O6 - Tool Steel - Oil-Hardening Cold-Work Steel (UNS T31506)

Topics Covered

Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations


Cold-work tool steels are high carbon steels that are further classified into three subgroups - oil-hardening steels, air-hardening steels, and high-carbon, high-chromium steels, Oil-hardening cold-work steels are also known as group O steels consisting of O1, O2, O6, and O7 types.

The following datasheet provides details about the O6 oil-hardening cold-work steel. Type O6 includes manganese, molybdenum, and silicon. The O6 tool steel reduces galling and seizing of fully hardened steel and improves machinability of annealed stock as graphite is present in its microstructure.

Chemical Composition

The following table shows the chemical composition of O6 tools steels.

Element Content (%)
Carbon, C 1.25-1.55
Silicon, Si 0.55-1.50
Manganese, Mn 0.30-1.10
Chromium, Cr 0.3
Nickel, Ni 0.3
Copper, Cu 0.25
Molybdenum, Mo 0.20-0.30
Phosphorus, P 0.03
Sulfur, S 0.03

Physical Properties

The physical properties of O6 tool steels are outlined in the following table.

Properties Metric Imperial
Density 7.67 g/cm3 0.277 lb/in3

Mechanical Properties

The mechanical properties of O6 steels are tabulated below.

Properties Metric Imperial
Poisson's ratio (25°C) 0.27-0.30 0.27-0.30
Elastic modulus 190-210 GPa 27557-30458 ksi
Bulk modulus 140 GPa 20300 ksi
Shear modulus 80.0 GPa 11600 ksi
Hardness, Rockwell C (As hardened, 64.0 HRC at 120°C temper, 63.0 HRC at 150°C temper, 62.0 HRC at 175°C temper, 61 HRC at 190°C and 205°C temper, 60 HRC at 230°C temper, 59.5 HRC at 260°C temper, 58.5 HRC at 315°C temper. Oil quenched from 800°C, tempered for 1 hour) 65 65
Hardness, Knoop (converted from Rockwell C hardness) 850 850
Hardness, Brinell (converted from Rockwell C hardness) 752 752
Hardness, Vickers (converted from Rockwell C hardness)    
Machinability (based on 100% machinability for 1% carbon tool steel) 130 130

Thermal Properties

The table below shows the thermal properties of O6 tool steels.

Properties Metric Imperial
Thermal expansion co-efficient (@25-250°C/77-482°F) 12.48 mm/m°C 6.933 min/in°F

Other Designations

Other designations that are equivalent to O6 tool steels include:

  • ASTM A681
  • FED QQ-T-570
  • SAE J437
  • SAE J438
  • UNS T31506


  1. Alastair Nicholson Alastair Nicholson United Kingdom says:

    Does anyone know where i could find a ballpark value for tensile and yield strength of this material? It's for some basic FEA

  2. Ronald Beilstein Ronald Beilstein United States says:

    will hardening an internal gear ring in this steel warp , crack or change size after hardening?

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