The ending line is solid, "Fire could only temper steel. It could never defeat it," and it's a theme that runs throughout the story (e.g. Ed can't snap flames with his right - metal - hand). However, the rest of the fic could use some revision. There's little passion or liveliness in the characters (liveliness in the sense of being 3D, living, breathing people rather than liveliness in the sense of vivacity, which is obviously not what's needed here) and the dialogue falls flat.
Suggestion: Rework the dialogue. Ed and Roy speak in full sentences and whole thoughts, which tends to sound like they're reading from a script rather than interacting with each other. If you try listening to people holding a conversation in real life, you'll find that they interrupt each other, they engage in repartee and give and take, and don't speak for more than a sentence at a time unless they're pontificating. Ed has a short temper, Roy is used to being in command, and neither is particularly dispassionate, and so their conversational style should reflect that. Think about how Roy would react to something Ed would say and try to put some of that into the style of the dialogue - short, long, terse, wordy.
The phrasing also tends to be too verbose: "You honestly didn't think I was gullible enough to belive that you were some sort of hotblooded young man with a crush on a commanding officer?"
Go for brevity and try to make your writing concise. That way, it'll pack a bigger punch, especially in a story like this that focuses on using people and intense, deeply personal issues. Try to stay away from cliches such as "felt rather like a man watching his wife kiss a stranger."