O1 Tool Steel - Oil-Hardening Cold-Work Steel (UNS T31501)

 

Introduction

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

Type O1 oil-hardening cold-work steel consists of chromium, manganese, and tungsten and is relatively inexpensive. The following datasheet provides details about O1 type.

Chemical Composition

The following table shows the chemical composition of O1 tools steels.

Element	Content (%)
C	0.85-1.00
Mn	1.00-1.40
Si	0.50
Cr	0.40-0.60
Ni	0.30
W	0.40-0.60
V	0.30
Cu	0.25
P	0.03
S	0.03

Physical Properties

The physical properties of O1tool steels are outlined in the following table.

Properties	Metric	Imperial
Density (hardened to 62 HRC)	7.81 g/cm3	0.282 lb/in3
Density (@399°C/ 750°F, hardened to 62 HRC)	7.70 g/cm3	0.278 lb/in3
Density (@191°C/ 375°F, hardened to 62 HRC)	7.75 g/cm3	0.280 lb/in3
Melting point	1421°C	2590°F

Mechanical Properties

The mechanical properties of O1 steels are tabulated below.

Properties	Metric	Imperial
Hardness, Brinell (soft annealed -delivery condition)	190	190
Hardness, Rockwell C (tempering temp 300-1200°F)	34.0-64.0	34.0-64.0
Elastic modulus (hardened to 62 HRC)	193 GPa	28000 ksi
Elastic modulus (@399°C/750°F, hardened to 62 HRC )	172 GPa	25000 ksi
Elastic modulus (@191°C/375°F, hardened to 62 HRC)	186 GPa	27000 ksi
Compressive yield strength (0.2%, hardened to 50 HRC)	1350 MPa	196000 psi
Compressive yield strength (0.2%, hardened to 55 HRC)	1800 MPa	261000 psi
Compressive yield strength (0.2%, hardened to 60 HRC)	2150 MPa	312000 psi
Compressive yield strength (0.2%, hardened to 62 HRC)	2200 MPa	319000 psi

Thermal Properties

The table below shows the thermal properties of O1 tool steels.

Properties		Conditions
		T (°C)	Treatment
Thermal expansion	10.6 x 10-6/ºC	20-260	-

Other Designations

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Other designations that are equivalent to AISI O1 tool steels include:

• DIN 1.2510
• B.S. BO 1
• ASTM A681
• FED QQ-T-570
• SAE J437
• SAE J438
• UNS T31501

Fabrication and Heat Treatment

Machinability

The machinability of O1 steels is very good with a rating of 90% that of water hardening low alloy steels.

Forming

O1 steels can be easily formed in the annealed condition using conventional methods.

Hardening

O1 steels should be heated uniformly to 780-820°C (1436-1508°F) until completely heated through. If needed, the steels can be preheated at 300-500°C (572-932°F). About 30 min/per 25 mm of ruling section is to be provided and then the steels should be immediately quenched in oil.

Welding

O1 steels are weldable but run the risk of crack formation but this can be avoided with certain specifications.

• During welding of soft annealed O1 tool steel, the steel should be preheated to 300 - 500°C (572-932°F), welded at same temperature and finally the stress should be relieved immediately. The Cr-Mo alloy electrode is preferable for welding structural steel.
• While welding in order to harden soft annealed O1 tool steel, the steel should be heated to hardening temperature, cooled to 500°C (932°F) and welded at same temperature. Finally quenching is performed. The hard-facing electrode is the preferable choice of electrode.
• When repair welding of the O1 tool steel in hardened and tempered condition, the steel should be preheated to a minimum tempering temperature of 200°C (93°F) and welded at the same tempering temperature. Finally, it should be heated immediately to a maximum tempering temperature of 300°C (572°F) and soaked for 2 hours. The hard-facing electrode is the preferable choice of electrode.

Heat Treatment

The heat treatment requires O1 steels to be slowly preheated to 649°C (1200°F) and then heated at 788-816°C (1450-1500°F). Then these steels should be held at the same temperature for 10 to 30 minutes and finally oil quenched.

Forging

Forging of O1 steels can be performed at 1038°C (1900°F) down to 857°C (1575°F) but not below 816°C (1550°F).

Cold Work

O1 steels can be easily cold-worked in the annealed condition using conventional methods.

Annealing

Annealing should be performed at 788°C (1450°F) followed by slow furnace cool at a temperature less than 4°C (40°F) per hour.

Tempering

Tempering of O1 steels is performed at 177-260°C (350-500°F) to realize Rockwell C hardness of 62 to 57.

Martempering

Martempering is a substitute hardening procedure which can be used along with suitable salt bath equipment.

Applications

O1 steels are mainly used for short run tooling for cold forming dies, blanking dies, and cutting tools operating at ambient temperature.