In welding P91 materials, at what state that it requires solution annealing?

[Pipoy04]

How solution annealing affect the microstructure of the P91 materials?

 

[metengr]

Solution annealing is really not applicable to creep strength enhanced ferritic steels, like Grade 91. Instead, you perform a normalization heat treatment to provide a 100% martensitic structure for subsequent tempering. The effect of normalization heat treatment is to ensure austenite formation at elevated temperature that can transform on cooling to martensite.