Materials for engineering, 3rd Edition - (Malestrom)

108
Materials for engineering
The progressive fall in hardness with increasing tempering temperature
arises because of the rapidly increasing diffusivity of carbon, allowing
progressively coarser particles of iron carbide to form in the steel. Highspeed
steels contain major additions of strong carbide-forming elements
such as Cr, Mo, W and V. When these steels are quenched and tempered,
alloy carbides form in the high-temperature range when the hardness of
plain-carbon steels declines, so secondary hardening (Fig. 3.25) is observed.
High-speed steels are so called because they maintain their hardness during
high-speed machining operations: appreciable softening does not occur until
the temperature exceeds about 550°C, which represents the maximum operating
temperature for these steels.
Pearlitic steels
As apparent from Fig. 3.21, air-cooled medium- to high-carbon steels undergo
a eutectoid decomposition to form a microstructure of islands of lamellar
pearlite (α + Fe 3 C) in an α matrix and their strength will increase as the
volume fraction of pearlite increases as illustrated in Fig. 3.26. Their
applications include rail steels, as well as high-carbon wire rod. The latter
are transformed from γ at a temperature near the ‘nose’ of the TTT curve
(Fig. 3.23) to form a microstructure of minimum interlamellar spacing. The
wire is then cold drawn, and the work-hardened product may have tensile
strengths in excess of 2 GPa.
Bainitic steels
Steel transformed at temperatures below the ‘nose’ of the TTT curve form a
non-lamellar aggregate of α + Fe 3 C known as ‘bainite’, which consists of
fine plates of ferrite containing fine Fe 3 C particles. Alloying with about
1.5wt% of silicon suppresses the precipitation of Fe 3 C during the bainite
transformation and the resulting microstructure of these silicon-rich carbidefree
steels consists of fine plates of α separated by carbon-enriched regions
of γ. These carbide-free bainitic steels show exceptional promise as rail
steels, since the brittle Fe 3 C phase is eliminated; they derive their strength
from the ultrafine α plates, which are much less than 1 µm in thickness. Such
grain refinement simultaneously improves both the strength and the toughness
of steel and these steels are relatively cheap to produce. They, therefore,
show considerable potential in terms of reduced maintenance and increased
safety with an expected reduction in rail fractures.
Maraging steels
Maraging steels may be termed ultra-high-strength steels and may exhibit