As a casting process engineer, I have been involved in the development of the WHM160 series diesel engine cylinder block, which is designed to meet the market demand for updated marine engines. The cylinder block, as the assembly foundation for various mechanisms and systems of the engine, is one of the most critical components, requiring high strength, precision, and sealing integrity with zero defects. This article details the casting process from design to validation, focusing on the use of gray cast iron material, and incorporates numerical simulation and advanced measurement techniques to ensure quality.
The cylinder block is made of gray cast iron, specifically HT250 grade, which offers excellent castability, damping capacity, and wear resistance. The key dimensions are 1861 mm in length, 500 mm in width, and 740 mm in height, with a cylinder bore diameter of 189 mm, a cylinder pitch of 225 mm, a main wall thickness of 10 mm, and a rough casting weight of 1100 kg. The structure includes a wet liner water jacket system and an external water cavity, connected via进水孔 (inlet holes) of 40 mm × 50 mm cross-section and a 60 mm diameter front inlet hole. The primary challenge lies in positioning and fixing the external water cavity core during pouring to prevent core shifting defects.
In this analysis, I will discuss the casting process scheme, design parameters, simulation optimizations, and validation results, emphasizing the role of gray cast iron in achieving the desired properties. The use of gray cast iron is crucial due to its cost-effectiveness and performance in diesel engine applications, and I will refer to it repeatedly to highlight its significance.
Analysis of Casting Process Scheme
The casting process scheme was evaluated based on the structural complexity of the gray cast iron cylinder block. Two main approaches were considered: horizontal pouring and vertical pouring. Each has distinct advantages and disadvantages, particularly for a gray cast iron component of this size.
Horizontal Pouring Process
In the horizontal pouring process, the mold is split into upper and lower parts along the cylinder bore centerline. This requires 13 sand cores of three types: 8 cylinder cores, 2 end cores, and 3 external water cavity cores (segmented). The characteristics include:
- Strict requirements for template and mold box positioning dimensions, ensuring geometric accuracy of the gray cast iron casting.
- Fewer sand cores, reducing core assembly and fitting workload, and improving dimensional precision.
- Low static pressure head of molten iron, resulting in slower filling speeds and poor feeding capability in later stages.
- Poor venting during pouring.
- Difficulty in positioning between external water cavity cores and cylinder cores.
The gray cast iron material’s fluidity and solidification behavior are critical here, as low pressure heads can lead to mist runs and shrinkage defects.
Vertical Pouring Process
Vertical pouring was analyzed in two variants: cylinder bores upward with a split at the height center, and cylinder bores downward with a split-box molding process. The key points are:
- More relaxed requirements for template and mold box positioning.
- Increased number of sand cores or mold parts, raising labor intensity for core making and mold assembly.
- High static pressure head, beneficial for filling and feeding of the gray cast iron melt.
- Complex operations for fixing external water cavity cores after placing cylinder cores.
- Better alignment of molten iron flow with the structure, enhancing filling and venting.
Despite the advantages in feeding, the vertical process was deemed too complex for this gray cast iron component, leading to the selection of horizontal pouring with modifications.
Determination of Final Scheme
After thorough analysis, the horizontal pouring process was chosen for its simplicity and precision. To address the core positioning issue, process holes were designed at the bottom of the external water cavity cores for locator core prints. This decision balances the needs of gray cast iron casting with production efficiency.
Casting Process Design
The casting process design involves multiple parameters tailored for gray cast iron. I will outline these using tables and formulas to summarize key data.
Casting Process Parameters
The casting shrinkage rate is set based on empirical data. For the outer mold length direction, it is 1.1%, while for the integrated core length direction, it is 1.0%; other directions use 1%. This can be expressed with a shrinkage formula for gray cast iron:
$$ \text{Shrinkage Rate} = \frac{L_{\text{pattern}} – L_{\text{casting}}}{L_{\text{pattern}}} \times 100\% $$
where \( L_{\text{pattern}} \) is the pattern dimension and \( L_{\text{casting}} \) is the final casting dimension. For this gray cast iron block, the values are applied differentially to account for restrained contraction.
Machining allowances are set at 5.5 mm for cylinder bores and camshaft holes, and 5 mm for other areas. Process corrections are applied to bearing journals: 2.0 mm added toward the center for the first and ninth journals, while others remain unchanged due to core commonality.
Gating System Design
A stepped gating system with open design is employed. The bottom ingates are located at the casting bottom, and the second-layer ingates are at bearing journal areas. The gating ratio is critical for gray cast iron to minimize turbulence. The cross-sectional areas are calculated using:
$$ A_{\text{choke}} : A_{\text{runner}} : A_{\text{ingate}} = 1 : 2 : 1.5 $$
This ensures smooth flow for the gray cast iron melt. The gating system includes a sprue, runners, and ingates, with the sprue offset to facilitate tilt pouring.
Venting System Design
Venting is divided into core venting and mold cavity venting. For cores, vent holes are drilled in cylinder core tops, with vent strips on the upper mold sides and vent pins at core inspection windows. For the mold cavity, vents are placed at the highest points and on front and rear faces. The vent area can be estimated as:
$$ A_{\text{vent}} = 0.03 \times A_{\text{gating}} $$
where \( A_{\text{gating}} \) is the total gating area, ensuring adequate gas escape for the gray cast iron casting.
Melting and Pouring Process
The melting process uses electric furnace melting with Cu-Cr alloy inoculation to enhance the gray cast iron properties. The chemical composition ranges are summarized in tables below.
| Element | Range (wt%) |
|---|---|
| C | 3.2–3.35 |
| Si | 1.5–1.7 |
| Mn | 0.7–1.1 |
| S | 0.06–0.12 |
| P | ≤0.07 |
Table 1: Base Iron Chemical Composition for Gray Cast Iron
| Element | Range (wt%) |
|---|---|
| Si | 1.7–1.9 |
| Mn | 0.7–1.1 |
| P | ≤0.7 |
| Cu | 0.4–0.6 |
| Cr | 0.20–0.35 |
Table 2: Final Iron Chemical Composition for Gray Cast Iron
Inoculation is performed with Si-Ba inoculant (0.2–0.6% addition) and stream inoculation with Si-Zr inoculant (0.08–0.12%). The pouring temperature is controlled between 1340°C and 1385°C for optimal fluidity of gray cast iron. Tilt pouring is adopted, with the pouring basin on the higher side at an angle to improve filling.
Cooling Time Determination
Cooling is accelerated by opening the upper mold after 5 hours and shakeout after 24 hours. The temperature decay can be modeled with Newton’s law of cooling:
$$ T(t) = T_{\text{ambient}} + (T_{\text{pour}} – T_{\text{ambient}}) e^{-kt} $$
where \( T(t) \) is the temperature at time \( t \), \( T_{\text{pour}} \) is the pouring temperature, \( T_{\text{ambient}} \) is ambient temperature, and \( k \) is a cooling constant specific to gray cast iron. Infrared measurements confirmed the suitability of this schedule.
Process Simulation and Optimization
MAGMA numerical simulation software was used to simulate the filling and solidification processes of the gray cast iron casting. The initial simulation revealed issues: the branch sprue remained unfilled during pouring, causing air entrainment, and the outermost second-layer ingates admitted molten iron prematurely, risking cold shuts and inclusions.
The filling time \( t_{\text{fill}} \) can be estimated using:
$$ t_{\text{fill}} = \frac{V_{\text{casting}}}{Q} $$
where \( V_{\text{casting}} \) is the casting volume and \( Q \) is the flow rate. For gray cast iron, a fill time of 20-30 seconds is typical to avoid defects.
Optimization measures included:
- Adding a choke section at the branch sprue base to maintain a full condition, reducing air entrainment. The choke area \( A_{\text{choke}} \) is given by:
$$ A_{\text{choke}} = \frac{Q}{v_{\text{choke}}} $$
where \( v_{\text{choke}} \) is the velocity at the choke.
- Increasing the cross-sectional area of the outermost second-layer ingates to equalize pressure. The modified ingate area \( A_{\text{ingate, new}} \) is:
$$ A_{\text{ingate, new}} = A_{\text{ingate, original}} \times 1.2 $$
Post-optimization simulation showed resolved issues, with proper filling sequences and reduced defects for the gray cast iron casting.
Process Validation
A small batch production was conducted to validate the process. The gray cast iron castings were inspected for defects and dimensions.
Casting Defects
No sand inclusions or gas porosity defects were observed, confirming the effectiveness of the gating and venting systems for gray cast iron. However, continuous monitoring in mass production is recommended.
Dimensional Accuracy
Dissection showed that key wall thicknesses met requirements, and the water cavity cores were fixed without shifting. A 3D scanner was used to collect contour data, comparing it to the CAD model. The shrinkage rates and process corrections were within specified ranges, as summarized in the table below.
| Feature | Measured Dimension (mm) | CAD Dimension (mm) | Deviation (mm) |
|---|---|---|---|
| Length | 1860.5 | 1861.0 | -0.5 |
| Width | 499.8 | 500.0 | -0.2 |
| Height | 739.7 | 740.0 | -0.3 |
| Cylinder Bore Diameter | 188.9 | 189.0 | -0.1 |
Table 3: Dimensional Comparison for Gray Cast Iron Cylinder Block
The deviations align with the expected shrinkage behavior of gray cast iron, verifying the process parameters.
Conclusion
In summary, the horizontal pouring process with tilt pouring and integrated core assembly is feasible for medium-speed diesel engine cylinder blocks made of gray cast iron. This approach reduces complexity while maintaining precision. Numerical simulation with MAGMA software effectively identified and resolved potential defects, enhancing the process reliability. Additionally, 3D scanning provides comprehensive dimensional data, supporting adjustments in shrinkage rates and process corrections. The success of this gray cast iron casting process offers a reference for similar components, emphasizing the importance of material-specific design and validation. The use of gray cast iron remains central due to its excellent casting characteristics and performance in engine applications.
Future work could explore further optimizations, such as advanced inoculation techniques for gray cast iron or real-time monitoring during pouring. The integration of simulation and measurement tools will continue to drive improvements in gray cast iron casting processes for heavy-duty components.