P20 Mold Steel: Comprehensive Study Guide

I. Overview of P20 Mold Steel

P20 mold steel is a versatile, low-alloy tool steel known for its good toughness at moderate strength levels.

It is predominantly used for plastic injection mold cavities, tooling, and die casting dies for zinc.

P20 is typically supplied in a pre-hardened condition, eliminating the need for initial hardening heat treatment.

 

II. Chemical Composition and Physical Properties

A. Chemical Composition

• Carbon (C): 0.33
• Manganese (Mn): 0.80
• Silicon (Si): 0.65
• Chromium (Cr): 1.75
• Molybdenum (Mo): 0.40

B. 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

C. Mechanical Properties

• Hardness, Brinell (typical): 300 HBW
• Hardness, Rockwell C (typical): 30 HRC
• Tensile strength, ultimate: 965-1030 MPa
• Tensile strength, yield: 827-862 MPa
• Elongation at break (in 50 mm (2″)): 20.00%
• Compressive strength: 862 MPa
• Charpy impact (V-Notch): 27.1-33.9 J
• Poisson’s ratio: 0.27-0.30
• Elastic modulus: 190-210 GPa

III. Heat Treatment Procedures

P20 mold steel is typically supplied in a pre-hardened condition (approx. 300 HBW / 30 HRC).

A. Stress Relieving

• Purpose: To relieve internal stresses after machining or intermittently during service.
• Procedure: Heat to 900ºF (482ºC), equalize, and hold for one hour per inch (25.4mm) of thickness. Cool in air to ambient temperature.

B. Hardening (Rare Circumstances – requires prior annealing)

• Critical Temperature (Ac1): 1405ºF (763ºC)
• Preheating: Heat at a rate not exceeding 400ºF per hour (222ºC per hour) to 1150-1250ºF (621-677ºC) and equalize.
• Austenitizing (High Heat): Heat rapidly from preheat to 1550ºF (843°C). Soak for 30 minutes for the first inch (25.4mm) of thickness, plus 15 minutes for each additional inch.
• Quenching: Pressurized gas, or interrupted oil to 150-125ºF (66-51ºC). For oil, quench until black (approx. 900°F / 482°C), then cool in still air to 150-125°F (66-51°C).
• Tempering: Immediately after quenching or cryogenic treatment. Hold for 1 hour per inch of thickness (2 hours minimum), then air cool to ambient temperature. Tempering at 482-593°C (900 to 1100°F) for a Rockwell C hardness of 37 to 28.

C. Annealing

• Purpose: To soften the steel for re-hardening or extensive machining, if not supplied in the pre-hardened state.
• Procedure: Heat at a rate not exceeding 400°F per hour (222°C per hour) to 1450°F (788°C). Hold at temperature for 1 hour per inch (25.4mm) of maximum thickness (2 hours minimum). Cool slowly with the furnace at a rate not exceeding 50ºF per hour (28ºC per hour) to 1150ºF (621ºC). Continue cooling to ambient temperature in the furnace or in air.

IV. Forging

• P20 steel is forged at 1093°C (2000°F) down to 899°C (1650°F).
• Forging below 871°C (1600°F) is not recommended.

V. Applications

• Plastic injection mold cavities and tooling
• Die casting dies for zinc
• Low temperature applications
• Die holders, backers, frames for plastic pressure dies, hydro forming mold tools, bolsters
• Other applications such as rails, shafts, and wear strips

VI. International Equivalents

• USA: AISI P20 (ASTM A681)
• Germany: DIN 1.2311 and 1.2312 (DIN EN ISO 4957)
• Japan: PX5 (New Standard P20)
• China: 3Cr2Mo (GB/T 1299)
• Also: P20+Ni (1.2738), P20+S (1.2312) are variants.

1. What are the primary applications of P20 mold steel?

Ans:- P20 mold steel is primarily used for plastic injection mold cavities and tooling, as well as for die casting dies for zinc. Its versatility also extends to other low-temperature applications like die holders and wear strips.

2. In what condition is P20 mold steel typically sold, and what is its approximate hardness in that state?

Ans:- P20 mold steel is typically sold in a pre-hardened condition. In this state, its approximate hardness is 300 HBW (Brinell Hardness) or 30 HRC (Rockwell C).

3. Why is stress relieving important for P20 mold steel, and at what temperature is it typically performed?

Ans:- Stress relieving is important for P20 mold steel to alleviate internal stresses that accumulate after machining or during extended service. It is typically performed by heating the steel to 900ºF (482ºC) for a specified duration based on thickness.

4. Describe the key steps involved in the hardening process of P20 steel, assuming it needs to be re-hardened.

Ans:- For re-hardening P20 steel, it must first be annealed. Then, it’s preheated to 1150-1250ºF, rapidly heated to 1550ºF for austenitizing, and finally quenched using pressurized gas or interrupted oil to achieve desired hardness.

5. What is the purpose of annealing P20 mold steel, and what are the general temperature parameters for this process?

Ans:- The purpose of annealing P20 mold steel is to soften it. This makes the steel more workable for extensive machining or prepares it for a re-hardening process. Annealing typically involves heating to 1400-1450°F (760-788°C) and then slow cooling in the furnace.

6. List three important physical properties of P20 mold steel.

Ans:- Three important physical properties of P20 mold steel include its density of 0.284 lb/in³, a specific gravity of 7.86, and a Modulus of Elasticity of 30 x 10⁶ psi. It also has a thermal conductivity of 24 Btu/ft/hr/°F.

7. How does the machinability of P20 mold steel compare to that of a 1% carbon steel?

Ans:- The machinability of P20 mold steel is rated at 60-65% of a 1% carbon steel. This indicates that it is moderately machinable, less so than a standard 1% carbon steel.

8. Identify two elements from the chemical composition of P20 steel and briefly mention their general role in steel properties.

Ans:- Chromium (Cr) and Molybdenum (Mo) are two elements in P20 steel. Chromium enhances toughness and hardness, while Molybdenum contributes to hardenability and strength at elevated temperatures.

9. If P20 steel needs to be forged, what is the recommended temperature range, and what temperature should be avoided?

Ans:- P20 steel is recommended to be forged at temperatures ranging from 1093°C (2000°F) down to 899°C (1650°F). Forging below 871°C (1600°F) is not recommended as it could negatively impact the material’s properties.

10. Name at least two international equivalent grades for P20 mold steel from different countries.

Ans:- Two international equivalent grades for P20 mold steel are DIN 1.2311 (Germany) and 3Cr2Mo (China). In Japan,

is the new standard equivalent.