The use of nanotechnology to improve the bulk and surface properties of steel for structural applications - Nanotechnology in Eco-efficient Construction - page 77

High-strength steels are often martensitic; as the C content increases,
their strength increases but ductility and weldability tend to decrease. Mar-
tensite can serve as an effective starting phase for acquiring ultrafi ne or
nano-grained microstructure with small strains. For instance, Tsuji and Maki
(2009) reported the formation of microstructure in steel characteristic of
equiaxed

phase. Austenitic stainless steels (SS) generally have low yield strength
(150-300 MPa) yet excel at corrosion and oxidation resistance, work-hard-
ening rate, and formability. Their strengthening by grain refi nement can be
achieved via reverse

α grains ( ∼ 200 nm), by cold-rolling and annealing of a starting α ′

transformation or SPD, and greatly enhances
strength and resistance to wear, pitting, cavitation, cavitation-erosion, and
radiation-induced damage (Lo et al. , 2009). The grain refi nement of

α ′

γ SS

may compromise ductility and work-hardening. The strengthening of SS

by martensite or dispersed precipitates can compromise their corrosion
resistance, formability and ductility (Lo et al. , 2009).
Alloying elements in steel 'may change the transformation temperature,
or form more stable special carbides instead of cementite, . . . (or) change
the sequence of nucleation and growth processes by affecting the interface
energies or the diffusion processes' (Eisenhüttenleute, 1992). In addition,
the microstructure of steel can be controlled by manipulating the kinetics
of its formation, via thermal and mechanical treatments. For instance, the
following solid-solid state transformation can occur (Fig. 5.1):

γ

Fe 3 C
(Branagan et al. , 2006). Each phase may feature multiple allotropes, i.e.,
various structural forms of the same chemical composition. For instance,
cementite can be in the form of 'pearlite, bainite, and/or tempered marten-
site, depending on steel composition and preceding cooling conditions'
(Eisenhüttenleute, 1992). The ultimate microstructure of a steel with given
alloy chemistry is often defi ned by the

γ → α +

-state grain size and homogeneity,
transformations induced by heat treatment (e.g., quenching and annealing),
and physical interactions induced by mechanical processing (e.g., SPD).

γ

￿ ￿ ￿ ￿ ￿ ￿

5.2.2 Infl uence of nano-modifi cation

In the last decades, grain refi nement has been confi rmed as an effective way
to concurrently enhance strength and toughness of polycrystalline materials
and has been used to remarkably improve the comprehensive mechanical
properties of steels. Zhao et al. (2011) found the grain size of large-grain-
size regions to be responsible for the ductile-to-brittle transition tempera-
ture of an ultrafi ne grained ferrite/cementite steel. As the average grain size
or weighted average grain size decreases, the yield strength and hardness
of polycrystalline materials (e.g., steels) are expected to increase greatly,
according to the well-known Hall-Petch relation (Lo et al. , 2009). This can
be attributed to the high concentration of GBs, which serve as barriers to