SPRING 2024

84
THE DISTRIBUTOR’S LINK
LAURENCE CLAUS HOW FASTENERS ARE MADE - PART 2: HOT HEADING AND SCREW MACHINING from page 8
But how much heat is needed and what is the
difference between warm and hot forming? Naturally,
some contemplate that question and reply that the
answer is obvious, “it’s the temperature.” Of course, they
are correct, the discriminator between warm forming and
hot forming is simply the magnitude of the temperature.
However, in technical terms it is not that simple. A more
precise answer to that question would be that the process
temperature is either above or below some “critical
temperature.” In fact, warm forming is conducted below
a critical temperature and hot forming above it. What,
though, is a critical temperature? The answer to this
will vary from material to material but generally means
a temperature where either an internal transformation
occurs, or an external reaction is triggered. Steel provides
an excellent example of a critical temperature where
internal transformation occurs. When fastener steels,
which almost always possess less than 0.5% carbon, are
heated up, they undergo their first transformation between
about 1300°F and 1650°F. When this critical temperature
is reached, if held there long enough, the steel will
transform from whatever structure it possesses (most
likely some mix of Ferrite and Pearlite) to the elevated
temperature structure known as Austenite. Austinite is
soft and very formable, and thus is an advantageous
structure to be working with if extreme forming is
necessary. Titanium provides an excellent example of a
metal with a critical temperature that triggers an external
reaction. When Titanium reaches temperatures of about
1600°F to 1800°F and is in contact with surrounding
air, it will react with the oxygen in the air and form a
brittle and undesirable oxide layer known as Alpha Case.
Thus, Titanium must be processed at lower temperatures
that will not trigger this undesirable condition. So, the
manufacturer’s choice of operating temperature becomes
a function of what the lowest critical temperature is for the
material and the consequences of exceeding it.
Whether warm or hot forming, however, the addition
of heat is sometimes necessary to facilitate a successful
forming process. In general, this process is employed for
one of two reasons. There are some materials that simply
will not form without the addition of heat. A couple of
common examples include Titanium, many of the Nickel
alloys (like Inconel and Hastelloy) , and some Stainless
Steel alloys. The other reason is that the parts are too big
to run on a cold header. In fact, the largest cold headers
usually reach maximum capacity between about 1 ¼ and
1 ½ inches in diameter. There are a couple of reasons
for this, but the most significant is a practical one, going
beyond this size increases the scale of the equipment
beyond what is efficient and feasible to work with.
Most warm and hot heading processes only address
a small section of the workpiece, usually the head. Surely
there are high speed hot formers that heat the entire
workpiece and present it to multiple dies like a multistation
cold heading parts former to create complex and
intricate part geometries. However, it is more common,
especially for larger diameter or very long parts to be
formed in single or double blow vertical presses and
horizontal upsetters. In these processes a blank is either
sheared or cut from long bar stock. The area of the part
to be formed (usually the head) is heated in an off-line,
stand-alone induction heater. It is allowed to reach the
appropriate temperature and then loaded into the press or
upsetter and struck by a punch (see Figure 1). The result is
a part with the desired head form (see Figure 2).
FIGURE 1: HEAD FIRMED IN VERTICAL PRESS
CONTINUED ON PAGE 126