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Hypothesis Regarding Formation of Carbides in Upper and Lower Bainites .

  1. Oct 8, 2011 #1
    I would like your expert opinion regarding my hypothesis on the Mechanism for formation of Carbides in Upper and lower Bainite. My Hypothesis is as follow:

    Upper Banite:
    • The carbide particles of upper bainite tend to be elongated and form between ferrite laths, areas to be which carbon has been rejected during the growth of the ferrite
    The above may be due the diffusion of carbon atoms with higher energy and removal of the shear strain of the lattice. Carbon atoms diffuse towards the edges and form Carbides on the lath boundary regions. As the temperature increases the Lath structure of Martensite tends to convert to pearlite , this mechanism allows the diffusion mechanism used in Pearlite to be active and thus carbon atoms diffuse out in the grain boundaries.

    Lower Banite:
    • fine carbides form within the plates rather than between plates and are significantly finer than the interlath carbides of upper bainite
    The above may be due to the fact that in lower bainite, the structure is highly tetroganal in nature and the energy required for carbon atoms to diffuse from its position towards the end of the grain is unavailable. Thus, the carbide forming particle diffuses towards the center of the plate and form Carbide which are formed on the mid-rib plane. The midrib is a plane that may face attraction from all ends and thus may have more space for diffusion and the subsequent energy for reaction.

    Other Points to Consider ;
    1. Higher the Temperature : higher the diffusion , lower tetragonality ,
    2. At Higher Temperature Mechanisms of diffusion like Pearlite structure is active at Lower Temperatures Mechanisms used in Martensite formation are applicable.

    This may be a vague theory but your opinion rebutting it would be highly helpful .
  2. jcsd
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