The transformation behavior of austenite is best studied by observing the isothermal transformation
at a series of temperatures below A1. The transformation progress is ordinarily followed metallographically
in such a way that both the time-temperature relationships and the manner in which the
microstructure changes are established. The times at which transformation begins and ends at a given
temperature are plotted, and curves depicting the transformation behavior as a function of temperature
are obtained by joining these points (Fig. ) Such a diagram is referred to as an isothermal transformation
(IT) diagram, a time-temperature-transformation (TTT) diagram, or, an S curve
Pearlite
at a series of temperatures below A1. The transformation progress is ordinarily followed metallographically
in such a way that both the time-temperature relationships and the manner in which the
microstructure changes are established. The times at which transformation begins and ends at a given
temperature are plotted, and curves depicting the transformation behavior as a function of temperature
are obtained by joining these points (Fig. ) Such a diagram is referred to as an isothermal transformation
(IT) diagram, a time-temperature-transformation (TTT) diagram, or, an S curve
Trends in Heat-Treated Products
Property Coarse-grain Austenite Fine-grain Austenite
Quenched and Tempered Products
- Hardenability Increasing Decreasing
- Toughness Decreasing Increasing
- Distortion More Less
- Quench cracking More Less
- Internal stress Higher Lower
Annealed or Normalized Products
- Machinability
- Rough finish Better Inferior
- Fine finish Inferior Better
The IT diagram for a eutectoid carbon steel is shown in Fig.In addition to the lines depicting
the transformation, the diagram shows microstructures at various stages of transformation and hardness
values. Thus, the diagram illustrates the characteristic subcritical austenite transformation behavior,
the manner in which microstructure changes with transformation temperature, and the general
relationship between these microstructural changes and hardness.
As the diagram indicates, the characteristic isothermal transformation behavior at any temperature
above the temperature at which transformation to martensite begins (the Ms temperature) takes place
over a period of time, known as the incubation period, in which no transformation occurs, followed
by a period of time during which the transformation proceeds until the austenite has been transformed
completely. The transformation is relatively slow at the beginning and toward the end, but much
more rapid during the intermediate period in which —25-75% of the austenite is transformed. Both
the incubation period and the time required for completion of the transformation depend on the
temperature.
The behavior depicted in this program is typical of plain carbon steels, with the shortest incubation
period occurring at ~540°C. Much longer times are required for transformation as the temperature
approaches either the Ae1 or the Ms temperature. This A1 temperature is lowered slightly during
cooling and increased slightly during heating. The 54O0C temperature, at which the transformation begins in the shortest time period is commonly referred to as the nose of the IT diagram. If complete
transformation is to occur at temperatures below this nose, the steel must be cooled rapidly enough
to prevent transformation at the nose temperature. Microstructures resulting from transformation at
these lower temperatures exhibit superior strength and toughness.
Pearlite
In carbon and low-alloy steels, transformation over the temperature range of ~700-540°C gives
pearlite microstructures of the characteristic lamellar type. As the transformation temperature falls,
the lamellae move closer and the hardness increases.
Bainite
Transformation to bainite occurs over the temperature range of ~540-230°C. The acicular bainite
microstructures differ markedly from the pearlite microstructures. Here again, the hardness increases
as the transformation temperature decreases, although the bainite formed at the highest possible
temperature is often softer than pearlite formed at a still higher temperature
Martensite
Transformation to martensite, which in the steel illustrated in Fig. 2.3 begins at ~230°C, differs from
transformation to pearlite or bainite because it is not time dependent, but occurs almost instantly
during cooling. The degree of transformation depends only on the temperature to which it is cooled.
Thus, in this steel of Fig. 2.3, transformation to martensite starts on cooling to 23O0C (designated as
the M5 temperature). The martensite is 50% transformed on cooling to ~150°C, and the transformation
is essentially completed at ~90°C (designated as the Mf temperature). The microstructure of martensite
is acicular. It is the hardest austenite transformation product but brittle; this brittleness can be
reduced by tempering as discussed below.
Pearlite
Pearlites are softer than bainites or martensites. However, they are less ductile than the lowertemperature
bainites and, for a given hardness, far less ductile than tempered martensite. As the
transformation temperature decreases within the pearlite range, the interlamellar spacing decreases,
and these fine pearlites, formed near the nose of the isothermal diagram, are both harder and more
ductile than the coarse pearlites formed at higher temperatures. Thus, although as a class pearlite
tends to be soft and not very ductile, its hardness and toughness both increase markedly with decreasing
transformation temperatures
Bainite
In a given steel, bainite microstructures are generally found to be both harder and tougher than
pearlite, although less hard than martensite. Bainite properites generally improve as the transformation
temperature decreases and lower bainite compares favorably with tempered martensite at the same
hardness or exceeds it in toughness. Upper bainite, on the other hand, may be somewhat deficient in
toughness as compared with fine pearlite of the same hardness
Martensite
Martensite is the hardest and most brittle microstructure obtainable in a given steel. The hardness of
martensite increases with increasing carbon content up to the eutectoid composition, and, at a given
carbon content, varies with the cooling rate.
Although for some applications, particularly those involving wear resistance, the hardness of
martensite is desirable in spite of the accompanying brittleness, this microstructure is mainly important
as starting material for tempered martensite structures, which have definitely superior properties
Tempered Martensite
Martensite is tempered by heating to a temperature ranging from 170 to 70O0C for 30 min to several
hours. This treatment causes the martensite to transform to ferrite interspersed with small particles
of cementite. Higher temperatures and longer tempering periods cause the cementite particles to
increase in size and the steel to become more ductile and lose strength. Tempered martensitic structures
are, as a class, characterized by toughness at any strength. The diagram of Fig. 2.4 describes,
within ± 10%, the mechanical properties of tempered martensite, regardless of composition. For
example, a steel consisting of tempered martensite, with an ultimate strength of 1035 MPa (150,000
psi), might be expected to exhibit elongation of 16-20%, reduction of area of between 54 and 64%,
yield point of 860-980 MPa (125,000-142,000 psi), and Brinell hardness of about 295-320. Because
of its high ductility at a given hardness, this is the structure that is preferred.
Transformation Rates
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