Материаловедение: свойства металлов. Матросова Т.А - 48 стр.

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48
Hardenability refers to the distance within a specimen normal to its surface
that appreciable hardness can be developed. Since the quenching rate of a steel is
limited by its heat diffusivity and the rate at which austenite can transform, the
hardenability of steels depends on the alloy content and the grain size of the original
austenite. Hot-work steel die blocks, which must be hardened throughout their mass,
are alloyed to such an extent that they can transform to a bainitic structure with an air
quench. The hardenability of a typical alloy steel and a plain carbon steel are
compared in fig. 17.2. Note that for all sections above 1/2-in. diameter the plain
carbon steel could not be quenched sufficiently rapidly to achieve full hardness even
at the outside diameter.
A standard test for checking
hardenability is the Jominy test. It
consists of heating in a 1-in. diameter
bar to its austenitizing temperature
and then setting it over a jet of water
that hits only the bottom face.
Consequently, there are various
cooling rates along the length of the
specimen; later, hardness values can
be measured along the side of the
bar, representing cooling rates that
vary from that of a full water spray
down to an air cool. This is a wide-
range test, yet its results can be
correlated with tests on fully
quenched bars, as well as with work
on actual parts.
Fig. 17.1. Maximum hardness versus carbon content
Fig. 17.2. Comparison of the hardenabilities
of 4140 and 1040 steels

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