Bill,
The purpose of the PWHT of the martensitic weldments is to temper or anneal the weld metal and HAZ to decrease the hardness, improve toughness, and to relieve any residual stresses incurred during the welding process. In the particular case of the low carbon martensitic materials NACE requires a maximum hardness of 23 Rockwell C for the F-6NM and 22 Rockwell C for the CA-6NM. These NACE requirements are to avoid cracking while in service. So, Bill, without the PWHT process the hardness in the weldment would be quite high with a greater possibility of cracking while in service.
Chuck