1.

1 Introduction
Many types of tool materials, ranging from high carbon steel to ceramics and diamonds,
are used as cutting tools in today’s metalworking industry. It is important
to be aware that differences do exist among tool materials, what these differences
are, and the correct application for each type of material.
The various tool manufacturers assign many names and numbers to their products.
While many of these names and numbers may appear to be similar, the applications
of these tool materials may be entirely different. In most cases the tool manufacturers
will provide tools made of the proper material for each given application.
In some particular applications, a premium or higher priced material will be justified.
This does not mean that the most expensive tool is always the best tool. Cutting
tool users cannot afford to ignore the constant changes and advancements that are
being made in the field of tool material technology. When a tool change is needed
or anticipated, a performance comparison should be made before selecting the tool
for the job. The optimum tool is not necessarily the least expensive or the most
expensive, and it is not always the same tool that was used for the job last time.
The best tool is the one that has been carefully chosen to get the job done quickly,
efficiently and economically.

A cutting tool must have the following

characteristics in order to produce
good quality and economical parts:
Hardness: Hardness and strength of
the cutting tool must be maintained at
elevated temperatures also called Hot
Hardness
Toughness: Toughness of cutting
tools is needed so that tools don’t chip
or fracture, especially during interrupted
cutting operations.
Wear Resistance: Wear resistance
means the attainment of acceptable tool
life before tools need to be replaced.
The materials from which cutting
tools are made are all characteristically
hard and strong. There is a wide range
of tool materials available for machining
operations, and the general classification
and use of these materials are of
interest here.
1.2 Tool Steels and Cast Alloys
Plain carbon tool steel is the oldest of
the tool materials dating back hundreds
of years. In simple terms it is a high
carbon steel (steel which contains about
1.05% carbon). This high carbon content
allows the steel to be hardened,
offering greater resistance to abrasive
wear. Plain high carbon steel served its
purpose well for many years. However,
because it is quickly over tempered
(softened) at relatively low cutting temperatures,
(300 to 500 degrees F), it is
now rarely used as cutting tool material
except in files, saw blades, chisels, etc.
The use of plain high carbon steel is
limited to low heat applications.
High Speed Tool Steel: The need for
tool materials which could withstand
increased cutting speeds and tempera-
Chap. 1: Cutting-Tool Materials
www.toolingandproduction.com Chapter 1/Tooling & Production 3
tures, led to the development of high
speed tool steels (HSS). The major difference
between high speed tool steel
and plain high carbon steel is the addition
of alloying elements to harden and
strengthen the steel and make it more
resistant to heat (hot hardness).
Some of the most commonly used
alloying elements are: manganese,
chromium, tungsten, vanadium, molybdenum,
cobalt, and niobium (columbium).
While each of these elements will
add certain specific desirable characteristics,
it can be generally stated that they
add deep hardening capability, high hot
hardness, resistance to abrasive wear,
and strength, to high speed tool steel.
These characteristics allow relatively
higher machining speeds and improved
performance over plain high carbon
steel.
The most common high speed steels
used primarily as cutting tools are divided
into the M and T series. The M series
represents tool steels of the molybdenum
type and the T series represents
those of the tungsten type. Although
there seems to be a great deal of similarity
among these high speed steels,
each one serves a specific purpose and
offers significant benefits in its special
application.
An important point to remember is
that none of the alloying elements for
either series of high speed tool steels is
in abundant supply and the cost of these
elements is skyrocketing. In addition,
U.S. manufacturers must rely on foreign
countries for supply of these very
important elements.
Some of the high speed steels are
now available in a powdered metal
(PM) form. The difference between
powdered and conventional metals is in
the method by which they are made.
The majority of conventional high
speed steel is poured into an ingot and
then, either hot or cold, worked to the
desired shape. Powdered metal is
exactly as its name indicates. Basically
the same elements that are used in conventional
high speed steel are prepared
in a very fine powdered form. These
powdered elements are carefully blended
together, pressed into a die under
extremely high pressure, and then sintered
in an atmospherically controlled
furnace. The PM method of manufacturing
cutting tools is explained in
Section 1.3.1 Manufacture of Carbide
Products.
HSS Surface Treatment: Many surface
treatments have been developed in
an attempt to extend tool life, reduce
power consumption, and to control
other factors which affect operating
conditions and costs. Some of these
treatments have been used for many
years and have proven to have some
value. For example, the black oxide
coatings which commonly appear on
drills and taps are of value as a deterrent
to build-up on the tool. The black oxide
is basically a ‘dirty’ surface which discourages
the build-up of work material.
One of the more recent developments
in coatings for high speed steel is titanium
nitride by the physical vapor deposition
(PVD) method. Titanium nitride is
deposited on the tool surface in one of
several different types of furnace at relatively
low temperature, which does not
significantly affect the heat treatment
(hardness) of the tool being coated.
This coating is known to extend the life
of a cutting tool significantly or to allow
the tool to be used at higher operating
speeds. Tool life can be extended by as
much as three times, or operating
speeds can be increased up to fifty percent.
Cast Alloys: The alloying elements
in high speed steel, principally cobalt,
chromium and tungsten, improve the
cutting properties sufficiently, that metallurgical
researchers developed the cast
alloys, a family of these materials without
iron.
Atypical composition for this class of
tool material was 45 percent cobalt, 32
percent chromium, 21 percent tungsten,
and 2 percent carbon. The purpose of
such alloying was to obtain a cutting
tool with hot hardness superior to high
Ceramics
Cast alloys
0 200 400 600 800 1000 1200 1400
60
55
65
70
75
80
25
30
35
40
45
50
55
60
65
70
20
85
90
95
100 300
Carbides
Carbon
tool
steels High-speed
steels
500 700
Temperature (°F)
Temperature (C°)
Hardness (H-Ra)
Hardness (H-Rc)
(a)
Diamond, CBN
Aluminum oxide (HIP)
Silicon nitride
Cermets
Coated carbides
Carbides
Strength and toughness
(b)