Fig. 1: Heat treatments of modern steel. (a) a schematic phase diagram of carbon composition range and phases expected in the microstructure of marine steel, (b & c) rolling and normalization (d) TMCP route involving reheating, accelerated cooling and self-temper or tempering at temperature below Ac1 (e) rolling, accelerated cooling and (f) rolling, direct-quenching and tempering at temperature below Ac1 [28][29].
Normalized steel (N -steel) is commonly obtained when the as-rolled steel is reheated to about 900°C or very little above Ac3, held for a desired period and allowed to cool naturally back to room temperature as shown in Fig. 1(c). When the finishing rolling operation is done at a temperature above about 900°C before cooling naturally to room temperature, aNR -steel is obtained. Here no reheating is necessary. Normalized and NR steels have practically the same properties and so are designated ‘N ’. The strength or other properties of the steel can be increased by addition of small amount of alloying elements and then quenched or accelerated-cooled and tempered as shown in Fig. 1(d). The tempering is then done below Ac1. TMCP is a microstructural control technique combining controlled rolling (CR ) and accelerated cooling to obtain exceptional strength-toughness combinations in low-alloy steels by grain refinement [29][30][31][32]. In general, controlled-rolling, controlled-cooling, accelerated cooling and direct-quenching are typical examples of thermomechanical processing [29]. Controlled rolling involves carrying out of the finishing deformation at temperature sufficiently low to prevent grain growth of the recrystallized austenite during cooling. The property obtained may not be different from those obtained after normalization [28][29]. The TMCP route is illustrated in Fig. 1(e & f) [28]. Traditional marine steels shown in Table 1(b) are produced byAR , N or NR . The TMCP steels production route is proprietary and so processing conditions and number of deformations are slightly different, producing what may appear as variation in the α and P sizes and morphologies.
Ideally, most common marine steels have microstructures consisting of very small volume fraction of P in a α matrix. It may be called α-P steels where volume fraction of P phase is significant. The common α morphologies are grain boundary allotriomorphic α , idiomorphic α , Widmanstätten α , and intragranular α or acicular α [33]. The αmorphologies commonly encountered or that dominate in marine steels are the allotriomorphic and idiomorphic α . In this study they will collectively be referred to as α . The microstructure shown in Fig. 2(a) is air-cooled of medium carbon steel and the relatively rapid method of cooling, limited the α grain growth. The α then appears to form layers which followed the austenite grain boundary contours. The prior austenite grain boundaries are completely covered by the α allotriomorphs and the residual austenite has transformed into P upon reaching the temperature, 723 oC, of eutectoid reaction. The α phase is soft and ductile, while theP is hard and brittle. P grows in colonies and each colony consists of a thin lamellar, alternating mixture of α (iron) and cementite, θ (iron carbide) or (α+Fe3C ). Fig. 2(b) shows a situation where the Fe - 0.4C steel is slowly cooled to room temperature, producing equiaxed grains. Several distinctP colonies can be seen (dark etching). The α phase (light etching) is soft and ductile, while the P is hard and brittle. As the volume fraction of P increases the bulk strength increases and ductility decreases. If the carbon content of the steel is decreased, the volume fraction of P in the microstructure will decrease while that of α increases as can be seen in Fig. 2(c). The morphology or shape of the α-P phases therefore depends on the processing route discussed above and electrochemical composition. These two phases are of major interest in this study. Any other phase that might be present for the purposes of clarity is neglected.