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Steel Designers' Manual - 6th Edition (2003)
230 Applied metallurgy of steel
ant properties can be obtained by combinations of mechanical work at appropriate
temperatures during manufacture of the basic steel.
In addition to the effect of cooling rate on microstructure, the grain size is significantly
affected by time at high temperatures and subsequent cooling rate. Long
periods of time at higher temperatures within a particular phase lead to the merging
of the grain boundaries and growth of larger grains. For ferritic crystal structures,
grain growth starts at temperatures above about 600°C, and hence long periods
in the temperature range 600°C to 850°C with slow cooling will tend to promote
coarse grain size ferrite/pearlite microstructures. Faster cooling through the upper
part of the C curves will give a finer grain structure but still of ferrite/pearlite
microstructure.
The type of microstructure present in a steel can be shown and examined by the
preparation of carefully polished and etched samples viewed through a microscope.
Etching with particular types of reagent attacks different parts of the microstructure
preferentially, and the etched parts are characteristic of the type of microstructure.
Examples of some of the more common types of microstructure mentioned
above are shown in Fig. 6.6. The basic microstructure of the steel is usually shown
by examination in the microscope to magnifications of from 100 to about 500 times.
Where it is necessary to examine the effects of very fine precipitates or grain boundary
effects, it may be necessary to go to higher magnifications. With the electron
microscope it is possible to reach magnifications of many thousands and, with specialized
techniques, to reach the stage of seeing dislocations and imperfections in
the crystal lattice itself.
6.3.2 Heat treatment in practice
In practical steelmaking or fabrication procedures cooling occurs continuously from
high temperatures to lower temperatures. The response of the steel to this form of
cooling can be shown on the continuous cooling transformation diagram (CCT
diagram) of Fig. 6.7. This resembles the isothermal transformation diagram, but the
effect of cooling rate can be shown by lines of different slopes on the diagram. For
example, slow cooling, following line (a) on Fig. 6.7, passes through the top part of
the C curve and leads to the formation of a ferrite/pearlite mixture. Cooling at an
intermediate rate, following line (b), passes through pearlite/ferrite transformation
at higher temperatures, but changes to bainite transformation at lower temperatures
so that a mixture of pearlite and bainite results. Rapid cooling following line (c)
misses the C curves completely and passes through the two horizontal lines to show
transformation to martensite. Thus in practice for any given composition of steel
different microstructures and resultant properties can be produced by varying the
cooling rate.
The microstructure and properties of a steel can be changed by carefully chosen
heat treatments after the original manufacture of the basic product form. A major
group of heat treatments is effected by heating the steel to a temperature such that