3Cr2Mo is a plastic mold steel under the Chinese GB/T system, and its equivalent steel is ASTM/AISI P20 steel in the United States and DIN 1.2311 in Germany. Its primary application is in mold manufacturing, particularly for plastic injection molds and die casting molds that handle lower-melting-point alloys.

1. Chemical Composition of 3Cr2Mo Steel(GB / T 1299—2000)

C	Si	Mn	Cr	Mo	P	S
0.28～0.40	0.20～0.80	0.60～1.00	1.40～2.00	0.30～0.55	≤0.030	≤0.030

2. Physical Properties of 3Cr2Mo Steel

2.1 Critical Temperatures of 3Cr2Mo Steel

Critical Point	Ac₁	Ac₃	Ar₁	Ar₃	Ms	Mf
Temperature (Approx.) /°C	770	825	640	760	335	180

2.2 Linear Expansion Coefficient of 3Cr2Mo Steel

Temperature/°C	18 ~ 100	18 ~ 200	18 ~ 300	18 ~ 400	18 ~ 500	18 ~ 600	18 ~ 700
Linear Expansion Coefficient α / x 10⁻⁶ °C⁻¹	11.90	12.20	12.50	12.81	13.11	13.41	13.71

2.3 Thermal Conductivity of 3Cr2Mo Steel

Temperature/°C	20	100	200	300	400
Thermal Conductivity λ / [W/(m·K)]	36.0	33.4	31.4	30.1	29.3

2.4 Other Physical Properties of 3Cr2Mo Steel

Density / (g/cm³)	Poisson’s Ratio μ	Elastic Modulus (Room Temp.) E/MPa	Shear Modulus (Room Temp.) G/MPa
7.81	0.288	212000	825000

3. Hot Forging Process Specification for 3Cr2Mo Steel

Item	Heating Temperature/°C	Initial Forging Temperature/°C	Final Forging Temperature/°C	Cooling
Steel Ingot	1180 ~ 1200	1130 ~ 1150	≥850	Pit cooling
Steel Billet	1120 ~ 1160	1070 ~ 1100	≥850	Sand cooling or slow cooling

4. Heat Treatment of 3Cr2Mo Steel

4.1 Preheating

Preliminary Heat Treatment Plan	Heating Temperature/°C	Isothermal Temperature	Hardness HBW
Isothermal Annealing	840 ~ 860°C × 2h	710 ~ 730°C × 4h, furnace cool to below 500°C then air cool	≤229
High-Temperature Tempering	720 ~ 740°C × 2h	Furnace cool to below 500°C then air cool	≤229

4.2 Quenching and Tempering Process

Quenching Specification			Tempering Specification
Quenching Temp./°C	Cooling Method	Hardness HRC	Tempering Temp./°C	Cooling Method	Hardness HRC
850 ~ 880	Oil cooling	50 ~ 52	580 ~ 640	Air cooling	28 ~ 36

Note: 3Cr2Mo steel can be carburized, nitrided, carbonitrided, or ion-nitrided before polishing. The surface roughness value Ra can be reduced to 0.03 μm. The surface hardness of the mold can be further improved, and the service life of the mold can be extended.

5. Mechanical Properties

3Cr2Mo pre-hardened steel has high hardenability, which allows steel with larger cross-sectional dimensions to obtain a more uniform hardness, and its comprehensive mechanical properties are good. This steel has excellent polishing performance, and the surface roughness of the molds made from it is low. The steel is pre-hardened at the factory to a hardness of 28 ~ 35 HRC. After cold working into a mold, it can be used directly, avoiding the deformation of the mold caused by heat treatment. After re-forging, it is re-heat-treated according to the performance requirements of the mold.

Effect of Tempering Temperature on the Mechanical Properties of 3Cr2Mo Steel

Tempering Temp./°C	Hardness HRC	Rₘ/MPa	Rₑₗ/MPa	A(%)	Z(%)	aₖ/(J/cm²)
450	42	—	—	—	—	50
500	41	1350	1200	11	52	60
550	38	1250	1200	14	58	80
600	33	1010	1140	17	65	115
650	26	900	720	20	58	150
700	21	790	600	23	69	180

6. Applications

3Cr2Mo steel has good comprehensive mechanical properties and high hardenability, enabling larger cross-sectional dimensions of steel to achieve uniform hardness. This steel exhibits excellent polishing properties, resulting in low surface roughness values for molds. When manufacturing molds from this steel, it is typically subjected to tempering treatment first, achieving a hardness of 28–35 HRC (i.e., pre-hardening), followed by cold working to form the mold, which can then be used directly, thereby avoiding deformation caused by heat treatment. Therefore, this steel grade is suitable for manufacturing large, medium-sized, and precision plastic molds, as well as die-casting molds for low-melting-point alloys (such as tin, zinc, and lead alloys).

For more demanding applications, such as high-volume aluminum die casting or extremely large and high-performance plastic molds, alternative grades such as 4Cr5MoSiV1 (H13) or 3Cr2MnNiMo (P20+Ni / 1.2738) may be better suited.