A4 Tool Steel - Air-Hardening, Medium-Alloy, Cold-Work Steels

Topics Covered

Chemical Composition
Physical Properties
Mechanical Properties
Other Designations


Steels of the type A2, A3, A4, A6, A7, A8, A9, and A10 form air-hardening, medium alloy, cold work steels or group A steels. Sufficient amounts of manganese, molybdenum and chromium present in group A steels provide full hardness in sections comprising a thickness of at least 100 mm (4 in.) in diameter when air cooled.

Carbon is present in the range of 0.5 to 2% in group A steels. These steels are provided with air hardening characteristics that allow minimum cracking and distortion on quenching.

The sections below will provide an overview of A4 air-hardening, medium-alloy, cold-work tool steels.

Chemical Composition

The chemical composition of A4 tool steels is outlined in the following table.

Element Content (%)
C 0.95-1.05
Mn 1.80-2.20
Si 0.50
Cr 0.90-2.20
Ni 0.30
Mo 0.90-1.40
Cu 0.25
P 0.03
S 0.03

Physical Properties

The following table shows the physical properties of A4 tool steels.

Properties Metric Imperial
Density 8.03 g/cm3 0.290 lb/in3

Mechanical Properties

The mechanical properties of A4 tool steels are displayed in the following table.

Properties Metric Imperial
Hardness, Rockwell C (as air quenched from 845°C, 61 HRC at 150°C temper, 60 HRC at 205°C temper, 59 HRC at 260°C temper, 58 HRC at 315°C temper, 57 HRC at 370°C temper, 56 HRC at 425°C temper, 54 HRC at 480°C temper, 53 HRC at 540°C temper, 50 HRC at 595°C temper, 40 HRC at 650°C temper.) 63 63
Bulk modulus (typical for steels) 140 GPa 20300 ksi
Machinability (based on carbon tool steel) 65% 65%
Shear modulus (typical for steels) 80.0 GPa 11600 ksi
Poisson's ratio 0.27-0.30 0.27-0.30
Elastic modulus 190-210 GPa 27557-30457 ksi

Other Designations

Equivalent materials to A4 tool steels are:

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

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