It looks like centre-line segregation that can effect the etching behavior of the steel. Higher Mn steels are prone to segregation, more of a problem in plates rather than rods, so it may be a cooling rate effect, with the core not fully hardened.
Thank you so much for your suggestion Dr. Kristin R. Carpenter. I'd still appreciate if you could explain how or why such a phenomena of not fully hardened core happens.
Can you post a higher magnification image of the center region? That will help providing the right answer. Thanks.
I have posted (in the attached file) micrographs of the centeral weld sone at different magnifications. Thanks.
Thanks for the pics, It does look like the rod was not fully hardened at the centre. which may be a cooling rate effect. The center is hottest , so the cooling rate will be slower, in this case, slow enough for ferrite to form. The steel is a boron grade, if the boron was lost during steelmaking, then hardenability would decrease.
Thanks a lot for such an elaborate explanation and extremely great help. With your help, I can now safely proceed to the next phase of the challenge, namely solving the scenario.
I think that one can either increase the contact surface area of the adjoining ends (so as to minimize the current intensity, hence avoid high temperature), or adjust the welding parameters accordingly.
I wonder if I could bother you one more time: given that the welds are air-cooled (see please the video-clip), why is the cooling rate becoming slower and what could possibly be done to alleviate the problem?
Thanks for the video. The microstructure is from the parent materials processing, not the welding. The band of ferrite curves up toward the surface due to the upset butt welding process.
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