts can easily be drilled
in new places as the need arises.
Of course, there are
disadvantages to using threaded fasteners as well.
The most common ones are listed below:
Weight: Bolts,
washers, and nuts add up quickly, and very often a
bolted joint will turn out to be much heavier than
another type of joint. One way to get around this,
of course, is to begin with a bolted joint and then,
when the design has been finalized, convert it to
another type by removing the bolts and fastening it
another way, such as brazing.
Dynamics:
Bolted joints tend to be susceptible to vibration
and fatigue, sometimes loosening or even coming
completely out when used in cyclic loading or
vibration conditions. There are a variety of schemes
that are used to prevent bolted joints from
"vibrating loose", such as safety wire, cotter pins,
and self-locking nuts. These features, however, make
the joint more expensive and time consuming to
install, taking away from the benefit of using this
type of joint in the first place.
Maintenance:
Bolted joints require that the joints be assembled
using wrenches, which can often become a pain when
space is tight. Bolts in awkward places that must be
re-tightened or installed can sometimes require that
other pieces be removed to allow access, when other
types of joints could be affected without such a
painful process.
Holes: The
holes that must be put into joined parts can
sometimes weaken the part substantially, causing it
to fail. Another, related problem with holes is
alignment. Bolting two parts together with many
bolts requires that the parts have matching holes,
which can also sometimes become a pain if you don't
have a drill press. ( A good way around this is to
temporarily clamp the parts together, and then drill
through both parts to be joined at the same time.)
Types of Threads:
The common thread types are unified national coarse
(UNC), unified national fine (UNF), unified national
extra fine (UNEF), UNJC, UNJF, UNR, UNK, and
constant-pitch threads.
Unified national coarse: UNC is the most
commonly used thread on general-purpose fasteners.
Coarse threads are deeper than fine threads and are
easier to assemble without cross threading. The
manufacturing tolerances can be larger than for
finer threads, allowing for higher plating
tolerances. UNC threads are normally easier to
remove when corroded, owing to their sloppy fit.
However, a UNC fastener can be procured with a class
3 (tighter) fit if needed (classes to be covered
later).
Unified national fine: UNF thread has a
larger minor diameter than UNC thread, which gives
UNF fasteners slightly higher load-carrying and
better torque-locking capabilities than UNC
fasteners of the same identical material and outside
diameter. The fine threads have tighter
manufacturing tolerances than UNC threads, and the
smaller lead angle allows for finer tension
adjustment. UNF threads are the most widely used
threads in the aerospace industry.
Unified national extra fine: UNEF is a still
finer type of thread than UNF and is common to the
aerospace field. This thread is particularly
advantageous for tapped holes in hard materials and
for thin threaded walls, as well as for tapped holes
in thin materials.
UNJC and UNJF threads: "J" threads are made
in both external and internal forms. The external
thread has a much larger root radius than the
corresponding UNC, UNR, UNK, or UNF threads. This
radius is mandatory and its inspection is required,
whereas no root radius is required on UNC, UNF, or
UNEF threads. Since the larger root radius increases
the minor diameter, a UNJF or UNJC fastener has a
larger net tensile area than a corresponding UNF or
UNC fastener. This root radius also gives a smaller
stress concentration factor in the threaded section.
Therefore, high-strength (180 ksi or more) bolts
usually have "J" threads.
UNR threads: The UNR external thread is a
rolled UN thread in all respects except that the
root radius must be rounded. However, the root
radius and the minor diameter are not checked or
toleranced. There is no internal UNR thread.
UNK threads: The UNK external thread is
similar to UNR, except that the root radius and the
minor diameter are toleranced and inspected. There
is no internal UNK thread.
Constant-pitch threads: These threads offer a
selection of pitches that can be matched with
various diameters to fit a particular design. This
is a common practice for bolts of 1-in. diameter and
above, with pitches of 8, 12, or 16 threads per inch
being the most common.
Classes of Threads:
Thread classes are distinguished from each other by
the amounts of tolerance and allowance. The
designations run from IA to 3A and IB to 3B for
external and internal threads, respectively. A class
I is a looser fitting, general-purpose thread; a
class 3 is the aerospace standard thread, and has a
tighter tolerance.
Forming of Threads:
Threads may be cut, hot rolled, or cold rolled. The
most common manufacturing method is to cold form
both the head and the threads for bolts up to one
inch in diameter. For bolts of larger diameter and
high-strength smaller bolts, the heads are hot
forged. The threads are still cold rolled until the
bolt size prohibits the material displacement
necessary to form the threads (up to a constant
pitch of eight threads per inch). Threads are cut
only at assembly with taps and dies or by lathe
cutting. Cold rolling has the additional advantage
of increasing the strength of the bolt threads
through the high compressive surface stresses,
similar to the effects of shot peening. This process
makes the threads more resistant to fatigue
cracking.
Fatigue-Resistant
Bolts:
If a bolt is cycled in tension, it will normally
break near the end of the threaded portion because
this is the area of maximum stress concentration. In
order to lessen the stress concentration factor, the
bolt shank can be machined down to the root diameter
of the threads. Then it will survive tensile cyclic
loading much longer than a standard bolt with the
shank diameter equal to the thread outside diameter.
Descriptions of
Basic Types of Rivets:
Rivets are one of the oldest and most time tested
methods of fastening. In general, riveting involves
placing the shank of the rivet through a hole in two
or more materials. The protruding end of this shank
is then "cinched" or closed so that the shank cannot
be removed and the materials cannot be separated
easily. Rivets which are not specified as
self-piercing require that a hole be drilled in the
material to allow the shank to penetrate the
materials. Most rivets discussed in this section
require a riveting machine of some sort to cinch the
protruding end of the rivet.
Solid Rivets:
Solid rivets use a
completely solid shaft. When pushed through a
pre-drilled hole in two or more materials, the
protruding end of the shaft is bent over, hammered,
or twisted to produce a strong connection. These
small rivets provide an extremely strong joint
strength but at the disadvantage of being slow and
hard to attach. Solid rivets generally require
powered machinery to form the connection while
providing tough, lasting hold.
Tubular Rivets:
Tubular Rivets are the most popular and plentiful of
the small rivets that are readily available. Tubular
rivets all involve a shaft which is at least
partially hollow throughout its length. When the
shaft is placed through the material (either through
pre-drilled holes or by a means of self piercing)
the formed shape - commonly called a cinch or clinch
- is produced when the shaft material is rolled back
against the surface of one of the materials being
joined. Tubular rivets are used in almost every
feasible area of manufacturing. (Note: small tubular
rivets are often referred to as "pop" rivets and are
easy to find at your friendly neighborhood hardware
store.)
Tubular rivets are further classified as full
tubular, semi-tubular, compression, and rare
self-piercing rivets.
Advantages of
Rivets:
- Aside from the
initial cost of a riveting gun, rivets are cheaper
than most threaded fasteners.
- Rivets require
very little training to use.
- Rivets can join
dissimilar materials of various thicknesses.
Rivets can join as many different parts as the
length of the shank allows.
- Since rivets are
available in a variety of finishes and styles,
they add to provide simple and aesthetically
pleasing fastening.
- Rivet quality
can be easily determined through visual inspection
(quality is apparent if cracks or looseness
exist).
Limitations of
Rivets:
- Lower tensile
strength than most bolts.
- Lower fatigue
strength than most bolts.
- High tensile
loads can cause loosening in the clinch.
- Rivets are
generally neither water or air tight.
- Disassembly of
Rivets is moderately difficult (usually requiring
drilling and cutting).
Hydrogen
Embrittlement:
If you think
hydrogen's detrimental effect on fasteners is just
induced from "processing", think again. It could be
the "environment".
Failure mechanisms
often viewed as synonymous are stress corrosion
cracking, hydrogen embrittlement, and
hydrogen-assisted stress corrosion. Reason is
understandable. Cause and effect similarities
outnumber identifiable differences.
Reality: Only
stress corrosion cracking and hydrogen-assisted
stress are corrosion related.
All cause failure -
actual breaking of the part. But, the fracture is
delayed. Sometimes it occurs within hours after load
is applied. Sometimes not for months, even years.
But, when failure occurs, it's sudden, with no
advance warning. Failures occurring in service can
be serious, costly, even catastrophic.
Hydrogen
embrittlement is associated with carbon and alloy
steel fasteners. Cause : absorption of atomic
hydrogen into the fastener's surface during
manufacture and processing - particularly acid
pickling and alkaline cleaning prior to plating. And
then, during electroplating where the deposited
metallic coating traps hydrogen against the base
metal.
If the hydrogen is
not diffused out by post-baking, the gas migrates
toward points of highest stress concentration when
stress is applied. Pressure builds until strength of
the base metal is exceeded and minute ruptures
occur.
Hydrogen is
exceptionally mobile. It will quickly penetrate into
any newly formed cracks. This pressure - rupture -
penetration cycle continues until part failure.
Hydrogen
embrittlement is non-corrosion-related. It can be
neutralized by proper processing before the
fasteners are released for service. And, while
hydrogen can be baked out before it embrittles, it's
not possible to bake out the micro cracks once
formed.
Stress
Embrittlement:
This is similar to
hydrogen embrittlement - with the generalized
exception that the presence of offending hydrogen is
chemical-reaction induced through the service
environment - not because of in-plant processing.
Example: Caustic materials (such as soaps,
detergents), in contact with nitrates and silicates,
chemically react to release hydrogen which can
diffuse into the surface of non-coated fasteners.
Steels with
high-carbon contents, and heat treated to high
strengths, are most susceptible to stress
embrittlement.
All hydrogen
embrittlement failures are intergranular - but not
all intergranular failures can be attributed to
hydrogen embrittlement. Note these examples:
- Three
self-drilling screws fastened a curved plastic
sill plate to a car door liner. The middle screw
frequently failed in factory assembly. Analysis
showed misalignment where the sill met the curved
metal frame under the middle screw. Also, a
factory wash test allowed moisture to hit the
misaligned screw, acting as an electrolyte in a
galvanic corrosion cell that then generated
hydrogen. The hydrogen gas migrated to the
over-stressed (due to bending) middle screw,
causing a hydrogen-assisted stress corrosion
failure.
- Case hardened
fasteners used to hold structural aluminum members
to steel beams in a U-channel configuration. These
channels, used to retain glass windows, had
drainage holes in their tracks. The fasteners were
coated with a zinc-bearing, corrosion-resistant,
organic compound. Environmental moisture from the
glass collected in the tracks, causing a galvanic
reaction which generated hydrogen and subsequent
fastener failure.
- Structural
aluminum stadium bleacher seats bolted to
concrete. During rain, calcium from the concrete
became the corrosion agent.
- Screws holding
turn signal lights on auto right rear quarter
panels were failing in the factory prior to car
shipment. Cause: Assembled cars were tested for
water leaks with high pressure jets from an
enclosed circulating system. Bacteria/sludge
problems in the wash were controlled by adding two
gallons of sodium hypochlorate daily. But,
concentration levels weren't monitored. After 3
months, the concentration built to a level high
enough to cause stress corrosion cracking of the
fasteners. Car design was such that the
pressurized wash could not penetrate signal light
joints on the left car side, hence only failures
on the right. Interestingly, the fasteners were
mechanically galvanized to guard against hydrogen
pickup during manu-facture. But, this could not
guard the joint from environmental hydrogen
pickup.
Mechanical Engineering
Schools
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