Civil Engineering Reference
In-Depth Information
size from millimeter scale to micron scale is achieved by increasing the rate
of grain nucleation while reducing the rate of grain growth. The grain refi ne-
ment in the liquid state, for instance, may involve the use of pulsed magnetic
fi elds, pulsed current, or ultrasonic energy to induce refi ned microstructure
of SS during its solidifi cation (Lo
et al.
, 2009). Okamura
et al.
(1995) reported
the formation of ultrafi ne grained bainite structure in new HT780 steels
(0.05-0.06% C; B-free) by direct quenching and tempering. These steels
exhibit outstanding mechanical properties: tensile strength (TS) of 825 MPa
and total elongation (El) of 26% at fracture. They also exhibit excellent
welding properties (e.g., low HAZ hardness) and good fatigue properties,
attributable to precipitation hardening and grain refi nement by Cu, Nb, and
V. Some alloying elements (e.g., Tb, Ni, and V) can form nano-precipitates
with the C or N in steel and greatly inhibit the growth of grains while
forming a high concentration of non-uniform grains. There are also alloying
elements (e.g., Mn and Cr) that can reduce the phase transformation tem-
perature and refi ne the grains during or after the phase transformation. Lei
et al.
(2007) suggested that the best way of grain refi ning was to couple
alloying with TMT or SPD.
While nano-grained or nano-amorphous steels may be synthesized by the
'bottom-up' approach such as gas condensation or chemical analysis (Aver-
back, 1993; Gonsalves
et al.
, 1994), these production processes do not lend
themselves to industrial manufacturing. In contrast, the 'top-down' approach
such as mechanical alloying (MA), TMT and SPD can be readily imple-
mented at large industrial scale. TMT can be as simple as the conventional
cold rolling followed by annealing, or may consist of advanced treatment
sequences. Bhadeshia (2008) reported the use of TMT to induce phase
transformation, forming nanostructured bainitic
20 nm) in
steels with a high C concentration. In addition to phase transformation,
TMT may induce other solid-solid reactions such as recrystallization and
precipitation. SPD involves intensive straining processes. SPD can induce
the formation of nanocrystalline grains in bulk material (with equal channel
angular pressing - ECAP, high pressure torsion, accumulative roll bonding,
repetitive corrugation and straightening, constrained groove rolling or
pressing, etc.) or in surface layer (100
α
and
γ
plates (
∼
m to 30 mm thick, with surface
mechanical attrition treatment - SMAT, ball milling, slide wearing, wire
brushing, ultrasonic or high-energy shot peening, supersonic fi ne particles
bombardment - SFPB, severe cold drawing, ultrasonic cold forging - UCFT,
etc.). SPD concurrently improves the strength and toughness of steels by
substantially refi ning the microstructure of steel, increasing the density of
dislocations, dislocation walls, and vacancies, forming non-equilibrium GBs,
and stabilizing austensite phases (Valiev, 2004). Tsuji and Maki (2009) dem-
onstrated that nanostructures in steel can be produced by different ways of
combining phase transformation and plastic deformation. They also revealed
μ
Search WWH ::
Custom Search