Hi,
Have you worked out how much power you are going to need, 550C is pretty high to keep something at.
What is the volume of the area and the mass/volume of object your want to heat stress relieve?
By the way 550C at the radiation point is not 550C at the work you are trying to heat.
Do you have the entire heating area in an enclosure, kiln, to keep the heat energy in as you increase input power.
Your nichrome wire will need to be on a ceramic former so it keeps its shape and does not short out on adjacent turns or the heatsink.
This is a good place to use PID, but interfacing heavy power control to Arduino will be the main concern.
Can ypou post a diagram of the layout of the heater boxes and the object being
Tom.... 