Bill: wise not to attempt a diagnosis from a small low resolution image, but it's probably the ribband edge weld.

It is less likely to be a decarburization line



BTW: this would be an appropriate application of magnetic particle inspection if one wanted to exclude a crack
Scroll down about 1/2 way here
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit