Fig. 1(a) is common Fe-C phase diagram showing important regions
in the heat treatment of marine steels. γ denotes austenite, αferrite and P pearlite. A1 is an equilibrium
lower transformation temperature. Eutectoid reaction occurs at
A1 which is 723◦C in the binary diagram. The
Ar1 temperature depicts the lower transformation
temperature upon cooling. Ac1 temperature depicts the
start point of the transformation between the α-ferrite and the
austenite γ upon heating. A3 is an equilibrium upper
transformation temperature where alpha iron (α-Fe ) transforms to
gamma iron (γ-Fe) and it is 910°C for pure iron and it is lowered with
addition of carbon. Ar3 is the temperature at which
austenite γ starts transforming to ferrite α during cooling.
Ac3 temperature depicts the upper transformation
temperatures upon heating. The line in Fig. 1(a) are straight - in fact
in the case above they are drawn arbitrarily. In reality, the shape and
position of the lines – straight or curved, depend on the steel
chemistry, cooling rate and rolling or deformation variables.
Austenitization is the first operation in many of the most important
deformation and heat treatment processes. After austenitization, upon
cooling the austenite decomposes into various morphologies. From the
austenite region, the bcc α-ferrite is reported to be the first to
nucleate at the austenite grain surfaces. Other microstructures that
form as the transformation temperature is lowered are pearlite
(P ), bainite, and martensite.
The nature of α-Pmicrostructure due to heat treatment can be explained by the type of
heat treatment process – commonly referred to as as-rolled (AR ),
normalized (N ), normalized-rolled (NR ), quenched and
tempered (Q&T ) and thermomechanical control process
(TMCP ). The common phases in these steels are ferrite (α )
and pearlite (α+Fe3C ). Fig. 1(b - f) are
schematic of heat treatment of modern steel. The heat treatments of
interest in this study are AR , N , NR andTMCP . Normalizing involves cooling of austenitized steel,
usually in air, to a temperature substantially below the transformation
range [27]. The austenitizing temperature for normalizing is
typically 30–80ºC above the Ac3 transformation
temperature (the upper critical temperature, 800 – 925) for
hypoeutectoid steels. As-rolled is obtained by austenitizing at say
about 1250 °C, holding for specified time and then deforming at roughing
and finishing mills within the austenite range. The finishing rolling
may, for example, be at some temperature above Ac3 and
the steel is allowed to cool naturally to room temperature as shown in
Fig. 1(b).