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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).

Iron (Fe) 87.3 to 90.3
Chromium (Cr) 8.0 to 9.5
Molybdenum (Mo) 0.85 to 1.1
Manganese (Mn) 0.3 to 0.6
Silicon (Si) 0.2 to 0.5
Vanadium (V) 0.18 to 0.25
Nickel (Ni) 0 to 0.4
Carbon (C) 0.080 to 0.12
Niobium (Nb) 0.060 to 0.1
Nitrogen (N) 0.030 to 0.070
Phosphorus (P) 0 to 0.020
Aluminum (Al) 0 to 0.020
Sulfur (S) 0 to 0.010
Titanium (Ti) 0 to 0.010
Zirconium (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