The ASTM A213 T91 seamless tubes are primarily used for boiler, superheater, and heat-exchanger. It is also an 9Cr-1Mo-V alloy steel with UNS designation K90901. Compared to its equivalent ASTM A335 P91, the chemical composition requirement of T91 steel is slightly different. On the other hand, the tensile test parameters of the former and the latter are the same. What makes A213 T91 distinct from  A335 P91 is that the former is usually applicable to tubing with sizes from 1/8″ [3.2 mm] in inside diameters to 5″ [127 mm] in outside diameters and minimum wall thicknesses from 0.015″ to 0.500″ [0.4 to 12.7 mm], inclusive. Besides, ASTM A213 T91 is only endorsed by ASME B31.1 Power Piping but not by ASME B31.3 process piping.

The Allowable Stress – Temperature of ASTM A213 T91

*Temp. °F	-20-100	200	300	400	500	600	650	700	750
*MAS-I, ksi	24.3	24.3	24.3	24.2	24.1	23.7	23.4	22.9	22.2
*MAS-II, ksi	24.3	24.3	24.3	24.2	24.1	23.7	23.4	22.9	22.2
*Temp. °F	800	850	900	950	1000	1050	1100	1150	1200
*MAS-I, ksi	21.3	20.3	19.1	17.8	16.3	14.0	10.3	7.0	4.3
*MAS-II, ksi	21.3	20.3	19.1	17.8	16.3	12.9	9.6	7.0	4.3

*Temp.: the metal temperature shall be interpreted as “not exceeding”.
*MAS-I: the maximum allowable stress for thickness less than 3 inches.
*MAS-II: the maximum allowable stress for thickness 3 inches or greater.
*The tabulated values are applicable to A213 T91 tubes used for power piping construction as per ASME B31.1.