SPRING 2022

96
THE DISTRIBUTOR’S LINK
LAURENCE CLAUS HOT FORGING FASTENERS from page 8
and another exists is known as the Recrystallization
Temperature or Critical Temperature. When a part is
heated to a temperature below this inflection point, it is
considered to be warm forming. When it is heated above
this inflection point, it is considered hot forming. This
is important because, although the higher temperature
makes it easier to form, it might trigger a full or partial
crystal rearrangement that is undesirable in the finished
part and must be addressed in a separate post forming
process.
FIGURE 2: BLANKS TRANSFERRING ACROSS MULTIPLE DIE STATIONS
Most everyday fasteners are formed cold, meaning
that the raw material enters the forming machine at
room temperature. The ability to actually form the part
becomes a function of machine capability and raw
material formability. In fact, manufacturers that utilize
cold forming processes pay a premium to buy raw
material that is especially formable. However, not all
fasteners can be formed cold. Some parts require heat
to be added to improve their chances of successful
forming. When this occurs the manufacturer is now
performing a warm or hot forming process.
There is a fine but very important difference between
warm and hot forming. Metals are crystals. This means
that their atomic structure exists in an organized and
repeated manner. When exposed to certain conditions,
such as reaching a specific temperature, the atoms will
rearrange themselves into a new crystal arrangement. It is
these different crystal arrangements that partially define
the properties a metal will exhibit. The temperature where
an inflection point between one crystal arrangement
When Are Parts Warm Or Hot Formed?
Although the addition of heat helps forming, most
fasteners do not need it. Heat is added when the parts
start to get too big for the available equipment to handle
them cold or when certain hard-to-form materials are
utilized.
There is no hard and fast rule regarding size, but
generally when thread sizes start to reach the 1-3/4” to
1-1/2” diameter they begin to exceed machine capability
for cold forming. Therefore, large diameter fasteners are
mostly hot formed or machined.
In addition to size, the formability of the fastener
material plays an important role. Carbon and Alloy
Steels form well and, within the constraints of typical
cold forming equipment, hardly ever require added
heat. Other materials, however, such as some stainless
steels, titanium, and many of the nickel alloys work
harden very quickly and cannot be formed cold. Added
heat is required to successfully form these materials.
The appropriate temperature to use depends on each
specific material. For example, titanium fasteners are
usually warmed formed at temperatures approximating
800°F rather than hot formed at 1600°-1800°F. The
reason for this is that at temperatures about 1000°F
in open atmospheric conditions, Titanium forms a
deleterious oxygen-rich scale known as Alpha-Case.
Manufacturers strike a balance, therefore, and give
up some forming improvement to prevent Alpha-case
formation which can be extremely difficult to remove
once formed.
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