Zhuojin Pipe Fitting Co., LtdSurfacing Performance of Crankshaft Die Materials RMD248 and RMD647
Literature Overview
The paper by Sun Jianli and Gao Wenliang from Chengdu Aviation Vocational and Technical College (published in Foundry Technology, Vol. 36, No. 3, 2015, pp. 755-758) investigates the surfacing performance of two transition layer materials, RMD248 and RMD647, selected for crankshaft die applications. The study compares the room temperature and high temperature wear resistance, crack resistance, and the effects of tempering temperature and time on hardness and microstructure of the surfacing deposits. The optimal tempering process is determined to be 550°C for 8 hours.
Material Selection and Application Context
Crankshaft dies are subjected to extreme conditions during the forging process: high contact stresses, cyclic thermal loading, and abrasive wear from the hot workpiece. The transition layer between the die base material and the surfacing layer is critical for ensuring metallurgical compatibility and preventing cracking at the interface. The selection of RMD248 and RMD647 as transition layer materials reflects the need for high strength and high toughness at the interface.
| Material | Key Property | Role in Die Construction |
|---|---|---|
| RMD248 | High strength | Structural transition layer |
| RMD647 | High toughness | Crack arrest and stress distribution |
| Base die material | Wear-resistant steel | Primary structural component |
| Surfacing layer | Hardfacing alloy | Wear and thermal protection |
Wear Performance Analysis
The study evaluates both room temperature and high temperature wear performance of the surfacing deposits. This dual-temperature assessment is essential because crankshaft dies operate at elevated temperatures (typically 500-800°C during forging operations), where wear mechanisms differ significantly from room temperature conditions.
At room temperature, wear is primarily governed by abrasive and adhesive mechanisms, with hardness being the dominant factor. At elevated temperatures, however, oxidative wear, thermal softening, and diffusion mechanisms become significant. The surfacing alloy must maintain adequate hardness and microstructural stability at operating temperatures to provide effective protection.
The comparison between RMD248 and RMD647 likely reveals differences in their wear resistance characteristics, with one material potentially offering better high temperature performance due to its specific alloy composition and carbide distribution.
Crack Resistance and Microstructural Evolution
Crack resistance is a critical property for transition layer materials in die applications. Cracks initiated at the interface can propagate rapidly under cyclic loading, leading to catastrophic die failure. The study examines the crack resistance of the surfacing deposits, which is influenced by:
- Matrix toughness and ductility
- Carbide size, shape, and distribution
- Residual stress state after welding
- Interface bonding quality
The microstructural evolution during tempering is a key focus of the study. As tempering temperature and time increase, the following transformations occur:
- Low temperature tempering (below 400°C): Carbon precipitation from martensite, slight hardness reduction.
- Intermediate tempering (400-550°C): Carbide coarsening, dislocation recovery, significant hardness reduction with improvement in toughness.
- High temperature tempering (above 550°C): Over-tempering, excessive carbide coarsening, potential phase transformation, substantial hardness loss.
The optimal tempering condition of 550°C × 8 h produces a microstructure of tempered martensite plus carbides. This condition represents a balance between hardness retention and toughness improvement, with the extended tempering time allowing for more complete stress relief and carbide coarsening to a stable size.
Tempering Process Optimization
| Tempering Parameter | Effect on Hardness | Effect on Microstructure | Effect on Toughness |
|---|---|---|---|
| Increasing temperature | Decreases | Carbide coarsening | Increases |
| Increasing time | Decreases then plateaus | Carbide redistribution | Increases then stabilizes |
| 550°C × 8 h (optimal) | Moderate reduction | Tempered martensite + carbides | Sufficiently improved |
The determination of the optimal tempering condition through systematic experimentation is a practical approach to process development. The 550°C × 8 h condition provides a good balance for crankshaft die applications, where both hardness and toughness are required. The extended tempering time of 8 hours ensures thorough stress relief, which is critical for preventing delayed cracking during die service.
Engineering Practice Considerations
For crankshaft die manufacturers, the following practical considerations emerge from this study:
- Material selection: RMD248 and RMD647 should be selected based on the specific wear and thermal conditions of the die application. RMD248 may be preferred for higher wear resistance, while RMD647 may be better for crack-prone applications.
- Process control: The tempering process must be tightly controlled to achieve the target microstructure. Temperature uniformity across the die is critical, as local variations can lead to non-uniform properties.
- Quality assurance: Post-tempering hardness testing and metallographic examination should be performed on representative samples to verify the achieved microstructure.
- Service monitoring: Regular inspection of die surfaces for wear and cracking should be conducted during production, with re-surfacing performed when wear limits are reached.
Key Questions and Reflections
The study provides valuable practical guidance but leaves several questions open:
- What are the specific alloy compositions of RMD248 and RMD647, and how do these compositions relate to their observed properties?
- How does the dilution rate from the base material affect the final properties of the surfacing layer?
- What is the long-term service life of dies with these surfacing materials under actual forging conditions?
- How does the tempering process affect the residual stress distribution in the die, and is additional stress relief required?
The lack of detailed alloy composition information limits the ability to fully understand the structure-property relationships. Future work should include comprehensive chemical analysis of the surfacing deposits and correlation with microstructural features.
Study Insights and Implications
This research provides practical guidance for crankshaft die manufacturers on the selection and processing of surfacing materials. The determination of the optimal tempering condition (550°C × 8 h) is a directly actionable result that can improve die performance and service life. The study also highlights the importance of microstructural control in surfacing applications, where the tempering process can be used as a tool to optimize the balance between competing properties. The work serves as a reminder that surfacing is not merely a matter of selecting the right alloy; the post-weld heat treatment is equally critical in determining the final performance of the surfacing layer.