1. Introduction and Industrial Background
The armored face conveyor, also known as the scraper conveyor, serves as the critical transportation equipment in underground fully mechanized coal mining faces. Its importance in the coal extraction process cannot be overstated, as it directly impacts the efficiency, safety, and reliability of the entire mining operation. The middle trough, as the core component of the scraper conveyor, bears the direct load of coal transportation and supports the scraper chain system. Within the middle trough assembly, the channel steel (also referred to as trough rail steel or channel side rail) represents the most complex and demanding structural component to manufacture.
For many years, the production of high-quality channel steel for mining scraper conveyors has been dominated by a few major coal machinery companies in China. The fundamental reasons for this technological monopoly lie in the extremely demanding manufacturing requirements. The channel steel exhibits complex geometric configurations, particularly at the connection ends where precise tolerances are essential for proper assembly. Furthermore, the underground mining environment exposes these components to harsh conditions including high impact loads, abrasive wear from coal and rock particles, corrosive mine water, and significant temperature variations. These operational conditions demand exceptional mechanical properties, including high yield strength, excellent impact toughness, good wear resistance, and superior fatigue life.
Traditional manufacturing approaches have proven inadequate for meeting these stringent requirements. The conventional hand molding sand casting technique cannot consistently achieve the necessary dimensional accuracy and mechanical performance indicators required for reliable underground operation. The dimensional tolerances achievable through manual sand molding typically range from ±3 to ±5 millimeters for components of this size, which falls short of the ±1 to ±2 millimeter precision required for proper mating of the connecting ends.
On the other hand, advanced fully automatic molding lines, while capable of producing high-quality castings, require enormous capital investment. The initial investment for a complete automatic molding system, including the molding machine, sand handling system, core making equipment, and ancillary facilities, typically ranges from 50 to 100 million RMB depending on capacity and automation level. For many coal mining enterprises, this level of capital expenditure is prohibitive, especially when considering the economic uncertainties inherent in the coal industry.
Faced with this dilemma, our company at Tiefa Energy Co., Ltd. made a strategic decision to leverage the existing lost foam casting technology to develop and manufacture channel steel for the SGZ800/800 type scraper conveyor. The lost foam casting process, also known as expendable pattern casting (EPC), offers a unique combination of advantages that make it particularly suitable for this application. Unlike traditional casting methods, lost foam castings eliminate the need for mold draft angles, enable the production of complex internal geometries, provide excellent dimensional accuracy, and require significantly lower capital investment compared to automatic molding lines.
This paper presents our comprehensive research and development efforts in utilizing lost foam castings for the production of channel steel. We will detail the entire manufacturing process, discuss the technical challenges encountered, and present the solutions developed to overcome these challenges. The successful implementation of this technology represents a significant breakthrough in the field of mining equipment manufacturing, offering a cost-effective alternative to traditional high-investment production methods.
2. Foam Pattern Manufacturing
2.1 Pattern Design and Dimensional Specifications
The research and development work focused on the channel steel for the SGZ800/800 type scraper conveyor middle trough. The dimensions of this component are considerable: 1507 mm in length, 303 mm in height, and 335 mm in width. The critical areas requiring precise dimensional control are the two end connections and the shifting lug (also known as the pusher dog or haulage bracket). These features must interface precisely with adjacent trough sections and the hydraulic support system.
In the lost foam casting process, the pattern quality is the decisive factor that determines the success or failure of the entire casting operation. The foam pattern must possess adequate density for structural integrity, appropriate surface finish for good coating adhesion, and sufficient strength to withstand the handling and sand filling processes without deformation. After extensive experimentation and iterative optimization, we developed an optimal manufacturing strategy that balances quality requirements with production economics.
2.2 Hybrid Pattern Manufacturing Strategy
Through systematic testing and evaluation, we arrived at a hybrid approach for pattern fabrication. The two end sections, which contain the complex geometric features such as the shifting lugs, recessed pockets, and precision alignment surfaces, are manufactured using Expandable Polystyrene (EPS) foam bead molding. This process involves pre-expanding EPS beads to the desired density, aging them to stabilize, and then steam-heating them within a male-female aluminum mold to fuse the beads into the desired shape. The EPS molding process offers superior dimensional reproducibility and excellent surface quality for complex geometries, achieving tolerances of approximately ±0.5 mm on critical dimensions.
For the middle section of the channel steel pattern, which has a relatively uniform cross-section, we employed a more economical approach using CNC-cut polystyrene foam board. The foam board is first cut into precise slices using hot-wire cutting equipment guided by CNC templates, and then these slices are bonded together using specialized foam adhesives. This approach dramatically reduces tooling costs, as the expensive aluminum molds are only required for the relatively small end sections rather than the entire 1507 mm length. The cost savings from this hybrid approach are substantial, reducing overall tooling investment by approximately 60% compared to a single-piece EPS mold.
After manufacturing the individual pattern components, they are assembled by precision bonding to form the complete pattern. The assembled patterns undergo quality inspection to verify dimensional accuracy and surface integrity. The final patterns exhibit a smooth surface finish, with the surface roughness Ra value maintained below 25 micrometers, which is essential for achieving good casting surface quality in lost foam castings. The density of the finished pattern is carefully controlled at 21 kg/m³, a critical parameter that balances pattern strength with foam gasification behavior during pouring. At this density, the pattern exhibits sufficient stiffness to resist deformation during handling and sand compaction, while remaining adequately porous to facilitate complete gasification and escape during metal pouring.
2.3 Gating and Riser System Design
The design of the gating and riser system in lost foam castings presents unique challenges compared to conventional casting. The presence of the foam pattern creates additional phenomena that must be considered, including carbon pickup, gas generation, and the need for controlled pattern decomposition. For the channel steel component, which is characterized by its considerable length (1507 mm) and non-uniform wall thickness, the risk of deformation, carbon segregation, and shrinkage defects is particularly pronounced.
After extensive modeling and experimental verification, we adopted a vertical pouring configuration with the convex end positioned at the bottom. This orientation was selected for several compelling reasons. First, the vertical arrangement promotes more uniform filling of the thin cross-sections. Second, it facilitates the upward flow of decomposition products, allowing them to escape through the riser rather than becoming trapped in the casting. Third, the vertical orientation provides natural directional solidification from the bottom upward, promoting sound feeding.
The gating system incorporates six stepped ingates (runner gates) arranged at different heights along the pattern. This configuration ensures progressive filling from bottom to top, maintaining a controlled metal front velocity. The uppermost gate is strategically positioned at the mid-height of the riser, an arrangement that provides several benefits. During the final stage of pouring, the hottest metal flows through this top gate directly into the riser, creating a steep thermal gradient that enhances feeding efficiency. The six-gate configuration also serves as an effective anti-deformation mechanism, as the multiple attachment points constrain the pattern during the critical early stages of filling before structural rigidity is established by the metal itself.
The riser, positioned at the top of the vertical arrangement, serves multiple functions. Beyond its primary role of feeding the solidification shrinkage, the riser acts as a collection chamber for slag, gas, and decomposition products from the foam pattern. The vertical pouring method maximizes the effectiveness of this slag collection, as the lower density contaminants naturally rise into the riser due to buoyancy. Our trials demonstrated consistently clean castings when using this arrangement, with minimal inclusion defects observed in the cast components.
The gating and riser design emerged as a critical success factor in producing sound lost foam castings for the channel steel application. The system effectively addresses the three primary challenges of lost foam castings for this geometry: uniform carbon distribution, effective shrinkage feeding, and the elimination of gas-related defects.
3. Coating Formulation, Application, and Drying
3.1 Coating Composition and Functional Requirements
The refractory coating applied to the foam pattern is widely recognized as one of the most critical factors influencing the quality of lost foam castings. The coating serves multiple essential functions in the process, and its formulation directly determines the success of the casting operation.
Primary among these functions is the separation of the molten metal from the unbonded sand mold. During pouring, the foam pattern gasifies and the resulting gas must escape through the coating layer into the surrounding sand. Simultaneously, the coating must prevent the penetration of liquid metal into the sand, which would cause sand adhesion, surface roughness, and potential sand inclusion defects. The coating must also provide thermal insulation to slow the rate of heat transfer from the metal to the sand, maintaining metal fluidity for complete filling of thin sections.
Based on extensive experimental evaluation, we developed an optimized coating formulation that produces excellent casting quality. The composition and proportions are presented in Table 1.
Table 1: Optimized Coating Formulation for Channel Steel Production
| Component | Function | Proportion (parts by weight) | Percentage (%) |
|---|---|---|---|
| Zirconium silicate powder (镐英粉) | Primary refractory aggregate | 60 | 52.6 |
| Quartz powder (石英粉) | Secondary refractory aggregate | 40 | 35.1 |
| Phenolic resin (酚醛树脂) | Binder | 5 | 4.4 |
| Vinyl acetate emulsion (乳白胶) | Secondary binder, improves low-temperature strength | 4 | 3.5 |
| Lithium-based bentonite (锂基膨润土) | Suspension agent, thixotropy control | 2 | 1.8 |
| Carboxymethyl cellulose (羧甲基纤维素) | Thickener, water retention | 2 | 1.8 |
| Water | Vehicle/solvent | Appropriate amount | — |
The zirconium silicate, with its high refractoriness of approximately 1900-2000°C, provides the primary thermal barrier. The quartz powder, with a melting point of approximately 1650-1700°C, complements the zirconium silicate and contributes to the overall refractory performance while reducing cost. The combination of these two refractory materials creates a coating that can withstand the elevated pouring temperatures characteristic of steel castings in lost foam castings.
The phenolic resin serves as the primary binder, providing high-temperature strength to the coating layer. As the metal front approaches, the phenolic resin decomposes and its carbonization contributes some additional thermal protection. The vinyl acetate emulsion enhances the room-temperature strength and green bonding of the coating, which is essential for withstanding the handling and sand filling operations without cracking or spalling.
Lithium-based bentonite and carboxymethyl cellulose (CMC) work synergistically to provide excellent suspension properties, preventing settling of the heavy refractory particles during storage and application. This suspension stability is crucial for achieving consistent coating thickness and uniformity across the entire pattern surface, including vertical surfaces where flow and sagging tendencies are most pronounced.
If the coating’s refractory properties are inadequate, the castings exhibit severe surface roughness, sand adhesion (burn-on), and potential sand inclusion defects. Conversely, if the coating’s permeability is insufficient, the decomposition gases generated during pouring cannot escape quickly enough, leading to gas porosity in the castings and excessive carbon pickup at the surface. The optimized formulation achieves a balance between these competing requirements, providing adequate permeability for gas escape while maintaining a sufficient thermal barrier.
3.2 Coating Application Process
The coating application procedure proved to be equally important as the formulation itself. Through systematic process development, we determined that four sequential coating layers with controlled drying between applications produced the optimal coating structure. Each successive layer partially penetrates and heals defects in the previous layer, building up a dense, continuous coating with minimal pathways for metal penetration.
The drying conditions were precisely controlled throughout the process. The drying temperature was maintained at 48°C, which provides an optimal balance between drying rate and the avoidance of foam degradation. Temperatures above this level risk causing distortion or even local collapse of the foam pattern, particularly in thin sections. Too low a temperature extends the processing time and risks incomplete drying in the interior of thick coating sections.
The drying schedule for the four coating layers was as follows:
Table 2: Coating Drying Schedule
| Layer | Coating Application | Drying Duration | Cumulative Drying Time |
|---|---|---|---|
| 1st layer | Primary base coat (higher viscosity) | 1 day at 48°C | 1 day |
| 2nd layer | Intermediate coat | 1 day at 48°C | 2 days |
| 3rd layer | Intermediate coat | 1 day at 48°C | 3 days |
| 4th layer | Sealing coat (lower viscosity) | 9 days at 48°C | 12 days total |
The extended drying time after the final coat, 9 days, was deliberately chosen to ensure complete removal of residual moisture from all coating layers. Residual moisture in the coating can cause blow defects as the water vaporizes explosively during pouring. This extended drying period, while increasing production lead time, proved essential for consistent casting quality. The final coating thickness, measured on cross-sections of representative samples, ranged between 1.5 to 2.5 mm, with a typical average of approximately 2.0 mm.
4. Sand Filling and Molding
4.1 Selection of Molding Sand
The unbonded sand used in lost foam castings must meet several critical requirements. It must provide adequate permeability for the escape of decomposition gases, possess high refractoriness to withstand steel casting temperatures without fusion, maintain dimensional stability under vibration, and be easily removed from the solidified casting.
For our channel steel production, we selected 20-mesh ceramsite sand (also known as pearlite sand or fused ceramic sand), which consists of essentially spherical ceramic particles manufactured by melting and atomizing natural aluminosilicate minerals. The specifications and properties of this sand are presented in Table 3.
Table 3: Properties of 20-Mesh Ceramsite Sand for Lost Foam Castings
| Property | Value | Significance |
|---|---|---|
| Mesh size (particle diameter) | 20 mesh (~0.85 mm) | Optimal permeability and surface finish balance |
| Refractoriness | > 1900°C | Withstands steel pouring temperatures |
| Bulk density | Approximately 1.9 g/cm³ | Provides adequate mold support |
| Thermal conductivity | Low-medium | Promotes directional solidification |
| Moisture content | < 1% | Prevents steam-related defects |
| Sand temperature | < 49°C during use | Prevents premature foam softening |
The spherical shape of ceramsite sand grains provides several significant advantages. The round particles achieve higher packing density with lower vibration energy, creating a more stable mold that resists sand movement during pouring. This stability is essential for maintaining dimensional accuracy in lost foam castings. Additionally, the spherical particles flow more easily into complex patterns and around intricate core features, ensuring complete support of the pattern geometry.
4.2 Sand Filling Procedure
The sand filling process for the channel steel pattern required careful control to ensure complete mold filling without pattern distortion. Since we employed a complete pattern assembly (as opposed to separate runner and riser components), the pattern was placed directly into the flask without additional assembly steps. This streamlined approach eliminated potential misalignment issues and simplified the overall molding process.
The sand was added in three sequential charges, with vibration applied between each charge. The vibration parameters and process sequence are detailed in Table 4.
Table 4: Sand Filling and Vibration Schedule
| Charge | Sand Addition (% of total) | Vibration Duration | Vibration Amplitude |
|---|---|---|---|
| 1st charge | Approximately 40% | 1 minute | 0.5 – 0.8 mm |
| 2nd charge | Approximately 35% | 1 minute | 0.5 – 0.8 mm |
| 3rd charge (final) | Approximately 25% | 5 minutes | 0.8 – 1.2 mm |
The extended final vibration period of 5 minutes ensures maximum sand compaction and stabilization around the entire pattern, including the intricate undercut areas at the end connections. This thorough compaction is essential for maintaining dimensional stability during pouring. The vibration frequency was maintained in the range of 50-60 Hz with the amplitude progressively increased for the final charge to enhance densification.
4.3 Mold Stability Criteria
Several critical stability criteria must be satisfied to ensure that the mold assembly can withstand the hydrostatic forces developed during pouring without premature movement of the coating or sand into the cavity. These forces originate from the metallostatic pressure of the liquid metal front, the pressure of decomposition gases accumulating between the coating and the advancing metal meniscus, and the reaction forces from the evolving foam pattern.
Condition 1: Coating and Sand Layer Stability
During pouring, the coating and adjacent sand layer must resist displacement into the unoccupied cavity space ahead of the advancing metal front. This condition is expressed mathematically as:
$$ p_{阻} + p_{气} \geq \left(\rho_{砂} g H_{S} + p_{0} – p_{型}\right) \cdot \frac{1-\sin\varphi}{1+\sin\varphi} + p_{型} \tag{1} $$
where \(p_{阻}\) is the unit-area resistance force of the coating and sand layer to displacement (MPa), \(p_{气}\) is the pressure of gases in the air gap between the metal and the coating (MPa), \(\rho_{砂}\) is the bulk density of the sand (kg/m³), \(g\) is the gravitational acceleration (m/s²), \(H_{S}\) is the distance from the air gap to the top surface of the mold (m), \(p_{0}\) is the atmospheric pressure (MPa), \(p_{型}\) is the internal pressure within the mold (MPa), and \(\varphi\) is the internal friction angle of the dry sand (degrees).
The term \(q_{z} = \rho_{砂} g H_{S} + p_{0} – p_{型}\) represents the vertical static pressure at the location of the air gap. The expression \((1-\sin\varphi)/(1+\sin\varphi)\) is the Rankine active earth pressure coefficient, which determines how much lateral pressure a granular material with internal friction angle \(\varphi\) transmits at a given vertical pressure. For dry ceramsite sand with an internal friction angle of approximately 30°, this coefficient equals:
$$ \frac{1-\sin 30^\circ}{1+\sin 30^\circ} = \frac{1-0.5}{1+0.5} = \frac{0.5}{1.5} = \frac{1}{3} \tag{1a} $$
This means that only one-third of the vertical pressure is transmitted laterally, which helps stabilize the sand layer against displacement.
Condition 2: Mold Filling and Lifting Prevention
After the mold cavity is completely filled with liquid metal, there is a tendency for the metal to lift the entire mold assembly or for the mold to separate at the parting line. The condition to prevent this lifting phenomenon is:
$$ p_{0} + \rho_{金} g \frac{H}{S} \geq (p_{0} – p_{型}) + p_{阻} + \rho_{砂} g \frac{H}{S} \tag{2} $$
where \(\rho_{金}\) is the density of the liquid metal (kg/m³), \(H\) is the height of the liquid metal column above the consideration point (m), and \(S\) is the area over which the pressure acts (m²).
This inequality indicates that the combined pressure of atmospheric pressure and the metallostatic head must exceed the sum of the mold internal pressure imbalance, the resistance of the sand, and the weight of the overlying sand column. In practice, for the channel steel casting with a vertical height of approximately 1500 mm and steel density of 7800 kg/m³, the metallostatic pressure is sufficient to satisfy this condition when proper venting is provided.
Condition 3: Continuous Metal Front Advancement
For the metal to continuously advance through the cavity during pouring, the pressure at the metal front must be sufficient to overcome the back-pressure created by the accumulated decomposition gases in the air gap. This condition is:
$$ \frac{p_{0} + \rho_{金} g H}{S} \geq p_{气} \tag{3} $$
This simplified relationship illustrates that the total metallostatic pressure available at the metal front must exceed the gas pressure in the gap ahead of the advancing metal. If this condition is not met, the metal front will slow, stall, or even reverse, leading to cold shuts, misruns, or incomplete filling—fatal defects in lost foam castings.
In practice, this condition is controlled by maintaining adequate coating permeability and proper gating design to ensure a continuous supply of molten metal to the advancing front. For the channel steel geometry, we confirmed through computational simulation that the pouring system provides a continuous positive pressure differential throughout the entire cavity filling sequence.
5. Melting and Pouring Operations
5.1 Material Selection
The selection of the appropriate steel grade for the channel steel requires careful consideration of the service conditions and the mechanical property requirements. The channel steel in a scraper conveyor middle trough must withstand high impact loads from coal blocks and rock fragments, abrasive wear from continuous coal flow, cyclic loading from the scraper chain, and potential overload conditions during stall or jam events.
After evaluating various candidate materials including ZG30Mn, ZG35CrMo, and ZG20MnMo, we selected ZG20MnMo, a low-carbon manganese-molybdenum alloy steel. The complete chemical composition is presented in Table 5.
Table 5: Chemical Composition of ZG20MnMo Steel (wt.%)
| Element | Weight Percentage (%) | Role |
|---|---|---|
| Carbon (C) | 0.17 – 0.23 | Strength, hardenability |
| Manganese (Mn) | 0.90 – 1.30 | Strengthening, hardenability |
| Molybdenum (Mo) | 0.20 – 0.35 | Hardenability, tempering resistance |
| Silicon (Si) | 0.30 – 0.60 | Deoxidation, solid solution strengthening |
| Sulfur (S) | ≤ 0.030 | Impurity, harmful to ductility |
| Phosphorus (P) | ≤ 0.030 | Impurity, harmful to toughness |
| Chromium (Cr) | ≤ 0.30 | Residual |
| Nickel (Ni) | ≤ 0.30 | Residual |
This material was selected because of its combination of reasonable strength, good toughness, excellent weldability, and superior response to heat treatment. The relatively low carbon content ensures good weldability, which is essential since the channel steel must be welded to the middle plate and other components during final fabrication of the trough. The manganese and molybdenum additions enhance hardenability, enabling through-thickness hardening during quenching, even in the thicker sections of the end connections.
5.2 Carbon Content Control Strategy
One of the most significant technical challenges in producing lost foam castings from steel is the phenomenon of carbon pickup. During pouring, the molten steel at temperatures of 1550-1650°C thermally decomposes the polystyrene foam pattern. The decomposition products include gaseous hydrocarbons, free carbon, and residual carbonaceous solids. At these elevated temperatures, carbon atoms are highly mobile and can diffuse into the surface and interior of the solidifying steel, causing a local increase in carbon content.
The severity of carbon pickup is inversely proportional to the initial carbon content of the steel. Steels with lower initial carbon content exhibit more pronounced carbon pickup because the concentration gradient drives greater carbon diffusion into the steel matrix. Additionally, the surface area-to-volume ratio of the casting and the pouring temperature influence the extent of carbon pickup.
To compensate for this phenomenon, we implemented a deliberate carbon-content reduction strategy in the melting operation. The target carbon content in the molten steel was established at 0.15% ± 0.1%, which is at the lower end of or slightly below the specification for ZG20MnMo. Through this preemptive reduction, even after carbon pickup during casting, the final carbon content would remain within the acceptable specification range of 0.13% to 0.27%.
For the experimental production runs, we precisely controlled the melting to achieve a carbon content of 0.15% in the steel melt. This required careful adjustment of the charge materials, including the proportion of returned scrap, pig iron, and steel scrap, considering the carbon contributions from each component. The actual carbon content measurements after casting are presented in Section 7.1.
5.3 Melting and Refining Conditions
For the production of high-quality lost foam castings, strict control of the melting and refining parameters is essential. Our melting operations utilized a 5-ton basic electric arc furnace for primary melting, followed by ladle refining with argon gas purging. The process parameters are summarized in Table 6.
Table 6: Melting and Refining Process Parameters
| Parameter | Value | Rationale |
|---|---|---|
| Steel tapping temperature | 1680°C | Compensates for temperature drop during degassing/ladle handling |
| Ladle refining temperature | 1600°C (at pouring start) | Optimal fluidity for foam gasification |
| Bottom argon purging | 8-12 minutes | Homogenizes temperature and composition |
| Deoxidation practice | Aluminum (0.8-1.2 kg/t) | Ensures low dissolved oxygen |
| Final oxygen activity | < 25 ppm | Minimizes oxide inclusions |
| Pouring temperature | 1600°C | 30-50°C higher than conventional casting |
| Casting quantity per heat | 5 channel steel pieces | Consistent batch production |
The elevated melting and pouring temperatures, compared to conventional sand casting, are necessary for several reasons. First, the thermal energy required to gasify the foam pattern consumes a significant portion of the heat content of the molten steel. Calculations indicate that the energy required to vaporize the EPS foam pattern is approximately 5-8% of the total heat content of the steel, representing a non-trivial thermal loss. Second, the decomposition gases form a boundary layer at the metal front that reduces the effective heat transfer to the surrounding sand, further extending solidification time and requiring higher superheat to maintain fluidity. Third, the higher temperature improves the fluidity of the metal, allowing it to fill thin sections and reproduce intricate details before solidification.
However, excessively high pouring temperatures bring their own risks, including increased oxidation, greater thermal shock to the coating, enhanced carbon pickup kinetics, and the potential for sand fusion. The selected pouring temperature of 1600°C represents an optimum balance, based on extensive experimental trials.
5.4 Pouring Process Characteristics
The pouring process in lost foam castings differs fundamentally from conventional casting in that the liquid metal must progressively decompose and replace the foam pattern. This is a dynamic process with several stages occurring simultaneously at the advancing metal front. The decomposition of the foam pattern consumes thermal energy, generates gases that must escape through the coating, and creates a transient low-pressure gap ahead of the metal front.
The filling behavior can be characterized by the metal front velocity and the pressure distribution within the cavity. For the channel steel, we targeted a filling rate of 5-10 kg/s, which corresponds to a metal front velocity of approximately 20-40 mm/s in the vertical orientation. This relatively slow filling rate is typical of lost foam castings and is essential for allowing adequate time for the decomposition gases to escape through the coating and sand.
The pouring was performed by experienced operators using a bottom-pour ladle to minimize slag carryover. The molten steel was tapped at 1680°C into a preheated teapot ladle, where argon gas was bubbled through a porous plug in the bottom to facilitate inclusion floatation and temperature homogenization. The bottom argon purging also helps to break up any clusters of deoxidation products and refine the steel by promoting the flotation of non-metallic inclusions to the slag layer.
The use of a ladle with a stopper rod mechanism enabled precise control of the pouring rate. During the initial phase of pouring, a slower rate is maintained to minimize thermal shock to the coating and prevent washing of the coating from the exposed pattern surfaces. The pouring rate is then increased as the metal head develops, providing the pressure needed to overcome the gas counter-pressure. The final phase of pouring is controlled to maintain positive pressure in the riser until complete solidification.
Each heat was poured into five channel steel molds arranged on a pouring floor with adequate clearance for operator movement and safety. The total pouring time per mold was approximately 80-120 seconds, with the exact duration depending on the pouring temperature and the degree of preheating of the gating system.
6. Heat Treatment Process
6.1 Heat Treatment Philosophy
The heat treatment of ZG20MnMo channel steel was designed to achieve the optimal balance of strength, toughness, and ductility required for the demanding service conditions in underground coal mining. The material specifications require a minimum yield strength, ultimate tensile strength, elongation, reduction of area, and impact energy, as detailed later in Table 9.
A conventional approach for ZG20MnMo would involve normalizing followed by tempering. However, given the exceptionally demanding service conditions and the need for optimal performance in lost foam castings, we adopted a more advanced heat treatment protocol. This protocol consists of two stages: a preliminary high-temperature complete annealing, followed by a quench-and-temper (quenching and tempering) treatment.
6.2 High-Temperature Complete Annealing
The first stage of the heat treatment was a complete annealing conducted at 920°C. This temperature is above the upper critical temperature (Ac3) for this steel, ensuring complete transformation to austenite and the dissolution of any carbides or segregation products formed during solidification.
The annealing serves three essential purposes:
First, it refines the as-cast grain size. The as-cast microstructure of steel castings typically exhibits coarse columnar grains with segregation of alloying elements at the grain boundaries. The complete transformation to austenite during heating, followed by slow cooling through the transformation range, promotes the nucleation of new, finer grains through the austenite-to-ferrite/pearlite transformation.
Second, the annealing eliminates or reduces internal stresses that develop during solidification and cooling in the mold. The complex geometry of the channel steel, with its varying section thicknesses, inevitably leads to differential cooling rates and the development of significant residual stresses. These stresses, if not relieved, can cause distortion, warpage, or even cracking during subsequent heat treatment operations.
Third, the slow cooling from the annealing temperature produces a relatively soft, uniform microstructure that is more machinable and less prone to cracking during the subsequent rapid quenching process. The annealed hardness is typically in the range of 150-180 HBW, providing good machinability for any pre-quench machining operations.
6.3 Quenching and Tempering (Q&T) Treatment
Following the annealing, the channel steel was subjected to a hardening treatment consisting of the following steps:
Step 1: Austenitizing at 890°C for a sufficient duration to achieve complete austenitization and homogenization of the carbon and alloying elements. This temperature, while slightly below the annealing temperature, is still well above Ac3, ensuring complete transformation to austenite without excessive grain growth.
Step 2: Water quenching. The steel was quenched in clean, circulating water at 20-40°C. The rapid cooling transforms the austenite to martensite, providing the high strength and hardness essential for wear resistance and structural integrity. For the low-carbon, alloyed steel ZG20MnMo, the as-quenched martensitic structure exhibits a hardness range of 380-420 HBW, with a characteristic lath martensite morphology.
The choice of water quenching represented a significant technical gamble, given the complex geometry and the varying section thicknesses of the channel steel. Water is a very severe quenchant, and for high-carbon or highly hardenable steels, it would risk catastrophic cracking. However, the low carbon content of ZG20MnMo (0.15-0.23%) provides excellent intrinsic toughness, reducing the cracking risk. Furthermore, the chromium and molybdenum additions enhance hardenability, enabling the formation of martensite even in the thicker sections without the need for even more severe quenchants.
Step 3: Tempering at 650°C. The tempered martensite structure combines high strength with excellent toughness. The tempering temperature of 650°C is in the range typically used to achieve a good strength-toughness balance. At this temperature, the martensite undergoes recovery and recrystallization, forming fine ferrite grains with finely dispersed spheroidized carbides. This microstructure, known as tempered martensite or tempered sorbite, provides an excellent combination of strength, ductility, and impact toughness.
The use of vertical quenching was a critical detail in the process. The channel steel, at 1507 mm in length, is a slender component that could easily distort or bend during quenching if not properly oriented. By quenching vertically, the thermal gradients are maintained symmetrically around the cross-section, minimizing differential contraction and avoiding distortion. This approach proved successful, with no cracking or warping observed in any of the quenched components.
6.4 Tempering Product Microstructure
The final tempered microstructure, observed through optical microscopy at 100× magnification, reveals a fully tempered structure. The microstructure consists of fine, equiaxed ferrite grains with uniformly dispersed spheroidal carbides, characteristic of tempered martensite at this tempering temperature. The absence of untempered martensite or retained austenite confirms that the tempering process was completely effective.
This microstructure provides the optimal combination of properties for the intended service. The fine grain size contributes to both strength and toughness through grain boundary strengthening. The spheroidal carbide particles provide precipitation strengthening while reducing the tendency for cleavage fracture initiation. The absence of large, elongated inclusions in the matrix reflects the clean steel-making practice employed.
7. Process Performance Verification
7.1 Carbon Content Analysis
To quantify the carbon pickup phenomenon and validate the effectiveness of the preemptive carbon adjustment strategy, we conducted systematic sampling of a representative casting. Sample locations were distributed across the entire length of the channel steel, from the ingate region to the riser contact, to map the carbon distribution. Table 7 presents the carbon content measurements from the 14 sampling locations.
Table 7: Carbon Content Measurements at Sample Locations
| Sample Location | Carbon Content (%) | Carbon Pickup (%) | Comment |
|---|---|---|---|
| 1 (bottom ingate region) | 0.18 | 0.03 | Minimum carbon pickup |
| 2 | 0.20 | 0.05 | Near ingate |
| 3 | 0.20 | 0.05 | |
| 4 | 0.19 | 0.04 | |
| 5 | 0.21 | 0.06 | |
| 6 | 0.22 | 0.07 | Maximum carbon pickup |
| 7 | 0.20 | 0.05 | |
| 8 | 0.22 | 0.07 | Upper region |
| 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 | Near riser |
| 14 (riser contact area) | 0.19 | 0.04 | Diluted by riser metal |
The measurements demonstrate a consistent carbon pickup ranging from 0.03% to 0.07%, with an average pickup of approximately 0.05%. This consistent and relatively narrow range indicates that the carbon pickup phenomenon is uniform and predictable for this casting geometry and process conditions. The observed pattern shows slightly higher carbon pickup in the mid-section regions (locations 5-8), which may correspond to the longer residence time of the foam decomposition products in these areas.
All measured carbon contents fall within the acceptance range of 0.13% to 0.27% specified for ZG20MnMo. More importantly, the control strategy of maintaining the melt carbon content at 0.15% ensured that even after the maximum pickup of 0.07%, the final carbon content remained below the upper specification limit. This confirms the effectiveness of the preemptive carbon reduction approach.
7.2 Mechanical Property Evaluation
To evaluate the mechanical properties of the lost foam castings channel steel, one piece from the six cast in the same heat was cut longitudinally using a band saw, then machined to prepare standard tensile and impact test specimens. The testing was conducted according to Chinese national standards for metallic materials. The results are compared with the conventional property requirements for ZG20MnMo steel in Table 8.
Table 8: Mechanical Properties of Lost Foam Castings Channel Steel
| Property | Our Test Results (Complete Annealing + Q&T) | Conventional Spec. (Normalized + Tempered)* | Improvement (%) |
|---|---|---|---|
| Yield strength σs (MPa) | 560 | 265 | +111 |
| Ultimate tensile strength σb (MPa) | 710 | 471 | +51 |
| Elongation δ5 (%) | 25 | 19 | +32 |
| Reduction of area ψ (%) | 50 | 40 | +25 |
| Impact energy Ak (J/cm²) | 55 | 50 | +10 |
*Note: Values shown are the minimum specified values for ZG20MnMo after normalizing and tempering per the applicable Chinese standard.
The mechanical property results are remarkably superior. The yield strength is more than double the minimum specification, and the ultimate tensile strength is 51% higher. Even more impressively, the elongation and reduction of area are also significantly improved. Traditionally, one would expect a trade-off between strength and ductility; achieving simultaneously higher strength and higher ductility indicates a remarkably refined and clean microstructure.
The yield-to-tensile ratio (YS/UTS) of 560/710 = 0.79 is reasonable for a quenched and tempered low-carbon steel, indicating good capacity for strain hardening and plastic deformation before fracture. The excellent impact energy of 55 J/cm² demonstrates outstanding toughness, which is critical for resisting fracture under impact loading from coal blocks and rock in the underground environment.
These results dramatically outperform the conventional material properties, demonstrating the significant metallurgical benefits achievable through the combination of clean steel, optimized heat treatment, and the process advantages of lost foam castings.
7.3 Metallographic Examination
Metallographic specimens were prepared from the gage section of the tensile test specimens. After mounting, grinding, polishing, and etching with 3% nital solution, the microstructure was observed using an optical microscope. The microstructure, shown at 100× magnification, exhibits a fully tempered structure characterized by:
– Equiaxed ferrite grains arranged in a lath-like pattern, inheriting the morphology of the prior lath martensite;
– Uniformly dispersed fine spheroidal carbide particles, primarily cementite (Fe₃C), precipitated during tempering;
– No evidence of blocky retained austenite or untempered martensite;
– Absence of significant microporosity, shrinkage, or non-metallic inclusions;
– Fine effective grain size, estimated at approximately 10-15 micrometers, the result of the full annealing followed by re-hardening.
This tempered martensite (tempered sorbite) microstructure is the ideal structure for achieving the combination of high strength and good toughness required for the service conditions. The evolution from the as-quenched martensite through tempering recovers the lattice distortions, promoting ductility while retaining the fine substructure for strength.
7.4 Simulated Underground Operation Tests
To validate the structural integrity of the channel steel under conditions simulating actual underground loading, we assembled the remaining four castings into a short middle trough section and subjected it to simulated service testing. The test setup is described below.
Table 9 summarizes the simulated loading conditions applied to the assembly.
Table 9: Simulated Operational Loading Conditions
| Load Parameter | Design Value | Test Value | Load Factor |
|---|---|---|---|
| Maximum hydraulic support pushing force | Design max (100%) | 200% of design max | 2.0× |
| Maximum hydraulic support pulling force | Design max (100%) | 200% of design max | 2.0× |
| Applied load direction | Both push and pull | Push and pull alternating | — |
| Load application cycles | — | 10 cycles per direction | — |
The simulated operational testing utilized a 150-ton hydraulic jack to represent the coal mining hydraulic support. Pushing and pulling forces equivalent to twice the design maximum were applied through the shifting lugs and the connecting ends. This 2× load factor provides a substantial safety margin, subjecting the components to stresses well beyond those they would experience in normal service. The 150-ton capacity of the jack was sufficient to generate these elevated test loads.
The results were excellent. After completing the full ten push-and-pull cycles in both directions, detailed inspection revealed:
No visible deformation or distortion was observed at any location on the channel steel;
No cracking, fracturing, or material separation was detected;
The shifting lugs showed no measurable wear, yielding, or enlargement of the mounting holes;
The convex and concave end connections maintained their original dimensions and demonstrated proper fit when mated with corresponding sections;
All surfaces, including the sliding surfaces and the mounting bores, retained their original surface finish with no visible appearance of damage.
This successful simulated test confirmed that the channel steel produced by lost foam castings with the optimized process parameters meets and exceeds the structural performance requirements for underground service.
7.5 Field Trials in Underground Coal Mine
Following the successful laboratory testing, we proceeded to field trials under real mine service conditions. Using the identical optimized process, we produced a total of ten channel steel sections. These were subsequently assembled and welded into complete middle trough sections. The completed sections were installed at the Xiaoming Mine of Tiefa Energy, in an active fully mechanized longwall coal mining face.
The ten fabricated sections were incorporated into the operating scraper conveyor, replacing the corresponding number of sections from the existing conveyor. The installation was carried out by the mine’s competent maintenance crew following standard procedures, ensuring proper alignment and connection with the existing trough sections. The conveyor was then operated under normal production conditions, including coal extraction, transportation, and the associated loading and unloading cycles.
After one complete longwall panel was extracted, the experimental sections were withdrawn, inspected, and compared to the control sections from the existing conveyor that had operated under identical conditions. The observations and results are summarized in Table 10.
Table 10: Field Trial Results Summary
| Evaluation Parameter | Lost Foam Castings Channel Steel Sections | Existing Supplier Sections (Control) |
|---|---|---|
| Wear depth on sliding surface | Comparable to control | Baseline (reference) |
| Deformation or distortion | None observed | None observed |
| End connection integrity | Excellent, precise fit maintained | Excellent |
| Weld quality (attachment points) | No weld cracking or fatigue | No weld cracking or fatigue |
| Overall functional performance | Met operational expectations | Met operational expectations |
The field trial results confirmed that the channel steel produced by lost foam castings performs fully comparably to conventionally manufactured channel steel from established external manufacturers. The wear pattern, deformation characteristics, and overall structural integrity were virtually identical to the control sections. The precise dimensional accuracy achieved through lost foam castings was particularly notable, as the end connections maintained their precise fit after one complete panel of service, with no loosening or misalignment of the trough sections.
8. Overall Conclusions
The research and development program successfully demonstrated the viability of using lost foam castings for the production of channel steel for scraper conveyor middle troughs. The work represents a significant technological achievement with several important implications for the mining industry and for the broader field of manufacturing.
Key conclusions from this work include:
First, the hybrid pattern manufacturing strategy, combining EPS bead molding for complex end sections with CNC-cut foam board for the middle sections, achieved an optimal balance of dimensional accuracy and tooling cost. This approach dramatically reduced the tooling investment required compared to single-piece EPS molding, making the process economically viable for relatively low-volume production runs. The overall pattern cost was reduced by over 50% while maintaining pattern quality.
Second, the optimized gating system with six stepped ingates and the strategic placement of the final ingate through the riser successfully addressed shape distortion, carbon segregation, and shrinkage feeding challenges. The vertical pouring arrangement with the convex end at the bottom and riser at the top promoted clean, directed solidification and effective slag and gas removal.
Third, the coating formulation developed in this study, comprising zirconium silicate, quartz powder, phenolic resin, vinyl acetate emulsion, lithium-based bentonite, and CMC, provided the essential combination of refractory properties, gas permeability, and mechanical strength. The four-layer coating application with controlled drying ensured adequate coating integrity and thickness for the demanding steel-pouring conditions.
Fourth, the careful control of steel carbon content in the melt, combined with the understanding of the systematic carbon pickup behavior in lost foam castings, enabled us to maintain the final carbon content within the required specification. The 0.05% average carbon pickup was successfully counteracted by adjusting the initial carbon to 0.15%.
Fifth, the advanced heat treatment protocol, involving high-temperature complete annealing followed by water quenching and high-temperature tempering, produced a microstructure of fine tempered martensite with excellent mechanical properties. The yield strength of 560 MPa, ultimate tensile strength of 710 MPa, elongation of 25%, and impact energy of 55 J/cm² significantly exceeded the conventional property requirements, while the vertical quenching approach successfully prevented quench cracking and distortion.
Finally, the successful simulated and field testing under realistic underground mining conditions has conclusively demonstrated that channel steel produced by lost foam castings meets the performance requirements for scraper conveyor service. The ability to produce castings with near-net shape accuracy, complex internal features, and excellent mechanical properties at competitive cost positions lost foam castings as the preferred manufacturing route for this critical component.
The project successfully solved the following practical production problems:
1. Avoided the massive capital expenditure of automatic molding lines, achieving a low-investment, high-yield production model implementable within existing casting facilities;
2. Established a firm technical foundation for our company’s expansion into the scraper conveyor manufacturing market, an area previously dominated by a few specialized manufacturers;
3. Systematically resolved the carbon pickup issue in lost foam castings of steel components, providing valuable knowledge for future applications;
4. Resolved the occurrence of slag and gas porosity defects in lost foam castings, improving product quality and consistency;
5. Demonstrated that the lost foam castings process overcomes the limitations of traditional hand molding when producing complex-shaped castings, offering a flexible and cost-effective approach.
The findings from this research extend beyond the specific application of channel steel, offering proven methodology, process parameter guidelines, and defect-prevention strategies applicable to the production of other complex steel castings using lost foam castings. As the mining industry continues to demand higher performance equipment at lower cost, the lost foam castings process provides a strategic manufacturing alternative well-positioned to meet these challenges.