My suggestion is to visit the comprehensive web site below by Special Metals for excellent technical background on Incoloy materials. They can answer all of your questions.
Be aware that if you use Inc.800 types and the sum of Ti+Al is more than 0,7%, PWHT is nescessary when the operation temperature is between 500 - 750 C.
Reason : prevention stress induced cracking during operation. A crack type that occurs within 2 years of operation. Recommended : PWHT 3 hours 875 C