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There is a wide choice of fastener materials
available to satisfy specific applications, selection
could depend on:
- Cost
- Product Strength Characteristics
- Environmental & corrosion-resistance requirements
- Application of the fastened joint
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The more generally used materials for fasteners are
listed below. Remember that each class of material will
be supplied in various grades appropriate for the required
strength property of the fastener.
Steel
Carbon steels are used for most types of fasteners.
This steel has good workability and offers a wide choice
for the required strength property of the fastener.
Carbon steels are available in a range of strengths
according to composition. These range from relatively
low strength up to those that can be used in critical
applications.
Stainless Steel
All stainless steels contain alloying elements of which,
Chromium, Nickel & Molybdenum are the most important.
Stainless steels are catalogued into three main groupings
and each has its own identity.
• Austentic stainless steels; best for corrosion
resistance and most commonly used for the production
of standard fasteners.
The International Organisation for Standardisation
(ISO) published in May 1979 a specification (Standard
3506) for stainless steel fasteners. This defines
a four digit product designation.
The first digit indicates the general classification
of the steel:
A = austentic.
The second digit indicates the type of alloying elements:
A2 = austentic 18/8 type (not free machining types),
(18% Chromium & 8% Nickel). Also known as 304
S15 (formerly EN 58E) to BS 970
A4 = austentic 18/10/3 type (18% Chromium, 10% Nickel
& 3% Molybdenum). Also known as 316 S31.
The third & fourth digits indicate the strength
(mechanical property) class and represent one tenth
of the tensile strength expressed in Newtons per millimetres
squared (N/mm2):
50 = lowest strength class which is usually for products
of softened steel & with machined (cut) threads.
70 = higher strength class resulting from cold forging
& thread rolling.
80 = highest strength attainable by multiple cold
forging thread rolled parts & when specially drawn
cold bars or wire are used.
Thus A2-70 = austentic 18/8 class 70 cold forged
with a minimum tensile strength of 700 N/mm2.
A2 & A4 are the most common steel grades used
for fasteners. Generally A4 has better corrosion resistance
especially for marine applications, although it is
advisable to be fully aware of the application when
giving advice.
• Ferritic stainless steels; not as good as
austentic stainless steels for corrosion resistance
but good strength properties.
• Martensitic stainless steels; high strength,
not as good or as versatile as austentic and ferritic
stainless steels.
Brass
Brass, an alloy of copper & zinc, has good corrosion
resistance but may tarnish under certain atmospheric
conditions. Brass fasteners are used for electrical,
water & general marine applications and where decorative
appearance is required, particularly home furnishings.
Bronze
Bronze is another copper based alloy but more complex
and expensive than brass. Because of its higher strength
and greater corrosion resistance it is specified for
more critical marine applications. There are many different
types of bronze formulations, which in addition to copper,
can contain nickel, silicon, zinc, tin, aluminium &
manganese in varying proportions.
Aluminium
Aluminium alloy fasteners are use din the aluminium
window industry and for decorative appearance in general,
where appropriate. They have a natural matt silver grey
finish, but a surface treatment such as anodising, can
be applied after manufacture to resist atmospheric corrosion.
high strength aluminium fasteners have recently been
developed and are being specified in automobile applications
where light component construction is required.
Plastics
The demand for fasteners manufactured from various plastic
compounds is growing. Most of the compounds used for
plastic fasteners have low strength characteristics
but are quite suitable for applications where lightness
or electrical non-conductivity is important.
The properties of the various plastic materials used
are many and variable. The more commonly used grades
of plastic are nylon type 6/6, polycarbonate and high
density polythene.
The method of manufacture is machine cut from extruded
rod.
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