What could be causing this microstructural difference?

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The discussion centers on two A706 grade rebar samples with identical chemical compositions but differing pearlite sizes observed under microscopy. Possible explanations for the differences include variations in cooling rates, with faster cooling leading to smaller crystals, and potential differences in carbon content. The dimensions of the bars, 19 mm and 13 mm, may also influence microstructure, as the smaller bar could develop finer structures due to longer working time and deformation. Additionally, grain alignment from the stretching process may affect the observed microstructure. Overall, the significant increase in pearlite size in the 19 mm bar suggests multiple factors at play beyond just cooling rates.
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Hello,

I have two rebar samples of virtually identical chemical composition of 0.27 wt%C, 1.2 wt%Mn and 0.033 wt%V that are A706 grade. The rolling mill parameters are essentially the same yet I am seeing differences in pearlite size upon microscopy. The billets used to make the bars are reheated in a furnace before they hit the mill. What are some possible explanations of why this is occurring?

Attached are the two micrographs taken at the same magnification.
5318 A core 20x.png
4534 core 20x.png
 

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That was my first thought. The images show that the pearlite colonies are significantly larger in the first image, so I am lead to believe there is a difference in both cooling rate and carbon composition. Also, I must add that the micrographs were taken at the core of the bar.
 
Quentin_alex said:
the micrographs were taken at the core of the bar.
Different bar dimensions?
 
The first and second image are of 19 and 13 mm bars, respectively. The bars were not subject to a quenching process, but rather mostly air cooled. Taking the dimensions of the bar into account we might see a slightly finer structure in the center of the 13 mm bar. I think based on the images this is something we can suggest. However, my concern is the dramatic increase in pearlite of the 19 mm bar. I wonder if something else could be causing this.
 
Quentin_alex said:
The first and second image are of 19 and 13 mm bars, respectively.
Pearlite forms as the billet is worked and as the bar cools. The smaller bar needs to be worked for longer, so it has more deformation and time to develop pearlite before the microstructure reactions were frozen.

It is also possible that grain alignment due to stretching is affecting the view. It looks like both sections were cut from the same section orientation across the core of the bar. The section of each grain will be reduced as the crystals are stretched more to make the smaller bar.
 
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