The electric roller roll excavator roller produced by China First Heavy Machinery Co., Ltd. is made from P&H107 material, a type of steel developed by the American P&H Company (material code 4350 under the American Iron and Steel Association). However, this specific material standard is not available in China. While the 50CrNiMo component in the JB/T6401-1992 standard is somewhat similar, its mechanical performance is significantly lower than that of P&H107. As a result, there are no established empirical parameters for production, making it challenging to meet the required specifications.
Based on previous domestic experience, it was possible to achieve close strength and hardness levels, but the plasticity and toughness—especially at low temperatures—did not meet the foreign design standards. This affected the overall performance and service life of the workpieces. To address this, the company sent technical experts to inspect domestic manufacturers in search of potential collaborations, but none could fully satisfy the rigorous inspection criteria of P&H products.
In this study, we focused on optimizing the heat treatment process for the roller shovel roller. Through extensive testing and analysis, we determined the most suitable heat treatment parameters for our equipment. The goal was to ensure that the final product met or exceeded the required mechanical properties while maintaining structural integrity and durability.
**1. P&H107 Roller Roll**
The chemical composition of the P&H107 roller is detailed in Table 1. The manufacturing process includes steelmaking, forging, rough machining, tempering heat treatment, and finishing. During forging, the rod is first formed, and then rough machining is performed according to the process requirements. The hardness limits are set based on the same batch of steel and heat treatment conditions. Mechanical property samples are taken from the roughed sections, with two tensile specimens and three longitudinal and tangential impact samples collected. If several consecutive furnace batches pass the test, the mechanical properties can be accepted. The technical requirements after quenching and tempering are outlined in Table 2.
The critical transformation point of the P&H107 material was measured using a DT-1000 automatic thermal dilatometer, with a post-forged annealed specimen. The sample size was φ3 mm × 50 mm, and the test conditions included a heating rate of 200°C/h, an austenitizing temperature of 880°C, and a holding time of 20 minutes. The vacuum level was maintained at ≤10â»Â³ Pa to prevent decarburization. The results of the critical point test are presented in Table 3.
**2. Formulation of Heat Treatment Process Parameters**
(1) **Selection of Process Parameters**
Currently, the cooling parameters for quenching and tempering of structural steels are based on the carbon equivalent. According to the regulations:
- If the carbon content (wC) in the positive segregation zone is ≤0.31% and the carbon equivalent is ≤0.75%, water quenching can be safely applied.
- If wC is between 0.32% and 0.36%, and the carbon equivalent is between 0.75% and 0.88%, water quenching is allowed with caution.
- If wC ≥ 0.36% and the carbon equivalent ≥ 0.88%, water quenching is prohibited unless special guidance is provided.
P&H107 has a carbon content of approximately 0.50%, which falls into the third category. Therefore, oil quenching is typically used. To achieve ultra-high strength, the austenitizing temperature was increased by 20–30°C above the standard (Ac3 + 50°C), enhancing hardenability. At the same time, the tempering temperature was reduced as much as possible. Raising the quenching temperature slightly improves the toughness of medium-carbon alloy steel by reducing flake martensite formation. However, excessive quenching temperature increases retained austenite, lowering hardness.
Other parameters were determined based on the effective cross-section size of the roller (200mm), considering the number of parts in the furnace and the attachment effect. The specific heat treatment parameters are shown in Figure 2.
(2) **Process Test Results**
Using the process parameters from Figure 2, heat treatment was conducted on two steelmaking furnaces (a total of 12 pieces). The actual chemical composition is listed in Table 4. After heat treatment, four hardness limits were tested for mechanical properties, and the results are shown in Table 5. Although the strength was slightly below expectations, the plasticity, especially the low-temperature impact toughness, did not meet the design requirements. Further experiments confirmed similar results.
**3. Optimization of Heat Treatment Process Parameters**
Increasing the quenching cooling speed leads to more martensite formation and a deeper hardened layer, improving both the comprehensive mechanical properties and low-temperature impact resistance of the workpiece. However, using water cooling for P&H107 carries significant risks, so strict control is necessary.
The design of the quenching tank and agitation system plays a crucial role in the effectiveness of water cooling. By carefully controlling the water-cooling conditions, we achieved the desired quenching effect for the P&H107 roller. Figure 3 shows the cooling capacity of water at different temperatures and N46 oil.
Due to the increased quenching speed, the austenitizing temperature could be slightly reduced, and the tempering temperature increased, which improved the plastic toughness of the roller. Considering the use of a trolley furnace, the time between quenching and entering the water tank must be minimized. The high-temperature austenitizing temperature was set to Ac3 + (60–70)°C. Our smallest trolley furnace measures 2600mm × 1300mm × 800mm, and each batch can hold over 12 rollers. The smallest quenching tank is 5000mm × 3000mm × 5000mm, with a maximum cooling capacity of 20 tons. To avoid overcooling, the water temperature before quenching was controlled at 30–40°C, providing a cooling capacity between water and oil. The cooling speed was adjusted during the film boiling and bubble boiling stages by oscillating the workpiece. The surface temperature after quenching was kept between 100–150°C. The specific process parameters are shown in Figure 4.
**4. Conclusion**
(1) Forging the long bar first results in noticeable forged fibers, which can negatively affect the radial performance of the roller after heat treatment.
(2) By precisely controlling the water quenching parameters and optimizing the process, the roller achieves comprehensive mechanical properties far exceeding domestic standards, fully realizing the material’s potential. After adjusting the forging process, the final product met all technical requirements of the US P&H company and received approval from the foreign partner.
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