The title of the movie Out of the Furnace first and foremost seems to be an allusion to the whole steel mill backdrop of the movie. But I have a strong feeling that is not all there is to it. The IMDb trivia section of the movie says
The title of the film went through a couple of iterations before settling on "Out of the Furnace". The original script was called "The Low Dweller" but when Scott Cooper was rewriting it, he considered changing the name to "Under a Black Sun". While shooting the working title of the film was "Dust to Dust". It was Terrence Malick and Sam Shepard who finally convinced Cooper that "Out of the Furnace" was an apt title for the film.
But this doesn't say much about its further significance. Is there anything more to the term "out of the furnace" in relation to the movie's story and themes apart from the obvious steel mill scenario?