ASTM A387 Grade 91 Class 2 (K90901) 9Cr-1Mo Steel
ASTM A387 grade 91 class 2 is an alloy steel formulated for primary forming into wrought products. Cited properties are appropriate for the normalized and tempered condition. ASTM A387 Grade 91 Class 2 is the ASTM designation for this material. K90901 is the UNS number.
It has a fairly low thermal conductivity among wrought alloy steels in the same category. In addition, it has a fairly high base cost and a fairly high embodied energy.
The graph bars on the material properties cards below compare ASTM A387 grade 91 class 2 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
200
Elastic (Young's, Tensile) Modulus
190 GPa 28 x 106 psi
Elongation at Break
20 %
Fatigue Strength
330 MPa 47 x 103 psi
Poisson's Ratio
0.28
Shear Modulus
75 GPa 11 x 106 psi
Shear Strength
420 MPa 61 x 103 psi
Tensile Strength: Ultimate (UTS)
670 MPa 98 x 103 psi
Tensile Strength: Yield (Proof)
470 MPa 69 x 103 psi
Thermal Properties
Latent Heat of Fusion
270 J/g
Maximum Temperature: Mechanical
600 °C 1100 °F
Melting Completion (Liquidus)
1460 °C 2660 °F
Melting Onset (Solidus)
1420 °C 2580 °F
Specific Heat Capacity
470 J/kg-K 0.11 BTU/lb-°F
Thermal Conductivity
26 W/m-K 15 BTU/h-ft-°F
Thermal Expansion
13 µm/m-K
Electrical Properties
Electrical Conductivity: Equal Volume
8.9 % IACS
Electrical Conductivity: Equal Weight (Specific)
10 % IACS
Otherwise Unclassified Properties
Base Metal Price
7.0 % relative
Density
7.8 g/cm3 490 lb/ft3
Embodied Carbon
2.6 kg CO2/kg material
Embodied Energy
37 MJ/kg 16 x 103 BTU/lb
Embodied Water
88 L/kg 10 gal/lb
Common Calculations
PREN (Pitting Resistance)
13
Resilience: Ultimate (Unit Rupture Work)
120 MJ/m3
Resilience: Unit (Modulus of Resilience)
580 kJ/m3
Stiffness to Weight: Axial
14 points
Stiffness to Weight: Bending
25 points
Strength to Weight: Axial
24 points
Strength to Weight: Bending
22 points
Thermal Diffusivity
6.9 mm2/s
Thermal Shock Resistance
19 points
Alloy Composition
Among alloy steels, the composition of ASTM A387 grade 91 class 2 is notable for including zirconium (Zr) and containing a comparatively high amount of chromium (Cr). Zirconium is used to improve ductility. Chromium is used to improve corrosion resistance and most mechanical properties (particularly at higher temperatures).
| Fe | 87.3 to 90.3 | |
| Cr | 8.0 to 9.5 | |
| Mo | 0.85 to 1.1 | |
| Mn | 0.3 to 0.6 | |
| Si | 0.2 to 0.5 | |
| V | 0.18 to 0.25 | |
| Ni | 0 to 0.4 | |
| C | 0.080 to 0.12 | |
| Nb | 0.060 to 0.1 | |
| N | 0.030 to 0.070 | |
| P | 0 to 0.020 | |
| Al | 0 to 0.020 | |
| S | 0 to 0.010 | |
| Ti | 0 to 0.010 | |
| Zr | 0 to 0.010 |
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
CRC Materials Science and Engineering Handbook, 4th ed., James F. Shackelford et al. (editors), 2015
Steels: Processing, Structure, and Performance, 2nd ed., George Krauss, 2015