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ASTM A387 Grade 21 (K31545) 3Cr-1Mo Steel

ASTM A387 grade 21 steel is an alloy steel formulated for primary forming into wrought products. ASTM A387 Grade 21 is the ASTM designation for this material. K31545 is the UNS number. It has a moderately high base cost among the wrought alloy steels in the same category in the database.

The properties of ASTM A387 grade 21 steel include two common variations. This page shows summary ranges across both of them. For more specific values, follow the links immediately below. The graph bars on the material properties cards further below compare ASTM A387 grade 21 steel to: wrought alloy steels in the same category (top), all iron alloys (middle), and the entire database (bottom). A full bar means this is the highest value in the relevant set. A half-full bar means it's 50% of the highest, and so on.

Mechanical Properties

Brinell Hardness

150 to 180

Elastic (Young's, Tensile) Modulus

190 GPa 28 x 106 psi

Elongation at Break

21 %

Fatigue Strength

160 to 250 MPa 23 to 36 x 103 psi

Poisson's Ratio

0.29

Shear Modulus

74 GPa 11 x 106 psi

Shear Strength

310 to 370 MPa 45 to 54 x 103 psi

Tensile Strength: Ultimate (UTS)

500 to 590 MPa 73 to 86 x 103 psi

Tensile Strength: Yield (Proof)

230 to 350 MPa 33 to 51 x 103 psi

Thermal Properties

Latent Heat of Fusion

260 J/g

Maximum Temperature: Mechanical

480 °C 890 °F

Melting Completion (Liquidus)

1470 °C 2670 °F

Melting Onset (Solidus)

1430 °C 2600 °F

Specific Heat Capacity

470 J/kg-K 0.11 BTU/lb-°F

Thermal Conductivity

41 W/m-K 24 BTU/h-ft-°F

Thermal Expansion

13 µm/m-K

Electrical Properties

Electrical Conductivity: Equal Volume

7.6 % IACS

Electrical Conductivity: Equal Weight (Specific)

8.8 % IACS

Otherwise Unclassified Properties

Base Metal Price

4.1 % relative

Density

7.9 g/cm3 490 lb/ft3

Embodied Carbon

1.8 kg CO2/kg material

Embodied Energy

23 MJ/kg 10 x 103 BTU/lb

Embodied Water

62 L/kg 7.4 gal/lb

Common Calculations

PREN (Pitting Resistance)

6.4

Resilience: Ultimate (Unit Rupture Work)

84 to 110 MJ/m3

Resilience: Unit (Modulus of Resilience)

140 to 320 kJ/m3

Stiffness to Weight: Axial

13 points

Stiffness to Weight: Bending

24 points

Strength to Weight: Axial

18 to 21 points

Strength to Weight: Bending

18 to 20 points

Thermal Diffusivity

11 mm2/s

Thermal Shock Resistance

14 to 17 points

Alloy Composition

Among alloy steels, the composition of ASTM A387 grade 21 steel is notable for containing a comparatively high amount of chromium (Cr). Chromium is used to improve corrosion resistance and most mechanical properties (particularly at higher temperatures).

Iron (Fe) 94.4 to 96
Chromium (Cr) 2.8 to 3.3
Molybdenum (Mo) 0.9 to 1.1
Manganese (Mn) 0.3 to 0.6
Silicon (Si) 0 to 0.5
Carbon (C) 0.050 to 0.15
Phosphorus (P) 0 to 0.025
Sulfur (S) 0 to 0.025

All values are % weight. Ranges represent what is permitted under applicable standards.

Followup Questions

Similar Alloys

Further Reading

ASTM A387: Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum

ASTM A182: Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service

Pressure Vessels: External Pressure Technology, 2nd ed., Carl T. F. Ross, 2011

ASM Specialty Handbook: Carbon and Alloy Steels, J. R. Davis (editor), 1996

Manufacture and Uses of Alloy Steels, Henry D. Hibbard, 2005

Steels: Processing, Structure, and Performance, 2nd ed., George Krauss, 2015