Optimization of Casting Process for Rotary Kiln Support Rollers
Optimization of Casting Process for Rotary Kiln Support Rollers
Feb 27, 2025
Optimization of Casting Process for Rotary Kiln Support Rollers
1 Introduction The support roller is a key component of the rotary kiln. During operation, it bears the entire rotational load of the kiln and supports the riding ring. Each roller carries a rotational load of several hundred tons under harsh working conditions, with significant tangential friction forces, making it a wear-prone part. The support roller and its shaft are interference-fitted, and the roller is subjected to substantial expansion forces from the shaft. Therefore, the roller must possess excellent comprehensive mechanical properties, including high internal strength, good plasticity, and sufficient surface hardness and wear resistance. It must also be free from casting defects such as cracks, shrinkage cavities, shrinkage porosity, and hard spots. With the significant increase in the capacity and rotational speed of rotary kilns, the working conditions have become even more severe, demanding higher performance, materials, and structural improvements for the support rollers. The materials and structures of the rollers have been greatly enhanced, with most connection parts now designed with smooth transitions, as shown in Figure 1, making the rollers more durable. However, the connection ends create significant casting hot spots. The material of the rollers has mostly shifted from carbon steel to alloy steel, increasing the rigidity and casting stress of the castings, thereby raising the difficulty of casting. Consequently, more measures are required to ensure the quality of the castings. 2 Main Domestic Casting Processes for Support Rollers 2.1 Process Analysis of Support Rollers The support roller (as shown in Figure 1) has a maximum diameter of 1600 mm and a rough weight of approximately 10,000 kg. The main end face is 160 mm thick, and the material is ZG42CrMo, with the following chemical composition: C 0.38%–0.45%, Si 0.30%–0.60%, Mn 0.60%–1.00%, P ≤ 0.035%, S ≤ 0.035%, Cr 0.80%–1.20%, Mo 0.20%–0.30%. Due to the high carbon content and multiple alloying elements, this material is brittle and prone to cracking, with poor weldability. The connection ends create significant casting hot spots, making the casting process challenging. Figure 1 Schematic Diagram of the New Structure of the Support Roll 2.2 Core Assembly Process (1) Process Plan The process plan is shown in Figure 1. There are two parting surfaces: the lower part is formed by sweep molding, and the upper part consists of an inner core and an outer core. Above the second parting surface are the cover core and risers. (2) Riser Design Risers are set directly on the casting hot spots for feeding. Three risers are used, each with dimensions of 600×900×750 mm. The standard mass of molten steel required to fill a single riser is 2360 kg, with a total of 7080 kg for all three risers. Due to the thick sections of the casting, the internal solidification time is long, and riser feeding alone cannot completely eliminate the hot spots. A certain amount of internal chills is required. In recent years, the traditional process of setting risers directly on casting hot spots for feeding has led to many issues in the production of new support rollers. The most critical problem is severe surface cracking and spalling during use, as shown in Figure 2, significantly reducing the service life. Figure 2 Main defect of Support Roll 3 Optimization of Casting Process 3.1 Iron Mold with Sand Coating Process The traditional core assembly process, which involves setting risers directly on casting hot spots for feeding, has the advantages of simplicity, shorter lead times, and lower tooling investment. However, it creates excessive process hot spots, worsening the cooling and solidification conditions of the casting, resulting in coarse grains and high casting stress. Prolonged annealing is required to relieve stress and refine the grain structure. When internal chills are used, their quantity and weight are limited, and their effectiveness is not significant. To ensure the cooling rate of the roller rim, achieve dense microstructure in all parts of the casting, prevent shrinkage cavities and porosity due to insufficient feeding at hot spots, and reduce the impact of gating and risers on casting cooling, the outer sand core is replaced with external chills. The external chills are made into a complete ring, known as an iron mold. The iron mold with sand coating process involves covering the inner cavity of the metal mold with a thin layer of sand to form the mold. The metal mold itself acts as an indirect chill, saving a significant amount of chills. The chilling effect of the iron mold on the outer ring of the casting eliminates the casting hot spots between the outer ring and the connection parts. Additionally, the rigidity of the iron mold allows for effective feeding of the molten steel, preventing bulging and shrinkage cavities or porosity. The sand coating between the chill and the molten steel acts as a vent and buffer, preventing surface gas holes and reducing the chilling effect of the iron mold on the molten steel. This lowers the surface hardness of the casting, extends the service life of the iron mold, and reduces manufacturing costs. Thus, the iron mold with sand coating process fundamentally addresses the defects caused by the original process (see Figure 4). Figure 3 "Sand Mold Core Assembly" Process Schematic Diagram Figure 4 "Iron Mold Coating Sand" Process Schematic Diagram (1) Iron Mold Design Designing the iron mold is a critical aspect of the iron mold with sand coating process. Factors such as the thickness of the sand coating and the shrinkage of the casting must be considered. The iron mold design formula is as follows: D_inner = (D_outer + 2a) × (1 + b) Where:
D_inner = Diameter of the iron mold
D_outer = Outer diameter of the casting (including machining allowance)
a = Thickness of the sand coating, typically 10–20 mm
b = Shrinkage rate of the casting The thickness of the iron mold T = (0.9–1.1) × casting wall thickness. After calculation, T = 200 mm. The height of the iron mold is the same as the casting. For ease of operation, the iron mold can be made in two sections. The iron mold can be made of ordinary gray cast iron. It is important to note that the thickness of the sand coating is a highly sensitive factor. If the coating is too thick, it will not provide sufficient chilling effect; if it is too thin, it may cause other defects. Therefore, it must be strictly controlled. Although the sand coating increases the solidification time, the iron mold's superior heat dissipation conditions enhance its continuous heat absorption capacity, reducing the radial temperature gradient of the casting and minimizing casting stress and hot tearing tendencies. The thickness of the iron mold and the sand coating will significantly influence the solidification rate of the casting. Based on empirical comparisons, the heat storage capacity of the iron mold should not be lower than that of direct chills in the original process. Considering the better heat dissipation of the iron mold, the thickness of the iron mold is taken as the thickness of the rim. The thickness of the sand coating is selected to achieve rapid cooling while meeting the requirements for direct rough machining. Through experiments, a sand coating thickness of 15–20 mm is suitable for support rollers weighing around 10,000 kg. To ensure uniform sand coating thickness, a specialized split wooden pattern is used. A wooden mold with a 15–20 mm gap from the iron mold is prepared, and the gap is filled with sand and manually compacted to ensure a dense and uniform sand layer. Based on the structure and size of the support roller, a 2500 mm × 2500 mm sand box is used, with the outer ring coated with sand and the inner ring formed by sweep molding. The core is placed in the middle for ease of operation. (2) Riser Design A cylindrical riser is set in the middle, and the gating system is bottom-poured. A circular open riser with insulation is selected, with specifications of Ø1000×1000 mm. The standard mass of molten steel required to fill the riser is 6500 kg. Riser feeding calculation: The feeding efficiency of a cylindrical open riser is 14%, and the volumetric shrinkage of the casting is 5.12%. According to the cylindrical open riser table, the maximum feeding weight of this riser is 13,380 kg. One riser can meet the feeding requirements of the casting. The casting process yield rate = (Casting weight) / (Casting weight + Riser total weight) = 10000 / (10000 + 6500) = 60.6%
3.2 Computer Solidification Simulation Results and Analysis (1) Solidification Simulation Plan The simulation was conducted using Huazhu CAE casting simulation software, employing the solidification heat transfer module with gravity feeding enabled. Three types of numerical calculations were performed: liquid phase distribution, casting temperature, and shrinkage cavity formation. The pouring temperature was set at 1560°C. The core assembly process is referred to as Plan 1, and the iron mold with sand coating process as Plan 2. The solidification time for Plan 1 was 48,360 seconds, and for Plan 2, it was 46,022 seconds. (2) Shrinkage Cavity Formation Analysis Plan 1 showed shrinkage cavities and porosity extending into the casting below the riser, as shown in Figure 5. Internal chills were required below the riser, which was also implemented in production. Plan 2 showed no shrinkage cavities or porosity extending into the casting, as shown in Figure 6. Both plans used the same riser weight, but Plan 2 outperformed Plan 1. Figure 5 Numerical visualization of shrinkage cavity formation at the end of solidification for Plan 1 Figure 6 Numerical visualization of shrinkage cavity formation at the end of solidification for Plan 2
(3) Casting Temperature Analysis From the continuous solidification numerical images, the casting temperature images of both plans at the same solidification time were compared, as shown in Figures 7 and 8. Plan 2 showed a clear chilled layer on the outer ring, with a relatively flat temperature gradient inward, achieving the goal of refining the grain structure without causing excessive casting stress.
Figure 7 Numerical visualization of the color temperature of the casting at 7792 seconds of solidification for Plan 1 Figure 8 Numerical visualization of the color temperature of the casting at 7877 seconds of solidification for Plan 2
(4) Liquid Phase Distribution Analysis In Figures 9 and 10, at the same solidification time, Plan 1 showed a clear solidification sequence from bottom to top, resulting in significant internal structural differences. The prolonged solidification time at the bottom of the riser could lead to inclusion segregation, stress concentration, and coarse grains, as verified in the casting's use (see Figure 2). Plan 2 showed a reasonable solidification sequence, from the outside inward, with the riser solidifying last, forming a favorable feeding sequence.
Figure 9 Numerical visualization of liquid phase distribution at 10569 seconds of solidification for Plan 1 Figure 10 Numerical visualization of liquid phase distribution at 10685 seconds of solidification for Plan 2
3.3 Iron Core Process A single-piece iron mold with sand coating during molding has the advantage of providing a moderate chilling effect on the casting surface while refining the grain structure. However, it has several drawbacks: 1) High tooling investment and large storage space requirements; 2) Complex sand coating operation, with uneven sand layer strength, leading to severe sand sticking on some casting surfaces, making cleaning difficult and often damaging the iron mold or casting; 3) Long production cycles, severely limiting its use. By combining the advantages of both plans and making multiple improvements, the iron core process was developed. Its main feature is dividing the single-piece iron mold into several pieces, similar to sand cores. This reduces tooling storage space, changes the production sequence to parallel processing, allowing core production during molding, shortening lead times, and simplifying operations. The sand coating can be applied with the iron core laid flat, ensuring uniform sand layer strength and reducing sand sticking defects. During casting cleaning, separating the single-piece iron mold from the casting is difficult, especially with severe sand sticking, requiring significant force and often damaging the tooling, leading to substantial losses. The iron core separates easily from the casting, and the loss from a single tooling failure is smaller. The key and challenging issues of the iron core process are the thickness of the sand coating and the adhesion between the sand coating and the iron mold. A specialized sand coating frame was designed, fitted to the iron core, with a wooden frame around it and rails on the sand coating surface. After compacting the sand layer, it is scraped flat along the rails to ensure uniform thickness. Through multiple experiments, a combination of core sand adhesive and water glass in a specific ratio was used, applied in layers, solving the adhesion issue between the sand coating and the iron mold. 4 Conclusion (1) The iron core process is stable and reliable, especially in achieving dense internal casting structures, saving a significant amount of internal and external chills, and reducing costs. The support rollers produced using the improved process have complete dimensions, and no shrinkage cavities or porosity were found during machining inspection. (2) The iron core process, with its specialized sand coating frame and adhesive technology, addresses the key challenges of sand coating thickness and adhesion between the sand coating and the iron mold. It retains the advantages of various process plans while overcoming their drawbacks. (3) The support rollers produced using this process have smooth surfaces, free from sand sticking or sand inclusions, and significantly reduced gas holes, shrinkage cavities, and porosity. Ultrasonic testing showed a substantial reduction in cracks, meeting customer requirements for part quality. The iron mold performs well, increasing the operational time of the rotary kiln and significantly reducing maintenance costs and kiln downtime losses.
