The level of mold manufacturing is an important indicator to measure a country’s product manufacturing level, and the mold itself is high in cost, known as “black gold”.
With the development of the automotive industry, especially the rapid development of new energy vehicles in China, the demand for molds has continued to increase, and the requirements for mold quality and service life have become higher and higher.
Therefore, it is necessary to choose the mold material reasonably, develop the correct heat treatment process, select the appropriate surface treatment method, and research and develop new mold materials.
In this article, we use the actual production of our company’s die-casting mold material as an example to analyze and discuss the process methods for extending the service life of molds.
Manufacturing of Die-casting Molds
Technical Requirements
The technical requirements for manufacturing die-casting molds are as follows:
- Material: H68M
- Dimensions: 400mm × 910mm × (3000 ~ 5000mm)
Production process: Electroslag Ingot → Cutting → Heating → Forging → Heat Treatment → Machining → Inspection → Shipping.
⑴ Surface hardness requirement: ≤229HBW, single hardness difference: ≤40HBW.
⑵ Forging ratio: ≥5.
⑶ Performance testing requirements: Notch impact energy: not less than 25J, no notch impact energy: not less than 350J.
⑷ Non-destructive testing UT individual defect equivalent: ≤1.0mm. Dense defects are not allowed.
⑸ Grain size: finer than level 6.
⑹ Inclusion requirements: Refer to Table 1.
Table 1: Inclusion Requirements for Mold Materials
| Class A | Class B | Class C | Class D | Ds Class (Level) | ||||
| Coarse | Fine | Coarse | Fine | Coarse | Fine | Coarse | Fine | |
| ≤0.5 | ≤0.5 | ≤1.0 | ≤1.0 | ≤0.5 | ≤0.5 | ≤1.0 | ≤1.0 | ≤1.0 |
⑺ Raw materials are made using electric arc furnace + refining + vacuum degassing + electroslag remelting. The chemical composition is shown in Table 2, with [H] ≤2.0PPm, [O] ≤25PPm, and [N] = 70~170PPm.
Table 2: Chemical Composition of Mold Materials (mass fraction, %)
| Element | C | Si | Mn | P | S | Cr | Mo | V |
| Min | 0.36 | 0.25 | 0.25 | – | – | 4.95 | 2.2 | 0.50 |
| Max | 0.42 | 0.45 | 0.45 | 0.015 | 0.005 | 5.25 | 2.8 | 0.80 |
Production Process:
⑴ Use a 12.5t electroslag ingot to remove the sprue and flash from the ingot plate, then load it into the gas heating furnace. The heating temperature is controlled at 1260℃. After 24 hours of insulation, forging is started using the two upset and two pull process with a forging ratio greater than 6, as shown in Figure 1.
⑵ After forging, adopt air-cooling control and timely put it into the annealing furnace for post-forging heat treatment at around 400°C. The heating temperature for heat treatment is 850°C for 20 hours of insulation, and then 730°C for 30 hours of insulation. The heating furnace is cooled at less than 30°C/h, and the mold is taken out of the furnace when the furnace temperature is below 450°C.
⑶ After heat treatment, perform machine processing according to the drawing requirements. After the processing is completed, perform UT testing. Qualified products will be shipped.
The main factors affecting the service life of die-casting molds are:
⑴ The purity of the material’s interior.
⑵ The reasonable selection of material manufacturing process.
⑶ The rationality of die-casting mold design and working conditions.
Process Improvement for New Die-Casting Mold Material H68M
⑴ According to the previous production process, the service life of H68M mold material was not significantly improved compared to other domestic materials.
Our technical personnel tracked and analyzed the smelting process, forging process control, and heat treatment control on site to obtain information on purity, structure, and grain size as shown in Figures 2, 3, and 4. This provided experience for further improving the material’s service life control in the future.
Regarding the extensive experimental data above, our technical research team has analyzed and implemented the following process improvements for smelting, forging, and post-forging heat treatment.
Improvement measure 1
The ideal base alloy microstructure was achieved by reasonable composition design. The improved chemical composition is shown in Table 3.
Table 3 Adjusted Chemical Composition (mass fraction, %) of H68M after Improvement
| Element | C | Si | Mn | P | S | Cr | Mo | V |
| Min | 0.36 | 0.20 | 0.30 | – | – | 4.95 | 2.350 | 0.55 |
| Max | 0.40 | 0.40 | 0.45 | 0.015 | 0.005 | 5.15 | 2.65 | 0.75 |
Improvement measure 2
Clean scrap steel and alloy were used as raw materials and effective measures such as LF refining, vacuum degassing, and electric slag under protective atmosphere were employed to reduce the content of impurity elements, harmful gases, and inclusions in the steel.
This resulted in obtaining pure forging billets. In the smelting process, the risk of material failure caused by this was controlled, and the material purity can be controlled to the level shown in Table 4. The high magnification image of inclusions is shown in Figure 5.
Table 4 Adjusted High Magnification Detection Data after Improvement
| Grade | Class A | Class B | Class C | Class D | Ds Class (Level) | ||||
| H68M | Coarse | Fine | Coarse | Fine | Coarse | Fine | Coarse | Fine | |
| ≤0.5 | ≤0.5 | ≤1.0 | ≤1.0 | ≤0.5 | ≤0.5 | ≤1.0 | ≤1.0 | ≤1.0 | |
Improvement measure 3
During heating, high-temperature diffusion at 1280℃ was employed. The forging process used three-drawing and three-sizing process control and cross-die roughing process with the final deformation amount controlled to be greater than 30%.
The combination of multi-directional compression forging and EFS (Ultrafine refining) process was used to obtain refined grains and uniform annealed microstructure.
This allowed for good hot treatment microstructure even after machining and conditioning, which provided a good foundation for mold use. The adjusted microstructure image is shown in Figure 6, and the grain size image is shown in Figure 7.
By implementing the above measures, our H68M material produced conforms to the standards of the North American Die Casting Association.
The die-casting mold material should be tested for V-notch impact value to inspect the toughness of the material.
At the same time, the material’s ductility should be tested by testing its impact energy without notches. H68M conforms to the Dievar test requirements and obtains the impact value under operating conditions hardened to 44-46HRC, as shown in Table 5.
Table 5 Impact Value of H68M Material Hardened to 44-46HRC Under Operating Conditions
| Grade | V-shaped (J) | No gap (J) | ||
| average value | min value | average value | min value | |
| 1.2367 | ≥19 | ≥14.9 | ≥300 | ≥269 |
| H68M | ≥25 | ≥18 | ≥350 | ≥300 |
Through optimized design of the material’s alloy composition and strict control of the batching, smelting, electric slag, forging, and post-forging heat treatment processes during production, H68M material exhibits excellent performance in high hardenability, high red hardness, high toughness, and high ductility.
Conclusion
(1) By adjusting the chemical composition design of H68M material, it has become a hot work mold steel grade with high hardenability, high red hardness, high toughness, and high ductility.
(2) H68M material has undergone electric slag, high-temperature homogenization, grain refinement treatment and three times of strong drawing and sizing process to exceed customer expectations. The lifespan of 10,000 molds required by customer contract orders can now be improved to more than 20,000, greatly reducing the cost of mold use and creating value for customers.
