Annealing

Full annealing is accomplished by heating a hypoeutectoid steel to a

temperature above the UCT (Upper Critical Temperature). In
practice, the steel is heated to about 100 oF above the UCT. It is then
cooled in the furnace very slowly to room temperature. The formation
of austenite destroys all structures that have existed before heating.
Slow cooling yields the original phases of ferrite and pearlite.

Figure 1. Annealing and Spheroidizing Temperatures

Hypereutectoid steels consist of pearlite and cementite. The cementite
forms a brittle network around the pearlite. This presents difficulty in
machining the hypereutectoid steels. To improve the machinability of
the annealed hypereutectoid steel spheroidize annealing is applied.
This process will produce a spheroidal or globular form of a carbide
in a ferritic matrix which makes the machining easy. Prolonged time
at the elevated temperature will completely break up the pearlitic
structure and cementite network. The structure is called spheroidite.
This structure is desirable when minimum hardness, maximum
ductility and maximum machinability are required.

Figure 2. Spheroidizing process applied at a temperature below the

LCT.

Figure 3. Spheroidizing process applied at a temperature below and

above the LCT.

Low carbon steels are seldom spheroidized for machining, because

they are excessively soft and gummy in the spheoridized conditions.
The cutting tool will tend to push the material rather than cut it,
causing excessive heat and wear on the cutting tip.

Figure 4. Spheroidized cementite in a ferrite matrix.

Stress-Relief Annealing is sometimes called subcritical annealing, is
useful in removing residual stresses due to heavy machining or other
cold-working processes. It is usually carried out at temperatures
below the LCT, which is usually selected around 1000oF.
The benefits of annealing are:
Improved ductility
Removal of residual stresses that result from cold-working or
machining
Improved machinability
Grain refinement
Full annealing consists of (1) recovery (stress-relief ), (2)
recrystallization, (3) grain growth stages. Annealing reduces the
hardness, yield strength and tensile strength of the steel.

Back to Table of Contents

Last Updated: October 28, 1999
Prepared by: Serdar Z. Elgun

Continuing with the series of tutorial articles on the subject of heat treatment, the next step is to
look at the basic heat treatment procedure, its principles and why it is necessary to heat treat the
steel. The previous articles have reviewed "What is Steel" and the "Influence of Alloying Elements
on Steel."
This article and the next three articles will focus on the basic principles of heat treatment such as:

The definitions of processes.

Their meanings and selection.
Quenching.
Surface treatments, including nitriding up to thin-film depositions.
Quality control.
Troubleshooting.

What Is Heat Treatment as Applied to Steel?

Heat treatment - as applied to steel - can be defined as the application of heat to change a
characteristic or condition of the steel. The amount of heat can be measured by the temperature
of the steel being treated.
Temperature can be either cold or hot to the touch, therefore the process treatment temperatures
can range from extremely cold to extremely hot. Or, in terms of temperature, the range can be
from a cold negative temperature to a hot plus temperature.
Heat treatment is the process of heating up to temperature, soaking at that temperature and then
cooling down from the temperature.

Why Do We Heat Treat Steel?

Steel can be categorized in a variety of manners (See Figure 1). The principle alloying element is
carbon, which influences the steel's hardness and its mechanical properties.
It is necessary to apply heat to steel to enable its condition and mechanical properties to be
changed to allow the steel to function either during manufacture or its operating life cycle.

Principles of Heat Treatment

If we consider the primary metal of steel, which is iron, there are a number of features that we can
observe immediately about the metal:

It is stable at room temperature.

It is magnetic.
It can have a shiny finish. In other words, it polishes to a bright shine.
It has a high density (it is heavy).
It is ductile (it can bend or be shaped easily).

Consider a straight vertical line as being an iron and temperature line combined. If we look at the
line in the vertical configuration at room temperature, the iron is stable.
To understand what happens to iron when heat is applied, you must first know about ferrite, a
condition in iron that has a number of inherent properties at room temperature such as a large
grain size, a low hardness, good ductility and is easily machinable.
The ferrite condition exists in a particular crystal form and will exist at low temperatures. Iron is
therefore made up of millions of tiny crystals much like sugar or salt. Those crystals are bound
together in what is called a lattice structure.
The ferrite condition of iron can be compared to water, or H 2O to be more precise. H2O exists in
three forms: ice, liquid and vapor. Each of the three phases or conditions is still H 2O, but they
each exist in a different crystal form. Similarly, ferrite is one of the conditions of iron.
As heat is applied to iron, a number of things begin to happen to the shiny piece of steel that is
being heated. The oxygen in the air will begin to react with the shiny surface of the iron to form
Iron Oxide. As the temperature is increased, the reaction between the iron and oxygen becomes
more aggressive, and a visible product begins to form on the surface of the iron, which is known
as scale.
Once the steel reaches an approximate temperature of 1,350xF, a change in the structure takes
place as well as the phase. The phase changes from ferrite to austenite. In addition to this, the
crystal structure changes from body-centered cubic to face-centered cubic.
An indication of the change occurring is that the ion will lose its magnetic property, which can be
seen by testing the surface of the steel with a magnet. Protect your hand and fingers when testing
for the loss of magnetism from the hot steel (see Figure 2)

What Does the Crystal Structure Look Like?

As the iron is heated up to above the lower transformation temperature of 1,350xF the crystal
structure transforms to a face-centered cubic structure. Therefore, we can say that in order to
make the phase of austenite, we need to apply sufficient heat to create the phase.
In addition to this, there is a growth that will occur due to the atomic structural change. Therefore,
the size of the mold will change as it is heated. This is called growth and should not be confused
with distortion.
So by definition, the lower transformation temperature (or the magnetic change line) is the
temperature that the ferrite phase (a body-centered cubic structure) begins to change to austenite
(a face-centered cubic structure).

What Then Happens to Steel?

Steel is simply an alloy of iron and carbon. If we look at Figure 2, it can be seen that the diagram
only addresses iron. We are now considering steel, therefore a horizontal carbon line is now
drawn and shown in Figure 3.
The line at 0.77 percent carbon is known as the eutectoid line. To the left of the line, the steels
are known as hypo eutectoid steels (ferrite condition) and to the right of the line, the steels are
known as hyper eutectoid steels (cementite condition). In order to establish what the upper
change temperature would be to ensure a complete phase change form ferrite to austenite, one
would need to know the carbon content of the steel. In other words, if we consider a steel at 0.40
percent carbon, we would look for a 0.40 percent on the horizontal carbon line and extend the line
vertically. At the point where the line intersects the upper change line, the intersect point would
extend a line horizontally to intersect the vertical temperature line. This would be the temperature
where the ferrite has changed fully to austenite. Once the austenitizing temperature has been
established, then approximately 50xF is added to that temperature to ensure that the steel is in
the austenite region for full transformation. If the steel is left at a temperature that is in the

transformation area of austenite + ferrite, then a mixed phase will exist and will not fully transform.
Both phases have different volumes.
Once the steel is in the austenite region, it is necessary to cool it down to create the particular
phase that is necessary for the steel to function, either for machining or for performance. The rate
at which the steel is cooled will determine the phase or microstructure. The cool down can be
slow or fast, depending on what is to be accomplished.
By controlling the soak temperature and the cool down rate of the steel, we can determine the
process to be accomplished. Those processes include annealing, normalizing, stress relieving,
hardening and tempering.

What Is Annealing?
Annealing is the process of heating the steel to a particular temperature in the austenite region
and cooling down the steel very slowly. There are many derivatives of the annealing process, but
generally the process is a slow cool process.
Another derivative of the annealing process is known as sub-critical anneal. This process involves
soaking at a temperature below the lower transformation line, in the region of 1,200xF to 1,300xF,
until the steel has equalized across its cross-section in temperature, followed by a slow cool.
Slow cooling can mean a cooling rate between 5xF per hour up to 50xF per hour.
As can be imagined, the cooling period can be a considerable amount of time. It should be noted
that the nickel alloyed steels and the A series tool steels should be cooled very slowly, as nickel
will cause an air-hardening effect.

Other Types of Annealing:

Bright Anneal. This method is a method of annealing which uses a protective

atmosphere to prevent the steel surface from oxidation.
Process Anneal. This procedure is done at a temperature close to the lower critical
line on the iron carbon diagram. Sometimes confused with sub-critical annealing, it is
used when considerable cold working is to follow.
Recrystallization Anneal. Once again, this is a process often mistaken for subcritical
annealing. It is used after cold working to produce a specific grain structure.
Sub-Critical Anneal. This method is used on cold-worked steel and is carried out
below the lower critical line on the iron carbon equilibrium diagram. It is sometimes
applied to tool steels that have been over tempered and require annealing before
hardening and tempering.
Spheroidize Anneal. This process is a controlled heating and cooling procedure to
produce spheroidal or globular cementite particles. It is usually applied on high carbon
steels for good machining characteristics such as high alloy steels and tool steels.
Isothermal Annealing. The process temperature of this procedure is determined by
knowledge of the steel's carbon content. The steel is then taken to that temperature
and cooled down to a holding temperature that allows the steel to transform
isothermally.
Full Anneal. This is a process that involves raising the steel's temperature up to the
sustenite region followed by a slow cool.

What Is Normalizing?
Normalizing is a process that makes the grain size normal. This process is usually carried out
after forging, extrusion, drawing or heavy bending operations.

When steel is heated to elevated temperatures to complete the above operations, the grain of the
steel will grow. In other words, the steel experiences a phenomenon called "grain growth."
This leaves the steel with a very coarse and erratic grain structure. Furthermore, when the steel is
mechanically deformed by the aforementioned operations, the grain becomes elongated.
There are mechanical property changes that take place as a result of normalizing - inasmuch as
the normalized steel is soft, but not as soft as a fully annealed steel. Its grain structure is not as
coarse as an annealed steel, simply because the cooling rate is faster than that of annealing.
Usually the steel is cooled in still air and free from air drafts. The process temperature is virtually
the same as for annealing, but the results are different due to the cooling rate.
The process is designed to:

Give improved machining characteristics.

Ensure a homogenous structure.
Reduce residual stresses from rolling and forging.
Reduce the risk of "banding."
Help to give a more even response to the steel when hardening.

What Is Stress Relieving?

Stress relieving is an intermediate heat treatment procedure to reduce induced residual stresses
as a result of machining, fabrication and welding. The application of heat to the steel during its
machining or fabrication will assist in removing residual stresses that will, unless addressed
during the manufacturing by stress relieving, manifest themselves at the final heat treatment
procedure.
It is a relatively low temperature operation that is done in the ferrite region, which means that
there is no phase change in the steel, only the reduction of residual stresses. The temperature
region is usually between 800xF to 1,300xF. However, the higher that one goes in temperature,
the greater the risk of surface oxidation there is. It is generally better to keep to the lower
temperatures, particularly if the steel is a "pre-hard" steel. The hardness will be reduced if the
stress relieve temperature exceeds the tempering temperature of the steel.
There is a general rule of thumb for time at temperature. It must be stated that the time is taken
when the part is at temperature, not when the furnace is at temperature. The time at temperature
for the processes of full anneal (not spheroidize anneal), normalize and stress relieve is 60
minutes at part temperature per one-inch of the maximum cross-sectional area.

From Wikipedia, the free encyclopedia

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A vacuum furnace is a type of furnace that can heat materials, typically metals, to very
high temperatures and carry out processes such as brazing, sintering and heat treatment
with high consistency and low contamination.
In a vacuum furnace the product in the furnace is surrounded by a vacuum. The absence
of air or other gases prevents heat transfer with the product through convection and
removes a source of contamination. Some of the benefits of a vacuum furnace are:

Uniform temperatures in the range 20002800F (11001500C)

Temperature can be controlled within a small area

Low contamination of the product by carbon, oxygen and other gases

Quick cooling (quenching) of product.

The process can be computer controlled to ensure metallurgical repeatability.

Heating metals to high temperatures normally causes rapid oxidation, which is

undesirable. A vacuum furnace removes the oxygen and prevents this from happening.
An inert gas, such as Argon, is typically used to quickly cool the treated metal back to
non-metallurgical levels (below 400 F) after the desired process in the furnace.[1] This
inert gas can be pressurized to two times atmosphere or more, then circulated through the
hot zone area to pick up heat before passing through a heat exchanger to remove heat.
This process is repeated until the desired temperature is reached.

[edit] Common uses

A common use of a vacuum furnace is for the heat treatment of steel alloys. Many
general heat treating applications involve the hardening and tempering of a steel part to
make it strong and tough through service. Hardening involves heating the steel to a predetermined temperature, then cooling it rapidly.
Vacuum furnaces are ideal for brazing applications. Brazing is another heat-treating
process used to join two or more base metal components by melting a thin layer of filler
metal in the space between them.
A further application for vacuum furnaces is Vacuum Carburizing, also known as Low
Pressure Carburizing or LPC. In this process, a gas (such as acetylene) is introduced as a
partial pressure into the hot zone at temperatures typically between 1600F and 1950F.
The gas disassociates into it's constituent molecules (in this case carbon and hydrogen).
The carbon is then diffused into the surface area of the part. This function is typically
repeated, varying the duration of gas input and diffusion time. Once the workload is
properly "cased", quench is induced typically using oil or high pressure gas (HPGQ)
typically, nitrogen or for faster quench helium.
This process is also known as case hardening.

[edit] References