The development and reliable production of high-performance channel steel components for the middle trough of scraper conveyors represent a significant technical challenge within the coal mining equipment sector. These components are subjected to extreme abrasive wear and high dynamic loads in underground environments. Traditional sand casting methods often struggle with the geometric complexity and stringent dimensional accuracy required, while investment in fully automated high-pressure molding lines entails prohibitive capital expenditure. This detailed research and development effort explored the application of lost foam casting (LFC) as a viable, cost-effective alternative to produce these critical parts. The entire technological chain, from pattern creation to final heat treatment, was systematically investigated and optimized to address the inherent challenges of the lost foam casting process and meet the rigorous performance standards for mining applications.
The core advantage of lost foam casting for this application lies in its ability to produce complex, near-net-shape castings with excellent dimensional repeatability without the need for conventional cores or parting lines. The process utilizes an expendable foam pattern that vaporizes upon contact with molten metal. For producing the long, structurally intricate channel steel, this method offered a compelling solution to overcome the limitations of our existing foundry capabilities.
Pattern Design and Assembly
The success of any lost foam casting project is fundamentally dependent on the quality and integrity of the foam pattern. The subject component was the channel steel for an SGZ800/800 type scraper conveyor, with critical dimensions of 1507 mm in length, 303 mm in height, and 335 mm in width. The most complex features are the convex and concave end profiles and the push ear sections.
To balance pattern quality with tooling cost, a hybrid pattern construction method was developed. The intricate end sections were produced using expandable polystyrene (EPS) bead molding in aluminum dies. This ensured high dimensional fidelity and excellent surface finish for these critical areas. The main body section, with its relatively simpler geometry, was fabricated from machined polystyrene foam boards. The EPS-molded ends and the machined center section were then meticulously assembled using a specialized hot-melt adhesive designed for foam. The final assembled pattern exhibited a smooth surface, precise dimensions, and a density of approximately 21-22 kg/m³, providing sufficient strength and rigidity for subsequent handling and coating processes.
Gating and Risering System for Lost Foam Casting
The design of the feeding system is critical in lost foam casting to manage defects such as non-uniform carbon pickup, inadequate feeding, slag inclusion from pattern decomposition, and distortion of the long, thin-walled casting. After extensive simulation and prototyping trials, a vertical gating approach was selected. The pattern was oriented with the convex end down. A multi-level step gating system consisting of six ingates was employed along the vertical height of the pattern. The topmost ingate was fed directly into a side riser. This design serves multiple purposes:
- Uniform Carbon Pickup: The sequential filling from multiple levels promotes more even thermal decomposition of the foam and distribution of pyrolysis products, leading to a more consistent carbon profile across the casting.
- Distortion Control: The multiple points of metal entry help balance the thermal stresses during filling, effectively restraining the tendency for the long casting to warp.
- Effective Feeding: By having the last and hottest metal enter the riser, thermal gradients are favorable for directional solidification towards the riser, enhancing its feeding efficiency.
- Slag Collection: The vertical orientation and the final metal flow into the riser aid in trapping low-density slag and debris from the decomposed foam in the riser cavity.
Coating Formulation and Application
The refractory coating in lost foam casting performs the essential functions of maintaining mold integrity, preventing metal penetration, and providing a permeable pathway for the gaseous and liquid products of foam degradation to escape. An inadequate coating can lead to surface roughness, burn-on, sand inclusion, gas porosity, and severe carbon defects.
Through iterative testing, an optimal coating composition was formulated. The key criteria were high refractoriness, good permeability, and sufficient adhesion and strength. A typical successful batch formulation is summarized below:
| Component | Function | Percentage (%) |
|---|---|---|
| Zircon Flour | Primary refractory, high thermal stability | 60 |
| Quartz Flour | Secondary refractory, cost-effective filler | 40 |
| Phenolic Resin | Binder, provides green and baked strength | 5 |
| Polyvinyl Acetate (PVA) adhesive | Binder, improves coating adhesion to foam | 4 |
| Lithium-based Bentonite | Thixotropic agent, suspension stabilizer | 2 |
| Carboxymethyl Cellulose (CMC) | Viscosity modifier, improves brushability | 2 |
| Water | Carrier medium | To desired viscosity |
The coating was applied via dipping and brushing in four separate layers. Each layer was dried at a controlled temperature of 48°C to prevent pattern distortion. The first three layers were dried for 24 hours each, while the final coating layer underwent a prolonged drying cycle of 9 days to ensure complete moisture removal and achieve optimum coating permeability, a crucial factor for successful lost foam casting.
Molding and Sand Compaction
The molding medium for lost foam casting must exhibit high permeability, good flowability, and thermal stability. Ceramic proppant sand (20 mesh) was selected for its superior properties: permeability exceeding 150 AFS, refractoriness above 1900°C, and low, consistent moisture content (<1%). The sand was cooled to below 49°C before use to prevent premature softening of the foam pattern.
The coated pattern cluster was placed in the flask, and dry sand was introduced in three stages with mechanical vibration to achieve uniform and dense compaction around the complex geometry. The vibration regimen was: first two stages for 1 minute each, and a final compaction stage for 5 minutes. Proper sand compaction is vital to resist metallostatic pressure and prevent mold wall movement during pouring.
The integrity of the mold during the lost foam casting process is governed by specific mechanical conditions. To prevent the coating and sand from collapsing into the advancing metal-foam interface gap, the following condition must be met:
$$p_{\text{res}} + p_{\text{gas}} \ge \left[ (\rho_{\text{sand}} g H_S + p_0 – p_{\text{mold}}) \frac{1 – \sin \phi}{1 + \sin \phi} + p_{\text{mold}} \right] = q_z \cdot \frac{1 – \sin \phi}{1 + \sin \phi} + p_{\text{mold}}$$
where:
$p_{\text{res}}$ = combined resistance per unit area from coating and sand movement,
$p_{\text{gas}}$ = gas pressure in the interface gap,
$\rho_{\text{sand}}$ = bulk density of sand,
$g$ = gravitational acceleration,
$H_S$ = height of sand column above the point of interest,
$p_0$ = atmospheric pressure,
$p_{\text{mold}}$ = pressure within the mold cavity,
$\phi$ = internal friction angle of the dry sand,
$q_z$ = vertical static pressure.
Furthermore, to prevent the flask from being lifted by buoyant forces once the mold is completely filled, the condition is:
$$p_0 + \rho_{\text{metal}} g H_{\text{metal}} \le (p_0 – p_{\text{mold}}) + p_{\text{res}} + \rho_{\text{sand}} g H_S$$
where $\rho_{\text{metal}}$ is the density of the molten metal and $H_{\text{metal}}$ is the metallostatic head.
Melting, Pouring, and Carbon Control in Lost Foam Casting
The choice of material, melting practice, and pouring parameters are exceptionally important in lost foam casting due to the interaction between the metal and the decomposing foam pattern.
1. Material Selection: ZG20MnMo low-alloy cast steel was chosen for its good combination of strength, toughness, and hardenability, suitable for the demanding service conditions.
2. Carbon Content Strategy: A significant challenge in lost foam casting of steels is carbon pickup. The thermal decomposition of the polystyrene foam generates a carbon-rich atmosphere at the advancing metal front, which can diffuse into the steel. To ensure the final casting chemistry meets the specified range (e.g., 0.13-0.27% C for ZG20MnMo), the initial melt carbon was intentionally set lower. Based on experimental data, the target melt carbon was established at $0.15\% \pm 0.01\%$. For this study, a melt carbon content of 0.15% was used.
3. Pouring Temperature: The pouring temperature in lost foam casting is typically 30-50°C higher than in conventional sand casting. The higher temperature compensates for the heat absorbed by the endothermic foam decomposition process, maintaining fluidity at the metal front. A tapping temperature of 1680°C was used. The ladle was treated with argon purging for refining. The pouring temperature was maintained at 1600°C. Each heat was used to cast five channel steel pieces for evaluation.
4. Pouring Dynamics: For the metal to continuously fill the mold cavity, displacing and vaporizing the foam, the pressure condition at the liquid front must satisfy:
$$p_0 + \rho_{\text{metal}} g H_{\text{metal}} \ge p_{\text{gas}}$$
where $p_{\text{gas}}$ is the pressure in the gap ahead of the metal, largely from foam decomposition gases. The designed gating system ensured this condition was met for stable filling.
Heat Treatment Development
A novel two-stage heat treatment was developed to optimize the microstructure and mechanical properties of the lost foam cast channel steel. Recognizing the component’s complex shape, variation in section thickness, and need for high strength and toughness, the following regimen was implemented:
- Full Annealing: The castings were first subjected to a full anneal at 920°C. This process served to homogenize the microstructure, refine the as-cast grain structure, and completely relieve internal stresses from the casting and cooling process, providing a sound foundation for subsequent hardening.
- Quenching and Tempering (QT): Following annealing, a quenching and tempering (QT) or “调质” treatment was performed. The castings were austenitized at 890°C and then quenched in water. To mitigate the risk of distortion or cracking during quenching of such a long, complex-shaped, low-carbon alloy casting, a specialized vertical quenching fixture was employed. Finally, the parts were tempered at 650°C to achieve the desired balance of strength and toughness. This QT treatment represented a significant advancement over the conventional normalization and tempering typically specified for this grade.
Comprehensive Performance Evaluation
1. Carbon Pickup Analysis: To evaluate the effectiveness of the carbon control strategy in the lost foam casting process, samples were taken from 14 distinct locations on one casting, including various points along the length, thick and thin sections, and near the gating/riser. The carbon content was analyzed via combustion analysis. The results are summarized below, demonstrating remarkable consistency.
| Sample Location | Carbon Content (%) | Carbon Pickup* (%) |
|---|---|---|
| 1 (Lower end) | 0.18 | 0.03 |
| 2 | 0.20 | 0.05 |
| 3 | 0.20 | 0.05 |
| 4 | 0.19 | 0.04 |
| 5 | 0.21 | 0.06 |
| 6 (Mid-section) | 0.22 | 0.07 |
| 7 | 0.20 | 0.05 |
| 8 | 0.22 | 0.07 |
| 9 | 0.21 | 0.06 |
| 10 | 0.19 | 0.04 |
| 11 | 0.20 | 0.05 |
| 12 | 0.20 | 0.05 |
| 13 | 0.21 | 0.06 |
| 14 (Upper end, near riser) | 0.19 | 0.04 |
*Carbon Pickup = Measured %C – 0.15% (Melt Carbon). The data shows a uniform carbon pickup between 0.03% and 0.07%, well within the acceptable material specification, validating the lost foam casting process controls.
2. Mechanical Properties and Metallography: Tensile and impact specimens were machined from the body of a heat-treated casting. The results, compared to the standard properties for ZG20MnMo under conventional heat treatment, are presented below.
| Property | Symbol | Lost Foam Casting + QT (Result) | ZG20MnMo (Standard N&T) |
|---|---|---|---|
| Yield Strength | $\sigma_s$ | 560 MPa | 265 MPa |
| Tensile Strength | $\sigma_b$ | 710 MPa | 471 MPa |
| Elongation | $\delta_5$ | 25% | 19% |
| Reduction of Area | $\psi$ | 50% | 40% |
| Impact Energy | $A_k$ | 55 J/cm² | 50 J/cm² |
The mechanical properties achieved through the lost foam casting and optimized QT process significantly exceed the standard requirements. The vertical quenching method successfully prevented distortion or cracking. Metallographic examination revealed a fine, equiaxed microstructure of tempered martensite (tempered troostite), characterized by a uniform matrix of $\alpha$-ferrite with finely dispersed spheroidized cementite particles, confirming a complete and effective transformation during heat treatment.
3. Simulated Service and Underground Trials: Four additional channel steels were assembled into a middle trough section. This assembly was subjected to a rigorous bench test simulating underground loading. Using a 150-ton hydraulic ram, push and pull forces equivalent to twice the maximum operational load of the hydraulic support were applied to the critical push ears and end connectors. No deformation, cracking, or failure was observed. Subsequently, a batch of ten lost foam cast channel steels was produced, assembled into complete middle troughs, and installed in an operational longwall face. After a full production cycle, the performance and wear of these troughs were comparable to commercially supplied units, confirming their suitability for harsh mining service.
Conclusion
The research and development project conclusively demonstrated that lost foam casting is a highly effective and economically viable manufacturing route for producing high-integrity channel steel components for mining scraper conveyors. The process successfully addressed the key challenges of geometric complexity and dimensional control without the need for massive capital investment in automated molding lines. Specific technical hurdles inherent to lost foam casting, such as carbon pickup control, slag/porosity formation, and distortion of long castings, were systematically overcome through optimized pattern and gating design, coating formulation, controlled melting practice, and a tailored heat treatment cycle. The resulting castings exhibited uniform chemistry, superior mechanical properties exceeding standard specifications, and excellent performance in simulated and real-world underground conditions. This work validates lost foam casting as a competitive and capable technology for expanding manufacturing capabilities in the heavy mining equipment sector.