You've either left it switched on unattended or set the temperature too high. Now the tip is coated in
black iron oxide which you need to carefully remove without puncturing the thin iron layer that protects
the copper bit.
The new tip you used needed to be carefully wetted with solder on first heating, which takes a bit of determination usually, and careful and repeated cleaning/wiping of the tip.
Never run an iron too hot, it rapidly oxidizes the tip like this, and oxide is hard to remove. In normal
use flux will prevent the build up of oxide, but leaving an iron for long periods or running too hot will
char the flux and stop it working.
I am worried by your description of "fry the flux" - if the flux is getting discoloured the iron is definitely
too hot. Rosin flux should melt and smoke a little, but not blacken.
If you need to leave an iron for a few minutes, always coat the bit with fresh flux (or flux-cored solder)
Sorry, fry is not the right word. The flux rapidly melts when touched by the iron and sometimes sorta boils off. As you say it smokes a little and boils offf. I normally dip both items to be soldered in flux before soldering. When I solder two wires together I normally tin both wires and then join and heat with iron.
Its a Weller 100 soldering station that I have been using for about two years. It has always workd like a champ. I never leave it on more than the time I am actually soldering. When done I turn it off. It can be set to a max setting of 5, I have been using it at about 4 and it worked fine.
Yesterday I tried my first solder of a protoboard. Not a very happy experience. I just wanted to connect three wires to an led. Getting the components soldered to the board wasn't too difficult. I watched several videos and they had some good ideas that seemed to work pretty well. Soldering a common wire to all three was a different story, that's when I noticed the problem with the iron. When I hold the iron to the board with a wire already soldered in place and try to apply the solder, it just doesn't want to melt the solder. If I take the iron off the board and place the solder on the tip, nothing happens, it doesn't melt like it used to. I am using the same solder and flux I have been using for two years with no problems.
I am almost at the point of buying another iron. I have no way to measure the tip temp, so have no way to actually trouble shoot the iron. But since the station has a place for a normal plug, I can probably just buy a good iron and plug it in.