This note describes a production practice aimed at reducing the energy consumption of P20 plastic mold steel heat treatment by replacing the conventional offline-normalizing route with controlled rolling and cooling plus low-temperature tempering. The steel used in the trial was produced by the medium-plate mill of a steel works, with the chemical composition below (mass fraction, %):
| C | Si | Mn | Cr | Mo | B / Ti |
|---|---|---|---|---|---|
| 0.35 | 0.38 | 1.12 | 1.70 | 0.40 | Trace |
1. Determination of Critical Points
The phase transformation points were measured with a dilatometer using a heating rate of 200 °C/h, following the applicable Chinese standard for the determination of steel critical points. The measured values were Ac1 = 740 °C and Ac3 = 800 °C.
2. Experimental Procedure
- Hot-rolled samples were quenched at 820, 860, 900 and 940 °C with a holding time of 40 minutes.
- Quenched samples were tempered at 400, 500, 550 and 600 °C.
- To simulate mass-production conditions, direct tempering trials were also conducted in the hot-rolled state.
- Metallographic samples were prepared, etched with nitric-acid alcohol, and examined under an optical microscope; Rockwell hardness was measured on each sample.
3. Results
(1) Adding a trace amount of boron prolongs the incubation period of ferrite transformation, inhibits ferrite formation and improves hardenability, making the through-thickness hardness of thick plates more uniform.
(2) After quenching at 900 °C, tempering at increasing temperatures gradually coarsens the martensite laths and precipitates dispersed fine carbides. At 550 °C some carbide particles begin to grow, and tempering hardness declines as the tempering temperature rises, with the rate of decline accelerating above 550 °C.
(3) The on-site trial adopted controlled rolling and cooling plus low-temperature tempering. The resulting structure is tempered bainite, which combines good strength, toughness and sufficient hardness - a Rockwell hardness between 32 and 36 HRC. Replacing offline normalizing with rolling-in-lieu-of-normalizing delivers significant energy and economic benefits.
4. Conclusion and Practical Implications
For P20 plastic mold steel, the low-energy route of controlled rolling and cooling with low-temperature tempering achieves the target hardness range of 32–36 HRC with a tempered bainite structure, while eliminating a full offline-normalizing heat treatment. The trial demonstrates that the process is suitable for mass production and offers a clear reduction in energy cost. Confirm the exact hardness and structure requirements of your application before selecting the process parameters.
Frequently Asked Questions
What is P20 plastic mold steel?
P20 is a chromium-molybdenum plastic mold steel (approximately C 0.35, Cr 1.70, Mo 0.40) supplied pre-hardened or in the annealed condition for plastic injection and die-casting molds.
What hardness does P20 reach with this low-energy process?
Controlled rolling and cooling plus low-temperature tempering produces a tempered bainite structure with a Rockwell hardness of 32–36 HRC.
How does the low-energy process reduce cost?
By replacing the conventional offline-normalizing heat treatment with rolling-in-lieu-of-normalizing, the energy-intensive reheating step is eliminated, lowering energy and processing cost.
What is the role of boron in the trial steel?
A trace boron addition prolongs the ferrite incubation period, inhibits ferrite formation and improves hardenability, giving more uniform through-thickness hardness in thick plates.
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