The structures formed during the continuous cooling of steel from above Ac₃ can be understood best by studying the constant-temperature (isothermal) transformation of austenite, thus separating the two variables: time and temperature.

One method consists of heating small specimens above Ac₃ to form austenite, then quenching into a suitable bath (e.g. liquid tin) at some constant sub-critical temperature. After holding for selected periods of time, the specimens are withdrawn from the bath and rapidly quenched in cold water. This converts any untransformed austenite into martensite the volume of which can be estimated microscopically. Another method consists in measuring length changes caused by the decomposition of austenite at the constant temperature by means of a dilatometer.

When carbon steel is quenched in the baths at constant temperatures, the velocity of austenite transformation is found to depend on temperature. The time for the beginning and completion of the transformation of the austenite is plotted against the temperature to give the Bain "S-curve", shown in Fig. 1, now called TTT-curve (time-temperature-transformation).

Figure 1. Ideal TTT-curve for 0,65% carbon steel depicting time interval required for beginning, 50% and 100% transformation of austenite at a constant temperature A= Austenite F= Ferrite P = Pearlite B = Bainite

The logarithmic scale of time is used to condense results into a small space. Ae1 and Ae3 lines represent the equilibrium transformation temperatures. Austenite is completely stable above Ae3 and partially unstable between Ae3 and Ae1. Below Ae1 austenite is completely unstable and transforms in time. Two regions of rapid transformation occur about 550° and 250°C. The form of each of the curves and their positions with respect to the time axis depend on the composition and grain size of the austenite which is transforming.

The TTT-curve is most useful in presenting an overall picture of the transformation behaviour of austenite. This enables the metallurgist to interpret the response of steel to any specified heat-treatment, to plan practical heat-treatment operations and to control limited hardening or softening and the time of soaking.

The decomposition of austenite occurs according to three separate but sometimes overlapping mechanisms and results in three different reaction products: pearlitic, bainitic, martensitic.

Pearlitic

The upper dotted curve in Fig. 1 represents the beginning of the formation of ferrite. The curve just below it indicates the beginnings of the breakdown of the austenite remnant into a ferrite-carbide aggregate. In the high-temperature pearlitic range in Fig. 1 the process resembles the solidification of crystals from a liquid by the formation and growth of nuclei of carbide followed by ferrite by side nucleation with side and edge growth, Fig. 2a and b.

At 700°C the formation of nuclei is slow (i.e. incubation period), then growth proceeds rapidly to form large pearlite colonies covering several austenite grains in some cases. As the transformation temperature is lowered to 500°C the incubation period decreases and the pearlite becomes increasingly fine.

Large numbers of nuclei form in the austenite boundaries, but growth is slower and this produces nodular troostite, Fig. 2a. In the case of medium carbon steels the excess ferrite decreases in volume and begins to show an acicular or Widmanstätten type of distribution. The relative amounts of free ferrite to be expected after a given heat-treatment is indicated by the size of the "austenite and ferrite" field and by the temperature interval between Ae₁ and Ae₃.

Bainitic

Between about 500° and 350°C initial nuclei are ferrite which is coherent with the austenite matrix. Cementite then precipitates from the carbon-enriched layer of austenite, allowing further growth of the ferrite as shown in Fig. 2c.

The carbides tend to lie parallel to the long axis of the bainite needle to form the typical open feathery structure of upper bainite. Below 350°C coherent ferrite, supersaturated with carbon, forms first and is then followed by the precipitation of carbide within the ferrite needle, transversely at an angle of 55°. A proportion of the carbide is Fe2,4C and the ferrite contains a little dissolved carbon. This lower bainite structure is somewhat similar to lightly tempered martensite (Fig. 2d).

Figure 2. (a) Effect of different speeds of nucleation and growth on formation of pearlite colonies; (b), (c), (d) diagrammatic representation of formation of pearlite, upper bainite and lower bainite

Martensitic

In quenching down to about 250°C, the temperature drops rapidly through the interval in which "nucleation" could take place, to a temperature so low that the molecular mobility, i.e. diffusion, becomes too small for the formation of nuclei.

In the third stage, therefore, the austenite changes incompletely into a distorted body-centred structure, with little or no diffusion of the carbon into particles of cementite, to form martensite the plates of which are formed at a high speed (less than 0,002 sec). This suggests that the mechanism of formation of this structure is not nucleation and growth but a shearing process. This resembles the process of mechanical twinning and involves very little atomic movement, but considerable internal stress due to the shear and to the position of the carbon atoms.

As the temperature decreases the elastic energy increases and eventually causes a shear in a part of the matrix, which stabilises the rest. Further shear can only occur when the temperature is lowered and more energy gained. The amount of martensite formed, therefore, is practically independent of time and depends principally on the temperatures at which the steel is held. Hence a proportion of austenite is usually retained in quenched steel which can be reduced in amount by a decrease in temperature. This fact is used in sub-zero quenching.

The temperature at which martensite begins to form (Ms) is progressively lowered as the carbon content of the steel increases, e.g.

C %	0,02	0,2	0,4	0,8	1,2
Ms °C	520	490	420	250	150

The temperature is also affected by the alloy content, but the following empirical formula (Steven and Haynes) can be used for calculating Ms from the chemical analyses, provided all carbides have been dissolved in the austenite:

Ms in °C = 561 - 474 (% C) - 33 (% Mn) - 17 (% Ni) - 17(% Cr) - 21 (% Mo).

Mf is about 215°C below the Ms.

Plastic and elastic stresses promote the formation of martensite, but it is retarded when cooling is interrupted. When cooling is resumed after such a stabilisation arrest martensite only begins to form again after cooling to a lower temperature.

The rate and extent of stabilisation (depression) depend on the temperature and time of holding, amount of prior transformation and alloy content.

Two forms of martensite have been identified depending on carbon content. In low carbon steels laths containing many dislocations are found, while in high carbon steels the plates are heavily twinned, Fig. 3(a) and (b).

Figure 3. (a) Lathe martensite formed in 0,08°C steel quenched in brine from 100°C (x20000),	b) Twinned martensite in Fe30%Ni (x110000)

Two groups of phase transformation are now given the name civilian, in which atoms move in a random manner (e.g. pearlite) and military because of its orderly disciplined manner, e.g. martensite. Martensite transformations also occur in non-ferrous alloys often differing greatly from the rather special case in steel.

In the case of steel castings full annealing is the only way for completely effacing the coarse grains and Widmanstätten structure, with its associated brittleness.

The following properties of a 0,3 % carbon steel illustrate the improvement obtained by annealing:

Table 1

	BH	YP MPa	TS MPa	El %	Cold Bend	Izod J
Cast	160	309	448	6	45°	12
Annealed 880°C, 6hr, furnace cooled	153	247	479	24	165°	32

The Widmanstätten structure can be modified into a "feathery" arrangement of the ferrite by the influence of silicon. This is shown in Fig. 1 which consists of a portion of the boundary-ferrite, Widmanstätten and feathery structures. Fig. 2 shows the macroform of the primary crystals, revealed by the segregation of the impurities. The effects of this segregation have to be effaced as much as possible by annealing and this necessitates temperatures higher than those used for worked steels.

Figure 1. Micro-structure of same steel sho-wing part of ferrite network, Widmanstätten and feathery structure. Ferrite-white. Pearlite-dark ( x 80)	Figure 2. Macro-structure of cast steel revea-ling large prirmary austenite crystals due to presence of impurities (x 4)

An imperfect anneal is illustrated by Fig. 3, in which the original cast structure is still outlined by the deposition of the ferrite in the old positions, especially around the inclusions. They can be prevented by double annealing.

Double annealing consists of heating the steel to a temperature considerably over the A3 point, cooling rapidly to a temperature below the lower critical range and then immediately reheating to a point just over the upper critical point (Ac3), followed by slow cooling.

This method is particularly useful for castings. The high temperature treatment effaces the strains, coalesces the sulphide films in the ferrite which embrittle the steel and produces homogenity by rapid diffusion. The quick cooling prevents the coarse deposition of the ferrite in the large grains, but tends to harden the metal. The second heating refines the coarse grains and leaves the steel in a softened condition. A typical structure, which is shown in Fig. 4, should be compared with Fig. 3.

Figure 3. Same steel imperfectly annealed ferrite formed in masses outlining original cast structure (x80)	Figure 4. Same steel properly annealed: ferrite and perlite uniform and fine (x80)

Softening Tool and Air-hardening Steels

To soften high carbon and air-hardening steels, in order to allow machining operation to be carried out, they are heated just below the lower change point (650-700°C), causing. the cementite to balls-up into rounded masses (i.e. spheroidising anneal). When the cementite is in this condition high carbon steels can be cold drawn; but too high a temperature causes pearlite to be reformed, with consequent high resistance to deformation.

It should be remembered, however, that coarse laminated cementite spheroidises extremely slowly, and the above treatment is therefore carried out on a "hardened" material, obtained by suitable cooling from above A3 or after cold working. A short cycle anneal consists of heating just above Ac1, cooling below Ar1 and then raising temperature just below Ac1 for 8 hours. Although the softest condition is obtained when the large globules of cementite are embedded in the ferrite, a smooth machined surface is difficult to obtain due to tearing. Groups of large globules cause failure of sharp-edged cutting tools.

During the subsequent hardening operations, the time required to dissolve fine spheroidised cementite is less than for the lamellar type. This property is being used in hardening thin sections, such as safety razor blades and needles, in order to reduce decarburisation.

Annealing and Hardening Temperatures for Tool Steels

The annealing or hardening temperatures of steels containing more than 0,9% carbon is just above the lower critical point (730-790°C) instead of above the upper range.

Fig. 5 shows the appearance of a 1,3% carbon steel cast, in which the cementite exists as brittle networks and plates. This type of structure must be replaced by rounded particles of cementite in a fine pearlite before hardening, otherwise cracks will propagate through the continuous masses of brittle cementite.

Figure 5. As cast: cementite network and plates in pearlite (x 100)	Figure 6. Heated to 1050°C and quenched in water. Large grains (x 100)
Figure 7. Cementite globules in properly hot-worked steel (x 200)	Figure 8. Cementite globules in martensite, in hardened steel (x 200)

The upper critical line rises steeply with increasing carbon content above 0,87% and an excessively high temperature is required to dissolve all the free cementite. This tends to develop coarse austenite crystals which cause the steel to become brittle and cracks to form on quenching. This grain growth is shown on Fig. 6, which is the structure of steel quenched from 1050°C. On the other hand, particles of cementite restrain grain growth.

Forging is, therefore, carried out through the critical range in order to disperse the free cementite. This is followed by annealing at about 760°C (just above Ac1), to ball-up the free cementite and to remove strains. Fig. 7 shows the structure formed. Even a fine cementite network structure would cause trouble when drastic quenching is used, such as for files. Fig. 8 shows the structure of a steel hardened from 760°C, consisting of particles of cementite dispersed in a matrix of martensite.

Products of Quenching: Cnstituents of Hardened Steel

The equilibrium diagram indicate the changes which occur under the slow cooling conditions of annealing. The rapid rates of cooling necessary to harden a steel cause the austenite to persist to a lower temperature and to transform into a variety of micro-constituents discussed below.

Austenite is sometimes present with martensite, in drastically quenched steels (Fig. 9). It cannot be completely retained in carbon steels by even drastic quenching, but suitable additions of alloying elements allow the retention of this constituent, for example 18/8 austenitic stainless steel. This austenite consists of polyhedral grains, showing twins (Figs. 9 and 10). It is non-magnetic and soft.

Martensite is the hardest constituent obtained in a given steel, but the hardness increases with the carbon content of the steel up to 0,7%. The micro-structure exhibits a fine needle-like structure, which becomes more pronounced when the steel is quenched from high temperatures. See Figs. 9 and 11.

Figure 9. Forms of carbide in micro-constituents in steel

Figure 10. Martensite needles (dark) in austenite (x1200)	Figure 11. Steel (C, 0,4) quenched from between A1 and A3. Undissolved ferrite arou-nd inclusion in martensite (x 100)
Figure 12. Martensite and quench crack. Steel (C, 0,5) quenched in water from 900°C(x400)	Figure 13. Nodular troostite in martensite (x400)
Figure 14. Sorbite in quenched and tempered (600°C) steel (C, 0,5) (x500)	Figure 15. Case-hardened screw. Cracked martensitic case (white), martensite and fer-rite core (x30)

The nature of martensite has not been definitely agreed upon, but for the present purpose it might be considered to be highly stressed a-iron which is supersaturated with carbon. The fc.c. lattice of g-iron is equivalent to a body centred tetragonal lattice with ratio 1,414. The tetragonal lattice of martensite is formed from this by compressing its height and increasing its cross-section. A slight further compression to give a ratio of 1 gives rise to a-iron.

Bainite, which occurs in alloy steels, has a rapidly etched needle-like structure, somewhat resembling tempered martensite. As the temperature of its formation becomes higher the acicular nature becomes less accentuated, the needles increasing in size

Troostite (nodular) rapidly etches black and is practically unresolvable under the ordinary microscope. Special microscopic technique has shown that nodular troostite is a mixture of radial lamellae of ferrite and cementite. Therefore it differs from pearlite only in degree of fineness and carbon content which is the same as that in the austenite from which it is formed. Figs. 9 and 13 show a typical martensite-troostite structure; nodules outline the boundaries of the original austenite grains. Troostite is softer than martensite and small amounts in the steel lessen the risks of cracking and distortion.

Some confusion arises as to the nomenclature of micro-constituents found in hardened and tempered steels. The terms troostite and sorbite are frequently used to indicate constituents formed during quenching and also during tempering. In the former case (quenching) the cementite always occurs in a laminated form, while in the latter (tempering) it has a granular form. Hence, the term troostite has been adopted in this book for constituents possessing a laminated structure. Sorbite is used to denote the granular structures.

The purpose of annealing may involve one or more of the following aims:

1. To soften the steel and to improve machinability.
2. To relieve internal stresses induced by some previous treatment (rolling, forging, uneven cooling).
3. To remove coarseness of grain.

The treatment is applied to forgings, cold-worked sheets and wire, and castings. The operation consists of:

1. heating the steel to a certain temperature,
2. "soaking" at this temperature for a time sufficient to allow the necessary changes to occur,
3. cooling at a predetermined rate.

Sub-critical Anneal

It is not always necessary to heat the steel into the critical range. Mild steel products which have to be repeatedly cold worked in the processes of manufacture are softened by annealing at 500° to 650°C for several hours. This is known as "process" or "close" annealing, and is commonly employed for wire and sheets. The recrystallisation temperature of pure iron is in the region of 500°C consequently the higher temperature of 650°C brings about rapid recrystallisation of the distorted ferrite Since mild steel contains only a small volume of strained pearlite a high degree of softening is induced. As shown, Fig. 1b illustrates the structure formed consisting of the polyhedral ferrite with elongated pearlite (see also Fig. 2).

Prolonged annealing induces greater ductility at the expense of strength, owing to the tendency of the cementite in the strained pearlite to "ball-up" or spheroidise, as illustrated in Fig. 1c. This is known as "divorced pearlite". The ferrite grains also become larger, particularly if the metal has been cold worked a critical amount. A serious embrittlement sometimes arises after prolonged treatment owing to the formation of cementitic films at the ferrite boundaries. With severe forming operations, cracks are liable to start at these cementite membranes.

Figure 1. Effect of annealing cold-worked mild steel

Figure 2. Effect of annealing at 650°C on worked steel. Ferrite recrystallised. Pearlite remains elongated (x600)

The modern tendency is to use batch or continuous annealing furnaces with an inert purging gas. Batch annealing usually consists of 24-30 hrs 670°C, soak 12 hrs, slow cool 4-5 days. Open coil annealing consists in recoiling loosely with controlled space between wraps and it reduces stickers and discoloration. Continuous annealing is used for thin strip (85% Red) running at about 400 m/min. The cycle is approximately up to 660°C 20 sec, soak and cool 30-40 sec. There is little chance for grain growth and it produces harder and stiffer strip; useful for cans and panelling.

"Double reduced" steel is formed by heavy reduction (~50%) after annealing but it suffers from directionality. This can be eliminated by heating between 700-920°C and rapidly quenching.

Full Anneal and Normalising Treatments

For steels with less than 0,9% carbon both treatments consist in heating to about 25-50°C above the upper critical point indicated by the Fe-Fe3C equilibrium diagram (Fig. 3). For higher carbon steels the temperature is 50°C above the lower critical point.

Figure 3. Heat-treatment ranges of steels

Average annealing and hardening temperatures are:

Carbon, %	0.1	0.2	0.3	0.5	0.7	0.9 to 1.3
Avg.temp. °C	910	860	830	810	770	760

These temperatures allow for the effects of slight variations in the impurities present and also the thermal lag associated with the critical changes. After soaking at the temperature for a time dependent on the thickness of the article, the steel is very slowly cooled. This treatment is known as full annealing, and is used for removing strains from forgings and castings, improving machinability and also when softening and refinement of structure are both required.

Normalising differs from the full annealing in that the metal is allowed to cool in still air. The structure and properties produced, however, varying with the thickness of metal treated. The tensile strength, yield point, reduction of area and impact value are higher than the figures obtained by annealing.

Changes on Annealing

Consider the heating of a 0,3% carbon steel. At the lower critical point (Ac1) each "grain" of pearlite changes to several minute austenite crystals and as the temperature is raised the excess ferrite is dissolved, finally disappearing at the upper critical point (Ac3), still with the production of fine austenite crystals. Time is necessary for the carbon to become uniformly distributed in this austenite. The properties obtained subsequently depend on the coarseness of the pearlite and ferrite and their relative distribution. These depend on:

a) the size of the austenite grains; the smaller their size the better the distribution of the ferrite and pearlite.
b) the rate of cooling through the critical range, which affects both the ferrite and the pearlite.

As the temperature is raised above Ac3 the crystals increase in size. On a certain temperature the growth, which is rapid at first, diminishes. Treatment just above the upper critical point should be aimed at, since the austenite crystals are then small.

By cooling slowly through the critical range, ferrite commences to deposit on a few nuclei at the austenite boundaries. Large rounded ferrite crystals are formed, evenly distributed among the relatively coarse pearlite. With a higher rate of cooling, many ferrite crystals are formed at the austenite boundaries and a network structure of small ferrite crystals is produced with fine pearlite in the centre.

Overheated, Burnt and Underannealed Structures

When the steel is heated well above the upper critical temperature large austenite crystals form. Slow cooling gives rise to the Widmanstätten type of structure, with its characteristic lack of both ductility and resistance to shock. This is known as an overheated structure, and it can be refined by reheating the steel to just above the upper critical point. Surface decarburisation usually occurs during the overheating.

During the Second World War, aircraft engine makers were troubled with overheating (above 1250°C) in drop-stampings made from alloy steels. In the hardened and tempered condition the fractured surface shows dull facets. The minimum overheating temperature depends on the "purity" of the steel and is substantially lower in general for electric steel than for open-hearth steel. The overheated structure in these alloy steels occurs when they are cooled at an intermediate rate from the high temperature. At faster or slower rates the overheated structure may be eliminated. This, together with the fact that the overheating temperature is significantly raised in the presence of high contents of MnS and inclusions, suggests that this overheating is conected in some way with a diffusion and precipitation process, involving MnS. This type of overheating can occur in an atmosphere free from oxygen, thus emphasising the difference between overheating and burning.

As the steel approaches the solidus temperature, incipient fusion and oxidation take place at the grain boundaries. Such a steel is said to be burnt and it is characterised by the presence of brittle iron oxide films, which render the steel unfit for service, except as scrap for remelting.

Steel is usually defined as an alloy of iron and carbon with the carbon content between a few hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5 wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels, stainless steels (>10.5%) and heat resisting CrNi steels (>18%). Steels can exhibit a wide variety of properties depending on composition as well as the phases and micro-constituents present, which in turn depend on the heat treatment.

The Fe-C Phase Diagram

The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig 1). Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe₃C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or metastable equilibrium) for different combinations of carbon concentration and temperature.

We distinguish at the low-carbon end ferrite (α-iron),which can at most dissolve 0.028% C, at 727°C (1341°F) and austenite -iron, which can dissolve 2.11 wt% C at 1148°C (2098°F). At the carbon-rich side we find cementite (Fe₃C). Of less interest, except for highly alloyed steels, is the δ-ferrite existing at the highest temperatures.

Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + δ-ferrite.

In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names:

• A₁, the so-called eutectoid temperature, which is the minimum temperature for austenite
• A₃, the lower-temperature boundary of the austenite region at low carbon contents, that is, the γ/γ + α boundary
• A_cm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe₃C boundary

The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content (0.77 wt% C). The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that degenerates into cementite particles dispersed with a ferrite matrix after extended holding close to A₁.

Fig. 1. The Fe-Fe3C diagram.

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. 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 the A₁, A₃, and A_cm boundaries and the eutectoid composition are changed. It suffices here to mention that

1. all important alloying elements decrease the eutectoid carbon content,
2. the austenite-stabilizing elements manganese and nickel decrease A, and
3. the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A₁.

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 is a mixture of ferrite and cementite. Martensite, the hardest constituent, forms during 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 elevated-temperature structure of austenite or austenite-carbide is also of importance in the heat treatment of steel.

One can conveniently describe what is happening during transformation with transformation diagrams. Four different types of such diagrams can 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, describing the decomposition of austenite
• Continuous heating transformation (CRT) diagrams
• Continuous cooling transformation (CCT) diagrams

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.

ITh Diagrams (Formation of Austenite). During the formation of austenite from an original microstructure of ferrite and pearlite or tempered martensite, the volume decreases with the formation of the dense austenite phase. From the elongation curves, the start and finish times for austenite formation, usually defined as 1% and 99% transformation, respectively, can be derived.

IT Diagrams (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. The cooling was started from 850°C (1560°F). The A₁ and A₃ temperatures are indicated as well as the hardness. Above A₃ no transformation can occur. Between A₁ and A₃ only ferrite can form from austenite.

CRT Diagrams

In practical heat treatment situations, a constant temperature is not required, but rather a continuous changing temperature during either cooling or heating. Therefore, more directly applicable information is obtained if the diagram is constructed from dilatometric data using a continuously increasing or decreasing temperature.

Like the ITh diagrams, the CRT diagrams are useful in predicting the effect of short-time austenitization that occurs in induction and laser hardening. One typical question is how high the maximum surface temperature should be in order to achieve complete austenitization for a given heating rate. To high a temperature may cause unwanted austenite grain growth, which produces a more-brittle martensitic microstructure.

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 rare can be very complex. 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. The more severe the cooling medium, the longer the times to which the C-shaped curves are shifted. The M, temperature is unaffected.

Fig.2. CCT (a) and TTT (b) diagrams.

It should be noted, however, that transformation diagrams can not be used to predict the response to thermal histories that are very much different from the ones used to construct the diagrams. For instance, first cooling rapidly to slightly above M_s and then reheating to a higher temperature will give more rapid transformation than shown in the IT diagram because nucleation is greatly accelerated during the introductory quench. It should also be remembered that the transformation diagrams are sensitive to the exact alloying content within me allowable composition range.