Heating a brass blade with soldering iron element
Your calculations are correct, but only if the blade is perfectly insulated thermally. Your biggest heat loss will be convection, and without knowing the blade dimensions there is no way to estimate that. With a large enough blade you won't be able ever to reach 300 C. 

Your "best" bet would be to use a much larger temperature-controlled iron, but this will obviously cost you. If you use a non-regulated iron your final temperature will depend on your blade size and how easily air will flow over it. 

From what you've shown I'm inclined to accept your intuition. 40 watts sounds pretty wimpy.