Steel Heat Treatment: Equipment and Process Design (Steel Heat Treatment Handbook, Second Edition)

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The pure white areas are the islands of free ferrite grains described previously. Grains that are white but contain dark platelets are typical lamellar pearlite. These platelets are cementite or carbide interspersed through the ferrite, thus conforming to the typical two-phase structure indicated below the BH line in Fig.

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Eutectoid Steels. A carbon steel containing approximately 0. All of the carbon is dissolved in the austenite. When this solid solution is slowly cooled, several changes occur at C F. This temperature is a transformation temperature or critical temperature of the iron-cementite system. At this temperature, a 0. This change occurs at constant temperature and with the evolution of heat. The new phases are ferrite an cementite, formed simultaneously; however, it is only at composition point G in Fig.

Steel Heat Treatment Equipment and Process Design Steel Heat Treatment Handbook, Second Edition

The microstructure of a typical eutectoid steel is shown in Fig. The white matrix is alpha ferrite and the dark platelets are cementite. All grains are pearliteno free ferrite grains are present under these conditions. Cooling conditions rate and temperature govern the nal condition of the particles of cementite that precipitate from the austenite at C F.

Under specic cooling conditions, the particles become spheroidal instead of elongated platelets as shown in Fig. Figure 5 c shows a similar two-phase structure resulting from slowly cooling a eutectoid carbon steel just below A1.

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This structure is commonly known as. There is no indication of grain boundaries in Fig. The spheroidized structure is often preferred over the pearlitic structure because spheroidite has superior machinability and formability. Combination structures that is, partly lamellar and partly spheroidal cementite in a ferrite matrix are also common. As noted previously, a eutectoid steel theoretically contains a precise amount of carbon. In practice, steels that contain carbon within the range of approximately 0. Hypereutectoid steels contain carbon contents of approximately 0.

Assume that a steel containing 1. When cooled, no. Effects of carbon content on the microstructures of plain-carbon steels. Source: Ref 4. At a temperature slightly below C F , the remaining austenite changes to pearline. No further changes occur as cooling proceeds toward room temperature, so that the room temperature microstructure consists of pearline and free cementite.

In this case, the free cementite exists as a network around the pearline grains Fig. Upon heating hypereutectoid steels, reverse changes occur. At C F , pearlite changes to austenite. Hysteresis in Heating and Cooling The critical temperatures A1, A6, and Acm are arrests in heating or cooling and have been symbolized with the letter A, from the French word arret meaning arrest or a delay point, in curves plotted to show heating or cooling of samples.

Such changes occur at transformation temperatures in the iron-cementite diagram if sufcient time is given and cab be plotted for steels showing lags at transformation temperatures, as shown for iron in Fig. However, because heating rates in commercial practice usually exceed those in controlled laboratory experiments, changes on heating usually occur at temperatures a few degrees above the transformation temperatures shown in Fig.

The c is from the French word chauffage, meaning heating.

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Thus, Ac1 is a few degrees above the ideal A1 temperature. Likewise, on slow cooling in commercial practice, transformation changes occur at temperatures a few degrees below those in Fig. These are known as Ar, or Ar3, the r originating from the French word refroidissement, meaning cooling. This difference between the heating and cooling varies with the rate of heating or cooling. The faster the heating, the higher the Ac point; the faster the cooling , the lower the Ar point. Also, the faster the heating and cooling rate, the greater the gap between the Ac and Ar points of the reversible equilibrium point A.

Going one step further, in cooling a piece of steel, it is of utmost importance to note that the cooling rate may be so rapid as in quenching steel in water as to suppress the transformation for several hundreds of degrees.

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This is due to the decrease in reaction rate with decrease in temperature. As discussed subsequently, time is an important factor in transformation, especially in cooling. Effect of Time on Transformation The foregoing discussion has been conned principally to phases that are formed by various combinations of composition and temperature; little reference has been made to the effects of time. In order to convey to the reader the effects of time on transformation, the simplest approach is by means of a time-temperature-transformation TTT curve for some constant iron-carbon composition.

Such a curve is presented in Fig. TTT curves are also known as S curves because the principal part of the curve is shaped like the letter S. The reason for plotting time on a logarithmic scale is merely to keep the width of the chart within a manageable dimension. In analyzing Fig. Above this temperature, austenite exists only for a eutectoid steel refer also to Fig.

When the steel is cooled and held at a temperature just below Ae1 C or F , transformation begins follow line Ps Bs , but very slowly at this temperature; 1 h of cooling is required before any signicant amount of transformation occurs, although eventually complete transformation occurs isothermally meaning at a constant temperature , and the transformation product is spheroidite Fig. Now assume a lower temperature C or F on line Ps Bs the line of beginning transformation ; transformation begins in less than 1 min, and the transfor-. Time-temperature-transformation TTT diagram for a eutectoid 0.

Next, assume a temperature of C F ; transformation begins in approximately 1 s and is completely transformed to ne pearlite in a matter of a few minutes. The line Pf Bf represents the completion of transformation and is generally parallel with Ps Bs. However, if the steel is cooled very rapidly such as by immersing in water so that there is not sufcient time for transformation to begin in the C F temperature vicinity, then the beginning of transformation time is substantially extended.

For example, if the steel is cooled to and held at C F , transformation does not begin for well over 1 min. It must be remembered that all of the white area to the left of line Ps Bs represents the austentic phase, although it is highly unstable. When transformation takes place isothermally within the temperature range of approximately to C to F , the transformation product is a microstructure called bainite upper or lower as indicated toward the right of Fig.

A bainitic microstructure is shown in Fig. In another example, steel is cooled so rapidly that no transformation takes place in the C F region and rapid cooling is continued note line XY in Fig. Under these conditions, martensite is formed. Point Ms is the temperature at which martensite begins to form, and Mf indicates the complete nish of transformation. It must be remembered that martensite is not a phase but is a specic microstructure in the ferritic alpha phase.

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Martensite is formed from the carbon atoms jamming the lattice of the austenitic atomic arrangement. Thus, martensite can be considered as an aggregate of iron and cementite Fig. It is also evident that the lower the temperature at which transformation takes place, the higher the hardness see Chapter 3, Hardness and Hardenability. Structure is martensitic. It is also evident that all structures from the top to the region where martensite forms Ae1 are time-dependant, but the formation of martensite is not time-dependent.

Each different steel composition has its own TTT curve; Fig. However, patterns are much the same for all steels as far as shape of the curves is concerned. The most outstanding difference in the curves among different steels is the distance between the vertical axis and the nose of the S curve. This occurs at about C F for the steel in Fig.

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This distance in terms of time is about 1 s for a eutectoid carbon steel, but could be an hour or more for certain high-alloy steels, which are extremely sluggish in transformation. The distance between the vertical axis and the nose of the S curve is often called the gap and has a profound effect on how rapidly the steel must be cooled to form the hardened structuremartensite.

Width of this gap for any steel is directly related to the critical cooling rate for that specic steel. Critical cooling rate is dened as the rate at which a given steel must be cooled from the austenite to prevent the formation of nonmartensitic products. Practical heat treating procedures are based on the fact that once the steel has been cooled below approximately C F , the rate of cooling may be decreased. These conditions are all closely related to hardenability, which is dealt with in Chapter 3. Boyer, Chapter 1, Practical Heat Treating, 1st ed. Boyer, Chapter 2, Practical Heat Treating, 1st ed.

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Steel Heat Treatment: Equipment and Process Design

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