Materials for engineering, 3rd Edition - (Malestrom)

106
Materials for engineering
Heat-treated steels
In many applications, the heat-treatment applied is that of quenching followed
by tempering. The steel is heated into the γ-phase field and then quenched in
order to form the hard, metastable α′-phase known as martensite. The
cooling-rate during quenching must be such that the entire cross-section of
the specimen is converted to martensite; a TTT curve is illustrated in Fig.
3.23, where it may be seen that the cooling-rate at the centre of a large crosssection
may be insufficient to suppress the nucleation of other phases. Small
quantities of Cr and Ni added to low-alloy steels have the effect of moving
the TTT curve to the right, thus enabling material of larger cross-section to
be fully transformed to martensite. This is known as increasing the
hardenability.
The martensite is finally tempered by heating in the range 200–600°C,
causing it to decompose to a mixture of carbide particles in α, known as
‘tempered martensite’: The fineness of the carbide dispersion depends upon
the tempering temperature: the lower the temperature, the finer the dispersion
of carbide and the harder the material (Fig. 3.24). The diagram also demonstrates
that, as the strength increases, the ductility decreases; this follows from the
Considère construction (Fig. 2.6), since the strain for plastic instability (i.e.
the point of contact of the tangent) decreases as the stress–strain curve is
raised.
Temperature
A A + F + C F + C
M S
M f
log time
3.23 A TTT curve; A austenite, F ferrite, C carbide and M martensite.