A Simplified Guide to Heat
Treating Tool Steels
When we consider that the greater overall costs of most tools and dies
are incurred prior to heat treatment and further that proper heat
critical to the successful application of tooling, this so-called "
hardening" process is placed in it's proper perspective of importance.
While there are a number of specialized procedures which can be employed
in the heat treatment of tool steels, the purpose of this discussion is to
review the basic heat treating process, it's steps and terms, and perhaps
offer some helpful hints along the way.
The Heat Treating Process
The process consists of:
A) PREHEATING the Annealed tool, typically at 1250 degrees F.
B) AUSTENITIZING (Soaking at High Heat).
C) QUENCHING - Quench to Hard Brittle (Martensite) condition.
D) TEMPERING (Drawing to desired hardness).
Tool steels are furnished in the annealed condition which is the soft,
machineable and necessary condition for proper heat treat response. The
exceptions to this are the prehardened steels such as P-20, Brake Die,
Holder Block and Maxel Tooling Plate which are furnished at 28/32 HRC and
used at that hardness.
Tool steels should always be annealed prior to re-hardening and annealed
steels should be re-annealed after welding. Unlike hardening which
requires a quench after soaking at the hardening temperature, the essence
of annealing is very slow cooling from the annealing temperature. By way
of example, A2 tool steel is annealed by heating to 1550 degrees F, soaking
for two hours at temperature, furnace cooling 50 degrees F per hour to
below 1200 degrees F followed by air cooling. Some shops anneal late in
the day after all other heat treating has been finished by simply soaking
the part to be annealed at the proper annealing temperature and then
turning the furnace off. The next morning the part is fully annealed and
ready for handling.
Preheating plays no part
in the actual hardening reaction and is often considered an unnecessary
preheating performs at least
major function, it minimizes thermal shock, thus reducing the danger
of excessive distortion, warping or cracking. As a matter of fact,
tools and particularly high speed steels, are often preheated
in two steps:
one below the transformation temperature and the second right
at the transformation temperature. Preheating does not require "soaking" but
tool should be equalized at the preheat temperature. Remember,
always bring your tools up with the furnace to the preheating temperature.
Austenitizing (High Heat)
Austenitizing depends upon time and temperature, thus the common term, soak
at high heat. Of the two, temperature is the most critical. Never exceed
the high heat range for the grade. Excessive temperature will cause erratic
results. Classic symptoms of overheating are low as-quenched hardnesses
and, depending upon the alloy content, shrinkage and loss of magnetic
properties. Soaking time should always be after the steel has caught up
with the furnace temperature. With the exception of high speed steels,
rule of thumb for soak time is one half hour per inch of thickness with
forty-five minute minimum and if in doubt over how long to soak a tool,
soak it longer - never less. High speed steels are essentially equalized
with the furnace temperature as opposed to soaking due to the fact that
high heat range is so close to the melting point. Essentially, high speed
steels are equalized at the austenitizing (High Heat) temperature in
minutes of furnace time, never soaked.
Quenching must be done promptly in the medium prescribed for the grade with
the exceptions discussed further. Actually, Water Hardening steels are
properly quenched in brine. A pound of salt to a gallon of water is a good
guide. Oil Hardening steels should be quenched in circulated commercial
quenching oil which has been heated to 100/125 degrees F. In either case,
the liquid quenching bath should contain sufficient volume to prevent the
bath from exceeding the proper bath temperatures. Air Hardening steels
will harden in still air in small sections. However, medium to large
sections may require a light, evenly distributed fan blast. A fan blast on
only one side of a section may cause uneven quenching which will result in
Large sections in certain air hardening grades will not develop full as-
quenched hardness unless they are started in oil. This process is commonly
called interrupted oil quenching. Simply stated, the tool is quenched into
oil until the section just turns black followed by air cooling. S7, an air
hardening -shock resisting tool steel, is a classic example of a steel
which will not develop full hardness in larger sections unless it is given
an interrupted oil quench.
High Speed steels are basically considered oil hardening, however, to
minimize distortion, high speed steels are commonly hardened with an
interrupted oil quench or quenched in hot quenching salt at 1000 degrees F
followed by air cooling.
Obviously, vacuum heat treating furnaces do not allow quenching outside
of the furnace unless the vacuum furnace has been equipped with an attendant
oil quenching zone. Small and medium sections of air hardening steel will
develop full hardness when quenched by the inert gas backfill. However,
larger sections will not develop full hardness unless the inert gas
backfill has sufficient capacity. Some vacuum furnaces have an adjustable
large capacity pressure pump to insure sufficient gas flow to develop
faster quenching rates. Do not temper until the tool reaches about 125
degrees F (handling temperature). If tools cannot be promptly tempered,
placing them in a container of boiling water (212 degrees F) will prevent
cracking due to quenching stresses until tempering can be accomplished.
Upon quenching, tool steels are in a highly stressed condition. To avoid
cracking, tools should be tempered immediately after quenching. As with
Austenitizing, tempering is dependent on temperature and time. The
temperature must be closely controlled, to develop the desired hardness
range. For tempering time, a rule of thumb is one hour per inch of
thickness with a two hour minimum. Longer tempering times are not
detrimental and it is essential that the steel is soaked at temperature
after the steel catches up with the furnace temperature. While one
thorough tempering cycle is sufficient for the lower alloyed tool steels
like Wl and 01, the more highly alloyed grades such as H13, S7, A2, D2 and
the High Speed steels require multiple tempering cycles. A rule of thumb
is that liquid quenched steels may be tempered once, air hardening steel
require a double temper and high speed steels should have a triple temper.
All steels must be cooled in air to about 125 degrees F prior to the next
Like preheating, stress relieving is not a part of the hardness reaction.
However, in order to minimize distortion, particularly in tools which
have had a considerable amount of machining and tools of intricate and/or
unbalanced design, stress relieving is an economical insurance. Annealed
tool steels are stress relieved about 150 degrees F below their Critical
Temperature. For most grades a 1200/1250 degrees F temperature is used
soak times are similar to tempering. Heat treated steels may be similarly
stress relieved at a temperature about 25 degrees F lower than the last
tempering temperature. This is a good policy for steels which have been
EDM'd in order to relieve stresses from the stressed "white layer zone".
It is very important to protect the surface of tools from carburization
(absorption of carbon) unless tools are to be intentionally carburized for
additional surface hardness. In the case of intentional carburizing, a
specific carburizing cycle is employed. Likewise, it is important to
protect tools from de-carburization (the loss of carbon from the steel's
surface) during the heat treating cycle.
Various methods are employed to prevent these detrimental changes to the
surface of tools during heat treating. Atmosphere controlled furnaces and
vacuum heat treating furnaces are two current methods. Stainless steel
foil is commonly used to wrap tools in a relatively air tight package.
This is a very effective method if atmospheric controlled or vacuum
furnaces are not available.
When using "stainless wrap" it
is good practice to cover sharp edges with a
small piece of wrap to prevent tearing out of corners.
Also, never quench tools in the wrap, it acts as an insulator
from the quench resulting
We at Gateway Metals, Inc. truly care about your tooling and have published
the above heat treating guide as a service to our customer-friends.
If you desire further information on heat
treating and/or tool steel grade recommendations for your specific
applications, call Gateway Metals, we'll be pleased to hear from