Steels_ Metallurgy and Applications, Third Edition

256 Steels: Metallurgy and Applications

However, vanadium carbide improves the abrasion resistance of high-speed steels

and is also beneficial as a grain-refining agent.

Although not a standard addition, up to 12% Co is incorporated in some high-

speed steels and, according to Hoyle, 2s the role of this element is less clearly

defined than that of the other major additions. Cobalt does not form carbides and

has a high solubility in austenite. For a given hardening temperature, it reduces the

level of retained austenite and also improves the secondary hardening response.

Cobalt also increases the thermal conductivity, particularly at high temperatures.

The net result is that cobalt increases the red hardness of high-speed steels and

improves their performance in fast-cutting operations.

Heat treatment

Given the high carbon content and complex alloy design, it might be anticipated
that the heat treatment of high-speed steels would involve rather complicated
procedures. However, these procedures are based on sound metallurgical princi-
ples and are designed to achieve specific microstructural features and properties.
A schematic illustration of the heat treatment of high-speed steels, including the
annealing, hardening and tempering cycles, is shown in Figure 3.37.
Because of their high alloy content, high-speed steels are air hardening and
will form martensite on cooling from the austenite temperature range. Therefore,
after forging or hot-forming operations, these steels must be annealed in order to
produce:

1. A softened condition for easier machining operations.


2. The relief of internal stresses.


3. A suitable microstructure for the subsequent hardening treatment.

Annealing is carded out by heating slowly to a temperature just above Acl which
involves temperatures in the range 850-900~ depending upon the particular
grade of steel. The material is held at the annealing temperature for two to
four hours and then furnace cooled to a temperature below 600~ The rate

1500

1250

1000

750

500

250

Annealing Hardening Tempering

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Figure 3.37 The heat treatment of high-speed steels