The exhaust housing is cast steel not cast iron. It'll weld fine without pre and post heat but if you have the facilities preheat is the go.
Heat it to 200 deg, weld it and then wrap it in heap of old welding gauntlets (you should have plenty being in the trade) and let it cool down at its own rate.
Ive done heaps with and without preheat and haven't had one crack yet. The temps the turbine housing gets to in operation is good for normalising and stress relieving the weld once it's in service