Why is this happening?

I've been using an old sketchy 5$ soldering for about 7 years already, and I finally decided it's time for an upgrade. I've picked this one. It should have 120-480 degrees Celsius controllable temperature. I found that 430 is perfect for my needs. But just under a month of use, it turned from shiny silver completely black. Is that a normal behaviour, and is there a way to prevent it? Never had such a controllable soldering iron, just the one which always heats at max. Maybe it's because of temperature. If it is what are the best temperatures to solder on?

I've been using an old sketchy 5$ soldering for about 7 years already, and I finally decided it's time for an upgrade. I've picked this one. It should have 120-480 degrees Celsius controllable temperature. I found that 430 is perfect for my needs. But just under a month of use, it turned from shiny silver completely black. Is that a normal behaviour, and is there a way to prevent it? Never had such a controllable soldering iron, just the one which always heats at max. Maybe it's because of temperature. If it is what are the best temperatures to solder on?