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With rapid heating, actual austenitizing temperature is raised as compared to the theoretical value obtained from iron-cementite phase diagram, whereas in practice, this temperature is lowered on fast cooling. Hence, during heat treatment practice, temperatures which are slightly higher than the temperature determined from the phase diagram are employed.
In the case of alloy steels, some alloying elements or their compounds do not dissolve or diffuse with ease. Such steels require higher heating temperatures for homogenization of austenite. Iron carbide, in general, dissolves readily in gamma-iron as compared to carbides of strong carbide-forming elements. Once the heat treatment temperature is decided, holding time is generally provided at a rate of 2 to 3 minutes per millimetre of section thickness. For an object with variable section thickness values, the holding time is determined on the basis of the thickest section.
As already mentioned, objects which are heated up with high heating rates require longer holding time. Heat treatment temperature and holding time are somewhat related in the sense that an increased heat treatment temperature results in reduction of holding time.
Similarly, lowering of heat treatment temperature demands an increase in holding time. High alloy steels and alloys, heavily enriched with alloying elements, are kept for more time at heat treatment temperature than plain carbon steels and thinly enriched alloys. In such cases, holding time may be increased by 25 to 40 percent. As far as cooling from heat treatment temperature is concerned, the mode of cooling as well as the rate of cooling are governed by those factors which control the heating mode and rate, as already discussed.
Heavy sections, complicated shapes, objects with variable section thickness, and highly enriched alloys are cooled slowly. Extremely slow cooling results in the development of a structure as is evident from the equilibrium phase diagram.
The size, shape, distribution and relative proportions of microconstituents can be controlled over a wide range by varying the cooling rates. Up to a certain limit, higher cooling rate results in a structure which should exist according to the equilibrium diagram.
By increasing the cooling rates beyond this limit, structures that are produced will consist of either non-equilibrium products or the high temperature phase s retained by sudden quenching. Introduction 5 1. How does heat treatment alter the mechanical properties of an alloy? Discuss the significance of holding time at heat treatment temperature. One can prepare a list of many items made of metals and alloys of daily life. The list will probably include aluminium pots, rolled sheet of mild steel for making the body of refrigerator, scooter and car, steels for making parts, such as gears, pistons, crank shafts of locomotives, copper wire conductors, agricultural implements made of steel and cast irons, aeroplanes made of aluminium and magnesium alloys, and many others.
In spite of knowing so much about metals, it is not an easy task to define a metal accurately. Chemists define metals in terms of acids and bases. The base is metal oxide or hydroxide. When metals react with acid, they liberate one or more atoms of hydrogen from acid. Physicists as well as chemists agree that metals have bright lustre ability to reflect light , good electrical and thermal conductivity, malleability and ductility. In general all metals show all these properties.
At the same time, there are non-metals which show one or more of the above mentioned properties. Besides this, metals among themselves show large variation with respect to these properties.
For example, lead at room temperature is very soft while tungsten at room temperature is very hard and brittle. The basic property associated with metals is high electrical and thermal conductivity. This distinguishes them from non-metallic substances. Amongst commonly used engineering metals the best conductor of electricity is copper.
However, silver is the best electrical conducting metal. Lead, on the other hand, has poor conductivity. It has electrical conductivity which is one-twelfth of copper. When we compare the conductivity of non-metals such as diamond with copper, it is observed that conductivity of diamond is 10—9 times that of copper.
Such a large difference in conductivity is due to electrons in metals which can move freely in conduction bands. In non-metals, free movement of electrons under the influence of external electrical field does not take place.
Thus, it can be concluded that in metals electrons can move freely and permit flow of current under the action of a potential difference. In general, the words metal and alloy are interchangeable.
However, for engineering and scientific work, metals are metallic elements which are almost pure. Each metal has its own characteristics such as atomic weight, atomic volume, atomic number, melting point and density. By chance it may be possible that an alloy may have the same colour, melting point and density as a pure metal.
However, X-ray diffraction techniques distinguish them clearly from parent metals. A very simple way of identification is chemical analysis as alloys contain more than one element. The element that is in major quantity should be a metal while the others may be metals, non-metals or combination of both.
The resulting material should have metallic characteristics. Brass, bronze and steel are typical examples of alloys. However, the interrelations between individual atoms and heat treatment is far fetched. For a sound understanding of the principles of heat treatment, it is sufficient to go up to the level of crystals rather than atoms. All metals are crystalline in nature, where atoms are arranged in a definite periodic order.
When a solid is composed of only one crystal, it is called single crystal. When a solid is composed of several crystals, it is called a polycrystalline material. In fact, the individual crystals get aggregated to form the solid mass of a polycrystalline substance.
The term grain is used to denote a single crystal in polycrystalline aggregate. In general grains do not have perfect outward form. On the basis of periodic arrangement of atoms, crystals are grouped into seven crystal structure systems.
Cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic and triclinic are the seven crystal structure systems. Most of the metals and alloys belong to cubic and hexagonal crystal structure systems and hence are being discussed here.
In a crystal structure, the smallest unit is the unit cell which characterizes the specific arrangement and location of atoms. When the unit cell is repeated in three directions, complete arrangement of atoms in crystal becomes apparent. Simple Cubic ln this case, atoms are located at all the corners of the unit cell. Figure 2. They help in transport of atoms.
For example, many industrial processes such as annealing, homogenization, precipitation of second phase particles, and case carburizing require large scale migration of atoms in the crystal lattice. Vacancies arise from thermal vibrations. They are also introduced during solidification.
There is slight distortion in lattice planes due to vacancy because atoms surrounding a vacancy have tendency to come closer. When vacancies are generated by displacement of atoms from crystal lattice site to the surface, a Schottky defect is formed.
Interstitialcies When an atom is displaced from a regular site and occupies an interstitial site, an interstitialcy is formed.
The atom may be a parent atom or a foreign atom. Interstitialcy also gives rise to lattice distortion because interstitial atom tends to push the surrounding atoms apart. The smaller the size of the interstitial atom relative to the parent atom, the smaller is the lattice distortion in the crystal.
When an ion is displaced from the crystal lattice into an interstitial site, a Frankel defect is formed as in Figure 2. Impurities lmpurities are foreign atoms which are present in the crystal lattice. Impurity atoms may occupy either interstitial or substitutional position. An interstitial impurity is a small atom which occupies an interstitial void space between atoms at lattice points of the crystal.
When a foreign atom substitutes a parent atom in the crystal lattice, a substitutional type of defect is generated. Both interstitial and substitutional impurities create distortions in the lattice.
Dislocation is the region of localized lattice disturbance between slipped and unslipped regions of a crystal. Due to lattice disturbances, elastic strain fields and stresses are associated with dislocations. Metals deform plastically by the mechanism of slip. Slipping is facilitated in the presence of dislocations.
Strain hardening, creep, fatigue and fracture are the mechanical phenomena, in which dislocations play a major role. Dislocations are of two types: A burger circuit is drawn around the dislocation line and the vector required to close the circuit, RS, is known as the burger vector of the dislocation. An edge dislocation moves in the direction of the burger vector slip direction.
It has an extra row of atoms either above or below the slip plane in crystal. When the extra row of atoms is below the slip plane, it is called negative edge dislocation and is represented by sign. In edge dislocation, the atoms above the edges are in compression and those below are in tension as shown in Figure 2.
An edge dislocation undergoes gliding in crystal and produces a slip step of one atom width at the edge of the crystal. S a Edge dislocation b Screw dislocation Figure 2. Here, the burger vector is parallel to the dislocation line and distortion is of shear type. The distortion follows a helical path, and it may follow right-hand positive or left-hand negative screw rule. Positive and negative dislocations are shown by clockwise and anticlockwise signs, respectively. Screw dislocation shows cross slip, where it moves from one slip plane to another.
Two dislocations either edge or screw of opposite signs, if present on the same plane, attract each other and can annihilate each other. Dislocations are introduced in a crystal due to growth accidents, thermal stresses, phase transformations and segregation of solute atoms. Perfect crystals, which are totally free from dislocations, are called whiskers and have very high strength.
During solidification of metal, nucleation and growth process starts. During this process, crystals nucleate and grow at different points. When the growing surfaces of these two crystals meet, some atoms are caught in the boundary region of two crystals and do not belong to any one of the two crystals. This region of atoms is called grain boundary see Figure 2. At grain boundaries, atomic packing is imperfect. Thickness of the grain boundary can be estimated by field ion microscopy.
This may be regarded as an array of parallel edge dislocations of same sign arranged one below the other. Twist boundaries are also low-angle grain boundaries but in this case an array of screw dislocations are present instead of edge dislocations.
When the arrangement of atoms on one side of a boundary is a mirror image of that of the other side, twin boundary is formed. In some instances, crystal structures of the two phases may be similar and the lattice parameters may be nearly equal. In such cases the boundary between the two grains called an interface is coherent. There is one-to-one correspondence of atoms at the interface.
When such matching does not exist at the interface, the interface is said to be incoherent. When there is a discrepancy in the packing sequence of the rows of atoms, stacking fault results. If the sequence changes to ABC AB ABC in practice, then stacking faults which are usually introduced by deformation of annealed metals and alloys occur. A stacking fault occurs when a metal crystal has two adjacent crystallo-graphic planes which do not fit in the normal geometric pattern within the system.
Stacking faults may be caused either by slip or some other mechanism. The role of imperfections in heat treatment is very important. Imperfections account for crystal growth, diffusion mechanism, annealing and precipitation.
Besides this, other metallurgical phenomena, such as oxidation, corrosion, yield strength, creep, fatigue and fracture, are governed by imperfections. Imperfections are not always harmful to metals. Sometimes they are generated to obtain the desired properties. For example, carbon is added in steel as interstitial impurity to improve the mechanical properties, and these properties are further improved by different heat treatment processes.
Give examples of solid solutions. Justify the above statement with the help of Hume Rothery Rules for the formation of solid solutions. Discuss the importance of metallography with special reference to heat treatment. The alloys of iron are very important for engineering applications.
Steel and cast iron are the important alloys of iron with carbon. Besides this, various alloying elements are also added to obtain alloy steels and alloy cast irons with required properties.
This chapter deals with the allotropic forms of iron, microstructure, and phase transformations in iron-cementite and iron- graphite system.
The effect of alloying elements on the Fe-Fe3C phase diagram is also discussed. Allotropy is characterized by changes in crystal structure of iron at a definite transformation temperature. Iron has three allotropic modifications. To study allotropic modifications, molten iron is allowed to cool slowly in an insulated crucible and the cooling curve is plotted, as shown in Figure 3.
The temperature remains constant till freezing is completed. Delta iron changes to gamma " iron with face centred cubic structure at this temperature. The temperature at which paramagnetic beta iron transforms to ferromagnetic alpha iron is termed as Curie temperature. These changes are reversible. Under normal rates of cooling, actual transformations take place at temperatures which are lower than the equilibrium temperatures indicated above.
Similarly, transformations occur at temperatures higher than the equilibrium temperatures while heating is carried out under normal rates.
These temperature lags are termed as thermal hysteresis in analogy with the hysteresis that is observed during magnetization and demagnetization cycles.
The following symbols are used for iron and steel: This is not important for heat treatment studies. Aem, Ae1, Ae3, Ae4 are the corresponding temperatures of phase changes at equilibrium. Iron-Cementite Phase Diagram The letters indicated above are from French words and have the following meaning: This is revealed by the reglowing of the metal surface due to the sudden increase in the temperature caused by the quick liberation of latent heat of transformation.
Decalescence is sudden decrease in the temperature due to reverse phase change in this system. This is also known as iron-carbon equilibrium diagram. But the term iron-carbon is incorrect in a strict sense because the phase which constitutes the binary phase diagram is cementite, and not carbon or graphite.
The term iron-cementite is also not strictly correct because the cementite phase is metastable. In metallurgical practice both the terms, namely, iron-carbon and iron-cementite phase diagrams are used. In the phase diagram, temperature is plotted against composition. Any point on the diagram therefore represents a definite composition and temperature. The phase diagram indicates the phase s present and the phase changes that occur during heating and cooling.
The relative amounts of the phases that exist at any temperature can also be estimated with the help of lever rule. The solubility of carbon varies in different forms of iron.
In delta iron, maximum solid solubility of carbon is 0. In gamma iron, the maximum solid solubility of carbon is 2. Austenite is a solid solution of carbon in face centred cubic iron and solute atoms occupy interstitial positions in this lattice. In -iron, carbon has a limited solid solubility of about 0.
The maximum solubility of carbon in ferrite is 0. The solid solution of carbon in -iron is known as ferrite. There are three reactions which occur in iron-cementite phase diagram see Figure 3. These reactions are now discussed. Peritectic Reaction In the alloy containing 0. Eutectic Reaction Alloy with carbon content 4. It contains 6. The eutectic of austenite and cementite is known as ledeburite. Eutectoid Reaction In iron-carbon alloy with 0. The eutectoid of ferrite and cementite Fe3C is known as pearlite.
The ferrite and cementite phases occur as alternate layers. Depending upon the carbon content and the reactions occurring in the iron-cementite phase diagram, the alloys of iron are broadly divided into two groups: Carbon in steels may be present up to 2.
These are: Steels with carbon content from 0. Steel with a carbon content of 0. Steels with carbon content greater than 0. Transformations in Hypoeutectoid Steels There are solid state transformations in these steels. They are the transformation of gamma iron to alpha iron and the decomposition of austenite. Line GS in the phase diagram shows the temperatures where transformation of austenite to ferrite starts. Below this line, ferrite is separated out of the austenite.
Line ES indicates the variation in the solubility of carbon in austenite with the temperature and, in cooling, corresponds to the temperature at which separation of cementite from austenite starts. Point S is called the eutectoid point and here the degree of freedom is zero. When point S 0. Pearlite is a eutectoid mixture of two phases: The critical point where austenite decomposes to pearlite is denoted by Ar1 in cooling and Ac1 in heating.
The line PQ shows the variation in solubility of carbon in alpha iron with the temperature and, in cooling, corresponds to starting of precipitation of surplus cementite out of ferrite. The limiting composition for getting pearlite is 0. With carbon content less than this amount, no pearlite will be formed. The alloy will contain only ferrite grains.
Steels containing carbon between 0. Let us take the case of heating upto austentic region followed by slow cooling of component X1 containing 0. In the austenitic range, this alloy consists of a uniform interstitial solid solution. Each grain contains 0. Figure 3. Proeutectoid -ferrite starts separating out along line GP, and the composition of austenite moves along line GS.
Since ferrite can dissolve very little amount of carbon, in those areas that are changing to ferrite the carbon must come out of solution before the atoms rearrange themselves to BCC structure.
The carbon which comes out of solution is dissolved in the remaining austenite, so that, as cooling progresses and the amount of ferrite increases and the remaining austenite becomes richer in carbon. Just above the PS line T2 , the microstructure consists of austenite and proeutectoid ferrite. At the eutectoid temperature, the remaining austenite, of the total material which contains 0.
Transformations in a Steel Containing Less Than 0. In this steel, carbon content is less than 0. Excess carbon, thus rejected, separates out as Fe3C. Hence, the microstructure is predominantly ferrite a with small amount of Fe3C. If the cooling rate is higher than the equilibrium rate, then enough time is not available for cementite to separate, and the microstructure is percent ferrite.
The ferrite is supersaturated as its carbon content is higher than the one according to the solvus line of the equilibrium diagram. Transformations in Hypereutectoid steels Consider a steel X3, containing 1. The phase present in the steel in the austenitic condition is FCC iron with carbon forming interstitial solid solution.
The composition is uniform throughout each grain. As the temperature is lowered from T6, phase transformation starts at temperature T7 which is the point of intersection of vertical line at X3 and the line ES.
At this temperature, austenite is fully saturated with carbon. As the temperature is lowered below T7 for example T8 solubility of carbon in austenite starts moving along ES. The excess carbon comes out in the form of proeutectoid cementite. This cementite is precipitated primarily along the grain boundaries. This leads to formation of grain boundary network of cementite in the hypereutectoid steel.
As the steel attains the eutectoid temperature A3 , eutectoid transformation takes place in the austenite and pearlite is formed. Below A3 for example, T9 the microstructure consists of pearlite with network of cementite.
Transformations in Eutectoid Steel Let us consider a steel containing 0. So, the microstructure at room temperature will reveal alternate layers of ferrite and cementite, called pearlite.
They are white cast irons and grey cast irons. In white cast irons, carbon is present in the combined form as cementite and in grey cast irons, it is present in free form as graphite.
Under normal conditions, carbon has a tendency to combine with iron to form cementite. However, under very slow rate of cooling, during solidification, carbon atoms get sufficient time to separate out in pure form as graphite.
In addition, certain elements promote decomposition of cementite. Their presence help in the formation of graphite. These elements are called graphitizers. Silicon and nickel are two commonly used graphitizing elements. Based on the Fe-Fe3C phase diagram, cast irons can be classified into three groups: Eutectic cast iron contains 4.
Therefore, at the eutectoid temperature, both eutectic and proeutectic austenite would contain 0. Hence, the final microstructure would be the same as the high temperature microstructure, except that wherever austenite was present, pearlite would now be present. Proeutectic cementite will remain as such since it does not undergo any further change on cooling. Austenite from eutectic rejects excess carbon as cementite. Thus, the final microstructure will reveal proeutectic cementite present as plate and transformed eutectic consisting of pearlite plus cementite.
As the temperature decreases, the solubility of carbon in -iron decreases, as indicated by the cementite line. Consequently, excess carbon is rejected from austenite and comes out in the form of cementite. The microstructure of eutectic cast iron at room temperature is identical to that at the high temperature structure.
However, there is a distinct difference. In place of austenite, the transformed product, namely, pearlite, will be observed. The eutectic structure consisting of austenite and cementite can be visualized as forming alternate layers of the two phases. Therefore, the final structure will be transformed ledeburite. The reactions during transformations in eutectic cast iron, on cooling, are as follows: We now give four examples of this.
Here, YX is the lever arm. Fulcrum is at point Z. The fraction of! YZ to the total length of the lever arm XY. At all temperatures, the following reaction takes place: At low temperatures, however, the reaction occurs so slowly that Fe3C remains as such.
At higher temperatures, the graphitization of the iron carbide occurs. In the presence of silicon, graphitization becomes faster. Aluminium and nickel, which are also graphitizers, form additional centres for crystallization of graphite flakes.
So, the stable phase, graphite, forms either by separating out of the liquid or solid solution, or as a result of decomposition of metastable cementite.
As the liquid alloy say having 3. This is called primary stage of graphitization. Separation of secondary graphite from the austenite is called the inter-mediate stage of graphitization. Formation of eutectoid graphite, as well as the decomposition of eutectoid cementite into graphite and ferrite, is called the secondary stage of graphitization. A major amount of graphite precipitates during the primary stage of graphitization. While during intermediate and secondary stage of graphitization, the additional graphite merely adds to the pre-existing graphite flakes and increases its size.
The prolonged heating at high temperatures of cast iron containing carbon in combined form also leads to graphitization, i. Composition and cooling rate are two factors which determine the type of structure formed on cooling of cast iron.
Rapid cooling inhibits precipitation of graphite partially or completely and promotes formation of cementite. The precipitation of graphite from the liquid phase is possible only at very slow cooling rates, i.
Since a cast iron containing coarse graphite flakes has low strength, various methods are used to improve the graphite distribution. Finer graphite tends to form when the alloy is superheated just before casting. A finer flake size can also be produced by adding inoculants like ferrosilicon and calcium-silicon in very small amounts which promote graphitization. Graphite nodules, rather than graphite flakes, will form if the molten alloy is treated with magnesium or cerium.
This results in formation of spheroidal graphite SG cast iron. This is called malleable cast iron. By knowing the behaviour of such elements on the Fe-Fe3C diagram, it is possible to evolve suitable heat treatment cycles to obtain alloy steels with desired properties.
Alloying elements affect the relative stabilities of alpha and gamma iron as also on critical points A4, A3, and A1 and carbon content. From the point of view of their effect on austenite and ferrite, the alloying elements can be grouped in two classes.
These are called ferrite stabilizers, e. Cr, W, Mo, V, and Si. These are called austenite stabilizers. Mn, Ni, Co, and Cu are some elements which fall under this category. Ferrite Stabilizers These elements stabilize ferrite. They are more soluble in alpha-iron than in gamma-iron. Most of these elements have the BCC crystal structure, the same crystal structure as that of alpha- iron.
They decrease the amount of carbon present in the gamma-iron, and thus favour formation of larger quantity of carbide in the steel for a given carbon content. These elements reduce the austenite region in the Fe-Fe3C phase diagram by lowering the A4 point and raising the A3 point until the A4 and A3 points meet and form a closed gamma loop.
The austenite phase may not appear in a steel at any temperature when sufficiently large quantities of such elements are added. For example, by adding Such steels cannot be heat treated in the conventional way because the austenite phase is not available for solid-solid phase transformation.
They can be strengthened only by cold working-annealing cycle. However, it is possible to reintroduce austenite phase in such a steel by addition of austenite stabilizers like nickel. The approximate compositions at which austenite phase altogether disappears due to addition of ferrite stabilizer elements are given in Table 3.
Table 3. All the elements except Si listed in Table 3. Silicon is diamond cubic in structure. Austenite Stabilizers Manganese, nickel, and copper are austenite stabilizing elements. These elements enlarge the austenitic region, make it stable phase even at room temperature, and shift the critical points. The A4 point is shifted upward and the A3 point downwards. As a result, the range of stable austenite is increased. Carbon has also similar effect, as can be observed from the Fe-Fe3C phase diagram.
Austenite stabilizers also restrict the separation of carbide. The general trend of effect due to addition of austenite stabilizing elements can be seen from Figure 3. The carbon content of the eutectoid composition is reduced and also the A1 temperature. Thus the austenite phase field is enlarged. The ferrite stabilizing elements e.
This is shown in Figure 3. The eutectoid temperature increases while the eutectoid composition shifts to lower carbon content. Effect on Eutectoid Temperature and Composition Different alloying elements may influence the eutectoidal transformation in one or more of the following ways: Effect on Temperature Figure 3. Being an austenite stabilizer, nickel lowers eutectoid temperature. When chromium, which is a ferrite stabilizer, is added, the eutectoid temperature is raised.
Ti and Mo are most effective in this context. Effect on Carbon Content of the Eutectoid Composition Both austenite and ferrite stabilizers lower the carbon content of eutectoid composition. In plain carbon steel, eutectoidal composition corresponds to 0. In general, steels may be classified into two broad groups: Plain carbon steel is that steel in which carbon is the main element which governs the properties of steel.
Other elements which are associated in the steel are kept below a certain limit so that they do not influence properties of steel. On the other hand, alloy steels are those steels in which one or more elements are added to impart some peculiar characteristics.
In such steels, carbon and alloying elements, contribute towards properties of steels. Plain carbon steel and alloy steels can be further subdivided on the basis of various factors. Plain carbon steel can be further classified on the basis of carbon content, structure at room temperature at slow cooling, quality and application. It may be recalled that a steel having 0. Hypoeutectoid steels contain proeutectoid ferrite and pearlite, while hypereutectoid steels contain proeutectoid cementite and pearlite.
These divisions are not so precise for many reasons. So, for all practical purposes, these can be classified into low, medium and high carbon steel: Low carbon steel up to 0. Plain carbon steel may contain several other elements like Al, Mn, Si, P, S, and so on, but their individual contents are below the critical limits above which the properties of steel are significantly affected. Low carbon steel is easily weldable and has good workability but show poor response to heat treatment. It is mainly used for sheets, strips, structural sections, case hardening, etc.
Medium carbon steel has good strength, toughness and its response to heat treatment is also good. It is mainly used for such applications as gear and connecting rods, laminated springs, and so on. High carbon steel has good strength and is used as tool steel, prestressed concrete wire, etc. The quality of any steel, in general, depends on S and P contents, the degree of the oxidation and cleanliness.
A quality steel has S and P contents, each 0. Steel with content of these elements less than 0. Steels having S and P between 0. Plain carbon steels are used successfully where strength and other requirements are not absolutely essential.
At ordinary temperature and in an atmosphere, which are not of a severely corroding nature, the performance of plain carbon steels is quite satisfactory. However, the relatively low hardenability of the plain carbon steels limits the strength that can be attained except in relatively small cross-sections. Following disadvantages limit the usefulness of plain carbon steels: The most common and practical method of overcoming the deficiencies of the plain carbon steel is to employ alloy steels.
Alloy steels are those in which one or more elements are added to steel for enhancing further the prominent characteristics of the plain carbon steels or to ensure specific properties.
These added elements are known as alloying elements. Based on their four features, alloy steels can be classified as follows: Depending on the alloying elements, alloy steels are classified into nickel steels, chromium steels, Cr-Ni steel and Cr-Ni-Mo steels.
They are classified according to the presence of various alloying elements. Various structures such as martensite, austenite, or bainite constitute steels. Accordingly, they are called martensitic steel, austenitic steel, and so on.
Alloying elements are added with a view to impart a number of desirable properties in steels. Some of these properties are i to increase hardenability, resistance to softening on tempering, resistance to corrosion and oxidation, and to increase resistance to abrasion; ii to improve high temperature properties; and iii to achieve the desired constituents e.
Alloy steels, however, are normally costly. They require more careful handling during manufacturing, heat treatment and mechanical working. Alloy steels have greater tendency for retention of austenite. The retained austenite decomposes at a later stage, leading to dimensional changes and introduction of internal stresses.
Certain grades of alloy steels are also prone to temper brittleness. What is the critical range? Discuss in brief the different reactions that take place in this system. As already mentioned, the properties of steel are related to its structural make-up. The desired levels of mechanical properties can be obtained by altering the size, shape and distribution of various constituents. This is achieved in practice by the process of heat treatment. In general, the structural make-up of any steel consists of transformed product s from austenite.
Depending on various parameters, transformed products from austenite may be pearlite, bainite or martensite. Not only the presence of these phases microconstituents but also the morphology of these products is of significance in deciding the resultant properties.
Therefore, it is very important, before proceeding for a heat treatment process, to know about the nature of austenite and its subsequent transformation behaviour.
In fact, such a study is essential in order to learn the theory of heat treatment practice. The following sections deal with various aspects of heat treatment theory. Formation of austenite is a preliminary step for any heat treatment process.
Austenite is formed on heating an aggregate of ferrite and pearlite, ferrite and cementite or cementite and pearlite, depending on whether the steel is of hypoeutectoid, eutectoid or hypereutectoid type, respectively.
Formation of austenite in eutectoid steel differs from that of hypoeutectoid and hypereutectoid steels in the sense that in the former case it occurs at a particular temperature Ac1 whereas for the latter it takes place over a range of temperature see Figure 4. Let us consider the formation of austenite from a mixture of ferrite-cementite in a eutectoid steel.
Normally, this mixture in eutectoid steel occurs as pearlite. Figure 4. The orientation of these layers varies for different grains. These layers are in direct contact with each other. The intermetallic compound cementite has 6. Though a carbon gradient exists, the structure is thermodynamically stable at room temperature and above.
As the temperature increases to higher values, but below eutectoid temperature, carbon atoms have a tendency to diffuse into ferrite. Maximum content of carbon in ferrite is about 0. Below this temperature, it is still less and decreases to a value of about 0. The maximum solubility of carbon in FCC iron is about 2 percent. Therefore, at this temperature, regions around the cementite layer will be enriched with carbon because of diffusion.
The maximum diffusion of carbon atoms will take place from the Figure 4. As sufficient number of interfaces are available, austenite nuclei will be formed at the interfaces.
These primary austenitic grains dissolve the surrounding ferrite and austenitic grains grow at the expense of ferrite see Figure 4. These processes, i. This explains the experimentally observed fact that dissolving of ferrite is completed before that of cementite. The austenite thus formed at eutectoid temperature is not homogeneous. The carbon concentration is higher in these regions which are adjacent to the original cementite lamellae than those which are adjacent to the ferrite mass.
Chemically homogeneous austenitic grains are obtained by holding steel above the eutectoid temperature. The holding time should be sufficient so that carbon atoms may diffuse and result in uniform distribution of carbon atoms. Transformation to austenite on heating hypoeutectoid and hypereutectoid steels is somewhat different from the above condition because of the presence of proeutectoid ferrite and proeutectoid cementite, respectively, along with pearlite.
Fe-Cementite phase diagram. On very slow heating, austenite nuclei are formed just above the eutectoid temperature. More nuclei will form with increase in temperature. Therefore, at first the austenitic grains will grow by the growth of initially formed austenitic grains and then by the growth of newly formed austenite nuclei.
The process will continue till the upper critical temperature A3 is reached. The austenite present at this temperature will be non-homogeneous due to the presence of embedded cementite particles within the austenitic grains.
For hypoeutectoid steels, growth of primary austenitic grains take place at the expense of proeutectoid ferrite. Further, austenite nuclei are also possibly formed at grain boundaries of ferritic grains. In hypoeutectoid steels, cementite dissolves into the ferrite which in turn transforms into austenite. In the case of hypereutectoid steels, the transformation proceeds in a similar way with the difference that austenitic grains grow by dissolving proeutectoid cementite. Theoretically, though pearlite must transform to austenite completely at eutectoid temperature, it does not happen so in practice.
Complete dissolution of cementite of pearlite into austenite takes place over a range of temperatures. On parallel lines, it has been experimentally observed that dissolution of proeutectoid ferrite or proeutectoid cementite is not completed at A3 or Acm respectively.
It, therefore, becomes essential to heat eutectoid, hypoeutectoid and hypereutectoid steels above A1, A3 and Acm, respectively, in order to get homogeneous austenite. The formation of austenite on heating always occurs at a temperature higher than that predicted by the Fe-Cementite phase diagram.
Heating of steel to austenitizing temperature is the first and foremost step of almost all heat treatment processes. Also, the grain size of austenite at heat treatment temperature largely controls the resultant mechanical properties after heat treatment. Therefore, the study of kinetics of formation of austenite is of great importance. A simple approach to the study the kinetics of austenite formation is to heat a number of steel samples to different temperatures above the eutectoid temperature.
The size of samples is restricted so that they may attain the required temperature in a very short period. Heating is done by immersing samples in constant temperature baths. A number of samples are immersed in a constant temperature bath and are taken out one by one after a definite interval of time, followed by immediate quenching which will result in the formation of martensite from transformed austenite.
The amount of martensite formed will depend on the amount of transformed austenite which in turn will depend on the temperature at which the steel sample has been heated and the holding time at that temperature. As already mentioned, the temperature in this case will be above the eutectoid temperature.
Thus, the percentage of transformed austenite with time for a given temperature can be known. The same sequence of operation is employed for different temperatures.
The results of such a process are shown in Figure 4. It can be easily concluded that the lower the transformation temperature, the more is the time required to complete the transformation. The results of Figure 4. The percentage of the transformed austenite on heating can also be determined by various other methods. Some examples are change in hardness value, volume, magnetic property and internal stresses. One important limitation of Figure 4.
In fact, under practical conditions of heating, transformation occurs at a temperature above the eutectoid temperature, and not at eutectoid temperature. It will be useful to remember that equilibrium temperatures are raised on heating and lowered on cooling.
The relationship has been derived by taking into consideration the effect of superheating. An analysis of Figure 4. The end of the transformation curve does not reveal any information about the nature homogeneity of austenite. The curved line only ensures that all the pearlite has been transformed into austenite.
In order to attain a homogeneous austenite, the steel has to be heated to still higher temperatures. Here it is important to emphasize on the austenite transformation temperature. There should not be any confusion regarding it on the basis of Figures 4.
Austenite is formed as soon as the eutectoid temperature is attained. Firm curved lines of Figure 4. The process of austenite formation on heating proceeds by nucleation and growth reaction.
Therefore, the factors which can vary either the rate of nucleation or the rate of growth or both will change the kinetics of austenite formation. Two such parameters are transformation temperature and holding time at transformation temperature. The effect of these parameters has already been described in this section.
In addition, there are some other factors which control the kinetic of transformation. We now discuss some of these factors.
As already mentioned, the austenite nuclei are formed at the interface of ferrite and cementite as soon as the eutectoid temperature is reached. Therefore, the kinetics of austenitic transformation is governed, to a great extent, by the nature of the pearlite. The number of possible austenite nuclei will increase with the increase in the interfacial area.
The interfacial area can be increased in two ways: The former condition, i. This is the reason why high carbon steels austenitize more rapidly than low carbon steels.
The closer the ferrite-cementite lamellae, the higher will be the rate of nucleation. Also, the carbon atoms have to diffuse for smaller distances in order to enrich low carbon regions. Therefore, the rate of growth of primary austenitic grains will also be higher in this case. This explains why pearlitic structure with less interlamellar spacing is transformed faster to austenite. The kinetics of austenitic transformation for coarse pearlitic structure is slow for the reason given above.
The kinetics of austenitic transformation from granular pearlite is slower than that of lamellar pearlite for same reasons. The kinetics of transformation will further decrease with increase in the size of globular cementite particles. Quenched structure will also transform to austenite more rapidly than the granular pearlitic structure. Principles of Heat Treatment of Steels The foregoing discussion was in relation to carbon steels. For alloy steels, the kinetics, in addition to the nature of pearlitic structure, is also dependent on the nature of alloying element s.
Kinetics of formation of austenite is slowed down in the presence of carbide forming elements. This is so because alloy carbides dissolve in the austenite with greater difficulty than iron carbide. Nuclei of austenite are formed as soon as a steel is heated to eutectoid temperature. These nuclei grow into primary austenitic grains. The process of nucleation and growth takes place simultaneously and continues till all the ferrite and cementite transform to austenite.
At the end of the transformation, only austenitic grains are present in the structure. The size of these austenitic grains is referred to as original grain size. In practice, a steel is heated above the critical temperature to ensure the homogeneity of austenite, This heating above the critical temperature results in considerable growth of original austenitic grains. The grain size of austenite thus obtained is known as the actual grain size of austenite.
The actual austenitic grain size is therefore dependent on the temperature to which steel has been heated up and holding time at this temperature. Until and unless specified, the grain size of steel, in general, means actual austenitic grain size and the same convention has been followed in this text.
In fact, many properties of steel are dependent on actual austenitic grain size or simply grain size of steel. Tensile strength, yield strength, toughness, hardenability and machinability can be altered considerably by varying the grain size of steel.
Depending on the tendency of steel to grain growth, steels can be classified into two broad groups: Inherently fine grained steel resists the growth of austenitic grains with increasing temperature. On the other hand, grains of inherently coarse grained steel grow abruptly see Figure 4.
Apparently, there is some mechanism which checks the austenitic grain growth in inherently fine grained steels and it is absent in inherently coarse grained steels. The terms inherently fine grained steels and inherently coarse grained steels do not essentially mean that the former will always possess finer grains than the latter.
On heating above a particular temperature which essentially depends on chemical composition and deoxidation practice, it is possible to get coarser grains in inherently fine grained steel as compared to inherently coarse grained steels.
This is possible only when the mechanism, which has been effective below this temperature, is no more effective above this temperature. It is now understood that it is the presence of ultramicroscopic particles of oxides, carbides and nitrides which prevent grain growth.
These particles are refractory in nature, i. Therefore, they act as barriers to the growth of austenitic grains. This explains the experimentally observed fact that steels which are either deoxidized with aluminium or treated with boron, titanium and vanadium, i. After dissolution, no particles are left to offer resistance to grain growth. Due to the limited effect of temperature on the austenitic grain growth, inherently fine grained steels can be hot worked at higher temperatures or can be heated to higher temperatures for heat treatment purpose without any danger of grain coarsening.
The most popular and widely accepted method for designating the austenitic grain size has been developed by the American Society of Testing Materials. In this designation system, grain size is expressed by grain size index, N. The grain size index is estimated by comparing the grain structure of the given specimen with the chart showing standard grain size structures.
As the grain size index number increases, grain size becomes fine. Table 4. ASTM No. The measurement of austenitic grain size is now becoming a regular practice. The diiference in the properties of two steels having the same chemical composition and subjected to similar heat treatment can be explained on the basis of austenitic grain size. Since austeniteis generally not the stable phase at room temperature, methods have been developed to reveal prior austenitic grain boundaries.
Revelation of austenitic grain boundaries is based on two distinctly different techniques, one involving change in chemical composition and the other in which change in chemical composition does not take place. When austenitic grain boundaries are revealed either by carburization or by oxidation, there is change in chemical composition. Carburization method is applicable to only those steels which can be case carburized. The principle involved in this method is the production of hypereutectoid case and cooling it in such a way that excess cementite forms network around the austenitic grains.
The steel remains packed in the carburization medium during cooling. The process is also referred to as the McQuaid-Ehn method. Longer carburizing period and slow cooling up to room temperature, involved in this process, result in the precipitation of carbides at the grain boundaries. This precipitation of carbides at grain boundaries facilitates the estimation of the grain size. Oxidation method is employed for plain carbon and low alloy steels.
The steel is oxidized under oxidizing atmosphere at an elevated austenitizing temperature. This is followed by quenching, resulting in the oxidation of austenitic grain boundaries which are clearly revealed at room temperature by etching. The methods which do not involve change in chemical composition consist of heat treating steel. In these methods, either network of ferrite or cementite or of transformed product is formed at austenitic grain boundaries.
Hypoeutectoid and hypereutectoid steels consist of pearlite and ferrite or pearlite and cementite, respectively. On slow cooling, depending on the carbon content, either ferrite or cementite will be formed as proeutectoid constituent.
Separation of the proeutectoid constituent generally takes place at austenitic grain boundaries. Under a specific set of favourable conditions, rejected ferrite or cementite may form a complete or almost complete network around the austenitic grains. On further cooling to eutectoid temperature, the austenite will transform to pearlite. Eutectoid steel has no excess constituents to be rejected. However, under specific conditions, a network of fine nodules of pearlite around the austenitic grains can be developed.
This network can be preserved and the remaining austenite can be transformed into martensite by quenching.
Now, prior austenitic grain boundaries can be easily revealed by etching with a suitable etchant. In the etched structure, light martensite and dark pearlite are clearly differentiated. Etching contrast method is generally employed for fully hardened, or hardened and slightly tempered steels consisting of martensite or tempered martensite, respectively.
This etchant develops an excellent contrast between differently oriented martensitic grains, thus enabling establishment of prior austenitic grain boundaries. With the knowledge of grain size, it is possible to predict the response of steel to heat treatment and behaviour during working or under various stress conditions to which it may be subjected during service. We now discuss the effect of grain size on various properties of steel.
Therefore, the finer the grain size, the higher will be the yield stress. In fact, grain refinement is the only commercially available conventional technique which improves strength and ductility at the same time.
The relationship between yield stress and grain size has been used extensively in the development of high strength low alloy HSLA steels. Grain size has a marked influence on the impact transition temperature. An increase in grain size raises the impact transition temperature, and thus makes the steel more prone to failure by brittle fracture.
The effect of grain size on the impact transition temperature is more pronounced in the case of low carbon steels. It is not possible to derive a mathematical relationship between creep strength and grain size since change in grain size for creep study, i. However, in general, a coarse grained steel has better creep strength above the equicohesive temperature. Below this temperature, fine grained steels exhibit superior creep strength. It has been observed that cast steels have improved creep strength over forged steels.
The basic coarse grain size of cast steel is believed to be responsible for this. Similar reasoning is given for better creep strength of silicon deoxidized steels as compared to aluminium deoxidized steels.
Fatigue strength, similar to creep strength, does not exhibit any basic relationship with grain size. However, fine grained steels have higher fatigue strength as compared to coarse grained steels. This is true provided the temperature of the test piece is not so high that creep is a predominant phenomenon.
Coarse grained steel has better hardenability than fine grained steel. The reason for this is that coarse grained steel has fewer grain boundaries. Grain boundaries are the region where the rate of diffusion is high. Consequently, formation of pearlite, which is a diffusion controlled process, starts at grain boundaries. With smaller grain boundary area in coarse grained steels, preferential formation of martensite from austenite takes place on cooling.
It will further be relevant to note that pearlite formation see Section 4. Austenitic grain boundaries act as nucleation sites for pearlite formation. Coarse grained steels have better machinability than fine grained steels. Coarse grained steel has reduced toughness, which is understood to be the source of providing small discontinuous chips during machining.
Austenite is a solid solution of carbon in gamma FCC iron. In the case of alloy steels, austenite may also have other elements dissolved in it in addition to carbon. As mentioned in Section 4. Below the A1 temperature, the mixture of ferrite and cementite is thermodynamically stable. On cooling a eutectoid steel below eutectoid temperature, austenite will decompose to an aggregate of ferrite and cementite. The transformation of austenite on cooling is a complex process in' the sense that compositional as well as configurational changes are involved.
Carbon present in the austenite adjusts itself in such a way that at one end it leaves behind an almost carbon-free phase ferrite and on the other it combines with iron to form cementite.
Also, the crystal structure of gamma iron will change to that of alpha iron. The process of austenitic decomposition becomes more and more complicated for hypoeutectoid and hypereutectoid steel as proeutectoid ferrite and proeutectoid cementite separate out in these two types of steel, respectively, before eutectoid decomposition of austenite. The process of decomposition of austenite to a ferrite-cementite aggregate is essentially a diffusion controlled process and proceeds by nucleation and growth mechanism.
Some of these factors are: The more the homogeneity of austenite, the better is the probability of getting a lamellar structure. A heterogeneous austenite may result in the transformation of austenite to spheroidized structure. With lowering of the transformation temperature, a harder and finer aggregate of ferrite and cementite results.
Diffusion is a time and temperature controlled process. The rate of diffusion decreases with decreasing transformation temperature.
At considerably low transformation temperature, the rate of diffusion decreases to such a level that austenite transforms by a diffusionless process. Cooling rate after the completion of austenitic transformation is of no metallurgical significance. Rapid cooling after completion of transformation will simply decrease the overall time. It can be concluded on the basis of the foregoing discussion that supercooled austenite transforms either to a mixture of ferrite and cementite or to a product formed by diffusionless process.
This product is known as martensite. Martensite possesses the same chemical composition as that of parent austenite but has a different crystal structure.
Martensite is a hard and brittle phase with entirely different properties than the other constituents commonly found in steels. It has been discussed in detail in Section 4. Ferrite- cementite mixture produced by the decomposition of austenite by diffusion process can be divided into two broad classes: