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.