Topics CoveredIntroductionChemical CompositionPhysical PropertiesMechanical PropertiesThermal PropertiesOther DesignationsFabrication and Heat Treatment Tempering Annealing Cold Working Welding Forging Forming Machinability Heat TreatmentApplications
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.
The chemical composition of A7 tool steels is outlined in the following table.
The following table shows the physical properties of A7 tool steels.
The mechanical properties of A7 tool steels are displayed in the following table.
|Hardness, Rockwell C
|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)
|Bulk modulus (typical for steels)
|Shear modulus (typical for steels)
|Machinability (based on carbon tool steel)
The thermal properties of A7 tool steels are given in the following table.
||12.4 x 10-6/ºC
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
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.
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.
Conventional machining operations are used for cold working of A7 tool steels.
A7 tool steels are difficult to weld as they are very high carbon alloys.
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 for A7 tools steels is performed using the forging and machining processes.
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%.
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).
A7 tool steels are used for thread rolling dies, slitters and complex die shapes.