Also, I am sure it was on ao3 and that it was a Tony/Bucky pairing,if this helps
From what I can remember from the story was that Tony had been given the dog tags from Bucky when he fell from the train when everyone thought he was 'dead'. Another thing i can remember is that Tony had the Arc Reactor from Afghanistan but had it removed before the first Avengers and that Bucky's dog tags that Tony was wearing had stopped Loki from controlling him.
It really would be helpfully with your help to find this story 😁. Also, if you wish to share some of the stories you read dont hesitate to drop them in you comment, I would love to know what other people's likes are! 😁