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).
| Fe | 94.4 to 96 | |
| Cr | 2.8 to 3.3 | |
| Mo | 0.9 to 1.1 | |
| Mn | 0.3 to 0.6 | |
| Si | 0 to 0.5 | |
| C | 0.050 to 0.15 | |
| P | 0 to 0.025 | |
| 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