A7 Tool Steel - Air-Hardening, Medium-Alloy, Cold-Work Steels (UNS T30107)

Topics Covered

Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
Fabrication and Heat Treatment
     Tempering
     Annealing
     Cold Working
     Welding
     Forging
     Forming
     Machinability
     Heat Treatment
Applications

Introduction

Steels of the type A2, A3, A4, A6, A7, A8, A9, and A10 are known as 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 of thickness of at least 100 mm (4 in.) in diameter when air cooled.

The 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.

A7 air-hardening, medium-alloy, cold-work tool steels are high carbon alloys comprising cobalt, chromium, vanadium and molybdenum. A7 tool steels have better wear resistance and low toughness rating.

The sections below will provide an overview on A7 air-hardening, medium-alloy, cold-work tool steels.

Chemical Composition

The chemical composition of A7 tool steels is outlined in the following table.

Element Content (%)
C 2.00-2.85
Mn 0.8
Si 0.5
Cr 5.00-5.75
Ni 0.3
Mo 0.90-1.40
W 0.50-1.50
V 3.90-5.15
Cu 0.25
P 0.03
S 0.03

Physical Properties

The following table shows the physical properties of A7 tool steels.

Properties Metric Imperial
Density 8.03 g/cm3 0.290 lb/in3
Melting point 1427°C 2600°F

Mechanical Properties

The mechanical properties of A7 tool steels are displayed in the following table.

Properties Metric Imperial
Hardness, Rockwell C 58.0-66.0 58.0-66.0
Hardness, Rockwell C (as air quenched from 955°C, tempered for 3 hours. 64-65 HRC at 150°C temper, 62-64 HRC at 260°C temper, 61-63 HRC at 315°C temper, 59-61 HRC at 425°C temper, 56-58 HRC at 540°C temper, 53-55 HRC at 595°C temper, 44-46 HRC at 650°C temper) 65.0-66.5 65.0-66.5
Bulk modulus (typical for steels) 140 GPa 20300 ksi
Shear modulus (typical for steels) 80.0 GPa 11600 ksi
Machinability (based on carbon tool steel) 65% 65%
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 A7 tool steels are given in the following table.

Properties Conditions
T (°C) Treatment
Thermal expansion 12.4 x 10-6/ºC 20-425 more -

Other Designations

Equivalent materials to A7 tool steels are:

  • ASTM A68i
  • FED QQ-T-570
  • UNS T30107
  • ASTM A681 (A-7)
  • UNS T30107

Fabrication and Heat Treatment

Tempering

A7 tool steels are tempered in the range of 149°C (300°F) for Rockwell C 67 to 538°C (1000°F) for Rockwell C 57.

Annealing

A7 tool steels are annealed at 885°C (1625°F) and then cooled in the furnace very slowly at a temperature less than 25°F per hour.

Cold Working

Conventional machining operations are used for cold working of A7 tool steels.

Welding

A7 tool steels are difficult to weld as they are very high carbon alloys.

Forging

A7 tool steels are forged between 1121 to 1010°C (2050 to 1850°F). Forging below 982°C (1800°F) is not recommended for these steels.

Forming

Forming for A7 tools steels is performed using the forging and machining processes.

Machinability

A7 tools steels have a high alloy and carbon content and hence their machinability is low. These steels have a machinability rating of about 80%.

Heat Treatment

A7 tools steels are preheated to 816°C (1500°F) in a slow manner and this is followed by an increase in temperature to 968°C (1775°F). These steels are then held at this temperature for 45 to 60 minutes and cooled in air (air quenched).

Applications

A7 tool steels are used for thread rolling dies, slitters and complex die shapes.

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