First of all that defect has created a weak spot..if the spring is going to break at some point in time if ever...that will probably be where it does.
Without looking at it in person...to me it looks like an imperfection caused by slag...when the steel was poured.
please see above posts that explain what you inaccurately identified as a defect and then as an imperfection
As a FYI, since we all make mistakes (myself more than most), the website does allow you to edit your post so that a person doesn't have to prove Mark Twain correct by "removing all doubt"