The tensile and yield strength of ASTM A335 P91 steel changes at different/ elevated working temperatures. According to ASME BPVC Section II Part D, their relationships are listed in below tables.

1. Tensile Strength – Temperature Ratings of ASTM A335 P91

Temperature, °F	-20~100	200	300	400	500	600	650
Tensile Strength, ksi	85.0	85.0	85.0	84.7	84.4	83.1	81.8
Temperature, °F	700	750	800	850	900	950	1000
Tensile Strength, ksi	80.0	77.6	74.7	71.1	66.9	62.2	57.0

*The temperature refers to “metal temperature, not exceeding” while the tensile strength shall be the minimum limit.

2. Yield Strength – Temperature Ratings of ASTM A335 P91

Temperature °F	-20~100	150	200	250	300	400	500	600
Yield Strength ksi	60.0	57.1	55.9	55.2	54.8	54.7	54.7	54.5
Temperature °F	650	700	750	800	850	900	950	1000
Yield Strength ksi	54.0	53.2	52.0	50.4	48.5	46.1	43.4	40.2

*The temperature refers to “metal temperature, not exceeding” while the yield strength shall be the minimum limit.

The two tables above are also applicable to other 9Cr-1Mo-V materials(UNS K91560 or K90901) including ASTM A213 T91, ASTM A182 F91, ASTM A369 FP91, ASTM A336 F91, ASTM A234 WP91, ASTM A387 Gr.91.