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This project work was carried out on steel of near eutectoid composition and samples were austenitized at 820°C for 30 minutes, quenched in a salt bath and isothermally cooled at constant temperature of 500°C for 2.5 hours in salt bath for transformation of austenite to fine pearlite, followed by air-cooling. Eutectic mixture of calcium chloride (50%), barium chloride (30%) and sodium chloride (20%) was used as quenching medium. Thereafter, different samples were subjected to different forging treatment such as forging at room temperature, 200°C, 400°C, 600°C and 950°C to give a deformation of 50% and 80%. Thorough examination of microstructure using various characterization techniques like optical microscopy, high resolution transmission electron microscopy, electron dispersive X-ray spectroscopy and microhardness testing was also carried out. Some interesting results were obtained which are as follows:
1. After deformation, pearlite structure was degenerated, cementite particles were broken from the pearlite colonies and carbon atoms concurrently entered into the matrix of ferrite.
2. It has been reported that if patented steel is heavily cold worked, interfacial dislocation density increases. There is also accumulation of extra dislocations towards ferrite side which is adjacent to high carbon containing region of cementite (Fe3C) lamella. When interaction energy between dislocations and carbon atoms exceeds the bond energy of Fe3C, Fe3C lattice disintegrates and carbon atoms migrate to ferrite resulting in high carbon containing ferrite. Thus, Fe3C under high strain conditions, behave as ductile phase.
3. At high temperature, Fe3C phase is unstable and it should go in solution in the austenite phase. But all the Fe3C does not go to the austenite and some of it is present in the matrix in the form of globular particles, known as fragmented Fe3C. Rest of the Fe3C goes to austenite and due to rapid cooling done after deformation, it results in the formation of high carbon ferrite. This supersaturated ferrite tends to reject the carbon which forms new carbide, i.e., ɛ-carbide (Fe2.4C) having hexagonal structure. Thus, the globular particles obtained after forging may be of fragmented Fe3C or ɛ-carbide.
4. The presence of some free carbon particles in the ferrite is observed after high temperature deformation (at 950°C), that means cementite (Fe3C) lattice collapses and breaks into Fe and C resulting in the presence of carbon particles in the matrix. In view of high deformation energy and temperature of deformation, it is more likely that collapse of cementite lattice has taken place rather than amorphisation of cementite which is ,till date , reported to have taken place by continuous deformation at low temperature.
5. Carbon concentration in ferrite is much higher after 80 % deformation than that observed after 50% deformation.
6. After forging at 200°C and 400°C, particles of ɛ-carbide having hexagonal structure were found to be distributed in the matrix of ferrite.
7. After forging at room temperature and at 600°C, presence of graphite particles was observed in the matrix of ferrite.
8. After forging at 950°C, particles of iron carbide (Fe3C) having orthorhombic structure were present in the matrix of ferrite.
9. The interlameller spacing in case of forged samples is finer than those of without forging resulting in an increase in strength and hardness.
10. In case of hot forging, the interlameller spacing in samples subjected to 80% deformation is higher than those deformed to 50% because due to higher strain rate of deformation, adiabatic heating occurs within the sample, thus post forging transformation takes place at higher temperature resulting in coarser interlamellar spacing.
11. For a given deformation of 50%, interlamellar spacing gradually decreased from 95.08 nm (forging at room temperature) to 78.87 nm (forging at 400°C), while in case of forging at 600°C, a sudden increase in interlamellar spacing to 228.27 nm was observed.
12. Hardness of ferrite phase increases with forging temperature during low temperature deformation due to increase in the amount of carbide present in the matrix of ferrite. It also increases with an increase in the amount of deformation from 50% to 80 % deformation because of increased diffusion of carbon into the ferrite matrix.
13. Hardness of pearlite phase decreases with forging temperature while it increases with an increase in the amount of deformation.
14. Overall, hardness obtained in case of low temperature deformation is higher than that obtained during high temperature deformation.
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