The use of nanotechnology to improve the bulk and surface properties of steel for structural applications - Nanotechnology in Eco-efficient Construction - page 79

grains and improve weld toughness is to add Mg and Ca nanoparticles in
the steel (Mann, 2006). Grain coarsening at elevated temperatures can be
inhibited via GB segregation and various drag effects (Malow and Koch,
1997; Matsuia et al. , 2006), or via GB modifi cation or manipulation of nano-
twin boundaries inside the grains (Lu et al. 2009; Li et al. 2010). All of these
mechanisms serve to preserve the ductility of high strength steel. Gonsalves
et al. (1994) suggested that carbide particles a few microns in diameter may
act as initiation sites of fatigue cracks. Lo et al. (2009) suggested that the
precipitation of carbides at GBs can cause localized depletion of Cr content
in SS and increase their risk of corrosion and sensitization They also
reported the use of nitrogen to inhibit the formation of the M 23 C 6 carbide
so as to minimize its undesirable effects on GB serration and creep-fatigue
resistance. As detailed in Table 5.1, the formation of nano-sized grains,
twins, or inclusions can mitigate such risks of microstructure ultrafi ning.
Nanotechnology provides a potential solution to considerably enhancing
the ductility of high strength steel, as the nanocomposite steels deform by
GB mechanisms, e.g., sliding and diffusion, rather than dislocation motion.
Grain refi nement can be coupled with phase transformation induced plas-
ticity (TRIP) to form grains of micron or nano-size, endowing the steel

with outstanding synergy in strength, ductility, and work-hardening ability.
While coarse MC carbides degrade creep-fatigue resistance and fracture
resistance of SS, fi ne MC carbides can inhibit the growth of grains and
greatly improve the impact property and strength of SS (Lo et al. , 2009).
Often, a second phase or nanostructure can be uniformly dispersed in the
relatively brittle bulk matrix in order to introduce plasticity. Alternatively,
nano-grains or nanotwin boundaries inside the grains can be introduced to
delocalize the micro- and nano-scale deformation and to improve tough-
ness. Zhao et al. (2011) suggested high-angle GBs to be most effective in
hindering the propagation of cleavage cracks. Manipulation of GB struc-
tures (i.e., GB engineering) can be utilized to enhance the bulk properties
of steels, e.g., by reducing carbide precipitation at GBs (Lo et al. , 2009).

γ

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5.2.3 Infl uence of processing approaches

Conventionally, there are four main pathways to achieve a fi ne-grained
microstructure (e.g., grain size of 1 mm): repetitive short austenitizing at
low temperatures; 'deformation in the austenite range followed by recrys-
tallization'; deformation of austenite at temperatures featuring a low recrys-
tallization rate; or cooling to induce transformation at low temperatures
(Eisenhüttenleute, 1992).
Recent decades have seen the increased use of metallurgical refi ning,
SPD, thermomechanical treatment (TMT), and liquid-state treatments to
produce steels with ultrafi ne grained microstructure. The reduction of grain