In this part, we discuss the formation of microstructures and their evolution during heat treatment, aspects of practical heat treatment (such as furnaces, fixtures, atmosphere and quenching media), and some important heat treatment processes. The discussion is cursory. The interested reader should refer to Boyer and Dossett for details on the practical aspects and Porter et al for the phase transformations aspects.
Hardness and hardenability:
Hardness and its measurement– Heat treatments are carried out to change the properties of materials by changing the microstructure of materials. The primary aim of majority of heat treatments is to change the mechanical properties of the given material; primarily, they are used either to harden (precipitation hardening, quenching, carburizing, nitriding, etc) or soften (tempering, annealing, stress relieving, etc). Hence, in majority of cases, the success or failure of heat treatment is decided by the mechanical property measurements; more specifically, hardness is the typical quantity that is measured. Hence, in this module, we discuss some of the standard hardness measurements in a brief manner.
Hardness is the resistance of a material to plastic deformation. It also correlates with the other mechanical properties such as strength (direct) and ductility (inverse). Further, hardness tests are easy to perform, and, if needed, can be performed without having to discard the sample after testing. This is the reason why hardness tests are usually employed after heat treatment processes.
Hardness can be measured using indentors such as Brinell, Rockwell and Vickers; in each of these cases, by measuring the geometric parameters of the indent (area, depth of penetration, mean diameter,etc). It is also possible to apply very low loads and carry out indentation experiments; such tests are known as microhardness tests.
Hardenability is the ability for a material to harden; it refers not to the highest value of hardness that can be obtained but to the capacity (depth or thickness over which such high hardness values can be achieved) to harden. Thus, hardenability is intimately related to the cooling rate that can be achieved (especially in steels). Typically, hardenability is tested using hardness penetration diagram test (in which, the hardness of a hardened sample is plotted as a function of depth from the surface), and Jominy end quench test (in which one end of a sample is quenched and the hardness at equal intervals from the quenched end is measured and plotted).
Effect of alloying elements on hardenability- In steels, alloying elements are added to increase hardenability; this is achieved by delaying the time required to transform austenite into ferrite and pearlite (and thus shifting the nose of the C curve in the TTT diagram to longer times), which helps in the formation of martensites even with slower cooling rates. The alloying elements delay the decomposition of austenite into ferrite and parlite by reducing either the nucleation or the growth. The alloying elements in steel are distinguished into two types: namely, austenite stabilizers (Mn, Ni, Cu) and ferrite stabilizers (Cr, Mo, Si). Even though both these additions help hardenability, the mechanisms involved are different.
In addition to heat treatments meant for hardening or softening a given material, there are also certain heat treatments such as homogenising and austenitizing, for example, which are used as the preliminary heat treatments for secondary heat treatments; and these secondary heat treatments are the ones which are used to achieve the desired microstructure.
Formation and evolution of microstructures
Phase diagrams and microstructures– Let us consider the Ph-Sn phase diagram shown in Figure 1. There are three compositions that are considered. These three compositions correspond to eutectic, hypo-eutectic and hyper-eutectic systems. In the case of eutectic system, the microstructure is as shown in Figure 2: it consists of a fine mixture of the two phases, namely, and , the tin-rich and tin-poor phases. In the case of hypo- and hyper-eutetic systems, on the other hand, the microstructures are as shown Figure 3. These type of microstructures arise, as shown schemicatally in Figure 4, due to the formation of the pro-eutectic (or )phase first making the composition of the remaining liquid shift towards the eutectic composition; the liquid that remains on achieving the eutectic point in terms of composition and temperature, then, phase separates into the eutectic mixture. Thus, from this exercise, it is very clear that the phase diagrams give us some indication of the microstructures that can result. However, phase diagrams assume equilibrium and hence gives an idea of microstructure that can form due to very slow cooling (small values of cooling rate). If we want to understand the formation of microstructures, taking into acocunt the cooling rates, we need to peruse TTT or CCT diagrams.
TTT diagrams and heat treatment:-
As seen in Part III, the overall transformation kinetics (in systems that undergo nucleation and growth) is represented using the TTT diagram. It is the TTT diagrams that are helpful in planning heat treatments.
For example, consider a typical TTT phase diagram shown in Figure 5. If this system is cooled at a rate smaller than , then, the second phase will form; however, if the quench is fast enough that the rate is greater than that given by , then, one can suppress the second phase formation completely.
Also, as one can see from the TTT diagram of eutectoid steel (Figure 6), it can contain phases which are not there in the phase diagram (martensite, for example) as well as give more information on the microstructure that is formed: bainite, pearlite, etc.
CCT and TTT diagrams and the rule of Scheil
Even though TTT diagrams are very useful for studying microstructures, in most of the cases of practical interest, the transformations take place not isothermally but during continuous cooling. Hence, the so-called Continuous Cooling Transformation diagrams (CCT diagrams are very important).
It is possible to go from TTT diagrams to CCT diagrams under certain circumstances, namely, if we assume that the kinetics of the transformation depends only one the fraction of the phases transformed and the temperature. In Figure 7 we show how to calculate the CCT diagram from the isothermal transformation curve, schematically; the steps involved are as follows:
- Divide the temperature range into steps of size ;
- Calculate the time spent in each temperature range; this calculation is carried out by dividing the time step by the time of the isothermal transformation which corresponds to the average temperature corresponding to the given ;
- The cumulative phase fraction then is given by
- The transformation begins when . Eq. 1 is known as the additive rule of Scheil.
Quenching is the process of rapdily cooling a material from high temperature. As noted in the earlier module, this rapid cooling is achieved using quenching media.
The thickness of the material to be quenched along with the rate of cooling required helps to choose the quenching medium. The quenching medium has to be chosen carefully. If a quenching medium that cools slower than the required rate is chosen, the quench is not effective in producing the required microstructures and hence properties. On the other hand, if a quenching medium that cools faster than the required rate is used, then that can sometimes lead to defects such as warping and cracking.
There are many different types of quenching: quenching in a fine vapour or mist is known as fog quenching; if quenching is carried out directly from some other heat treatment operation (carburizing for example), it is known as direct quenching; if only some portions of a workpiece is quenched, it is known as selective quenching; and so on.
As noted earlier, quenching is also a method used to determine hardenability of materials.
Probably, the most common example of quenching is what is used in steels; an alloy quneched past the nose of the C-curve in the isothermal tranaformation diagram will undergo martensitic transformation, which, as noted will lead to high hardenss in the material.
It is also possible to rapidly quench molten metallic liquids to retain the liquid-like structure; such materials are commonly known as metallic glasses.
Quenching is generally carried out to freeze the high temperature structure or phase in the material; however, it is not always possible for the structure to be retained. As noted earlier, during quenching there could be mechanisms such as vacancies diffusing to the grain boundaries leading to PFZs (Precipitate Free Zones) that become operative.
standard heat treatments:-
Consider the TTT diagram as shown and the cooling curves imposed on the TTT diagram shown in Figure 9. The curve 1 in Figure 9 , where the cooling is very slow is called annealing; annealing, in this case, results in coarse pearlite, as indicated.
Annealing is, in general, an operation that results in softening of the given material; such softening might be a result of annihilation of defects, recrystallisation (if the material is sufficiently cold worked), and growth of grains (and, in general, coarser microstructural features). When annealing process removes the stresses in a cold worked material, such a heat treatment process is known as stress relieving operation.
Normalising is a type of annealing with relatively faster cooling rates; normalising, in steels, as shown, leads to fine pearlite.
onsider a schematic phase diagram of an age hardenable alloy as shown in Figure 10 . The heat treatment process of cooling of the system from the single phase region to the two phase region as shown and holding at the two phase region for long times is known as age hardening; this is because, with increasing times, the hardness of the system changes as shown in Figure 11. As we have seen in Part IV, such hardening behaviour is due to precipitation; the decrease in strength after peak aging is a result of coarsening of precipitates (to be discussed in the next part, namely, Part VII).
Tempering is a heat treatment process in which the hardenss of a hardened alloy is reduced by the appropriate heat treatment process; for example, a steel hardened by the formation of martensite formation can be tempered using the heat treatment process shown in Figure.
Similarly, a steel sample can be qunched past the nose of the C curve and held just above the martensitic start temperature till the entire sample attains an uniform temperature and then air cooling to achieve tempered martensites. This process is known as martempering.
Heat treatments for mass diffusion
There are several heat treatments which are carried out to achieve controlled mass diffusion. Here we list a few cases.
There are several processes in which by diffusing carbon or nitrogen into steels (along with appropriate heat treatments, if need be) can be used to obtain hardened surfaces. These processes are carburizing, nitriding, carbonitriding, cyaniding, and so on.
The opposite process of removal of carbon atoms from the surface of a steel sample is known as decarburizing.
There are processes similar to carburizing which is used in the semi-conductor industry to carry out doping. Similarly, an effect similar to that of decarburization takes place in alclad (duralumin with pure aluminium layer on top) due to the diffusion of copper from duralumin to pure aluminium layer.
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