Материаловедение: свойства металлов. Матросова Т.А - 37 стр.

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37
Topic 13. Grain Size and Austenitizing
An important factor in the heat treatment of steel is the grain size, by which
is meant the size of the microscopic grains that are established at the last temperature
above the critical range to which the piece of steel has been treated. Fine-grained
steels show better toughness at high hardness. Grain-size-controlled steels will show
less warpage. Coarse-grained steels harden better.
Grain-size-controlled steel can be secured from the steel mill within certain
limits. If a given grain size is specified, it must be obtained by suitable control of the
recrystallization process and by prevention of excessive grain growth.
A relatively large dendrite grain size may be developed in the process of
solidification as a result of the freezing rate or section size of casting. Large castings
and ingots freeze with coarse grains. Thin-sectioned castings develop a fine grain
size owing to rapid freezing. Normally, a fine grain size is desirable, since higher
ductility and impact strength values are obtained at a given tensile strengths level
with fine grain size.
Fig. 13.1. The relation between temperature vs. soak time
and the amount of stress relief
Most ferrous heat treatments require that austenite be produced as the first
step in a heat-treating operation. The iron-carbon phase diagram (fig. 13.2) shows
the minimum temperature at which austenite can form. Austenitizing is a function of
both time and temperature. In practice, a soaking time of 1 hour/in, of cross section
is considered to be adequate for austenitizing a carbon steel, although temperature
and initial carbide particle size are both important factors.
Steels and cast irons contain, in addition to iron and carbon, other elements
that shift the boundaries of the phase fields in the Fe-C diagram. Some alloying
elements such as manganese and nickel are austenite stabilizers and extend the
temperature range over which austenite is
stable, while elements such as chromium
and molybdenum are ferritic stabilizers and restrict the range of austenitic
stability.

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