Abstract: This article presents the research and development process of high hardness roller teeth steel castings. Roller teeth, as the core component of drum crushers, are subjected to severe working conditions and require excellent mechanical properties, dimensional accuracy, and hardness. The article discusses the technical challenges faced during the development, including the complex structure of the castings, high hardness requirements, and stringent dimensional tolerances. Through a combination of casting process design, heat treatment optimization, and stringent process control measures, a solution was found to address these challenges. The use of simulation software, strict chemical composition control, and innovative heat treatment methods were crucial in ensuring the quality of the roller teeth steel castings. The successful development of this product has not only met customer requirements but also provided valuable experience for steel castings manufacturers in similar applications.
1. Introduction
Roller teeth, serving as the key component in drum crushers, play a vital role in the crushing process of materials. These teeth are arranged in pairs and rotate in opposite directions, with their teeth interlocking to break down materials. Due to the harsh working environment and the need for non-replaceable wearing parts, roller teeth must possess excellent mechanical properties, including high hardness, good wear resistance, and dimensional accuracy.
This article focuses on the research and development of high hardness roller teeth steel castings for a foreign client. The project faced multiple challenges, such as the complex structure of the castings, high hardness requirements, and strict dimensional tolerances. Through a combination of casting process design, heat treatment optimization, and process control measures, we successfully developed a product that met the client’s requirements.
2. Technical Requirements and Challenges
2.1 Product Parameters
The roller teeth steel castings have a gross weight of 2,500 kg, with dimensions of ϕ700 mm in diameter and 2,550 mm in height. The main body has a wall thickness of 55 mm, and there are 864 teeth distributed around its circumference. The material used is MCL400, a custom-made steel grade developed by our company based on the client’s specified chemical composition.
Table 1 shows the detailed chemical composition requirements for the roller teeth.
Table 1: Chemical Composition Requirements of Roller Teeth (mass fraction, %)
| Element | C | Mn | Si | P | S | Cr | Ni | Mo |
|---|---|---|---|---|---|---|---|---|
| Client Requirement | 0.25~0.29 | 1.00~1.20 | 0.20~0.40 | ≤0.025 | ≤0.015 | 1.25~2.00 | 3.20~4.00 | 0.25~0.50 |
2.2 Main Technical Requirements
2.2.1 Quality Assurance
Due to the inability to conduct UT (Ultrasonic Testing) on the roller body, the client requires a CAE (Computer-Aided Engineering) simulation of the casting process to ensure internal quality. Additionally, the entire surface of the casting must undergo magnetic particle inspection in accordance with GB/T 9444-2019, with a grade 2 acceptance standard.
2.2.2 Mechanical Properties
The hardness of the roller teeth must not be less than 400 HBW. This high hardness requirement ensures the castings can withstand the rigors of the crushing process.
2.2.3 Dimensional Tolerance
The dimensional tolerance is specified as CT12 in accordance with GB/T 6414-2017. The tooth positions must be verified using a sample plate provided by the client, ensuring all 864 teeth can pass the inspection simultaneously.
2.3 Technical Challenges
2.3.1 Complex Casting Structure
The roller teeth castings exhibit a complex structure with distributed hot spots, making it challenging to achieve proper feeding and shrinkage compensation.
2.3.2 High Hardness Requirements
The demand for high hardness necessitates precise control of the chemical composition and an effective heat treatment process.
2.3.3 Stringent Dimensional Tolerances
The high aspect ratio and tight dimensional tolerances of the roller teeth castings, combined with the small operating area for core assembly and mold preparation, make it difficult to ensure proper filling of the tooth positions and prevent pouring defects such as swelling and leakage.
3. Casting Process Design
3.1 Solution to Shrinkage Problems
To address the issue of shrinkage pores, the following measures were taken:
- Heating Risers: Three heating risers were placed at the top of the casting to ensure effective feeding.
- Buffered Stepped Gating System: A buffered stepped gating system with five layers of horizontal runners and four variable-diameter bricks per layer was used. External chillers were placed at the corresponding positions of the ingates to prevent shrinkage defects at the junction of the ingates.
The gross weight of the roller teeth was 2,500 kg, with a pouring weight of 3,200 kg, resulting in a process yield of 78.1%. After pouring, 2.5 kg of heating cover was evenly applied to each riser.
ProCAST casting simulation software was employed to model the solidification process. The simulation results indicated that no significant isolated liquid phases were present, satisfying the requirement for simultaneous solidification. The analysis of shrinkage porosity revealed only a small number of pores with an equivalent diameter of no more than ϕ5 mm, meeting the technical specifications.
3.2 Measures to Ensure High Hardness of Roller Teeth Steel Castings
3.2.1 Composition Control
To meet the high hardness requirement of the roller teeth steel castings, stricter control over the chemical composition was implemented:
- Chromium (Cr): Cr enhances the strength, hardness, wear resistance, and hardenability of steel. Therefore, the Cr content was controlled at the mid-to-upper limit, ranging from 1.6% to 2.0%.
- Nickel (Ni): Ni improves the strength of steel without compromising its ductility, enhances low-temperature toughness, and enhances hardenability. The Ni content was controlled at the mid-to-upper limit, ranging from 3.6% to 4.0%.
- Molybdenum (Mo): Mo increases the strength, hardness, and hardenability of steel. The Mo content was controlled at the mid-to-upper limit, ranging from 0.4% to 0.5%.
- Manganese (Mn): Mn enhances the strength, hardness, wear resistance, and hardenability of steel. The Mn content was controlled at the mid-to-upper limit, ranging from 1.1% to 1.2%.
- Silicon (Si): Si improves the strength, hardness, hardenability, corrosion resistance, and heat resistance of steel. The Si content was controlled at the mid-to-upper limit, ranging from 0.3% to 0.4%.
Table 2: Controlled Chemical Composition of Roller Teeth Steel Castings
| Element | Target Range (%) |
|---|---|
| C | 0.25~0.29 |
| Mn | 1.1~1.2 |
| Si | 0.3~0.4 |
| P | ≤0.025 |
| S | ≤0.015 |
| Cr | 1.6~2.0 |
| Ni | 3.6~4.0 |
| Mo | 0.4~0.5 |
3.2.2 Heat Treatment Process Design
Conventional normalization and tempering heat treatment alone could not meet the high hardness requirements of the product. Quenching and tempering, while effective, carries high costs and a risk of deformation or cracking due to the complex structure of the product. Instead, our company adopted a unique heat treatment process involving rapid furnace discharge, followed by intense air blowing and spraying, which successfully met the client’s mechanical property requirements.
3.3 Key Process Control Measures During Casting
3.3.1 Wood Pattern Making Control
- Core Box Design: Iron frame templates were used to increase the rigidity and strength of the core boxes, preventing deformation of the sand cores.
- Precision of Gear Position Dimensions: The dimensions of the gear positions in the core boxes were precise, and the movable blocks for the gear positions were designed reasonably to ensure smooth mold release without damaging the gear profiles.
- Mold Inspection: Strict inspection of mold dimensions was conducted during mold manufacturing.
3.3.2 Molding Operation Control
- Core Sand Material: Pearl furan resin sand was used for the cores. External chillers were placed accurately according to the process on the solid samples and core boxes.
- Gate Brick Placement: When combining the two semicircular sand cores, the core bones were firmly welded together, which is crucial for preventing swelling and leakage.
- Coating Application: The sand molds and cores were coated with alcohol-based paint twice, followed by inspection to ensure uniform coating, especially in the gear positions, to prevent accumulation.
- Mold Assembly Control: After placing the sand cores in position, flat steel bars were used to secure the backs of the cores to the flask bands to prevent swelling.
- Cavity Inspection: The mold cavity was inspected before closing the flasks to remove any floating sand or debris, and the cavity was checked again before pouring to ensure cleanliness.
3.3.3 Pouring Process Control
The pouring temperature was set at 1,560 °C, with a moderate initial pouring speed to facilitate the floatation of impurities and better filling of the gear positions. The pouring speed was reduced later to prevent false fullness in the risers.
3.3.4 Post-Process Finishing
The roller teeth steel castings were inspected using a sample plate and subsequently finished and polished.
4. Development Results
Due to thorough preparation, the high hardness roller teeth steel castings were successfully developed on the first trial. Subsequently, over a dozen more castings were produced, all meeting the client’s requirements. Additionally, several roller teeth of different specifications are currently under trial preparation.
5. Conclusion
The successful development of the high hardness roller teeth steel castings represents a significant technological innovation and has positively impacted our company’s expansion into the export market. The following key lessons were learned during the trial production and subsequent mass production:
- Casting Process: The application of the simultaneous solidification principle, combined with meticulous process planning and ProCAST simulation optimization, allowed us to meet the stringent quality requirements without compromising the process yield.
- Hardness and Mechanical Properties: Precise control over the alloying elements such as Cr, Ni, Mn, and Mo ensured the mechanical strength of the castings. Additionally, the unique heat treatment process involving rapid furnace discharge, intense air blowing, and spraying effectively enhanced the hardness of the castings.
- Dimensional Control: We developed a comprehensive method for controlling product dimensions throughout the process, effectively addressing the impact of various complex factors.
- Process Control: By establishing a comprehensive quality plan, clarifying responsibilities, and implementing preventive measures and continuous monitoring throughout the process, we effectively mitigated quality issues.
The successful development of the roller teeth steel castings not only met the client’s requirements but also provided valuable experience for steel castings manufacturers in similar applications, showcasing the importance of advanced casting technology, strict process control, and innovative solutions in addressing complex challenges.