ASTM A387 GRADE 21 CLASS 2|A387Gr.21CL.2|A387GR21CL2 STEEL

ASTM A387 Grade 21 Class 2 (also written A387 Gr.21 CL.2 or A387GR21CL2, and often ordered as SA387 Grade 21 Class 2 under ASME) is a chromium-molybdenum plate with about 3% Cr and 1% Mo. It is used for welded boilers and pressure vessels that run at elevated temperature. We supply to order: thickness about 6 to 300 mm, width about 1500 to 4050 mm, length about 3000 to 15000 mm, cut to size. EN 10204 3.1 is standard, 3.2 when you need it. Put HIC, NACE, through-thickness (Z), impact, or other extras on the inquiry.

The chemistry and mechanical values below follow ASTM A387/A387M and the matching ASME SA387 rules, so you can check them on the mill test certificate. Read heat analysis and product analysis as separate columns. The edition named on the contract and any PO supplements are what apply.

Chemical Composition — Heat Analysis

 

Element	C	Mn	P	S	Si	Cr	Mo
Grade 21, heat, %	0.05 to 0.15	0.30 to 0.60	≤0.025	≤0.025	≤0.50	2.75 to 3.25	0.90 to 1.10

Class 1 and Class 2 share the same chemistry. For plates over 125 mm, product carbon may be 0.17 max per ASTM footnote.

Chemical Composition — Product Analysis

 

Element	C	Mn	P	S	Si	Cr	Mo
Grade 21, product, %	0.04 to 0.15	0.25 to 0.66	≤0.025	≤0.025	≤0.50	2.63 to 3.37	0.85 to 1.15

 

Mechanical Properties — Class 2

 

Requirement	Class 2 (Grade 21)
Tensile strength	75 to 100 ksi [515 to 690 MPa]
Yield strength, min	45 ksi [310 MPa]
Elongation in 50 mm, min	18%
Reduction of area, min	45% (round) / 40% (flat)

 

Delivery Condition and Extras

Heat treatment follows the standard. For Grade 21, tempering is generally not below 675 °C. Certificate 3.1 is standard, 3.2 on request. Sizes as above. Optional HIC, NACE, Z15 to Z35, and other supplements as ordered.

Related Grades / Equivalent Grades

 

Standard	Comparable grade	Note
ASTM A387	ASTM A387 Grade 21 Class 1	Same grade, lower tensile class
ASME	SA387 Grade 21 Class 2	ASME twin of ASTM A387 Grade 21 Class 2
EN 10028-2	EN 10028-2 10CrMo9-10	Related Cr-Mo family reference (3Cr vs 2.25Cr)
ASTM A387	ASTM A387 Grade 2 Class 2	Related Cr-Mo grade on site
ASTM A387	ASTM A387 Grade 5 Class 2	Related Cr-Mo grade on site
ASTM A387	ASTM A387 Grade 9 Class 2	Related Cr-Mo grade on site
ASTM A387	ASTM A387 Grade 11 Class 2	Related Cr-Mo grade on site
ASTM A387	ASTM A387 Grade 12 Class 2	Related Cr-Mo grade on site

Dual certification only when the purchase order states the stricter combined limits. Full list: ASTM A387 Steel Plate.

Density

About 7.85 g/cm³; use the project value when the design requires it.

FAQ

What is A387 Gr.21 CL.2 / A387GR21CL2?

It is ASTM A387 Grade 21 Class 2 chromium-molybdenum pressure-vessel plate (about 3% Cr and 1% Mo) for elevated-temperature welded boilers and vessels. A387GR21CL2 is a compact mill/search writing of the same grade and class.

What is the difference between Class 1 and Class 2?

Chemistry is the same for Grade 21. Class sets the tensile level: Class 1 is the lower band; Class 2 is the higher band. Order the class named on the drawing and code calculation. See the mechanical table above for Class 2 values.

Any welding / preheat note?

3Cr-1Mo plate typically needs controlled preheat, interpass temperature, and PWHT per the project WPS and thickness. This page does not replace a qualified welding procedure.

What certificate does Gangsteel supply?

EN 10204 3.1 as standard; 3.2 with third-party inspection when ordered. The MTC lists heat analysis, mechanical tests, heat-treatment condition, and any optional UT / Z / HIC results.

Ordering Notes

Welding: Preheat, interpass, and PWHT follow the project WPS; this page is not a welding procedure.
Dual cert: Apply only the stricter combined limits written in the contract.
Before you order: Confirm standard edition, Class, delivery condition, impact, UT, and any extras.

On the inquiry, please include: thickness, width, length, Class, delivery condition, certificate (3.1 or 3.2), and whether you need HIC, NACE, Z, impact, or other extras. Gangsteel