Introduction
Cold-work tool steels include the high-carbon, high-chromium steels or group D steels. These steels are designated as group D steels and consist of D2, D3, D4, D5, and D7 steels. These steels contain 1.5 to 2.35% of carbon and 12% of chromium. Except type D3 steel, all the other group D steels include 1% Mo and are air hardened. Type D3 steel is oil-quenched; though small sections can be gas quenched after austenitization using vacuum. As a result, tools made with type D3 steel tends to be brittle during hardening. Type D2 steel is the most commonly used steel among the group D steels.
Chemical Composition
The following table shows the chemical composition of D2 tool steels.
| Element | Content (%) |
|---|---|
| C | 1.40 – 1.60 |
| Mn | 0.60 |
| Si | 0.60 |
| Co | 1.00 |
| Cr | 11.00 – 13.00 |
| Mo | 0.70 – 1.20 |
| V | 1.10 |
| P | 0.03 |
| Ni | 0.30 |
| Cu | 0.25 |
| S | 0.03 |
Physical Properties
The physical properties of D2 tool steels are outlined in the following table.
| Properties | Metric | Imperial |
|---|---|---|
| Density | 7.7 x 1000 kg/m3 | 0.278 lb/in3 |
| Melting point | 1421°C | 2590°F |
Mechanical Properties
The mechanical properties of D2 steels are tabulated below:
| Mechanical Properties | Metric | Imperial |
|---|---|---|
| Hardness, Knoop (converted from Rockwell C hardness) | 769 | 769 |
| Hardness, Rockwell C | 62 | 62 |
| Hardness, Vickers | 748 | 748 |
| Izod impact unnotched | 77.0 J | 56.8 ft-lb |
| Poisson’s ratio | 0.27-0.30 | 0.27-0.30 |
| Elastic modulus | 190-210 GPa | 27557-30457 ksi |
Thermal Properties
The following table shows the thermal properties of D2 steels.
| Properties | Conditions | ||
|---|---|---|---|
| T (°C) | Treatment | ||
| Thermal expansion | 10.4 x 10-6/ºC | 20-100 | – |
Other Designations
Equivalent materials to D2 tool steels are:
- DIN 1.2379
- NI KU
- B.S. BD 2
- ASTM A681
- FED QQ-T-570
- SAE J437
- SAE J438
- UNS T30402
Fabrication and Heat Treatment
Heat Treatment
D2 steels should be preheated very slowly to 815ºC (1500ºF) and then temperature can be increased to 1010ºC (1850ºF). They are then held at 1010ºC (1850ºF) for 20 to 45 minutes and air cooled (air quenched).
Forging
Forging of D2 steels can be done from 1065ºC (1950ºF) down to 954ºC (1750ºF). Do not forge below 926ºC (1700ºF).
Annealing
Annealing of D2 steels should be done at 871 to 898ºC (1600 to 1650ºF) followed by slow furnace cooling at 4.4ºC (40ºF) per hour or less.
Tempering
D2 steels can be tempered at 204ºC (400ºF) for achieving Rockwell C hardness of 61 and at 537ºC (1000ºF) for a Rockwell C hardness of 54.
Applications
D2 tool steels are used for long run tooling applications, where wear resistance is important, such as blanking or forming dies and thread rolling dies.
Tool steels are a variety of carbon and alloy steels that are particularly well-suited for making tools due to their hardness, resistance to abrasion, and ability to retain a cutting edge. Here are some common types of tool steels and their applications:
- Hot-Work Tool Steel:
- Types: H13 Loha
- Applications: Die casting, extrusion dies, hot forging, hot shearing, and plastic molds.
- High-Speed Tool Steel:
- Plastic Mold Tool Steel:
- Types: P20 Loha, P20 Ni Loha
- Applications: Plastic molding dies, die-casting dies, and extrusion dies.
Each type of tool steel is chosen based on its specific properties and the requirements of the application, such as hardness, toughness, wear resistance, and ability to withstand high temperatures.