Steels_ Metallurgy and Applications, Third Edition

218 Steels: Metallurgy and Applications

temperature in order to obtain a better combination of properties. In carburized
gears, the tempering treatment might be carried out at a temperature as low as
180~ the purpose being to achieve relief of internal stresses without causing
any significant softening in either the case or core of the components. On the
other hand, medium-carbon steels containing about 0.4% C might be tempered
at temperatures up to 650~ which results in a significant decrease in strength
compared with the as-quenched condition but which is necessary in order to
produce adequate toughness and ductility.
The mechanism of tempering involves the partial degeneration of martensite
via the diffusion of carbon atoms out of solid solution to form fine carbides. If
the tempering treatments are carried out at sufficiently high temperatures for long
periods, then the breakdown of martensite can be complete, the microstructure
consisting of spheroidized carbide in a matrix of ferrite. However, treatments that
are carried out to achieve this type of microstructure would be termed annealing
rather than tempering.
Alloying elements affect the tempering process by retarding or suppressing the
formation of Fe3C, either by stabilizing the e-carbide (Fe2.4C) which is formed
initially in the breakdown of martensite or by forming carbides that are more
stable and grow more slowly than Fe3C. Alloying elements therefore provide
the opportunity of tempering at higher temperatures in order to obtain a higher
ductility for a given strength level. Alternatively, improved tempering resistance
might be employed to allow a component to operate at a higher temperature
without softening.
Smallman ~l has produced the information given in Table 3.1 on the effect
of alloying elements on tempering. In this table, the negative value ascribed to
carbon indicates an acceleration in the tempering process which is due presumably
to the increased supersaturation/driving force effect.
Work by Grange and Baughman 12 established the following rank order of
potency in promoting tempering resistance:

Vanadium

Molybdenum

Chromium decreasing effect

Manganese

Silicon

Copper

Nickel

Thus there is broad agreement between the two sets of data, although Smallman
indicates a relatively higher effect for silicon.
Although alloying elements such as vanadium and molybdenum are effective
in promoting both hardenability and tempering resistance, they are expensive
and the design of composition in engineering steels is dictated as much by costs
as property requirements. Thus hardenability is achieved most cheaply through
additions of manganese, chromium and boron but with little contribution to
tempering resistance. However, such a condition may be perfectly acceptable
in a case-carburized component where the tempering treatment is carried out
at a low temperature. On the other hand, when the property requirements or