Explain objectives of annealing? Discuss the various annealing processes? Is spheroidising different from annealing? Illustrate.
A slow cooling rate from eutectoid temperature yields course pearlite - a mixture of comparatively coarse crystals of ferrite and cementite. Such a slow cooling is known as annealing. Here, sufficient time is available for carbon in the austenite to diffuse and redistribute itself to 0.02% in ferrite and 6.67% in cementite. During annealing, effective transformation temperature is low, where rate of growth is rapid compared to rate of nucleation. Hence, spheroidising is obviously different from annealing.