Introduction to the Casting Scheme for 6.6 Meter Girth Gears
Introduction to the Casting Scheme for 6.6 Meter Girth Gears
Mar 12, 2025
Introduction to the Casting Scheme for 6.6 Meter Girth Gears
1. Project Overview Girth gears are large cast steel components widely used in industries such as mining, metallurgy, cement, building materials, coal, and power. The gear ring material is ZG42CrMo, which has a wide solidification temperature range. Its solidification characteristics make the casting prone to microporosity, leading to excessive magnetic particle indications after inspection. With increasingly stringent quality requirements and inspections in recent years, casting defects have frequently occurred during production, especially after precision machining. Defects such as sand holes, porosity, shrinkage, and cracks on the tooth surfaces have resulted in a large number of rough castings requiring rework. This not only affects quality and increases costs but also impacts delivery schedules. Therefore, the production of large girth gears is directly related to the quality, safety, and progress of national key engineering projects, holding significant importance for the national economy and people's livelihoods. Part List
No.
Name
Material
Quantity
Weight（Kg）
Outline Dimensions (mm)
1
Cast Steel Girth Gear
ZG42CrMo
1
41000
Φ6604*558.8
2. Main Structural Features and Technical Requirements 2.1 Structural Features Figure 2-1 Schematic Diagram of Gear Blank Structure The girth gear is produced in two halves (see Figure 2-1), with an outer diameter of approximately 6604 mm. It features a double-web structure, with each half weighing about 20.5 tons. 2.2 Technical Requirements
The girth gear is produced in two halves and assembled with bolts before rough machining. During casting, the entire ring is molded to prevent deformation.
The gear material is ZG42CrMo, which must meet the technical requirements of GB/T37400.6-2019. Chemical composition and mechanical properties must be tested, with a yield strength of ≥510 N/mm², tensile strength of 690-830 N/mm², elongation of ≥11%, impact energy of ≥15 J, and hardness (HB) of 250-280.
After desanding, the cast steel must be free of oxide scale. Magnetic particle inspection must be performed on non-machined surfaces according to GB/T9444-2007, with a Level 2 acceptance criterion.
After rough machining, magnetic particle inspection must be performed on machined surfaces according to GB/T9444-2007, with a Level 2 acceptance criterion.
After rough machining, ultrasonic testing must be performed on machined surfaces according to GB/T 7233.1-2009, with a Level 2 acceptance criterion.
After desanding, the casting must be inspected for dimensions and shape according to GB/T6414-1999. Dimensions must comply with CT13 tolerance, and sufficient machining allowance must be ensured.
The wall thickness of the gear rim must be uniform, and the casting surface must be smooth, free of defects such as porosity, cracks, shrinkage, and cold shuts.
The gear blank must undergo normalizing and tempering heat treatment.
3. Casting Process Design 3.1 Determination of Pouring Position and Parting Surface Considering the usage and technical requirements of the casting, and following the principle of directional solidification for cast steel, the pouring position is determined to be vertical, with the tooth surface facing upward. The parting surface is selected on the plane of the gear end face. The molding method is pattern molding with core assembly. 3.2 Process Parameters Considering the mechanical hindrance of the mold and the structural influence of the casting during solidification shrinkage, a pattern shrinkage rate of 1.8% is selected. To ensure machining accuracy and allowance, a 35 mm machining allowance is selected for the tooth surface, and 30 mm for other surfaces. 3.3 Riser and Gating System Design To ensure rapid and smooth filling of the mold cavity and avoid defects such as sand inclusions and slag, a bottom gating system is designed. Based on the wall thickness and structure of the casting, risers and gating systems are set up to ensure directional solidification toward the risers, resulting in dense internal structure and qualified inspection. The riser and gating system setup is shown in Figure 3-1.
Figure 3-1 Casting Process Diagram 3.4 Computer Solidification Simulation Results To ensure internal quality, the casting process is simulated using Angcasting software to further optimize the process. The simulation results are shown in Figure 3-2. Figure 3-2 Angcasting Simulation and Defect Prediction Results The Angcasting simulation shows no isolated liquid regions inside the casting, with the steel solidifying last in the risers. The analysis indicates no tendency for shrinkage porosity in the casting body, confirming the rationality of the girth gear casting process design.
4. Casting Production Process Control 4.1 Pattern Making According to the casting process requirements, the girth gear is produced using pattern molding with core assembly. All wooden patterns are made according to the "Class II" requirements of JB/T7699-1995. Core boxes are CNC-machined to form the casting shape, with smooth surfaces and sufficient strength. Casting fillets are made in the core boxes, and the positions of gates and chills are marked. For areas where pattern removal is difficult, the pattern should use a tongue-and-groove form for easy removal and positioning. The pattern surface is painted twice, and the core boxes are marked with the contract number, part number, and core box number. 4.2 Molding (Core Making) Before core making, study the drawings, process, standards, and any temporary changes to understand the technical requirements. Prepare the tools and equipment for molding and core making. The core coating is an alcohol-based zirconium powder coating, applied three or more times to ensure a coating thickness of over 1.2 mm. The coating thickness should be uniform, with no cracks or peeling. Chills are placed according to the process requirements. The gating system uses special ceramic tubes, and care is taken to prevent loose sand from entering the gating system.
4.3 Core Assembly Before core assembly, remove loose sand and dust from the mold cavity, gating system, and cores. Inspect the core quality, and do not use cores that are damaged, damp, or have poor surface quality or coating peeling. When using weights for tightening, ensure the weights are placed symmetrically and firmly, without obstructing the pouring process. The weights should be placed on the box bars or walls with padding. Use a hot air blower for drying, with a drying time of over 12 hours. Check the surface to ensure it is dry; if not, rework until dry. 4.4 Steel Melting and Pouring Medium-frequency and refining furnaces are used, with high-quality furnace materials. The steel undergoes secondary refining to reduce non-metallic inclusions and harmful gases (H, O, N). The content of harmful elements is strictly controlled, and the S and P content is minimized. The alloy is fully deoxidized before tapping. The tapping temperature is controlled at 1570°C~1580°C. The ladle uses two Φ80 sliding nozzles. After tapping, the steel is held at 1555°C~1565°C before pouring. The initial pouring speed should not be too fast, followed by full-speed pouring. After reaching the risers, the flow rate is reduced, and the risers are topped up as required. 4.5 Shakeout and Desanding After pouring, the casting is slowly cooled in the mold and held for a period. The mold is opened when the temperature is below 300°C. During desanding, the casting must not be blown, sprayed with water, or impacted. The sand is thoroughly removed, and the gates, flash, and burrs are cleaned. Non-machined surfaces are finished using carbon arc gouging and grinding. The risers are hot-cut when the casting temperature is above 350°C, leaving the riser root as required. Samples are cut from the casting, leaving a 10~20 mm margin at one end to keep the sample attached to the product. The oxide slag is cleaned, and the casting is roughly shaped and inspected for surface quality. 4.6 Heat Treatment The heat treatment process is normalizing + tempering, as shown in Figure 4-3. Figure 4-3 Normalizing and Tempering Process Diagram
4.7 Finishing After heat treatment, the casting is ground and finished to meet the roughness requirement of Ra≤100 according to JB/T5000.6-1998.
4.8 Inspection Samples are taken from the casting for mechanical property testing. The casting's appearance and dimensions are inspected. Magnetic particle inspection is performed according to GB/T9444-2007, with a Level 2 acceptance criterion. A magnetic particle inspection certificate is provided. The casting is inspected for dimensions and machining allowance by marking. After passing the inspection, the quality control department issues a certificate of conformity, including the manufacturer's name, part number, material grade, heat number, chemical composition, mechanical properties, heat treatment report, and inspection report. 5. Conclusion Based on the process scheme submitted by the foundry, and considering the structural characteristics of the girth gear, the casting process was carefully designed to meet both the design requirements and the factory's actual conditions. Through multiple discussions with foundry technicians and analysis of key challenges, the process was optimized. The chemical composition, dimensions, UT, and MT results all meet the relevant standards. The computer simulation analysis confirms that the casting process is reasonable and feasible.
