Abstract: This article delves into the research and development process of high hardness roller teeth steel castings, addressing the technical challenges posed by their complex structure, high hardness requirements, and stringent dimensional tolerances. Through meticulous process planning, the application of advanced simulation software, strict composition control, and innovative heat treatment processes, we successfully produced roller teeth steel castings that met customer specifications. The experience gained from this project has significant implications for the casting industry.
1. Introduction
Roller teeth serve as the core component in drum-type crushers, tasked with crushing materials through the interaction of two side-by-side, counter-rotating rollers with interlocking teeth. The harsh operating environment, combined with the variability in material hardness and composition, necessitates roller teeth with dense internal quality and high hardness to ensure longevity. Additionally, the teeth must maintain a complete and precise tooth profile to guarantee seamless engagement during operation. This paper presents the research and development efforts undertaken to produce such roller teeth steel castings for an international client.
2. Technical Requirements of Roller Teeth Steel Castings
2.1 Product Parameters
The roller teeth steel castings weigh 2,500 kg, with an overall dimension of ϕ700 mm in diameter and 2,550 mm in height. The main body wall thickness is 55 mm, and there are 864 teeth around its circumference. The material used is MCL400, a proprietary cast steel grade formulated based on client-specified chemical compositions (Table 1).
Table 1. Chemical Composition Requirements of Roller Teeth (Mass Fraction, %)
| Element | Client Requirement |
|---|---|
| C | 0.25 – 0.29 |
| Mn | 1.00 – 1.20 |
| Si | 0.20 – 0.40 |
| P | ≤ 0.025 |
| S | ≤ 0.015 |
| Cr | 1.25 – 2.00 |
| Ni | 3.20 – 4.00 |
| Mo | 0.25 – 0.50 |
2.2 Main Technical Requirements
- Quality Assurance: Due to the inability to conduct ultrasonic testing (UT) on the roller body, the client relies on casting process CAE simulations to ensure internal quality. Magnetic particle inspection is performed on the entire casting, adhering to GB/T 9444-2019, with acceptance criteria at Level 2.
- Mechanical Properties: The hardness of the casting body must not be less than 400 HBW.
- Dimensional Tolerance: Dimensional tolerances follow GB/T 6414-2017, CT12 grade, with all 864 teeth passing a go/no-go gauge inspection.
2.3 Technical Challenges
- Complex Casting Structure: The roller teeth feature a complex structure with dispersed hot spots, making it challenging to achieve effective feeding and shrinkage compensation.
- High Hardness Requirements: Meeting the high hardness specifications necessitates precise control of chemical composition and tailored heat treatment processes.
- High Aspect Ratio and Dimensional Precision: The high aspect ratio and stringent dimensional tolerance requirements, coupled with limited access for core assembly and mold matching, make casting operations extremely difficult.
3. Casting Process Design Scheme
3.1 Addressing Solidification and Shrinkage Issues
- Heating Risers: Three heating risers are placed at the top of the casting to ensure effective feeding.
- Pouring System: A buffered stepped gate with five layers of horizontal runners and four variable-diameter slot gates per layer is employed. External chills are placed at the gate interfaces to prevent shrinkage defects.
- Yield Rate: With a gross weight of 2,500 kg and a pouring weight of 3,200 kg, the process yield rate is 78.1%.
- Exothermic Covering Agent: After pouring, 2.5 kg of exothermic covering agent is evenly applied to each riser.
Using ProCAST simulation software, the solidification process was modeled, revealing no significant isolated liquid phases and achieving simultaneous solidification. Analysis of shrinkage porosity showed only a few small pores with an equivalent diameter not exceeding ϕ5 mm, satisfying technical requirements.
3.2 Measures to Ensure High Hardness
3.2.1 Composition Control
To meet the high hardness requirements, stricter control over chemical compositions was implemented (Table 2).
Table 2. Adjusted Chemical Composition Range for High Hardness (Mass Fraction, %)
| Element | Adjusted Range |
|---|---|
| Cr | 1.6 – 2.0 |
| Ni | 3.6 – 4.0 |
| Mo | 0.4 – 0.5 |
| Mn | 1.1 – 1.2 |
| Si | 0.3 – 0.4 |
3.2.2 Heat Treatment Process Design
Conventional normalizing + tempering heat treatment was inadequate for achieving the required hardness. Quenching and tempering, while effective, was deemed too costly and risked causing deformation or cracking due to the casting’s structure. Instead, a rapid furnace exit followed by intense air blowing and mist spray during normalization was adopted, successfully meeting the client’s mechanical property specifications.
4. Key Safeguards During Casting
4.1 Wood Pattern Production Control
- Iron Frame Core Boxes: Iron frame core boxes increase rigidity and strength, preventing core deformation.
- Precise Core Box Dimensions: Ensure accurate tooth dimensions and reasonable movable part settings for smooth mold stripping without damaging tooth profiles .
4.2 Molding Operation Control
- Core Material: Baozhu furan resin sand is used for cores. External chills are accurately positioned, numbered, and sized based on the process.
- Pouring Gate Bricks: Carefully place gate bricks and securely weld core bones together after assembling the two semicircular cores, crucial for preventing leakage.
- Coating Application: Coat sand molds and cores with alcohol-based paint twice, ensuring even application, especially on tooth positions.
- Mold Assembly Control: Position sand cores accurately and use flat steel to brace the core backs against the mold box to prevent swelling.
- Cavity Inspection: Inspect the cavity before and after pouring to ensure it is clean.
4.3 Pouring Process Control
The pouring temperature is set at 1,560 °C, with a moderate initial pouring speed to facilitate impurity removal and better tooth filling. The pouring rate is slowed down later to prevent false fullness in the risers.
4.4 Post-Process Finishing
Use inspection gauges to verify roller teeth dimensions and perform finishing polish on the castings.
5. Development Results
Thanks to thorough preparation, our company successfully produced the high hardness export roller teeth steel castings on the first try, followed by over a dozen subsequent successful productions. Additional roller teeth of different specifications are currently under preparation.
6. Conclusion
The successful development of roller teeth represents a significant technological innovation, positively impacting our company’s export market expansion. The project accumulated valuable experience:
- Casting Process: By adhering to the principle of simultaneous solidification and leveraging meticulous planning and ProCAST simulation optimization, we met stringent quality requirements without compromising yield rates.
- Hardness and Mechanical Properties Control: Precise control over alloy elements like Cr, Ni, Mn, and Mo ensured mechanical strength, while tailored heat treatment processes, such as rapid furnace exit, intense air blowing, and mist spray, enhanced hardness.
- Dimensional Control: Developed an effective method for controlling product dimensions across complex influencing factors.
- Process Control: Implemented a comprehensive quality plan, clarifying personnel responsibilities, preventing quality issues, and tracking and monitoring throughout the process, effectively mitigating quality risks.
The experience gained from this project underscores the importance of advanced simulation, stringent composition control, and innovative heat treatment processes in producing high-quality steel castings.