About Fastener Materials
General
Fasteners are manufactured in a wide range of materials
from common steel to titanium, plastic and other exotic materials.
Many materials are further separated into different grades
to describe specific alloy mixtures, hardening processes,
etc. In addition, some materials are available with a variety
of coatings or plating to enhance the corrosion resistance,
or appearance of the fastener.
Fastener material can be important when choosing a fastener
due to differences between materials in strength, brittleness,
corrosion resistance, galvanic corrosion properties, and of
course cost.
When replacing fasteners, it is generally best to match what
you are replacing. Replacing a bolt with a stronger one is
not always safe. Harder bolts tend to be more brittle and
may fail in specific applications. Also some equipment is
designed so that the bolts will fail before more expensive
or critical items are damaged. In some environments such as
salt water galvanic corrosion must also be considered if changing
fastener materials. For more information see our About
Galvanic Corrosion page.
Materials
Stainless Steel
Stainless steel is an alloy of low carbon steel and chromium
for enhanced corrosion characteristics. Stainless steel is
highly corrosion resistant for the price and because the anti-corrosive
properties are inherent to the metal, it will not loose this
resistance if scratched during installation or use.
It is a common misconception that stainless steel is stronger
than regular steel. In fact, due to the low carbon content,
stainless steel can not be hardened. Therefore when compared
with regular steel it is slightly stronger than an un-hardened
(grade 2) steel fastener but significantly weaker than hardened
steel fasteners.
Stainless steel is also much less magnetic than regular steel
fasteners though some grades will be slightly magnetic.
18-8 Stainless
18-8 is a specific grade of stainless steel, and is the
most common grade used in fasteners. For those familiar
with the 300 series of stainless steel 18-8 is similar to
304 stainless. For more specific information on 18-8 stainless
steel please see our Material
Grade Identification and Properties Chart.
Steel
Steel is the most common fastener material. Steel fasteners
are available plain as well as with various surface treatments
such as zinc plating, galvanization, and chrome plating.
Steel fasteners are commonly available in 4 grades. Many
other grades exist but are used far less often. The most common
grades are Grade 2, Grade 5, Grade 8, and Alloy Steel. Grade
2, 5, and 8 are usually plated with a silver or yellow zinc
coating or galvanized to resist corrosion.
Determining Bolt Grade
Bolts of different grades are marked on the head to show
what grade bolt they are. For a list of the most common
grade markings see our Material
Grade Identification and Properties Chart.
Grade 2
Grade 2 is a standard hardware grade steel. This is the
most common grade of steel fastener and is the least expensive.
Grade 2 bolts have no head marking (sometimes a manufacturer
mark is present).
Grade 5
Grade 5 bolts are hardened to increase strength and are
the most common bolts found in automotive applications.
Grade 5 bolts have 3 evenly spaced radial lines on the head.
Grade 8
Grade 8 bolts have been hardened more than grade 5 bolts.
Thus they are stronger and are used in demanding applications
such as automotive suspensions. Grade 8 bolts have 6 evenly
spaced radial lines on the head.
Alloy Steel
Alloy steel bolts are made from a high strength steel alloy
and are further heat treated. Alloy steel bolts are typically
not plated resulting in a dull black finish. Alloy steel
bolts are extremely strong but very brittle.
Silicon Bronze
Silicon bronze, often referred to simply as bronze, is an
alloy made mostly of copper and tin with a small amount of
silicon. Bronze is used primarily in marine environments.
It is preferred over stainless in wooden boat construction
and re-fastening due to its superior corrosion resistance,
and over brass due to its higher strength. Bronze is similar
to copper in color and is also sometimes seen in fine woodworking
where it is used for its appearance. The main drawback of
bronze is its high cost.
Brass
Brass is an alloy of primarily copper and zinc. Brass is highly
corrosion resistant and electrically conductive. However,
its use as a fastener is somewhat limited due to its relative
softness. It is used primarily for its appearance.
Coatings
Zinc Plating
Many steel fasteners are electro-plated with zinc for better
corrosion resistance. Fasteners that have been zinc plated
have a shiny silver or golden appearance referred to as clear
or yellow zinc respectively. They are fairly corrosion resistant
but will rust if the coating is destroyed or if exposed to
a marine environment.
Hot Dip Galvanizing
Galvanizing is another coating involving the application of
a layer of zinc. Hot dipped galvanizing puts the thickest
possible coating on the metal resulting in superior corrosion
resistance. Due to the thickness of the coating hot dipped
galvanized bolts are not compatible with other nuts. Galvanized
nuts are tapped slightly larger than other nuts to accommodate
this coating.
Hot dipped galvanized fasteners are frequently seen in coastal
environments and in pressure treated lumber where the chemicals
in the lumber may corrode other fasteners.
Chrome
Chrome is used in plating fasteners for its appearance. It
provides similar corrosion resistance to zinc plating. The
main drawback of chrome is the extremely high cost. If more
corrosion resistance is required stainless steel may be chrome
plated, preventing any corrosion should the chrome be penetrated.
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