Sep 14 2012
Topics Covered
IntroductionChemical CompositionPhysical PropertiesMechanical PropertiesThermal PropertiesOther DesignationsFabrication and Heat Treatment Tempering Annealing Cold Working Welding Forging Forming Machinability Heat TreatmentApplications
Introduction
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 thickness of at least 100 mm (4 in.) in diameter when air cooled.
Carbon content in 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.
A2 air-hardening, medium-alloy, cold-work tool steels are a type of group A steels containing molybdenum and chromium. The carbon content in A2 tool steels is high. A2 tool steels can be deep hardened in order to minimize distortion.
The data below will provide an overview of A2 air-hardening, medium-alloy, cold-work tool steels.
Chemical Composition
The chemical composition of A2 tool steels is outlined in the following table.
| Element |
Content (%) |
| C |
0.95-1.05 |
| Mn |
1 |
| Si |
0.5 |
| Cr |
4.75-5.50 |
| Ni |
0.3 |
| Mo |
0.90-1.40 |
| V |
0.15-0.50 |
| Cu |
0.25 |
| P |
0.03 |
| S |
0.03 |
Physical Properties
The following table shows the physical properties of A2 tool steels.
| Properties |
Metric |
Imperial |
| Density |
7.86 g/cm3 |
0.284 lb/in3 |
| Melting point |
1424°C |
2595°F |
Mechanical Properties
The mechanical properties of A2 tool steels are displayed in the following table.
| Properties |
Metric |
Imperial |
| Hardness, Rockwell C (as air-hardened (63-65 HRC average), 60-62 HRC at 205°C, 59-61 HRC at 260°C, 58-60 HRC at 315°C, 57-59 HRC at 370°C and 425°C and 480°C, 56-58 HRC at 540°C, 50-52 HRC at 595°C, 42-44 HRC at 650°C) |
64 |
64 |
| Bulk modulus (typical for steels) |
140 GPa |
20300 ksi |
| Machinability (based on carbon tool steel) |
65% |
65% |
| Shear modulus |
78.0 GPa |
11300 ksi |
| 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 A2 tool steels are given in the following table.
| Properties |
Conditions |
| T (°C) |
Treatment |
| Thermal expansion |
10.7 x 10-6/ºC |
20-100 |
- |
Other Designations
Equivalent materials to A2 tool steels are:
- AFNOR Z 100 CDV 5
- DIN 1.2363
- JIS SKD 12
- SS 2260
- B.S. BA 2
- ASTM A681
- FED QQ-T-570
- SAE J438
- SAE J437
- UNS T30102
Fabrication and Heat treatment
Tempering
A2 tool steels are heat treated at temperatures ranging from 177°C (350°F) for Rockwell C 62 to 538°C (1000°F) for Rockwell C 56 and then tempered.
Annealing
A2 tool steels are annealed at 857°C (1575°F) and then cooled slowly in the furnace at a rate of 4°C (40°F) per hour or less.
Cold Working
A2 tool steels are cold worked in the annealed condition.
Welding
A2 tool steels are alloy steels and hence they are capable of being welded. However, welding of A2 tool steels are not much preferred due to the risk of crack formation. The following guidelines can be followed during the welding of A2 steels:
A. Welding in order to harden the soft annealed A2 tool steel
- Heat to austenitizing temperature
- Cool to approx. 500°C (932°F)
- Weld at approx. 500°C (932°F)
- Cool to approx. 100°C (212°F)
- Hard facing electrode
B. Repair welding of A2 tool steel in hardened and tempered condition
- Preheat to the tempering temperature (min. 200°C) (392°F)
- Weld at tempering temperature
- Heat immediately to tempering temperature, but max. 300°C (572°F). Soak the steels for 3 hours
- Cool in air to approx. 80°C (176°F)
- Heat to tempering temperature. Soak for 2 hours
- Hard facing electrode
C. Welding of soft annealed A2 tool steel
- Preheat to 300-500°C (572-932°F)
- Weld at 300-500°C (572-932°F)
- Stress relieve immediately.
- Cr-Mo alloy electrode for welding structural steel
Forging
A2 tool steels are forged between 1093 to 913°C (2000 to 1675°F). These tool steels should not be forged below 899°C (1650°F).
Forming
Forming of A2 tool steels is carried out by machining or hot forging.
Machinability
A2 tool steels have medium machinability.
Heat Treatment
A2 tool steels are heated slowly to 788°C (1450°F) and this is followed by increasing the temperature to 954°C (1750°F). A2 tool steels are further held at 954°C (1750°F) for 30 to 45 minutes and finally removed from the furnace and cooled.
Applications
A2 tool steels are used in slitters, die shapes and other such applications where wear resistance is significant.