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 products from austenite. Depending on various parameter, transformed products from austenite may be pearlite, bainite or martensite. Not only presence of these phases micro constituents but also the morphology of these products is of significance in deciding the resultant properties.
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.
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Formation of austenite on heating:
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 steel in 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.
Kinetics of formation of austenite:
In practice, it is not possible to heat a steel with equilibrium rate of heating. 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 in the study of this kinetics of formation of austenite is to heat a number of steel samples to different temperature above the eutectoid temperature. Heating is done by immersing in a 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 transformed austenite which in turn will depend on the temperature at which steel sample has been heated and the holding time at that temperature. The percentage of transformed austenite with time for a given temperature can be known. The same sequence of operation is employed for different temperatures.
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Effect of alloying elements on transformation:
Transformation of austenite to pearlite to alloy steel is more complicated than in carbon steel. Almost all alloying elements, whether ferrite stabilizers or austenite stabilizers, except cobalt, decrease both the rate of nucleation and the rate of growth. As compared to carbon, alloying elements diffuse very slowly.
Carbon and alloying elements partition to ferrite and carbide phases on decomposition on austenite. The partitioning approaches equilibrium value at higher transformation temperatures, say in the vicinity of eutectoid temperature. As transformation temperature decreases, the amount of carbon and alloying element in ferrite and carbide phases deviate from equilibrium partitioning. Stable carbides, in general, process higher metal to carbon ratio. As the diffusion rate for metallic atom is much slower than carbon atom, the formation of stable carbide during pearlite transformation will be feasible only at higher transformation temperature.
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Pearlite is a eutectoid mixture of two phases, namely ferrite and cementite. It consists of alternate layers of ferrite and cementite. It is an important constituent of steel and is present in almost all the steels which have been slowly cooled. The mechanical properties of annealed or normalized hypoeutectoid steel mainly depend on the constituent. Pearlite is formed by cooling austenite eutectoid composition just below the lower critical temperature.
Another aspect to be considered in this context is: which one two phases (ferrite and cementite) nucleates first? The nucleus which is formed first referred to as active nucleus for pearlitic transformation, provided it is present in the transformed product and has lattice orientation relationship with parent austenite. It has not been possible till that to specify with certainty as to which one of the two phases nucleate first. Also there is no theory which can explain all the characteristics associated with pearlite transformation such as lamellar nature. Temperature dependence of interlamellar spacing and kinetics of transformation and finally, the effect of alloying elements on interlamellar spacing and on kinetics of transformation. However, it is very established fact that active nuclei for austenite to bainite transformation are ferrite crystals. Active nuclei for pearlitic transformation are generally taken to be cementite platelets. This assumption can offer an explanation for the lamellar nature of pearlite. An orientation relationship between cementite and austenite has also been fount which confirms that cementite platelets are active nuclei for pearlitic transformation.
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At relatively larger super saturations (austenite cooled below the nose of the pearlite transformation), there is another eutectoid product that develops known as bainite; bainite is also a mixture of ferrite and cementite; however, it is micro structurally quite distinct. In the next two subsections, we discuss these Microstructural features.
Upper bainite: At the higher end of the temperatures (350-550°C), the microstructure consists of needles of laths of ferrite nucleated at the grain boundary and grown into one of the grains with cementite precipitates between the ferrite. The ferrite formed is Widmanstatten; it has a Kurdjumov-Sachs orientation relationship with the austenite grain into which it is growing; it is in this respect, namely the orientation relationship between the ferrite/cementite and the austenite grain in which they grow, that the bainite differs from pearlite.
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Lower bainite: At low enough temperatures, the bainitic microstructure changes to that of plates of ferrite and very finely dispersed carbides; since the diffusion of carbon is very low at these temperatures (especially in the austenite phase as compared to ferrite), the carbides precipitate in ferrite (and, with an orientation relationship). These carbides that precipitate could be the equilibrium cementite or metastable carbides (such as ε carbide for example).
The transformation temperature for the austenite to pearlite and to bainite is such that diffusion of carbon is quite intense. The diffusion rate of carbon decreases with the lowering of temperature, and at about 200˚C, the diffusion rate of carbon becomes negligible. When under cooled below the temperature 200˚C, transforms to a product known as martensite.
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Martensite is formed by quenching of austenite. On quenching, the rate of diffusion is lowered to such an extent that no phase transformation dependent on diffusion can take place. Hence basically, martensitic transformation is a diffusionless transformation. It is displaceable transformation that takes place by cooperative movement of a large number of neighboring atoms. Each atom moves over a distance which is less than one interatomic distance. In this process, the atoms maintain their neighborhood undisturbed. A large driving force required for the reaction to take place. The driving force is provided by free energy change accompanying the transformation. This magnitude of the driving force increases with decrease in the temperature of transformation. This is achieved by quenching operation. Although the displacement of individual atoms is less than one interatomic distance, the total displacement increases as one moves away from the inter phase boundary. Such build-up of displacements finally results in macroscopic slip. The slip can be observed as relief structure on the surface of martensite.
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Conclusion: In this article, you read an introduction about principle of heat treatment process. Thank you for reading this article hope it may help you to understand about basics about heat treatment. And visit for more article related to bio materials, electronic materials, advance materials, magnetic materials and metallurgical based.
(References: Heat Treatment: Principles and Techniques by C P Sharma, Physical Metallurgy: Principles and Practice by V Raghawan)
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