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Heat Treatment of 3Cr2NiMo(P4410) Plastic Mould Steel

1. Chemical Composition and Critical Phase Transition Point.

3Cr2NiMo steel is an improved type of 3Cr2Mo steel. It is 0.8%~1.2% nickel added to 3Cr2Mo steel. Its chemical composition is shown in Table 1. The actual composition of the domestic trial-produced P4410 steel is consistent with the improved 718 steel P20 steel produced in Sweden.

Critical phase transition point: Ac1 725℃, Ac3 810℃, Ms 280℃.

C	Mn	Si	Cr	Mo	Ni	P	S
0.28～0.40	0.60~1.00	0.20～0.80	1.40~2.00	0.30～0.55	0.80～1.20	≤0.020	≤0.015

Table 1 The chemical composition of P4410 (mass fraction/%)

2. Production Process

Alkaline open-hearth furnace roughing→vacuum degassing, ladle powder spraying refining→hydraulic forging→rough processing→ultrasonic flaw detection→quenched and tempered heat treatment→inspection and delivery. The steel produced by this process achieves high cleanliness, fine structure, good mirror polishing performance, and surface roughness Ra can reach 0.05~0.025μm.

3. Performance Characteristics

After quenching at 860°C and tempering at 650°C, the mechanical properties at room temperature and high temperature are shown in Table 2.

Experiment Temperature/℃	σb/MPa	σs/MPa	δ( 010)	ψ(%)	ak/(J/cm²)	HRC
Room temperature	1120	1020	16	61	96	35
200	1006	882	13.6	56	–	–
400	882	811	14.0	67	–	–

Table 2 Mechanical properties of P4410 steel

The hardness value of P4410 steel is in the range of 32~36HRC, and it has good processing properties such as turning, milling and grinding.

P4410 steel can also be quenched by flame partial heating, heating temperature 800~825℃, cooling in air or compressed air, the local surface hardness can reach 56~62HRC, which can extend the service life of the mold. The surface of the mold can also be chrome-plated, and the surface hardness can be increased from 370~420HV to 1000HV, which significantly improves the wear resistance and corrosion resistance of the mold.

The mold made of P4410 steel can also be repaired by repair welding after partial damage. The welding quality is good and can be processed.

4. Practical Application

P4410 steel is used in the pre-hardened state (30~36HRC) to prevent heat treatment deformation, and is suitable for manufacturing large, complex and precise plastic molds. The steel can also be subjected to chemical heat treatment such as nitriding and boronizing, and higher surface hardness can be obtained after treatment, which is suitable for making high-precision plastic molds.