In the field of diesel engine manufacturing, the crankshaft serves as a critical transmission and safety component, subjected to torsional fatigue, bending fatigue, and continuous impact loads. Traditionally, crankshafts have been produced from forged steel, but the advent of nodular cast iron has offered a cost-effective alternative with superior mechanical properties. Nodular cast iron crankshafts exhibit enhanced bending load capacity and wear resistance compared to their steel counterparts, making them a preferred choice for many engine applications. However, the casting process for nodular cast iron crankshafts presents challenges, particularly in preventing defects such as shrinkage porosity and cavities due to the volume changes during solidification. This study focuses on the application of shell casting technology filled with iron shot for producing S1100 diesel engine crankshafts using domestically available equipment and materials. The research aims to optimize the melting, molding, shell-making, and pouring processes to achieve high-quality, high-precision crankshafts with reduced investment and production costs.
The solidification of nodular cast iron involves complex volume changes, including liquid contraction, solidification contraction, and graphite expansion. The net volume change can be expressed as:
$$ \Delta V = V_{\text{liquid contraction}} + V_{\text{solidification contraction}} + V_{\text{graphite expansion}} $$
Where liquid contraction is approximately 1.6% per 100°C superheat, solidification contraction is about 3%, and graphite expansion contributes 2% per 1% carbon graphitization. For medium-sized crankshafts, with a carbon equivalent (CE) of 4.4–4.5%, the total volume change ranges from -0.4% to -1.2%, indicating that graphite expansion alone may not compensate for contraction, necessitating effective feeding systems. The shell casting process, which involves creating a resin-coated sand shell filled with iron shot, offers high dimensional accuracy, surface finish, and mold rigidity, making it suitable for crankshaft production. This study investigates key aspects such as chemical composition selection, feeding mechanisms, gating system design, and process parameters to enable the widespread adoption of this technology.
The shell casting process for nodular cast iron crankshafts leverages the advantages of phenolic resin-coated sand, which provides high strength, low gas evolution, and excellent dimensional stability. In this research, I designed a vertical pouring and cooling system for the S1100 crankshaft, with a casting weight of 16.5 kg and dimensions of 305 mm × 192 mm × 192 mm. The process utilizes a 1.5-ton medium-frequency induction furnace for melting, a two-station shell molding machine for shell production, an iron shot-filled circular pouring line for molding, and integrated cooling systems. Key considerations include optimizing the carbon equivalent to prevent graphite flotation, enhancing mold rigidity through iron shot compaction, and implementing effective chilling with cold irons to mitigate shrinkage defects. The following sections detail the process analysis, experimental trials, and results, highlighting the feasibility and benefits of using domestic resources for shell casting of nodular cast iron crankshafts.
Process Analysis and Design for Crankshaft Shell Casting
To produce high-quality nodular cast iron crankshafts via shell casting, several critical factors must be addressed: the chemical composition of the nodular cast iron, the design of feeding and gating systems, the selection of shell thickness, and the compaction of iron shot. The S1100 crankshaft casting design minimizes machining allowances, particularly at the web sections, to reduce thermal mass and improve cooling. The web inner sides are allocated a unilateral machining allowance of 1–1.5 mm, with a web opening dimension of 36 mm in the casting, compared to 38 mm in the final part. A shrinkage allowance of 0.8–1% is applied, similar to iron mold coated sand processes. The vertical pouring system incorporates a bottom gating approach with a riser for feeding, as illustrated in the design schematic.
The feeding system is crucial for compensating volume changes in nodular cast iron. The theoretical riser weight is calculated based on the casting weight and riser radius:
$$ W_{\text{riser}} = \frac{W_{\text{casting}}}{6.5} + \frac{R^3}{25} \, \text{kg} $$
where \( R \) is the riser radius in centimeters. For the S1100 crankshaft, the calculated riser weight is 4.25 kg, with a casting-to-riser weight ratio of 3.88:1. However, to improve yield, a blind riser design is adopted, reducing the riser weight to 2.49 kg and achieving a ratio of 6.6:1. The gating system includes a sprue of Ø35 mm, two ingates of 15 × 20 mm² each, and a pouring cup height of 30 mm to maintain metallostatic pressure. The total gating system weight is 40 kg. To enhance cooling at critical sections, chill irons are placed at the web and main journal areas. The web chill is wedge-shaped and positioned near the crankpin, while the main journal chill consists of three combined pieces (1/2 and 1/4 segments) with a thickness of 15 mm and width of 27 mm, facilitating removal during cleaning.
Shell thickness is designed to balance cooling and strength: 6–8 mm at the casting sections for rapid chilling and 12–14 mm at the riser and sprue for thermal retention. The shell is produced using a K9407EK shell molding machine with two stations for left and right halves, employing vertical parting. The machine parameters include a mold temperature of 180–240°C, shooting pressure of 0.3–0.4 MPa, and a cycle time of 150–240 seconds. The coated sand used has a tensile strength of 4.0–4.8 MPa and a gas evolution of 14–16 ml/g. To ensure proper iron shot compaction, the shell design integrates the web sections into a single piece, eliminating gaps that could lead to incomplete filling. A circular pouring line with vibration compaction is used, with iron shot of diameters Ø5.5 mm, Ø4 mm, and Ø3 mm mixed in a 40:30:30 ratio. The vibration time is optimized to 5–8 seconds to prevent shell cracking while achieving dense packing.
| Parameter | Shell Casting | Green Sand Casting | Iron Mold Coated Sand |
|---|---|---|---|
| Dimensional Accuracy (CT) | 6-7 | 8-11 | 6-7 |
| Surface Roughness (Ra, μm) | 12.5-25 | 25-50 | 12.5-25 |
| Mechanical Properties | Excellent | Moderate | Good |
| Investment Cost | Moderate | High | Low |
| Production Cost | Low | Low | Moderate |
The chemical composition of nodular cast iron is pivotal for achieving desired properties. The carbon equivalent (CE) is defined as:
$$ \text{CE} = \%\text{C} + \frac{1}{3}\%\text{Si} $$
For the S1100 crankshaft, the target CE is 4.4–4.5%, with carbon at 3.4–3.85% and silicon at 2.0–2.75%. This range optimizes fluidity and graphite expansion while avoiding graphite flotation. Residual magnesium and rare earth elements are controlled to ensure spheroidization: Mgres at 0.04–0.06% and REres at 0.025–0.045%, with a Mg:RE ratio above 1. The melting process uses 90% pig iron and 10% steel scrap, with a spheroidizing agent containing 10.5% Mg and 1.2% RE added at 1.5% via the sandwich method. Inoculation is performed with 75% ferrosilicon, totaling 0.8–1.5%, to promote graphite nucleation and reduce chilling tendency.
Experimental Trials and Process Optimization
The experimental setup includes a 1000 kW medium-frequency induction furnace for melting, a K9407EK shell molding machine, a Z9405E core shooter for oil plug cores, and a circular pouring line for iron shot filling and vibration. The nodular cast iron composition is monitored through spectroscopic analysis and quick metallographic tests. Initial trials focused on identifying defects such as shrinkage porosity, leakage, and gas holes. The first batch of six crankshafts, poured without chills at 1420°C, exhibited severe shrinkage at the webs and leakage due to inadequate iron shot compaction. Analysis revealed that the web gaps (16–18 mm) prevented proper filling, leading to mold deformation during pouring.
To address these issues, the shell design was modified to integrate the webs, and wedge-shaped chills were added at the web and main journal areas. The gating system was enlarged: the sprue diameter increased to Ø35 mm, and ingate dimensions were adjusted to 15 × 28 mm² to improve feeding. A connection channel between the sprue and riser was incorporated to enhance liquid supplement. The vibration time for iron shot compaction was reduced to 5–8 seconds to prevent shell cracking. Additionally, venting channels were added to the riser and core to facilitate gas escape during pouring. These modifications significantly reduced defects, as shown in subsequent trials.
| Element | Base Iron (%) | Final Nodular Cast Iron (%) | Target Range (%) |
|---|---|---|---|
| Carbon (C) | 3.78 | 3.7–3.9 | 3.4–3.85 |
| Silicon (Si) | 1.21 | 2.31 | 2.0–2.75 |
| Manganese (Mn) | 0.42 | 0.44 | ≤0.4 |
| Phosphorus (P) | 0.050 | 0.052 | ≤0.11 |
| Sulfur (S) | 0.028 | 0.020 | ≤0.02 |
| Rare Earth (RE) | – | 0.020 | 0.015–0.045 |
| Magnesium (Mg) | – | 0.056 | 0.04–0.06 |
| Copper (Cu) | – | 0.51 | 0.3–0.6 |
The shell-making process employs a phenolic resin-coated sand with a melting point of 95±5°C and grain size of 140/70 mesh. The shell thickness is controlled at 6–8 mm for casting areas and 12–14 mm for feeding systems. To prevent shell breakage during ejection, additional ejector pins and vent holes are incorporated in the mold, along with a release agent applied every 3–4 cycles. The shell halves are bonded using a hot adhesive in a pneumatic clamping machine, ensuring alignment and strength. The oil plug core is produced with the same coated sand, with a tensile strength ≥1.2 MPa and gas evolution <10 ml/g.
Pouring trials were conducted at temperatures of 1340–1360°C, with a pouring time of 8–10 seconds to minimize gas entrapment. Inoculation was initially attempted as stream inoculation, but unmelted inoculant particles accumulated near chills, so it was switched to ladle inoculation. The cooling time in the mold is 20–30 minutes, after which the casting is shaken out and cleaned. Ultrasonic testing is used to assess the integrity of nodular cast iron, with sound velocities of 5600 m/s for spheroidal graphite indicating good quality. The experimental results from multiple batches are summarized below, showing improvement in defect rates after process adjustments.
| Batch No. | Material | Casting Count | Acceptable Count | Defect Rate (%) | Pouring Temp. (°C) | Carbon Equivalent (%) |
|---|---|---|---|---|---|---|
| 1 | FCD55 | 14 | 5 | 64 | 1400 | 4.3 |
| 2 | FCD55 | 14 | 4 | 71 | 1380 | 4.3 |
| 3 | FCD55 | 12 | 2 | 83 | 1380 | 4.4 |
| 4 | FCD55 | 14 | 8 | 43 | 1400 | 4.3 |
| 5 | FCD55 | 14 | 4 | 71 | 1380 | 4.3 |
| 6 | FCD55 | 4 | 3 | 25 | 1420 | 4.5 |
The optimization of process parameters, including carbon equivalent, chilling, and gating design, led to a significant reduction in shrinkage and leakage defects. The nodular cast iron microstructure showed spheroidal graphite with grades 1–3 and nodule size of 6–8, meeting the requirements for crankshaft applications. The successful application of this technology was extended to other crankshafts, such as the KND-5B gasoline engine crankshaft, using a bottom-gating system with a pressurized riser, demonstrating the versatility of the shell casting process for nodular cast iron components.
Results and Economic Analysis
The shell casting technology for nodular cast iron crankshafts proves technically feasible, producing castings with dimensional accuracy of CT6-7, surface roughness of Ra 12.5–25 μm, and mechanical properties exceeding 700 MPa tensile strength after normalizing. The nodular cast iron exhibits a graphite spheroidization rate above 90%, ensuring high fatigue resistance. Compared to green sand and iron mold coated sand processes, shell casting offers superior repeatability, lower cleaning costs, and higher productivity. The investment for a domestic shell casting line is estimated at 230–300 thousand USD for an annual output of 300,000 crankshafts, significantly lower than imported systems (750–850 thousand USD) and green sand lines (2000–3000 thousand USD).
The direct production cost of shell casting is 10–20% higher than green sand due to resin-coated sand consumption, but the overall cost is reduced by 15–20% when considering lower machining allowances and defect rates. The process yield reaches 80%, with a casting-to-riser weight ratio optimized to 6.6:1. The use of iron shot filling enhances mold rigidity, preventing mold wall movement and minimizing shrinkage in nodular cast iron. The table below compares key economic and technical aspects of different casting methods for nodular cast iron crankshafts.
| Aspect | Shell Casting | Green Sand Casting | Iron Mold Coated Sand |
|---|---|---|---|
| Defect Rate (%) | ≤0.4 | 1.67 | 1.39 |
| Process Yield (%) | 80 | 62 | 88 |
| Machining Allowance (mm) | 2.5–3.5 | 3.5–4 | 3.5–4 |
| Graphite Spheroidization (%) | ≥90 | ≥80 | ≥90 |
| Production Flexibility | High | Very High | Low |
| Mechanization Level | High | Very High | Moderate |
The formula for calculating the economic benefit of shell casting can be expressed as:
$$ \text{Savings} = (C_{\text{green sand}} – C_{\text{shell}}) \times Q + (R_{\text{shell}} – R_{\text{green sand}}) \times L $$
where \( C \) is the unit production cost, \( Q \) is the annual quantity, \( R \) is the rejection rate, and \( L \) is the loss per defective casting. For nodular cast iron crankshafts, the lower rejection rate and reduced machining contribute to overall cost efficiency. The shell casting process also minimizes environmental impact by reducing sand waste and energy consumption compared to traditional methods.
Conclusions and Future Perspectives
This study demonstrates that shell casting technology using domestic equipment and materials is viable for producing high-quality nodular cast iron crankshafts. Key conclusions include:
- The carbon equivalent of nodular cast iron should be maintained at 4.4–4.5% to optimize fluidity and graphite expansion without causing flotation.
- Residual magnesium and rare earth elements must be controlled within 0.04–0.06% and 0.025–0.045%, respectively, to ensure spheroidization.
- Shell thickness of 6–8 mm at casting sections and 12–14 mm at feeding systems, combined with iron shot compaction at vibration times of 5–8 seconds, enhances cooling and mold rigidity.
- Chill irons at webs and main journals are essential to prevent shrinkage porosity in nodular cast iron.
- Gating design with a sprue of Ø35 mm and ingates of 15 × 28 mm² improves feeding, while venting channels reduce gas defects.
- Pouring temperature of 1340–1360°C and fast pouring within 8–10 seconds minimize gas entrapment.
- The machining allowance on web inner sides can be reduced to 0.5–0.7 mm, lowering material and machining costs for nodular cast iron crankshafts.
The shell casting process offers advantages such as high dimensional accuracy, excellent surface finish, and low investment, making it suitable for crankshafts with up to four cylinders. However, challenges remain, including shell deformation during heating and pouring, which may affect the straightness of multi-cylinder crankshafts. Future work should focus on developing multi-station shell molding machines, automated adhesive application, and computer simulation of solidification to optimize process parameters. Additionally, research on advanced inoculants and exothermic risers could further enhance the quality and efficiency of nodular cast iron crankshaft production. The successful implementation of this technology paves the way for broader adoption in the automotive and machinery industries, contributing to the advancement of casting techniques for nodular cast iron components.