Jul 9 2012
Topics Covered
IntroductionCompositionSpecifications and PropertiesApplications
Introduction
The American Iron and Steel Institute (AISI) classification system has designated the letter ‘P’ to refer to mold steels or Group P steels. The main alloying elements in the mold steels are chromium and nickel. Mold steels are all low-carbon tools steels with one medium-carbon tool steels. The P-series steels include P2, P3, P4, P5, P6, P20, and P21 steels. All steels belonging to this series, except for P4 and P21, exhibit low resistance to softening at high temperatures, while P4 and P21 have medium resistance. The surface of low-carbon steel types can be hardened by carburizing.
Composition
The following table shows the composition limits of low carbon mold steels.
| Designation |
Compositiona, % |
| AISI |
UNS |
C |
Mn |
Si |
Cr |
Ni |
Mo |
W |
V |
Co |
| P2 |
T51602 |
0.10 max |
0.10-0.40 |
0.10-0.40 |
0.75-1.25 |
0.10-0.50 |
0.15-0.40 |
- |
- |
- |
| P3 |
T51603 |
0.10 max |
0.20-0.60 |
0.40 max |
0.40-0.75 |
1.00-1.50 |
- |
- |
- |
- |
| P4 |
T51604 |
0.12 max |
0.20-0.60 |
0.10-0.40 |
4.00-5.25 |
- |
0.40-1.00 |
- |
- |
- |
| P5 |
T51605 |
0.10 max |
0.20-0.60 |
0.40 max |
2.00-2.50 |
0.35 max |
- |
- |
- |
- |
| P6 |
T51606 |
0.05-0.15 |
0.35-0.70 |
0.10-0.40 |
1.25-1.75 |
3.25-3.75 |
- |
- |
- |
- |
| P20 |
T51620 |
0.28-0.40 |
0.60-1.00 |
0.20-0.80 |
0.40-2.00 |
- |
0.30-0.55 |
- |
- |
- |
| P21 |
T51621 |
0.18-0.22 |
0.20-0.40 |
0.20-0.40 |
0.50 max |
3.90-4.25 |
- |
- |
0.15-0.25 |
1.05-1.25A1 |
aAll the steels (except group W) contain 0.25 max Cu, 0.03 max P, and 0.03 max S; Group W contains 0.20 max Cu, 0.025 max P, and 0.025 max S. Where mentioned, sulfur can be increased from 0.06 to 0.15% to enhance machinability of group A, D, H, M, and T steels.
Specifications and Properties
Owing to market demand, the P-series tool steels are now manufactured with all the specifications required for plastic molds such as cleanliness, hardenability, and soundness, vacuum degassing, electric furnace melting, and special deoxidation treatments.
The P2 and P6 low carbon mold steels are carburizing steels that are manufactured according to tool steel quality standards. Their low resistance to hardening in the annealed condition and low hardness make it possible to manufacture a mold impression using the cold hubbing process. Once the impression is made, the mold is then carburized, hardened, and tempered to about 58 HRC. The P4 and P6 low carbon mold steels are deep hardening. Full hardness of the P4 can be achieved in the carburized case by air-cooling. The P20 and P21 low carbon mold steels are carburized or nitrided. These can be heat treated to a hardness of 30 to 60 HRC, which ensures that the steel can be easily machined into large, intricate molds and dies. For plastic molds, P20 is carburized and hardened once the impression is machined. P21 is a precipitation-hardening steel that includes aluminum. It is usually supplied after prehardening to 32 to 36 HRC.
Applications
The major applications of low carbon mold steels are as follows
- Compression and injection molds for plastics
- Low-temperature die-casting dies
The P21 steel is particularly suited for critical-finish molds due to its superior polishability.