Section 13. qxd


Chromium is added to increase wear resistance and toughness; the content ranges from 0.25 to 4.5 percent.  Cobalt



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machining processes 1

Chromium
is added to
increase wear resistance and toughness; the content ranges from 0.25 to
4.5 percent. 
Cobalt
is commonly used in high-speed steels to increase hot
hardness so that tools may be used at higher cutting speeds and still
maintain hardness and sharp cutting edges; the content ranges from 5 to 
12 percent. 
Molybdenum
is a strong carbide-forming element and increases
strength, wear resistance, and hot hardness. It is always used in conjunc-
tion with other alloying elements, and its content ranges to 10 percent.
Tungsten
promotes hot hardness and strength; content ranges from 1.25 to
20 percent. 
Vanadium
increases hot hardness and abrasion resistance; in
high-speed steels, it ranges from 1 to 5 percent.
High-speed steels
are the most highly alloyed group among tool steels
and maintain their hardness, strength, and cutting edge. With suitable
procedures and equipment, they can be fully hardened with little dan-
ger of distortion or cracking. High-speed steel tools are widely used in
operations using form tools, drilling, reaming, end-milling, broaching,
tapping, and tooling for screw machines.
Cast alloys
maintain high hardness at high temperatures and have
good wear resistance. Cast-alloy tools, which are cast and ground into
any desired shape, are composed of cobalt (38 to 53 percent), chromium
(30 to 33 percent), and tungsten (10 to 20 percent). These alloys are rec-
ommended for deep roughing operations at relatively high speeds and
feeds. Cutting fluids are not necessary and are usually used only to
obtain a special surface finish.
Carbides
have metal carbides as key ingredients and are manufac-
tured by powder-metallurgy techniques. They have the following prop-
erties which make them very effective cutting-tool materials: (1) high
hardness over a wide range of temperatures; (2) high elastic modulus, 
2 to 3 times that of steel; (3) no plastic flow even at very high stresses;
(4) low thermal expansion; and (5) high thermal conductivity. Carbides
are used in the form of inserts or tips which are clamped or brazed to a
steel shank. Because of the difference in coefficients of expansion,
brazing should be done carefully. The mechanically fastened tool tips
are called 
inserts
(Fig. 13.4.8); they are available in different shapes,
such as square, triangular, circular, and various special shapes.
There are three general groups of carbides in use: (1) tungsten 
carbide with cobalt as a binder, used in machining cast irons and non-
ferrous abrasive metals; (2) tungsten carbide with cobalt as a binder,
plus a solid solution of WC-TiC-TaC-NbC, for use in machining steels;

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