Материаловедение: свойства металлов. Матросова Т.А - 53 стр.

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53
Topic 19. TTT diagrams of Heat-treated Steel
Figure 19.1 shows the M
s
temperature as a function of carbon content and
illustrates the transformation time in seconds required by austenite at different
temperatures. This chart is known as a TTT curve (time, temperature,
transformation). This isothermal transformation diagram is very useful as it shows
the various structures formed by a steel during its cooling period or formed while
the steel is held at a temperature to which it was cooled.
A study of this diagram will indicate the importance of cooling time in order to
avoid the formation of the soft pearlite. Small parts that cool rapidly (i.e., dwell in
the 1000°F zone for less than 1 to 3 s) are readily transformed to either bainite or
martensite and escape the pearlite formation. However, in large sections, where the
cooling period is longer, it would be necessary to use a steel that had the nose of its
TTT curve moved to the right so there would be a longer period of cooling time in
order to avoid the formation of pearlite. When steel is alloyed with certain element
such as chromium or molybdenum, thick sections can be hardened even in the
center. Such steels are referred to as having greater hardenability.
From this, it can be seen that the effect of reducing the carbon content is to
induce more rapid transformation rates. The effect of alloying is to shift the entire
isothermal transformation diagram to the right; that is, transformation at all
temperature levels starts later and is slower to go to completion. It should be
recognized that although this is a general characteristic of alloying, various alloys
differ substantially in both the magnitude and the nature of their effect. That is, the
shape of the isothermal curve will vary considerably as well as its location with
respect to the time-temperature axes. Several heat treatments are shown
schematically on TTT diagrams.
There are two forms of martensite in carbon steels: lath and plate. These forms
are based on the morphology and microstructural characteristics of the martensite.
The lath morphology forms in low- and medium-carbon steels and consists of
regions or packets where many fine laths or board-shape crystals are arranged
parallel to one another. The plate morphology forms in high-carbon steels and
consists of martensite plates that form at angles with respect to each other.
As-quenched martensite is supersaturated with carbon; it has a very high
interfacial energy per unit volume associated with the fine laths and/or plates.
Glossary
№ English word Translation
1. carbon углерод
2. content содержание
3. austenite аустенит
4. chart диаграмма, график

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