Hypothesis Regarding Formation of Carbides in Upper and Lower Bainites .

[ovais.abowath]

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 .

 

[Navin A S]

Your hypothesis is an interesting one and the points you have raised are certainly worth considering. It is important to note that the mechanism for formation of carbides in upper and lower bainite is not fully understood. Therefore, it is difficult to definitively answer your question. However, there are several points to consider when discussing the formation of carbides in upper and lower bainite. First, it is important to consider the structure and composition of the material in which the carbides are forming. The structure of lower bainite is typically more tetragonal than that of upper bainite, meaning that the grain boundaries and lattice strain may be different, potentially leading to different carbide formation mechanisms. Additionally, the composition of the material can influence carbide formation, as certain elements can cause preferential carbon diffusion or act as nucleation sites for carbide particles. Second, it is also important to consider the temperature at which the carbides form. At higher temperatures, the diffusion rate of carbon atoms increases, leading to a higher likelihood of carbide formation. Also, at higher temperatures, the tetragonality of the material decreases, likely leading to increased carbon diffusion and thus increased carbide formation. Finally, it is important to consider the kinetics of the carbide formation process. The rate of diffusion of carbon atoms, the rate of nucleation of carbide particles, and the rate of growth of the particles all affect the overall process of carbide formation. Understanding the kinetics of this process is essential to understanding the mechanism of carbide formation in upper and lower bainite. Overall, your hypothesis is an interesting one and it certainly raises some interesting points that should be considered when discussing the formation of carbides in upper and lower bainite. However, further research is needed to fully understand the mechanism of carbide formation in these materials.