A9 Tool Steel (UNS T30109) - Air-Hardening, Medium-Alloy, Cold-Work Steels

Topics Covered

Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations


Group A steels are A2, A3, A4, A6, A7, A8, A9, and A10 steels. They are also called air-hardening, medium alloy, cold work steels. Carbon content of group A steels range from 0.5 to 2%. These steels are provided with air hardening characteristics that allow minimum cracking and distortion on quenching.

Sufficient amounts of manganese, molybdenum and chromium present in group A steels provide full hardness in sections comprising thickness of at least 100 mm (4 in.) in diameter when air cooled.

The data below will provide an overview of A9 air-hardening, medium-alloy, cold-work tool steels.

Chemical Composition

The chemical composition of A9 tool steels is outlined in the following table.

Element Content (%)
C 0.45-0.55
Mn 0.5
Si 0.95-1.15
Cr 4.75-5.50
Ni 1.25-1.75
Mo 1.30-1.80
V 0.80-1.40
Cu 0.25
P 0.03
S 0.03

Physical Properties

The following table shows the physical properties of A9 tool steels.

Properties Metric Imperial
Density 7.78 g/cm3 0.281 lb/in3

Mechanical Properties

The mechanical properties of A9 tool steels are displayed in the following table.

Properties Metric Imperial
Hardness, Knoop (converted from Rockwell C hardness) 662 662
Hardness, Rockwell C (tempered at 500°C) 57 57
Hardness, vickers 641 641
Tensile strength, ultimate (tempered at 500°C) 2200 MPa 319000 psi
Tensile strength, yield (tempered at 500°C) 1780 MPa 258000 psi
Elongation at break (in 50 mm; tempered at 500°C) 5.00% 5.00%
Izod impact (smooth bar; tempered at 500°C) 18.0 J 13.3 ft-lb
Poisson's ratio 0.27-0.30 0.27-0.30
Elastic modulus 190-210 GPa 27557-30457 ksi

Thermal Properties

The thermal properties of A9 tool steels are given in the following table.

Properties Conditions
T (°C) Treatment
Thermal expansion 12 x 10-6/ºC 20-425 -

Other Designations

Equivalent materials to A9 tool steels are:

  • ASTM A681
  • FED QQ-T-570
  • UNS T30104

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