The development of large hydroelectric turbine runner body castings
The development of large hydroelectric turbine runner body castings
Jul 22, 2025
In recent years, with the optimization and adjustment of national energy policies, the proportion of fossil fuel-based power generation has gradually decreased, shifting focus toward clean energy generation. As the primary force in clean energy power generation, hydropower has demonstrated strong growth momentum. At the same time, in response to ecological protection concerns, China has been vigorously promoting the development of hydropower equipment toward large-scale, clean, and high-efficiency solutions. Large axial-flow hydroelectric generator units are among the mainstream products in the hydropower market. Among their components, the runner body casting serves as the "heart" of the entire hydro-turbine generator unit. As a critical load-bearing and torque-transmitting component, it demands high internal quality and dimensional precision. This paper investigates the T-shaped hot spot extension method for runner body castings, formulates an optimized casting process, and successfully manufactures large hydroelectric turbine runner body castings.
Technical Quality Requirements
The runner body casting manufactured by our company for a client is made of JIS G5102 SCW480 (similar to G20Mn5), with maximum dimensions of Sφ3200mm × h2790mm and a net weight of 58t. It requires high non-destructive testing standards (CCH70-3 Inspection Specification for Hydraulic Machinery Steel Castings, Grade II), with ultrasonic testing covering nearly all critical areas of the casting. The runner body casting features a complex structure with poor natural feeding taper. Additionally, the "oil cylinder" contains five built-in reinforcing ribs, further increasing feeding difficulty and making quality assurance challenging. The specific chemical composition is listed in Table 1, mechanical properties in Table 2, and non-destructive testing requirements in Figure 1.
2 Structural Analysis of the Casting
The main wall thickness of the runner body casting is ≥240mm, with a maximum hot spot diameter of 397mm. The overall structure is complex, featuring scattered hot spot distribution and significant feeding challenges. The casting contains six isolated hot spot zones (see Figure 2), with the primary difficulties lying in the T-shaped hot spot extension (No. ② hot spot) and ensuring internal quality in the bore area.Based on the layer-by-layer feeding characteristics of steel castings, the main design approach of the casting process adopts a dispersed riser feeding system. In the gravity direction, properly designed feeding channels ensure smooth feeding from bottom to top for isolated hot spot zones. Circumferentially, appropriate feeding distances are set according to the material's feeding capacity to meet zonal feeding requirements. This ensures that the final solidification areas of the runner body casting are concentrated in the riser zones of each partition, preventing internal shrinkage defects.
3 Casting Process 3.1 Determination of Casting Process Scheme Based on the structural characteristics and non-destructive testing requirements of the runner body casting, two preliminary casting process schemes were designed: the rib-up scheme and the rib-down scheme, as shown in Figure 3.From an economic perspective, when both schemes adopt a 5-dispersed-riser design, the rib-up scheme saves 5 tons of molten steel compared to the rib-down scheme.From a quality perspective, MAGMA simulation software analysis revealed that the rib-down scheme carries a risk of centerline porosity in the thickened sections.From a molding operation perspective, the rib-down scheme requires full core assembly for the outer mold, which increases operational difficulty and adversely affects dimensional control of the casting.From a cleaning perspective, the rib-down scheme features longer external thickened sections, resulting in larger thermal cutting areas and increased cleaning workload.In conclusion, the rib-up scheme was selected for the casting process.For the rib-up scheme, thickening the No. ② hot spot from the inner cavity instead of the outer surface saves approximately 10 tons of molten steel and facilitates feeding in the gravity direction. Considering both quality and cost factors, the structure of the runner body casting was optimized. After communication and agreement with the customer, the five internal ribs were modified to welded components (see Figure 4).
3.2 Hot Spot Extension The key to ensuring internal casting quality lies in achieving smooth feeding channels. Mastering extension methods for different structural hot spots helps improve casting process design efficiency and better guarantees casting quality.The runner body casting primarily contains two types of hot spots: straight-line and T-shaped. Straight-line hot spots are easier to extend, while T-shaped hot spots require consideration of poor heat dissipation at the initial hot spot location due to structural factors, necessitating an increased extension angle.Hot spots ③ and ④ on this runner body casting are straight-line type, with no intersecting hot spots at their initial locations. The hot spots are extended at 1.05 times the initial hot spot diameter, as shown in Figure 5.
The MAGMA simulation software was used to analyze the straight-line hot spot extension method. The simulation results (see Figure 6) demonstrate that when using the 1.05x extension ratio, no isolated liquid pools were present and the feeding channels remained unobstructed.
The No. ② hot spot in this runner body casting is of T-shaped configuration. This location features intersecting hot spots, with the downward branching section having comparable wall thickness to the thickened side wall. Consequently, the initial hot spot exhibits poor heat dissipation, and the actual hot spot typically exceeds the geometric hot spot in size.Three extension methods were implemented for this hot spot:
1.05x extension ratio
1.1x extension ratio
1.1x extension ratio with upper taper
(as illustrated in Figure 7).
MAGMA simulation was conducted to evaluate three extension methods for the T-shaped hot spot. The results (Figures 8 and 9) show that both the 1.05x and 1.1x extension methods exhibited significant isolated liquid pools with obstructed feeding channels and noticeable shrinkage defects.In contrast, the 1.1x extension with upper taper method demonstrated smooth feeding channels without shrinkage defects. This confirms that for T-shaped hot spots, the 1.1x extension with upper taper approach is more effective. Additionally, it is essential to ensure the width of the thickened section reaches at least 1.5 times its maximum thickness.
3.3 Riser and Gating System Design The size of the top risers was determined using the modulus method and feeding volume verification method. By setting appropriate feeding distances, circumferential feeding requirements were met, ultimately resulting in the selection of 3 risers.A two-layer open step gating system was adopted, facilitating smooth molten steel filling to prevent splashing and turbulence that could cause secondary oxidation slag. It also avoids overheating of the bottom-layer steel, which could lead to feeding-related defects.Based on the molten steel rise velocity, the size and quantity of the pouring cups were determined. The total cross-sectional area ratio was set as:
Pouring cups : Runner : Ingates = 1 : 2 : 2.2. 3.4 Simulation Results The final casting scheme was simulated using MAGMA software for solidification analysis. The results show smooth feeding channels in the casting, with all shrinkage defects concentrated in the riser zones and no shrinkage defects in the casting body, confirming the rationality of the process design. Solidification simulation images are shown in Figure 10.
4 Process Control Producing high-quality castings requires not only sound process design but also standardized operations. The wooden pattern for this product was CNC-machined to ensure dimensional accuracy. During molding, core positioning was precisely controlled using template gauges.The foundry floor conducted rigorous inspections of chill dimensions/placement and mold cavity cleanliness to maintain process consistency.Steel melting employed LF(S) (slag refining), with argon purging through the sliding gate during pouring. After shakeout but before riser cutting, the casting underwent low-temperature annealing to relieve residual casting stresses. Following riser removal, stress-relief heat treatment was performed to eliminate thermal cutting stresses and reduce deformation/cracking tendencies. The performance heat treatment employed quenching + tempering to refine grain structure, develop pearlite, and significantly enhance strength/toughness balance.Prior to final machining, another stress-relief treatment addressed welding/finishing stresses to prevent distortion and reduce hardness in repaired areas. Test coupons confirmed all mechanical properties and casting hardness met technical specifications.Given the ribs' heavy weight, tight tolerances, and confined welding space, strict welding controls were implemented:
Marking weld locations
MT/UT approval of weld zones
Rib quality inspection
Rib assembly
Dimensional verification
Preheating
Welding
Post-weld NDT/dimensional check
Stress relieving
Weld MT/PT/UT/hardness testing
Self-inspection
Joint inspection
5 Quality Status To comprehensively evaluate the internal quality of this runner body casting, in addition to conducting Grade II NDT inspection on bore areas and upper/lower flanges per CCH70-3 requirements, exploratory Grade II testing was performed on all other regions. Inspection results confirmed excellent internal casting quality with zero UT defects.The runner body casting exhibited satisfactory dimensional accuracy throughout, with no instances of dimensional expansion requiring weld repairs.
6 Conclusions (1) For T-shaped hot spots, adopting a 1.1x extension ratio with upper taper, while ensuring the thickened section width reaches at least 1.5 times its maximum thickness, achieves unobstructed feeding channels and eliminates shrinkage porosity risks. (2) Through optimized casting process design and precise operational control, the large runner body casting achieved zero UT defects, with internal quality meeting CCH70-3 UT Grade II requirements.
