The goal of heat treatment of steel is very often to attain a satisfactory hardness. The important microstructural phase is then normally martensite, which is the hardest constituent in low-alloy steels. The hardness of martensite is primarily dependent on its carbon content as is shown in Fig. 13.
If the microstructure is not fully martensitic, its hardness is lower. In practical heat treatment, it is important to achieve full hardness to a certain minimum depth after cooling, that is, to obtain a fully martensitic microstructure to a certain minimum depth, which also represents a critical cooling rate. If a given steel does not permit a martensitic structure to beformed to this depth, one has to choose another steel with a higher hardenability (the possibility of increasing the cooling rate at the minimum depth will be discussed later).
There are various ways to characterize the hardenability of a steel. Certain aspects of this will be discussed in the following article in the Section and has also been described in detail in previous ASM Handbooks, formerly Metals Handbooks (Ref 23). The CCT diagram can serve this purpose if one knows the cooling rate at the minimum depth. The CCT diagrams constructedaccording to Atkinsor Thelning presented above are particularly suitable.
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CCT Diagrams
As for heating diagrams, it is important to clearly state what type of cooling curve the transformation diagram was derived from. Use of a constant cooling rate is very common in experimental practice. However, this regime rarely occurs in a practical situation. One can also find curves for so-called natural cooling rates according to Newton's law of cooling.
These curves simulate the behavior in the interior of a large part such as the cooling rate of a Jominy bar at some distance from the quenched end. Close to the surface the characteristics of the cooling rate can be very complex as will be described below. In the lower part of Fig. 9 is shown a CCT diagram (fully drawn lines) for 4130 steel. Ferrite, pearlite, and bainite regions are indicated as well as the Ms temperature. Note that theMs temperature is not constant when martensite formation is preceded by bainite formation, but typically decreases with longer times.
The effect of different cooling curves is shown in Fig. 10. Each CCT diagram contains a family of curves representing the cooling rates at different depths of a cylinder with a 300 mm (12 in.) diameter. The slowest cooling rate represents the center of the cylinder. As shown in Fig. 10, the rate of cooling and the position of the CCT curves depend on the cooling medium (water produced the highest cooling rate followed by oil and air, respectively). The more severe the cooling medium, the longer the times to which the C-shaped curves are shifted. The Ms temperature is unaffected.
These curves simulate the behavior in the interior of a large part such as the cooling rate of a Jominy bar at some distance from the quenched end. Close to the surface the characteristics of the cooling rate can be very complex as will be described below. In the lower part of Fig. 9 is shown a CCT diagram (fully drawn lines) for 4130 steel. Ferrite, pearlite, and bainite regions are indicated as well as the Ms temperature. Note that theMs temperature is not constant when martensite formation is preceded by bainite formation, but typically decreases with longer times.
The effect of different cooling curves is shown in Fig. 10. Each CCT diagram contains a family of curves representing the cooling rates at different depths of a cylinder with a 300 mm (12 in.) diameter. The slowest cooling rate represents the center of the cylinder. As shown in Fig. 10, the rate of cooling and the position of the CCT curves depend on the cooling medium (water produced the highest cooling rate followed by oil and air, respectively). The more severe the cooling medium, the longer the times to which the C-shaped curves are shifted. The Ms temperature is unaffected.
Decomposition of Austenite
The procedure starts at a high temperature, normally in the austenitic range after holding there long enough to obtain homogeneous austenite without undissolved carbides, followed by rapid cooling to
the desired hold temperature (Fig. 5). An example of an IT diagram is given in Fig. 6.
The cooling was started from 850 °C (1560 °F). TheA1 andA3 temperatures are indicated as well as the hardness. AboveA3 no transformation can occur. BetweenA1 andA3 only ferrite can form from austenite. In Fig. 6, a series of isovolume fraction curves are shown; normally only the 1% and 99% curves are reproduced. Notice that the curves are C-shaped.
This is typical for transformation curves. A higher-temperature set of C-shaped curves shows the transformation to pearlite and a lowertemperature set indicates the transformation to bainite. In between is found a so-called austenite bay, common for certain low-alloy steelscontaining appreciable amountsof carbide-forming alloying elements such aschromium or molybdenum.
the desired hold temperature (Fig. 5). An example of an IT diagram is given in Fig. 6.
The cooling was started from 850 °C (1560 °F). TheA1 andA3 temperatures are indicated as well as the hardness. AboveA3 no transformation can occur. BetweenA1 andA3 only ferrite can form from austenite. In Fig. 6, a series of isovolume fraction curves are shown; normally only the 1% and 99% curves are reproduced. Notice that the curves are C-shaped.
This is typical for transformation curves. A higher-temperature set of C-shaped curves shows the transformation to pearlite and a lowertemperature set indicates the transformation to bainite. In between is found a so-called austenite bay, common for certain low-alloy steelscontaining appreciable amountsof carbide-forming alloying elements such aschromium or molybdenum.
Formation of Austenite
During the formation of austenite from an original microstructure of ferrite and pearlite or tempered martensite, the volume (and hence the length) decreases with the formation of the dense austenite
phase (see Fig. 3). From the elongation curves, the start and finish times for austenite formation, usually defined as 1% and 99% transformation, respectively, can be derived. These times are then conveniently plotted on a temperature-log time diagram (Fig. 4).
Also plotted in this diagram are the Ac1 and Ac3 temperatures. Below Ac1 no austenite can form, and between Ac1 and Ac3 the end product is a mixture of ferrite and austenite. Notice that a considerable overheating is required to complete the transformation in a short time. The original microstructure also plays a great role. A finely distributed structure like tempered martensite is more rapidly transformed to austenite than, for instance, a ferriticpearlitic structure. This is particularly true for alloyed steels with carbide-forming alloying elements such as chromium and molybdenum. It is important that the heating rate to the hold temperature be very high if a true isothermal diagram is to be obtained.
phase (see Fig. 3). From the elongation curves, the start and finish times for austenite formation, usually defined as 1% and 99% transformation, respectively, can be derived. These times are then conveniently plotted on a temperature-log time diagram (Fig. 4).
Also plotted in this diagram are the Ac1 and Ac3 temperatures. Below Ac1 no austenite can form, and between Ac1 and Ac3 the end product is a mixture of ferrite and austenite. Notice that a considerable overheating is required to complete the transformation in a short time. The original microstructure also plays a great role. A finely distributed structure like tempered martensite is more rapidly transformed to austenite than, for instance, a ferriticpearlitic structure. This is particularly true for alloyed steels with carbide-forming alloying elements such as chromium and molybdenum. It is important that the heating rate to the hold temperature be very high if a true isothermal diagram is to be obtained.
Isothermal Transformation Diagrams
This type of diagram shows what happens when a steel is held at a constant temperature for a prolonged period. The development of the microstructure with time can be followed by holding small specimens in a lead or salt bath and quenching them one at a time after increasing holding times and measuring the amount of phases formed in the microstructure with the aid of a microscope. An alternative method involves using a single specimen and a dilatometer which records the elongation of the specimen as a function of time. The basis for the dilatometer method is that the microconstituents undergo different volumetric changes (Table 3). A thorough description of the dilatometric method can.
Transformation Diagrams
The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels.
The metastable phase martensite and the morphologically metastable microconstituent bainite, which are of extreme
importance to the properties of steels, can generally form with comparatively rapid cooling to ambient temperature, that
is, when the diffusion of carbon and alloying elements is suppressed or limited to a very short range. Bainite is a eutectoid
decomposition that isa mixture offerrite and cementite. Martensite,the hardestconstituent, formsduring severe quenches
from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to
about 0.7 wt%. If these unstable metastable products are subsequently heated to a moderately elevated temperature, they
decompose to more stable distributions of ferrite and carbide. The reheating process is sometimes known as tempering or
annealing.
The transformation of an ambient temperature structure like ferrite-pearlite or tempered martensite to the elevatedtemperature structure of austeniteor austenite + carbideisalso of importance in the heat treatment of steel.
One can conveniently describe what is happening during transformation with transformation diagrams. Four different
typesof such diagramscan be distinguished. These include:
· Isothermal transformation diagrams describing the formation of austenite, which will be referred to as
ITh diagrams
· Isothermal transformation (IT) diagrams, also referred to as time-temperature-transformation (TTT)
diagrams, describingthe decompositionof austenite
· Continuous heatingtransformation (CHT) diagrams
· Continuous cooling transformation (CCT) diagrams
The metastable phase martensite and the morphologically metastable microconstituent bainite, which are of extreme
importance to the properties of steels, can generally form with comparatively rapid cooling to ambient temperature, that
is, when the diffusion of carbon and alloying elements is suppressed or limited to a very short range. Bainite is a eutectoid
decomposition that isa mixture offerrite and cementite. Martensite,the hardestconstituent, formsduring severe quenches
from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to
about 0.7 wt%. If these unstable metastable products are subsequently heated to a moderately elevated temperature, they
decompose to more stable distributions of ferrite and carbide. The reheating process is sometimes known as tempering or
annealing.
The transformation of an ambient temperature structure like ferrite-pearlite or tempered martensite to the elevatedtemperature structure of austeniteor austenite + carbideisalso of importance in the heat treatment of steel.
One can conveniently describe what is happening during transformation with transformation diagrams. Four different
typesof such diagramscan be distinguished. These include:
· Isothermal transformation diagrams describing the formation of austenite, which will be referred to as
ITh diagrams
· Isothermal transformation (IT) diagrams, also referred to as time-temperature-transformation (TTT)
diagrams, describingthe decompositionof austenite
· Continuous heatingtransformation (CHT) diagrams
· Continuous cooling transformation (CCT) diagrams
TheFe-CPhaseDiagram 2
The Fe-C diagram in Fig. 1 is of experimental origin. The knowledge of the thermodynamic principles and modern
thermodynamic data now permits very accurate calculations of this diagram (Ref 4). This is particularly useful when
phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to
develop.
If alloying elements are added to the iron-carbon alloy (steel), the position of theA1,A3, andAcm boundaries and the
eutectoid composition are changed. Classical diagrams introduced by Bain (Ref 5) show the variation ofA1 and the
eutectoid carbon content with increasing amount of a selected number of alloying elements (Fig. 2). It suffices here to
mention that (1) all important alloying elements decrease the eutectoid carbon content, (2) the austenite-stabilizing
elements manganese and nickel decreaseA1, and (3) the ferrite-stabilizing elements chromium, silicon, molybdenum, and
tungsten increaseA1. These classifications relate directly to the synergisms in quench hardening as described in the
articles "Quantitative Prediction of Transformation Hardening in Steels" and "Quenching of Steel"in this Volume.
Modern thermodynamic calculations allow accurate determinations of these shifts that affect the driving force for phase
transformation (see below). These methods also permit calculation of complete ternary and higher-order phase diagrams
including alloy carbides(Ref 6). Reference should be made to the Calphad computer system (Ref7).
thermodynamic data now permits very accurate calculations of this diagram (Ref 4). This is particularly useful when
phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to
develop.
If alloying elements are added to the iron-carbon alloy (steel), the position of theA1,A3, andAcm boundaries and the
eutectoid composition are changed. Classical diagrams introduced by Bain (Ref 5) show the variation ofA1 and the
eutectoid carbon content with increasing amount of a selected number of alloying elements (Fig. 2). It suffices here to
mention that (1) all important alloying elements decrease the eutectoid carbon content, (2) the austenite-stabilizing
elements manganese and nickel decreaseA1, and (3) the ferrite-stabilizing elements chromium, silicon, molybdenum, and
tungsten increaseA1. These classifications relate directly to the synergisms in quench hardening as described in the
articles "Quantitative Prediction of Transformation Hardening in Steels" and "Quenching of Steel"in this Volume.
Modern thermodynamic calculations allow accurate determinations of these shifts that affect the driving force for phase
transformation (see below). These methods also permit calculation of complete ternary and higher-order phase diagrams
including alloy carbides(Ref 6). Reference should be made to the Calphad computer system (Ref7).
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