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Heat Treatment of 3Cr2Mo (P20) Plastic Mold Steel

3Cr2Mo steel is a commonly used grade of imported plastic mold steel from the United States, and it is also the only plastic mold steel formally accepted in the GB/T 1299-2000 Alloy Tool Steel.

1. Chemical Composition and Critical Phase Transition Point

The chemical composition is shown in Table 1.

Critical phase transition point: Ac1 770℃, Ac3 825℃, Ar1 640℃, Ar3 760℃, Ms 300℃, Mf 120℃.

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

Table 1 Chemical composition of 3Cr2Mo steel (mass fraction/%)

2. Process Performance

Forging: heating temperature 1100~1150℃, initial forging temperature 1050~1100℃, final forging temperature ≥850℃, air cooling after forging.

Annealing: heating temperature 850℃, heat preservation 2~4h, isothermal temperature 720℃, heat preservation 4～ 6h, the furnace is cooled to 500°C, and the furnace is air-cooled

Quenching and tempering: quenching heating temperature is 860~870°C, oil quenching, and tempering at 540~580°C. The pre-hardening hardness is 30~35HRC

Chemical heat treatment: P20 steel has good hardenability and certain toughness, and can be carburized. After carburizing and quenching, the surface hardness can reach 65HRC, which has high thermal hardness and wear resistance. sex.

3. Mechanical properties

The room temperature mechanical properties of P20 steel quenched at 850°C and tempered at 550°C are shown in Table 2.

HRC	σb/MPa	σ0.2/MPa	δ(%)	ψ(%)	ak/(J/cm²)
30	1250	1040	14	58	11.5

4. Practical Application

P20 steel is suitable for the manufacture of large plastic molds such as the casing of TV sets, large cassette recorders and washing machine panel covers. Its machinability and polishing performance are significantly better than 45 steel. Under the same polishing conditions, the surface roughness is 1~3 lower than that of 45 steel. class.