Steels_ Metallurgy and Applications, Third Edition

140 Steels:MetallurgyandApplications

such as toughness and weldability. Therefore detailed attention was given to

identifying the strengthening mechanisms which were most cost-effective or that

provided the best combination of properties.

The practical options for increasing the strength of steels are:

1. Refining the ferrite grain size.


2. Solid solution strengthening.


3. Precipitation strengthening.


4. Transformation strengthening.


5. Dislocation strengthening.

Whereas work hardening or dislocation strengthening can result in very high

levels of strength, these are achieved at the expense of toughness and ductility.

For this reason, little use is made of this method of strengthening but, as illustrated

later in this chapter, work hardening is used in the production of high-strength

reinforcing bars.

Ferrite grain refinement

In the early 1950s, work published by Hall I and Petch 2 laid the foundation for the
development of modem, high-strength structural steels. The Hall-Petch equation,
perhaps the most celebrated in ferrous metallurgy, is as follows:

1

Cry -~ cr i q-. kyd-'2

where try = yield strength

tri = friction stress which opposes dislocation movement

ky = a constant (often called the dislocation locking term)

d = ferrite grain size

Thus refinement of the ferrite grain size will result in an increase in yield strength
and the relationship is shown in Figure 2.1.
Whereas a strengthening effect usually leads to a decrease in toughness, it
was shown that refinement of the ferrite grain size also produced a simultaneous
improvement in toughness. The Petch equation linking toughness to grain size is
given below:

/ST = In t5 - In C - In d-

where/5 and C are constants, T is the ductile-brittle transition temperature and
d is the ferrite grain size. Therefore, as illustrated in Figure 2.1, the impact
transition temperature decreases as the ferrite grain size is reduced.
Refinement of the ferrite grain size can be achieved in a number of ways. Tradi-
tionaUy, fine-grained steels contain about 0.03% A1 which is soluble at normal
slab or bloom reheating temperatures of around 1250"C and which remains in
solution during rolling and after cooling to ambient temperature. However, on
subsequent reheating through the ferrite range to the normalizing or solution
treatment temperature, the aluminium combines with nitrogen in the steel to