P20 Mold Steel-A Low Alloy Tool Steel

Introduction

Low-carbon mold steel, also known as Group P mold steels, covers multiple grades including P2, P3, P4, P5, P6, P20 and P21. Chromium and nickel are the primary alloying elements in Group P mold steels.

P20 tool steel can undergo nitriding or carburizing treatment. These steels can be machined into large, complex dies and molds. P20 steels are mostly used in the carburized condition. The addition of chromium and nickel improves the toughness and hardness of P20 steels.

The following sections present an overview of P20 tool steel.

1. P20 Tool Steel

1.1Chemical Composition

The chemical composition of P20 tool steels is outlined in the following table.

1.2 Supply Range of P20 Steel

Round Bar: diameter 200mm – 1000mm

Steel Plate: thickness 16-800mm x width 200-2300mm

Surface Finish: Black, Rough Machined, Turned or as per given requirements.

1.3 Physical Properties

The following table shows the physical properties of P20 tool steels.

1.4 ASTM P20 Mold Tool Steel Properties

Physical Properties

• Density: 0.284 lb/in3 (7861 kg/m3)

• Specific Gravity: 7.86

• Modulus of Elasticity: 30 x 106 psi (207 GPa)

• Thermal Conductivity:

• 24 Btu/ft/hr/°F

• 41.5 W/m/°K

• Machinability: 60-65% of a 1% carbon steel

2. Forging of P20 Steel Plate Mold Steel

AISI P20 steel plate and round bars materials are forged at 1093℃(2000°F) down to 899℃ (1650°F). Forging below 871℃(1600°F) is not recommended for these steels P20 material.

2.1Heat Treatment of P20 Tool Steel

●Preheating: Since

P20 mold steel plates are supplied in the pre‑hardened condition, hardening heat treatment is not required. After machining and intermittently during service, the steel shall be thermally stress‑relieved by heating to 900ºF (482ºC), holding for one hour per inch (25.4 mm) of thickness after temperature equalization, followed by air‑cooling to ambient temperature.

In rare cases where AISI P20 steel has to be re‑hardened, the steel must be annealed prior to hardening.

●Hardening

Critical Temperature: Ac1: 1405ºF (763ºC)

Preheating for P20 metal: Heat at a rate no greater than 400ºF per hour (222ºC per hour) to 1150‑1250ºF (621‑677ºC) and allow temperature equalization.

Austenitizing (High‑Heat Treatment):

Heat rapidly from the preheating temperature to 1550ºF (843°C). Soak for 30 minutes for the first inch (25.4 mm) of thickness, plus 15 minutes for each additional inch (25.4 mm) of thickness.

●Quenching:

Use pressurized gas or interrupted‑oil quenching down to 150‑125ºF (66‑51ºC).

For oil quenching: quench until the surface turns black at approximately 900°F (482°C), then cool in still air to 150‑125°F (66‑51°C).

●Tempering:

P20 tool steel is tempered at 482‑593°C (900‑1100°F) to achieve Rockwell C hardness ranging from 28 to 37 HRC.

●Annealing:

AISI P20 tool steel shall be annealed at 760‑788°C (1400‑1450°F). The steel is then slowly cooled inside the furnace at a cooling rate lower than 4°C (40°F) per hour.

3.Applications

P20 tool steel is suitable for low-temperature applications, including injection molds and die-casting molds. The main applications of P20 steel plates and bars are as follows:

Plastic molds, mold bases, backer blocks, plastic compression mold frames, hydroforming die tools, support blocks, mold bases. It is also applicable for other uses such as steel rails, shafts and wear strips.

4.Conclusion

This paper summarizes the properties, heat treatment and applications of P20 mold steel. As a low-alloy tool steel, P20 has balanced hardness, toughness and machinability for large complex molds. It has equivalent grades under different national standards. Proper heat treatment and surface treatments optimize its performance. P20 remains a cost-effective material widely used in medium-precision mold production.