Improvement in the Casting Process of Large Rotary Kiln Support Rollers
Improvement in the Casting Process of Large Rotary Kiln Support Rollers
Feb 27, 2025
Improvement in the Casting Process of Large Rotary Kiln Support Rollers
Abstract
Large rotary kiln support rollers, made of cast steel, are critical components widely used in cement and metallurgical mining machinery. Due to the enormous alternating compressive stresses and loads of hundreds of tons during operation, as well as harsh working conditions, the quality requirements for these rollers are extremely stringent. The material used is ZG35CrMo, with a rough casting weight of 18.5 tons, an overall dimension of Φ1,900 mm × 1,000 mm, and a main wall thickness of 610 mm, making it a large thick-walled casting. Previously, our company encountered severe surface sand adhesion issues, particularly in the small holes of the intermediate blank and the central machining holes, leading to sintering and cracks during cleaning. In severe cases, welding repairs were impossible, resulting in scrap. Additionally, shrinkage porosity beneath the riser affected the casting quality. Therefore, necessary process improvements were implemented to prevent these defects and meet customer technical requirements. 1. Technical Specifications The support roller material is ZG35CrMo, with the chemical composition shown in Table 1. Ultrasonic testing is required for the castings, following the JB/T 5000.14-2007 standard ("Heavy Machinery General Technical Conditions for Steel Castings - Non-Destructive Testing"), with a requirement of Grade III. 2. Defect Cause Analysis (1) Shrinkage Porosity Beneath the Riser: In previous production processes, the support roller's flat surface was used as the parting plane, with a large central riser and three circumferential risers with extensions beyond the circumference by 150 mm. After cutting the circumferential risers, shrinkage porosity was found beneath them. The analysis indicated that the riser extensions increased the thermal section beneath the risers, resulting in insufficient feeding and causing shrinkage porosity. (2) Surface Sand Adhesion: Due to the thick-walled nature of the support roller, localized overheating of the mold sand during pouring caused severe sand adhesion, especially in the core of the intermediate blank hole and the central machining hole. The strong squeezing effect during solidification and poor heat dissipation conditions exacerbated the issue. (3) Casting Cracks: During cleaning, 2-3 longitudinal cracks appeared in the intermediate blank hole and central machining hole. The analysis revealed that the low temperature during hot cutting of the risers and insufficient insulation after cutting caused uneven heating during prolonged cleaning, leading to cracks. 3. Process Improvements 3.1 Riser and Special Chill Design The parting plane remains the flat surface of the support roller. The riser design principle ensures that the riser solidifies later than the casting, providing sufficient liquid steel for feeding. The modulus method was used for riser design. A large central riser with an outer diameter of Φ1,200 mm and an inner diameter of Φ320 mm was adopted to ensure effective feeding. The three circumferential risers and their extensions were removed, and three 280 mm wide feeding channels were introduced from the central riser to feed the circumferential direction. Nine Φ80 mm vent holes were placed between the feeding channels to ensure proper venting. Circumferential chills, 800 mm high and 280 mm thick, were used with a spacing of 120-150 mm to promote directional solidification. The chills were required to have smooth, rust-free surfaces and uniform thickness. Riser Design: The support roller wall thickness is 610 mm, treated as a thick circular plate.
Modulus of the casting (M~casting~) = T/2 = 610/2 = 305 mm
Modulus of the riser (M~riser~) = 1.2 × M~casting~ = 1.2 × 305 = 366 mm
For insulated risers, the modulus increase factor is 1.7: M~insulated~ = M~riser~ / 1.7 = 366 / 1.7 = 215 mm
For cylindrical risers: M~riser~ = 0.187 × D~riser~, D~insulated~ = M~insulated~ / 0.187 = 215 / 0.187 = 1,149 mm
The riser outer diameter was set to 1,200 mm, and the height was determined to be 1,400 mm to ensure sufficient pressure head.
3.2 Mold Sand Selection and Core Making Ester-hardened sodium silicate chromite sand and ceramsite sand were used for the top and bottom surfaces of the support roller, with a thickness of 20-40 mm, to reduce sand adhesion. For the intermediate blank hole and central machining hole, which are prone to sintering, furan resin chromite sand and ceramsite sand were used for the core. The excellent collapsibility of furan resin sand, combined with the high refractoriness of chromite and ceramsite sand, effectively prevented sintering and facilitated core removal. The proportions of chromite sand, ceramsite sand, furan resin, and hardener were optimized. During core making, compaction was enhanced using fine wooden sticks to ensure tightness in every corner. A Φ50 mm rod was inserted into the core center and removed after hardening to create a cavity, reducing casting contraction resistance. The core was wrapped with a 20 mm thick straw rope to further minimize contraction resistance. The central machining hole core was made of sodium silicate self-hardening sand, coated with chromite and ceramsite sand. The core extended to the top of the central riser to ensure proper fixation and reduce overheating, thereby minimizing sand adhesion. 3.3 Elimination of Casting Cracks After shakeout, the risers were hot-cut at temperatures above 250°C, and the riser roots were covered with insulating asbestos cloth for 24 hours to prevent cracks caused by premature riser detachment. During core removal, preheating with gas was applied to ensure uniform heating and avoid thermal stress. 3.4 Double-Pour Gate and Pouring Temperature Control Two Φ70 mm pouring gates were used to ensure a fast pouring rate, reducing the thermal impact on the mold surface. The pouring temperature was controlled at (1,535 ± 5)°C to avoid mold overheating and sand adhesion. 4. Results of Process Improvements The improved process is shown in Figure 2. Two support rollers were produced using the improved process, resulting in improved surface quality and elimination of shrinkage porosity. The surface quality, especially in the intermediate blank hole and central machining hole, was significantly enhanced, reducing core removal difficulty. Chemical composition and mechanical property test data are shown in Tables 3 and 4. The condition of the casting after shakeout is shown in Figure 3, and the finished casting is shown in Figure 4. The figures demonstrate excellent surface quality, with complete removal of sand from the intermediate blank hole and central machining hole, significantly reducing cleaning efforts and eliminating the risk of cracks caused by prolonged cleaning of sintered sand. 5. Conclusion By optimizing riser and feeding channel design, selecting appropriate mold sand, improving core-making methods, ensuring proper riser hot-cutting, maintaining uniform heating during cleaning, and controlling pouring speed and temperature, high-quality support roller castings meeting technical requirements were successfully produced.
