In this study, we focused on the application of shell casting technology using domestically produced equipment and materials to manufacture nodular cast iron crankshafts for the S1100 diesel engine. Our research encompassed comprehensive investigations into melting, molding, shell (core) making, and pouring processes, with an emphasis on practical工艺 trials and optimizations. The goal was to establish a viable and推广-worthy production method for high-quality nodular cast iron crankshafts, particularly for single-cylinder engines, by addressing key challenges such as shrinkage defects, dimensional accuracy, and cost-effectiveness.
Introduction and Research Context
The crankshaft is a critical and safety-intensive component in engines,承受ing torsional fatigue, bending fatigue, and multiple冲击 loads. Nodular cast iron has emerged as a preferred material over forged steel due to its favorable cost-performance ratio and adequate mechanical properties. However, producing defect-free nodular cast iron crankshafts, especially via casting, requires precise control over the solidification process to prevent shrinkage porosity and缩孔. Traditional methods like green sand molding and iron mold覆砂 casting have limitations in terms of dimensional精度, surface finish, and consistency. Shell casting, particularly with iron shot backing, offers a promising alternative by combining high铸件 accuracy with improved cooling control.
Our research was motivated by the need to adapt shell casting for single-cylinder engine crankshafts using国产 resources, thereby reducing reliance on进口 equipment and lowering production costs. The study aimed to optimize the entire process chain, from熔炼 chemistry to final浇注, ensuring that the inherent advantages of shell casting—such as enhanced重复精度 and superior surface quality—could be realized economically.
Process Design and Analysis
The S1100 diesel engine crankshaft casting has a weight of 16.5 kg and dimensions of 305 mm × 192 mm × 192 mm. We adopted a vertical shell making, vertical pouring, and vertical cooling approach to facilitate mass production. Key aspects of the process design included:
- Melting and Treatment: We used a 1.5-ton medium-frequency induction furnace, with冲入法 for nodularization using a Mg-RE alloy and subsequent inoculation.
- Shell Making: A modified K9407EK shell molding machine was employed to produce two-part shells from coated sand (resin-coated sand).
- Molding and Pouring: A环形 line with iron shot filling and vibration compaction was utilized. The shells were backed with iron shot to enhance cooling and rigidity.
- Gating and Feeding System: A top暗冒口 system was designed to provide liquid feeding, with careful attention to sprue and runner dimensions.
The fundamental challenge in nodular cast iron casting lies in managing volume changes during solidification. The total volumetric change (ΔV) can be expressed as:
$$ \Delta V = V_{\text{liquid contraction}} + V_{\text{solidification contraction}} – V_{\text{graphitization expansion}} + V_{\text{mold deformation}} $$
Where:
- Liquid contraction is approximately 1.6% per 100°C superheat.
- Solidification contraction for nodular cast iron is around 3%.
- Graphitization expansion contributes about 2% volume increase per 1% carbon converted to graphite.
- Mold deformation depends on the rigidity of the shell and backing material.
For typical浇注 conditions (e.g., pouring temperature ~1300°C, carbon equivalent ~4.5%), the net volume change can be slightly negative, necessitating effective feeding via risers. The carbon equivalent (CE) is a crucial parameter for nodular cast iron, calculated as:
$$ \text{CE} = C\% + \frac{1}{3} Si\% $$
We targeted a CE of 4.4–4.5% to balance fluidity and minimize shrinkage without causing graphite floatation.
Key工艺 Parameters and Equipment Selection
Our experimental setup involved several custom-selected or modified pieces of equipment to suit local production conditions. The following table summarizes the main equipment and their roles:
| Equipment | Model/Specifications | Primary Function |
|---|---|---|
| Shell Molding Machine | K9407EK, two-station, vertical parting | Production of crankshaft shell molds (2 per cycle) |
| Core Shooting Machine | Z9405E, vertical parting | Making oil plug hole cores from coated sand |
| Shell Bonding Machine | Pneumatic, multi-point clamping | Hot bonding of two shell halves |
| Iron Shot Molding Line | 环形, with vibration station | Filling and compacting iron shot around shells |
The shell thickness was designed differentially: 6–8 mm at the casting sections for rapid cooling, and 12–14 mm at the gating and riser sections for thermal insulation. To ensure proper filling of iron shot in narrow gaps (e.g., between crank webs), the shell was designed as a continuous block in those areas, eliminating deep recesses that could lead to incomplete compaction.
Chills were strategically placed at thermal hotspots, such as the main journal and web regions, to accelerate solidification and reduce shrinkage tendency. The gating system featured a bottom-feed design with runners entering at the webs, and a connecting channel between the sprue and暗冒口 to enhance feeding.
Melting and Metallurgical Control for Nodular Cast Iron
Achieving high-quality nodular cast iron requires precise control over chemical composition and treatment practices. Our trials emphasized the following aspects:
Chemical Composition: The base iron was melted from 90% pig iron and 10% scrap steel. Target final compositions for the nodular cast iron were set as shown in the table below, which also compares typical ranges for珠光体 and铁素体 grades.
| Element | Base Iron (%) | Final Nodular Cast Iron (%) | Typical Range for Crankshafts |
|---|---|---|---|
| C | 3.78 | 3.7–3.9 | 3.4–3.9 |
| Si | 1.21 | 2.2–2.4 | 2.0–2.75 |
| Mn | 0.42 | 0.4–0.6 | <0.8 |
| P | 0.050 | <0.06 | <0.07 |
| S | 0.028 | <0.02 | |
| Mgres | – | 0.04–0.06 | 0.04–0.07 |
| REres | – | 0.02–0.04 | 0.015–0.045 |
We maintained a residual magnesium to residual rare earth ratio (Mgres/REres) above 1 to ensure good nodularization while countering trace干扰 elements.
Nodularization and Inoculation: A Mg-RE alloy (containing ~8% Mg, ~5% RE) was added at 1.5% via the冲入法. Inoculation was performed with 75% ferrosilicon, primarily in the ladle, to promote graphite nucleation. We avoided随流 inoculation in vertical pouring due to incomplete melting and segregation issues observed in initial trials.
Pouring Temperature: The optimal range was found to be 1340–1360°C. Lower temperatures risked mist runs and gas defects, while higher temperatures increased liquid收缩 and mold erosion.
Shell Making and Core Making Processes
The quality of shells and cores directly impacts casting accuracy and defect formation. We used commercially available coated sand with the following properties: tensile strength ≥4.0 MPa, bending strength ≥6.8 MPa, and gas evolution ≤16 mL/g at resin content of ~3%. The shell making cycle involved pre-heating molds to 180–240°C, shooting sand at 0.3–0.4 MPa pressure, and curing for 4–6 minutes. To prevent shell breakage during ejection, especially in the web regions, we implemented additional ejector pins, vent holes, and regular application of parting agents.
For the oil plug hole cores, a separate射芯机 was used, with similar parameters but shorter cure times (2–4 minutes). Core vents were integrated to facilitate gas escape during pouring.
Molding, Pouring, and Experimental Trials
The iron shot backing process was critical for providing rigidity and cooling. The iron shot mixture comprised particles of diameters 5.5 mm, 4 mm, and 3 mm in a 40:30:30 ratio. The molding sequence involved: pre-filling shot, placing the bonded shell, filling shot around it, and vibration compaction. We optimized the vibration parameters to achieve high compactness without damaging the shell; a短时间, high-frequency vibration of 5–8 seconds proved effective.
Pouring was conducted quickly (within 8–10 seconds per mold) to minimize temperature loss and prevent slag entrainment. The table below summarizes the results from a series of experimental batches, highlighting the impact of process adjustments.
| Batch No. | Material Grade | Number Cast | Sound Castings | Defect Rate (%) | Main Adjustments |
|---|---|---|---|---|---|
| 1 | FCD55 | 14 | 5 | 64 | No chills; basic gating |
| 2 | FCD55 | 14 | 4 | 71 | Added web chills |
| 3 | FCD55 | 12 | 2 | 83 | Increased riser height |
| 4 | FCD55 | 14 | 8 | 43 | Revised gating, added main journal chills |
| 5 | FCD55 | 14 | 4 | 71 | Optimized vibration time |
| 6 | FCD55 | 4 | 3 | 25 | Integrated shell design, improved venting |
Initial trials suffered from shrinkage porosity at the main journals and web junctions, as well as leaks due to shell破裂. Through iterative improvements—including chill placement, gating modifications, and shell design changes—we significantly reduced缺陷 rates. The final工艺 configuration yielded nodular cast iron crankshafts with satisfactory dimensional tolerances (comparable to CT6-7), surface roughness (Ra 6.3–12.5 µm), and mechanical properties (tensile strength ≥700 MPa, elongation ≥4%).
Results and Economic Analysis
The technological feasibility of the shell casting process for nodular cast iron crankshafts was confirmed through successful pilot production. Key performance metrics are compared against traditional methods in the table below.
| Aspect | Shell Casting (Our Method) | Green Sand Casting | Iron Mold覆砂 Casting |
|---|---|---|---|
| Dimensional Accuracy (CT) | 6–7 | 8–11 | 6–7 |
| Surface Roughness Ra (µm) | 6.3–12.5 | 25–50 | 6.3–12.5 |
| Tensile Strength (MPa) | ≥700 | ≥700 | ≥800 |
| Graphite Nodularity (%) | ≥90 | ≥80 | ≥90 |
| Scrap Rate (%) | ~2 | ~5.7 | ~4 |
| Metal Yield (%) | ~80 | ~62 | ~88 |
From an investment perspective, the shell casting approach using国产 equipment requires significantly lower capital expenditure compared to进口 lines or even large-scale green sand systems. For an annual output of 300,000 crankshafts, estimated investments are:
- Shell casting (domestic equipment): ~2.3–3.0 million USD.
- Green sand (imported line): ~2.0–3.0 million USD.
- Iron mold覆砂: ~0.35–0.45 million USD.
Although shell casting has slightly higher direct production costs than green sand, the overall cost is reduced due to lower machining allowances, fewer defects, and less清理 effort. The process is particularly suitable for crankshafts with up to four cylinders, offering a balance of quality and economy.
Conclusions and Future Directions
Our research demonstrates that shell casting with iron shot backing is a viable and advantageous method for producing nodular cast iron crankshafts, especially for single-cylinder engines. The main conclusions are:
- Chemical Composition: A high carbon equivalent (4.4–4.5%) is beneficial for reducing shrinkage in nodular cast iron, provided graphite floatation is avoided. Residual Mg and RE should be controlled within optimal ranges (e.g., Mgres 0.04–0.06%, REres 0.02–0.04%).
- Process Design: Effective feeding through risers and gating, combined with strategic use of chills, is essential to counteract solidification收缩. The gating system should be designed to remain液态 longer than the casting to enable补缩.
- Shell and Molding: Shell thickness should be varied to balance cooling and insulation. Iron shot must be compacted densely around the shell to provide rigidity and rapid heat extraction; vibration parameters (e.g., 5–8 s at high frequency) are critical.
- Pouring Practice: Fast pouring (within 8–10 s) helps prevent gas defects. Adefficient venting of shells and cores is necessary to expel gases generated during pouring.
- Economic Viability: The use of国产 equipment and materials makes shell casting cost-competitive, with lower initial investment and favorable overall production costs compared to进口 alternatives.
For future work, several areas warrant attention to further enhance the shell casting process for nodular cast iron components:
- Development of multi-station or rotary shell molding machines to increase productivity.
- Integration of automated hot bonding and coating systems to reduce manual labor and improve consistency.
- Exploration of advanced feeding aids like exothermic riser sleeves or filters to improve yield and soundness.
- Implementation of computer simulation tools to optimize gating and solidification patterns before physical trials.
- Extension of the工艺 to multi-cylinder crankshafts, addressing challenges related to shell distortion and dimensional stability over longer spans.
In summary, this study provides a comprehensive framework for applying shell casting technology to nodular cast iron crankshaft production, leveraging local resources to achieve high quality and economic efficiency. The findings underscore the potential of this method to meet the growing demands for precision cast components in the automotive and machinery sectors.