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"Heat Treatment of
Tool Steels"
Most
punches, dies, stamps, and other metalforming tools
require high hardness, usually above about 55 Rockwell
C, to perform successfully. However, they are supplied
to tool builders in the annealed condition, around
200/250 Brinell (about 20 Rockwell C), to facilitate
machining. They must be heat treated to develop their
characteristic properties, making the tool capable of
withstanding the pressure, abrasion, and impacts
inherent in metalforming. Each step of the heat treating
cycle performs a specific function. Like the links in a
chain, the final product is only as good as its weakest
component. Heat treating typically accounts for less
than 10% of the cost of a tool, but it may be the single
most important factor in determining the performance of
the tool. There is no such thing as an acceptable
shortcut in heat treating tool steels.
The high hardness structure of tool steels, called
martensite, cannot be produced directly from the soft
annealed structure. Tool steels must undergo 2
intermediate structure changes during the heat treating
process. First, the annealed structure, called ferrite,
must be heated to become a high-temperature structure
called austenite. Then, the austenite must be cooled
relatively quickly to become martensite. In order to
accomplish these structural changes in tool steels, 4
primary heat treating steps are used: preheating,
austenitizing, quenching, and tempering. The table shows
variations among those steps for 10 typical grades of
tool steel. Each step is discused in detail in the
following sections.
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Preheating, or slow heating, of tool steels provides 3
practical benefits, although it does not directly affect
the final properties of the steel. First, most tool
steels are sensitive to thermal shock. A sudden increase
in temperature up to 1500/2000øF may cause tool steels
to crack. Preheating to one or more intermediate
temperatures allows more gradual heating.
Second, tool steels undergo a change in density when
they transform from ferrite to austenite. If this volume
change occurs non-uniformly, it can cause unnecessary
distortion of tool, especially where differences in
section cause some parts of a tool to transform before
other parts have reached the required temperature. Tool
steels should be preheated to just below the temperature
at which austenite forms (called the critical
temperature), and then held at that temperature long
enough to allow the full cross-section to reach a
uniform temperature. Then on further heating, the tool
will transform more uniformly, causing less distortion
to occur.
Third, steels conduct heat faster at higher
temperatures. Thus, a preheated tool may require
slightly less furnace time at the hardening temperature
to reach uniformity. Minimizing time at temperature is a
good practice for maximizing the toughness (impact
resistance) of the tool. In some cases, particularly
involving hardening temperatures over 2000F, multiple
preheating steps may be used prior to reaching the final
austenitizing temperature.
Once tool steels have been satisfactorily preheated,
they are raised to their austenitizing temperature. The
desired hardened structure, called martensite, cannot
form directly from the annealed structure, called
ferrite. The ferrite must first be transformed to an
intermediate structure called austenite. Tool steel
becomes austenite when it is heated above its critical
temperature. The critical temperature for most tool
steels is about 1500F.
After the structure changes to austenite, further
heating is required to properly distribute the alloy
content of the steel. Most of the useful alloy content
of tool steels exists as microscopic carbide particles
in the soft matrix of the annealed steel. These carbide
particles must be at least partially dissolved into the
matrix of the steel during the hold at the austenitizing
or hardening temperature. The temperature used varies
greatly from grade to grade, depending primarily on the
composition of the steel. Typical temperature ranges are
shown in the table.
In some grades, the austenitizing temperature may be
varied to tailor the properties of the steel, in order
to provide a little extra wear resistance or toughness
for specific applications. Higher temperatures allow
more alloy content to dissolve into the matrix, allowing
slightly higher hardness and better wear resistance.
Lower temperatures dissolve less alloy content into the
matrix, and favor increased impact resistance, although
the attainable hardness is slightly lower. The use of
lower than maximum hardening temperatures, called
underhardening, is often the most effective way to
achieve the maximum toughness in the high alloy grades,
where a range of hardening temperatures are possible.
The hold times used depend primarily on the
austenitizing temperature. Diffusion of alloy content
occurs faster at higher temperatures, and hold times are
decreased accordingly. The hold times shown in the table
are typical for small sections (under about 2"), and
represent the total hold time after the tool has reached
the aim temperature, regardless of part size or other
factors. Larger sections require longer hold times to
allow them to be heated through to the aim temperature.
Extended hold times depend on the type of furnace
equipment, load size, and heat treat experience.
Once the alloy content has been dissolved as desired
into the steel matrix during austenitizing, the steel
must be cooled fast enough to keep the alloy content in
place and transform the austenite to the high hardness
martensite. How fast a steel must be cooled to fully
harden depends on the chemical composition. Low alloy
tool steels (O1, S5, L6 etc.) ol order to cool fast
enough. This drastic quench can cool some portions of
the tool much faster than other portions, increasing
risk of distortion or even cracking in severe cases.
Higher alloy tool steels (A2, D2, M4, 10V, etc.) may be
cooled in air or inert gas. Air-hardening steels cool
more uniformly, so distortion and risk of cracking are
lower.
For tool steels which are quenched from over about
2000øF, the quench rate from about 1800øF down to about
1300øF must be rapid enough to avoid undesirable
reactions which can impair toughness and hardness
response. The actual transformation of austenite to
martensite does not begin until the steel cools below
about 700øF. The specific temperature at which martenite
starts to form is called the "martenite start" or "MS"
temperature. In tool most steels, the martensite forms
between about 600øF and about 200øF. The amount of
martensite depends principly on how close to the lower
temperature, (martesnite finish or "MF" temperature),
the steel gets. 100% martensite is not formed until the
steel cools below the martesnite finishing temperature.
No matter how tool steels are quenched, the resulting
martensite is extremely brittle, and under great stress.
If put into service in this condition, most tool steels
would shatter. Some tool steels may even crack
spontaneously if allowed to remain for any time in this
condition. For this reason, as soon as tool steels have
been quenched to about 125/150øF, they should be
immediately tempered.
Tempering is performed to stress relieve the brittle
martensite formed during the quench. Most steels have a
fairly wide range of acceptable tempering temperatures
(see table). For best relief of quenching stresses, use
the highest tempering temperature which will give the
desired hardness.
Most tool steels must be tempered at least 2 times, with
triple tempering recommended for high alloy grades or
high hardening temperatures. The rate of heating to or
cooling from the tempering temperature is not critical,
except sudden drastic temperature changes should be
avoided. Tool steel should be allowed to cool completely
to room temperature (50/75øF) between tempers.
Tool steels should be held at temperature a minimum of 2
hours for each temper. A rule of thumb is to allow one
hour per inch of thickest section for tempering, but in
no case less than 2 hours.
When surface treatments are used, (nitriding, TiN
coating, etc.), the heat treating process should be
discussed with the surface treater. For best results, it
is important that heat treating temperatures and
processes are compatible with subsequent surface
treatment temperatures.
The heat treat process results in an unavoidable size
increase in tool steels due to the changes in
microstructure. Most tool steels will grow between about
0.0005" and 0.002" per inch of original length during
heat treatment. This will vary somewhat based on a
number of theoretical and practical factors. Many heat
treaters will have a "feel" for what to expect from
typical processes.
In certain cases, a combination of variables including
high alloy content, long austenitizing time or high
temperature, discontinuing the quench process too soon,
or other factors in the process may cause the MS
temperature to become depressed to below room
temperature. In this case some of the high temperature
structure, austenite, will be retained at room
temperature (martensite is not completely formed). This
retained austenite condition is usually accompanied by
an unexpected shrinkage in size, and sometimes by less
ability to hold a magnet. This condition can often be
corrected simply by exposing tools to a low temperature
(cryogenic or refrigeration treatments), to encourage
continuation of the transformation to martensite by
cooling the steel to below its MS.
Tool steels transform to martensite during quenching
from about 600øF down to about 200øF. In some cases, as
described above, the transformation to martensite may
not be complete at the end of the quench (125øF). In
such cases, some austenite may still be retained after
the normal heat treatment. This retained austenite can
sometimes lead to unexpected growth in service, causing
loss of accuracy. A2 and D2 are two grades commonly
prone to retained austenite after heat treating. By
cooling the steel to sub-zero temperatures, this
retained austenite may be transformed to martensite. The
newly formed martensite is similar to the as-quenched
martensite, and must be tempered. Cryogenic or
refrigeration treatment should include a temper after
freezing. The cold treatment is often performed between
normally scheduled tempers. By minimizing retained
austenite, certain kinds of dimensional stability
problems can be avoided.
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