Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 181

[~] Unnotched 1 Notched

bars bars

80

6O

C a) 40

"8 .e

8

20

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i

!

I

Cold worked Hot rolled Mild steel

high yield high yield

Figure 2.20 The effect of notching reinforcing bars on the energy absorbed in concrete
beams (After Armstrong et al. 27)

cheaply by eliminating the costs associated with cold twisting or micro-alloy
additions. The Tempcore process has been very successful, both technically and
commercially, and has been licensed throughout the world.
The Tempcore process is illustrated schematically in Figure 2.21. On leaving
the last finishing stand, the bar passes through a water cooling station which cools
the outer region of the bar sufficiently quickly for the formation of martensite.
At the end of the cooling process, the bar has an austenite core surrounded by a
mixture of austenite and martensite, the amount of martensite increasing towards
the surface of the bar. On leaving the cooling station, the bar is exposed to the
atmosphere and the temperature gradient between the core and quenched surface
begins to equalize. This leads to the tempering of the martensitic rim, providing
an adequate balance between strength and ductility. During this second stage in
the process, untransformed austenite in the outer layers of bars also transforms
to bainite. The final stage of the process takes place as the bars lie on the cooling
bed, namely the transformation of austenitic core. Depending upon factors such
as composition, finishing temperature and cooling rate, the core can transform
to ferrite and pearlite or to a mixed microstructure which includes some bainite.
Thus the Tempcore process produces a variety of microstructures throughout the
cross-section, ranging from tempered martensite in the outer layers to a region
which is essentially ferrite and pearlite in the core, with an intermediate zone
which may be predominantly bainitic.
Depending on the size of the bars, water cooling may be applied before the bars
enter the finishing stands in order to reduce the length of the cooling station. This