SA 387 Gr 11 vs Gr 22 vs SA 516 Gr 70 — Pressure-Vessel Plate Compared

Short answer

SA 516 Gr 70 is carbon steel for vessels at moderate and lower temperatures. SA 387 Gr 11 (1¼Cr-½Mo) takes over when the vessel runs hot or in hydrogen. SA 387 Gr 22 (2¼Cr-1Mo) is the step up for higher temperatures and hydrogen partial pressures. Chrome-moly plate costs more and needs controlled welding and PWHT.

The three plates at a glance

	SA 516 Gr 70	SA 387 Gr 11	SA 387 Gr 22
Alloy	Carbon–manganese	1¼Cr-½Mo-Si	2¼Cr-1Mo
Standard	ASTM A516	ASTM A387 Grade 11	ASTM A387 Grade 22
Classes	—	Class 1 and Class 2	Class 1 and Class 2
Typical service	Vessels at moderate and lower temperatures	Hot reactors, drums, exchangers; moderate hydrogen	Higher temperature and hydrogen partial pressure
ASME P-No.	P-No. 1	P-No. 4	P-No. 5A
Matching filler	E7018 / ER70S	E8018-B2 / ER80S-B2	E9018-B3 / ER90S-B3

Why chromium and molybdenum

Carbon steel loses strength and creep resistance as temperature rises, and in hot hydrogen it can suffer high-temperature hydrogen attack. Chromium and molybdenum form stable carbides, which keep strength at temperature and resist that attack. Refinery designers check the operating temperature and hydrogen partial pressure against the API RP 941 (Nelson) curves to decide whether carbon steel, 1¼Cr-½Mo or 2¼Cr-1Mo is needed.

Class 1 vs Class 2

Both Grade 11 and Grade 22 come in two strength classes with the same chemistry. For Grade 11, Class 1 is 415–585 MPa tensile with 240 MPa minimum yield; Class 2 is 515–690 MPa tensile with 310 MPa minimum yield. The higher class lets designers reduce wall thickness on heavy vessels. See the SA 387 Gr 11 Cl 2 plate page for the full table.

Fabrication: where the cost is

A516 Gr 70 welds with ordinary low-hydrogen practice, with preheat and PWHT set by thickness. Chrome-moly plate needs controlled preheat and interpass temperatures and, in most thicknesses, post-weld heat treatment under ASME VIII (vessels) or B31.3 (piping). Because PWHT can lower strength, buyers order simulated PWHT of the test coupons (ASTM A20 supplementary requirement S3) so the mill certifies properties after the fabricator's heat-treatment cycle. Impact testing (S5) and ultrasonic testing (S8 / S12) are also supplementary — put them on the order.

Matching pipe, fittings and flanges

Plate	Pipe	Fittings	Flanges
SA 516 Gr 70	A106 Gr B	A234 WPB	A105
SA 387 Gr 11	A335 P11	A234 WP11	A182 F11 Class 2
SA 387 Gr 22	A335 P22	A234 WP22	A182 F22 Class 3

Questions buyers ask

When should I use SA 387 Gr 11 instead of SA 516 Gr 70?

When the design temperature or hydrogen service is beyond carbon steel — check API RP 941 for hydrogen service and your code's allowable stresses at temperature.

Is SA 387 Gr 22 better than Gr 11?

It has more chromium and molybdenum for higher temperature and hydrogen partial pressure, but it costs more and has its own fabrication requirements. Choose by design conditions.

What is the P-Number of SA 387 Gr 11?

P-No. 4 under ASME Section IX. Gr 22 is P-No. 5A; SA 516 Gr 70 is P-No. 1.

Do chrome-moly plates need PWHT?

Usually — under ASME VIII for vessels (or B31.3 for piping), depending on thickness. Order simulated PWHT (A20 S3) so properties are certified after your cycle.