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).