STEEL TREATMENTS FOR MOLD COMPONENTS
“THE KNOWLEDGE ADVANTAGE”
Ken Rumore, Senior Design Engineer
Progressive Components International Corporation, Wauconda, IL
Abstract
Cementite (Figure 3) is an iron carbide or a
Heat Treatment is defined as the controlled heating
and cooling, of metals, in order to alter their physical
and mechanical properties.
First, before the mold is designed, the Engineering
department reviews the quote then employs proven
methods of design, material selection, and heat
treatment.
This understanding is the key to selecting the best
material, for a particular component. Additionally,
the ability to specify this, to your outside services,
will provide the end result you desire.
All Engineers do not have metallurgy knowledge,
therefore, the heat treatment processes defined below
will include some of the language used by a
Metallurgist. This will help when specifying
treatments, on mold design components, and to better
relate to the processes used.
Introduction
Heat treating is generally known for increasing steel
strength, however, it is also used to achieve certain
manufacturing objectives such as improving
machining (cutting), forming, and also restoration of
properties that may have changed due to metal
working.
Here, we’ll focus mainly on tool steels since they are
the majority of what’s currently used, in the mold
industry. I’ll start by offering basic definitions, of the
stages of transformation, which may be reached
during the heat treating process. Each definition
refers to the micro structure, of the steel, and in order
to be seen, must be viewed under a metallurgical
microscope.
chemical compound of iron and carbon and is mixed
with ferrite when found in steel. Its hard, brittle
material state is much like a ceramic and is formed
from Austenite during cooling or from Martensite
during tempering. This material is in very small
distribution and known to be unstable.
Austenite (or Gamma-Phase Iron) (Figure 4), in
plain carbon steel, is made from Ferrite and
Cementite. During the quenching process, the steel is
rapidly cooled allowing the transformation to
Martensite to take place. Austenite may be retained,
during this process, if the steel’s carbon percentage
lowers the temperature at which the Martensite
transformation
should
take
place.
Large
concentrations of retained Austenite (12% or more)
will cause deformation of the part and the possibility
of cracking. To prevent retained Austentite, consult
your steel provider’s Metallurgist for processing
information and optimal steel selection.
Martensite (Figure 5) has a very hard steel
crystalline structure and is formed in carbon steels by
the required rapid quenching from Austenite. Rapid
quenching, at such a high rate, causes carbon atoms
to become trapped in the structure and in large
enough quantities forms Cementite. The result is a
weakened steel structure. Steels with more than 1%
carbon, viewed under the microscope, will have a
plate like looking structure called plate Martensite.
Bainite (Figure 6) is a slightly harder alloy phase of
steel that is formed under more rapid cooling than
Pearlite. Its process range is between where Pearlite
and Martensite are formed as well as the level of
hardness that is achieved. If Austenite is processed
correctly, and Bainite is formed, it will possess the
extreme hardness of Martensite and the tough
structure of Pearlite. Bainitic steels enhance creep
resistance and are less likely to deform under stress.
Phases of Steel Treatment (Figure 1)
Pearlite (Figure 7) has a plate like (laminar)
(Hard) Ferrite (Figure 2) is a solid solution of iron
used as an alloy in steel and may give steel its
magnetic properties. Ferrite can be harder and
stronger than a diamond.
structure that looks similar to mother of pearl under
the microscope. Composed of Ferrite and Cementite,
and under slow cooling from the Austentite phase,
Pearlite is formed when two molten minerals
crystalize at the same time. This steel transformation
and plate like geometry make it hard and strong.
Steel advertised as ‘Pearlite free’ usually refers to the
likelihood of less cracking.
Why Heat Treat Steel?
The three main reasons for heat treatment processing
are: softening, hardening, and modifying material
properties.
Methods of Treating Steel
Softening of Steel
Softening is done to reduce hardness, stress relieve,
add toughness, improve ductility, and refine the
structure. The following processes fall in this
category and a brief description of each should be
learned to effectively communicate with your heat
treat service.
(Full) Annealing (Figure 8) of a work piece is
accomplished by raising the temperature, over time,
above the Austenitic temperature until it transforms
into the Austenite-Cementite phase and then is slowly
cooled into the Ferrite-Cementite range. Then, it is
cooled in room temperature air. In general, this
changes the steel to a coarse grain structure causing it
to become soft.
Spheroidizing is a widely misunderstood process
that is quite simple. It is the annealing process that
applies to steels identified as carbon steels. The
reason for its name is that once the part is processed,
the Cementite structure left is all in the form of small
spheres, or spheroids. This process improves
machining and is commonly used for screw machine
parts made of carbon steel. The process involves
heating to just below the Austenite range and holding
that temperature for a prolonged time, followed by
slow cooling.
Normalizing is achieved by raising the temperature
into the Austenitic range to convert the structure to
Austenite then cooled at room temperature. This
changes the grain to a fine Pearlite leaving the
material soft; how soft is determined by the cooling
conditions. It’s different than total Annealing because
Normalizing takes effect a distance in depth from the
surface, not all the way through. This can be
beneficial, yet less expensive, than Full Annealing if
the surface machining operations that follow are
shallow from the surface.
Tempering (Figure 9 and 10) is an important
process that is done immediately after quench
hardening to relieve brittleness. The main cause of
this brittleness is the abundance of Martensite in the
steel structure. A part is tempered, in general, until
the preferred hardness specifications are met. The
hardness specification has an effect on many others
specifications such as toughness, wear resistance,
strength, and stability. Tempering is performed
before the steel reaches room temperature, usually
when the steel has cooled to about 100°F and will
result in a lower Rockwell hardness. The process
controls what the resulting hardness will be and the
relative stability of the work piece. Uniformity and
stability of the heat envelope are important when
tempering. It’s performed in various ways such as
electric ovens and oil baths (for lower temperature
tempering) but it’s common to use a molten salt bath
with nitrate salts.
Austempering, most don’t realize, is actually a
quenching technique. The steel is quenched above the
temperature where Martensite forms, usually above
600°F. The quenching is held, at this temperature,
until the Austenite transforms into Bainite (which is
so unusually tough) the work piece will be crack
resistant and additional tempering is not required.
Martempering compares to the Austempering
process above, except the work piece is slowly
cooled through the range where Martensite will form.
When Martensite has formed, it requires tempering
just as if it had formed if the part were quenched
rapidly as in the hardening process. The advantage to
Martempering is the reduction of distortion and crack
potential of the work piece.
Hardening of Steel
Hardening is done to increase the strength and wear
resistance of steel. If the steel selected has sufficient
carbon content, then it may be directly hardened. If
not, the surface of the part has to have carbon added
by diffusing it into the surface.
Direct Hardening of steel (Harden Ability) is due
to the carbon content allowing a change in the
structure, of the steel, once it is heated to Austenitic
temperatures. Upon the sudden quench, Martensite is
formed, having a strong but brittle structure. The
depth of full hardness is measured and its maximum
is achieved partially by the amount of alloying
elements. The hardness will vary, depending on the
alloying elements, as these act to increase the steels
hardenability. Nondestructive hardness testing will
check the hardness at the surface. Direct Hardening,
however, takes place well below the surface of the
steel and to check this properly, the part may need to
be sectioned to allow access to the core of the part.
Adding alloys increases the hardness that can be
achieved throughout the steel, reducing the need for
sectioning and destroying what could be a usable
part. Alloys added to steel can reduce the need for
quenching, as rapidly, and also reduce the distortion
and cracking that could take place after quenching.
Diffusion Hardening can occur if the steel to be
hardened doesn’t have more than .25% carbon
content and the part can still be hardened by adding
carbon to the surface. This method allows the surface
to become wear resistant and the core to maintain a
high degree of toughness. There are several common
methods to perform this transfer of carbon and
hardening.
Carbonitride is a process that allows both carbon
and nitrogen to be diffused into the surface. The
furnace uses an atmosphere of propane and ammonia
heated to 1500°F (higher than Nitride). The parts are
then mildly quenched in an oxygen free atmosphere.
Depending on their mass, the elevated temperatures
may cause distortion, of the parts, resulting in a lower
hardness surface of around 62 Rc. and will usually
require tempering, after processing, due to the steel
transformation temperature being reached. This hard
case is made up of Nitrides and Martensite that
perform well in Die Casting dies for Aluminum and
lower temperature casting materials.
Selective Hardening are some of the most
interesting processes as most are used for production
treating of parts. The amount of wear resistance that
is achieved from such simple and quick processes is
amazing.
Carburizing is performed in a furnace by adding
carbon to the surface, via a carbon rich atmosphere of
any kind. This atmosphere will transfer carbon onto
the surface of steel only if the steel has a low carbon
content.
Pack Carburizing is when the part, to be treated,
is packed in a carbon carrying media as flakes or
powder, and is usually wrapped in a heat treating foil
to completely surround the part. This is a lengthy
process that may take up to 3 days, in the furnace, at
over 1600°F. A carbon monoxide gas is produced
that mimics the carbon atmosphere mentioned above.
Once the carbon has diffused into the part, it is
hardened at the surface because of the carbon that
was added.
Flame Hardening (Figure 11) occurs when an
oxy-acetylene torch flame is applied to the area to be
hardened. The temperature is high enough to promote
the Austenite transformation. The length of time to
apply the flame is determined by sampling the
hardness and is monitored by the operator based on
the color of the steel. The overall heat transfer is
specifically limited to the surface as the interior never
reaches the same temperature and the work piece is
then quenched to achieve the desired hardness.
Tempering may be done to eliminate brittleness.
The depth of hardening can be increased by
increasing the heating time. As much as .25” of depth
can be achieved.
Gas Carburizing is similar to the pack carburizing
Induction Hardening (Figure 12) occurs as the
mentioned above, but in this case there is no need for
the carbon flakes or powder to surround the part.
Carbon monoxide gas is injected into the tightly
sealed furnace atmosphere (it could be lethal if it
escapes) and diffuses carbon directly to the part
surface.
steel part is passed through an electrical coil with
alternating current running through it. This
alternating current energizes the work piece, and it is
heated. Heating is controlled through frequency and
amperage, and can be high enough to harden the
steel. The voltage, the rate of heating, and the depth
of heating can also be controlled. Hence, this is well
suited for surface heat treatment. The details of heat
treatment are similar to flame hardening.
Nitriding is the process performed to diffuse
Nitrogen into the surface of the steel; no additional
heat treating can be done after Nitride. The parts are
first through hardened, then thoroughly cleaned to
increase absorption to the surface. Nitrogen is
delivered to the part due to the furnace having an
ammonia atmosphere at about 900° to 1700°F
causing Nitrides to form, at the surface of the part.
The process works best if the steel contains
Vanadium, Aluminum or Chromium. The low
process temperature allows the parts to remain free of
distortion.
Laser Beam Hardening (Figure 13) is another
variation of flame hardening. A phosphate coating is
applied over the steel to facilitate absorption of the
laser energy. The selected areas, of the part, are
exposed to laser energy causing the selected areas to
heat. By varying the power of the laser, the depth of
heat absorption can be controlled. The parts are then
quenched and tempered. This process is very precise
in applying heat selectively to the areas that need to
be heat treated. Furthermore, this process can be run
at high speeds, producing very little distortion.
Electron Beam Hardening is similar to laser
beam hardening. The heat source is a beam of highenergy electrons and is manipulated using
electromagnetic coils. The process can be highly
automated but needs to be performed under vacuum
conditions since the electron beams dissipate easily in
air. As in laser beam hardening, the surface can be
hardened very precisely both in depth and in location.
Modifying Material Properties
There are many processes that will modify material
properties. The two below are commonly used to
offer advantages to the machinist and end user in
order to improve the stability of the work piece. They
are also very inexpensive processes that have a short
processing time.
Stress Relieving is the working of steel by
machining. Cutting or forming can leave stress
behind causing parts to distort when the stresses are
released during heat treatment. Stress relieving is
done roughly 200°F below the transformation
temperature and only takes about an hour to process.
This depends on the alloys in the steel; your steel
company metallurgist should be consulted for best
practices.
If a part has been sawed and milled to remove a
significant amount of material, it can be sent prior to
finishing, for stress relieving. A “clean up” amount of
material will be left for final finishing; this can be
estimated by the mass of the part and the amount of
reduction of mass in certain areas. It’s important to
note your expectations on the heat treating order, for
flatness and straightness, as corrections to any large
distortion that takes place can be manually reversed
by the heat treating company.
Cryogenic Treatment (Figure 14) or Cryo
treatment (sub-zero) is an immersion, usually in
liquid Nitrogen, to make sure there is no retained
Austenite during the quench. Austenite is the
remainder of what was not converted to Martensite
during the hardening or quenching process and will
add brittleness to heat treated parts. With high alloy
or high carbon steels, Cryogenic treatment can
eliminate all of the Austenite that will cause
instability or cracking of the work piece, over time,
that can occur on the shelf or in use.
Understanding the terms used for heat treating and
what they mean is important to the designer or those
that create shop work instructions. This guide can
also be used by the toolmaker, machinist, shop
manager or anyone associated with manufacturing as
it doesn’t only cover the type of heat treating, but
also processes for modification, prior to finish
manufacturing.
My previous paper focused on Tool Steel selection,
the important first step…next comes planning the
heat treating for the manufacturing process. My goal
is to offer definitions and advantages making it easier
for the non-Metallurgist to understand, and why each
is important.
The heat treating knowledge “bridge” between
manufacturing and metallurgy, in our industry, has
been missing for quite a long time.
I always encourage an engineer to speak with the
Metallurgist at the steel company. They can offer
recommendations and advantages for specific part
geometry, as well as variations of the steel
formulation, culminating in a higher degree of
success. Additionally, the Metallurgist can make
recommendations to assist the Engineer, when
writing shop work instructions, where heat treating is
concerned.
Documenting these processes is essential! Any
variations made along the way require the work
instructions are updated for the next part order,
yielding an even higher degree of success, possibly
saving labor costs each time they are refined.
Figure 1 (Simple Phase Diagram)
Figure 2 (Ferrite)
Figure 3 (Cementite)
Figure 4 (Austentite)
Figure 5 (Martensite)
Figure 6 (Bainite)
Figure 7 (Pearlite)
Figure 8 (Annealing)
Figure 9 (Tempering)
Figure 10 (Tempering)
Figure 11 (Flame Hardening)
Figure 12 (Induction Hardening)
Figure 13 (Laser Beam Hardening)
Figure 14 (Cryogenic Treatment)