Group 1.15 Pressure-Temperature Ratings

9Cr-1Mo-V alloy steel. The modern high-energy steam material, with far better creep strength than Group 1.10 but a narrow PWHT window that makes it unforgiving to weld.

Standard: ASME B16.5
Group: Group 1.15
Materials: A182 F91, A217 C12A, A387 Gr. 91 Cl. 2
Class 150 at 100 °F: 290 psig (20.0 bar)
Class 150 at 600 °F: 140 psig (9.7 bar)

ASME B16.5 pressure-temperature ratings for Group 1.15 materials, in psig.
Class	100 °F	200 °F	300 °F	400 °F	500 °F	600 °F	650 °F	700 °F	750 °F	800 °F	850 °F	900 °F	950 °F	1000 °F
Class 150	290	260	230	200	170	140	125	110	95	80	65	50	35	20
Class 300	750	750	730	705	665	605	590	570	530	510	485	450	385	365
Class 400	1000	1000	970	940	885	805	785	755	710	675	650	600	515	485
Class 600	1500	1500	1455	1410	1330	1210	1175	1135	1065	1015	975	900	775	725
Class 900	2250	2250	2185	2115	1995	1815	1765	1705	1595	1525	1460	1350	1160	1090
Class 1500	3750	3750	3640	3530	3325	3025	2940	2840	2660	2540	2435	2245	1930	1820
Class 2500	6250	6250	6070	5880	5540	5040	4905	4730	4430	4230	4060	3745	3220	3030

Interpolation between listed temperatures is permitted; extrapolation is not.

How far this table has been checked. The Class 400 row rests on a single secondary source. The other classes agree in two independent sources. None of the values on this site has been checked against the standard itself; see how the figures are checked.

What Group 1.15 is used for

This is the 9Cr-1Mo-V group, commonly called Grade 91. It is the strongest of the chrome-moly steels on this site and is used for main steam and hot reheat lines in power plants. It depends on correct heat treatment after welding to reach its strength.

Reading the ratings

A Class 300 flange in Group 1.15 is rated 750 psig at 100 °F, 605 psig at 600 °F and 365 psig at 1000 °F, which is 49% of its cold rating. Every other class in the group follows the same curve, scaled by the class. It is at or above Group 1.1 carbon steel at every temperature in the table: at 1000 °F carbon steel is down to 85 psig in the same class.

Each figure is the highest gauge pressure the flange may see at that metal temperature. To use the table, fix the material group and the design temperature first, read down to find the lowest class whose rating is at or above the design pressure, and interpolate between columns if the temperature falls between them. The rating of the flange is often what limits a line, since the pipe wall is usually good for more.

Guides and comparisons for this page

Pressure-temperature deratingReading a P-T table and interpolating between temperatures.Hydrostatic test pressureWhat a line is tested at, and what its flanges allow.Pipe material selectionA53, A106, A312 and A335: which grade suits which duty.Carbon steel vs stainless steel pipeStrength, temperature and corrosion trade-offs.Class 150 vs Class 300Dimensions, bolting and ratings of the two commonest classes.

Common questions

Which materials are in ASME B16.5 Group 1.15?

A182 F91, A217 C12A, A387 Gr. 91 Cl. 2.

What is a Class 300 Group 1.15 flange rated at 600 °F?

605 psig (41.7 bar). At 100 °F the same flange is rated 750 psig (51.7 bar).

Other material groups

Group 1.1 Group 1.2 Group 2.1 Group 2.2 Group 2.3 Group 1.9 Group 1.10