Here is a typical schedule for forging high-Cr alloy steels (courtesy of Cartech):
Large sections should be preheated to 1200/1400°F (649/760° C) or heated slowly in the furnace. Initial forging should be conducted in the 1900/2150°F (1038/1177°C) range. Billets should not be finish forged below 1600/1650°F (871/899°C). Avoid long heating cycles as this can result in excessive decarburization. Forgings should be cooled slowly and annealed or tempered as soon as possible after reaching room temperature. Do not harden without performing an intermediate anneal or preheating in the temperature range 1200/1400°F (649/760°C).
WARNING – This alloy is not typical of high Cr alloy materials. Grade 91 is a creep strength enhanced ferritic steel, and as such, must be controlled during forging and subsequent heat treatment operations to achieve the desired microstructure for creep strength.
As far as forging temperature, the information above is an acceptable temperature range for hot working. Forgings can be left in the hot worked condition BUT must be heat treated separately to achieve the desired martensitic microstructure using sufficient cooling - quenching and tempering.
