Abstract
This paper delves into the casting process of the cylinder block for steam turbines, focusing on the challenges posed by the material properties and complex structure of the ZG15Cr1Mo1 steel used in the turbine casting. By adopting innovative casting techniques, such as horizontal pouring, incorporating risers and chill blocks, and setting up slag-collecting gates, we were able to address issues related to cooling sensitivity and crack propensity. This comprehensive approach, coupled with numerical simulations, resulted in the production of high-quality turbine castings that meet stringent technical specifications.
1. Introduction
The turbine industry, particularly the manufacture of gas turbines, stands as a testament to a country’s technological prowess and scientific capabilities. Among the critical components of gas turbines, the cylinder block (also known as the combustion cylinder) plays a pivotal role. This component, characterized by its intricate design and thin walls, poses significant challenges during the casting process. Specifically, the material ZG15Cr1Mo1, used in turbine castings, exhibits sensitivity to cooling rates during heat treatment, which can lead to uneven mechanical properties and increased crack formation. Therefore, the study of casting techniques for turbine cylinder blocks is crucial to ensure product quality.
2. Product Specifications and Technical Requirements
The turbine cylinder block in question is the upper half of the cylinder, weighing approximately 2,250 kg. Made of ZG15Cr1Mo1 steel, its chemical composition and mechanical properties, respectively. The maximum dimensions of the casting are 1,600 mm x 1,100 mm x 1,100 mm, with a maximum wall thickness of 90 mm and a minimum of 30 mm. Non-destructive testing (NDT) requirements include 100% ultrasonic testing (UT) at level 2 and magnetic particle testing (MT) at level 2.
3. Structural Analysis of the Casting
A detailed analysis of the casting’s structure revealed several casting challenges:
- Thin Wall and Thick Flange Issues: The intake chamber features a minimum wall thickness of 30 mm, with a maximum thickness of 90 mm at the back flange. This large variation in thickness leads to potential problems such as sticking sand, slag inclusion, inadequate feeding, and difficult cleaning.
- Hot Spot Challenges: The large cross-ribs on the back flange create significant hot spots, making it difficult for molten steel to feed properly along the circumferential direction.
- Thin Wall and Thick Rib Intersection: The middle section of the cylinder, where thin walls intersect with thick ribs, is prone to thermal cracking and deformation.
- Large Circular Holes: The three large circular holes at the large end of the cylinder are difficult to feed, and the ribs between the holes are thicker than the hole locations, leading to potential porosity and shrinkage issues.
4. Casting Process Solutions
To address these challenges, several measures were implemented:
- Horizontal Pouring: Adopted to facilitate better feeding and reduce defects.
- Riser and Chill Block Arrangement: Additional risers and chill blocks were placed, including a sub-riser at the large end and a central dark riser. The three circular holes were internally supplemented with risers, and the ribs between them were subject to centralized feeding.
- Ventilation and Slag Collection: Ventilation holes or slag collection troughs were set on the flange surface.
- Pouring System Design: A bottom-pouring system with a slag-collecting gate was designed using the “dichotomy” method to prevent secondary oxidation and slag inclusion caused by turbulent flow in the pouring system.
5. Process Design
5.1. Riser Design
The positions, sizes, and number of risers were determined through computer simulation of the casting’s hot spot distribution, combined with horizontal feeding distance and modulus calculations. The use of risers, subsidies, and chill blocks ensured smooth feeding of molten steel. The efficiency of the insulating sleeve for feeding was verified, and the riser size was further adjusted based on the feeding volume.
5.2. Pouring System Design
Given the pouring temperature of the steel casting at 1,520-1,620°C, the cavity surface heated by the molten steel’s thermal radiation expands, potentially causing separation between the surface and inner layers, leading to sand collapse issues. Larger castings require rapid pouring to minimize harmful effects such as thermal radiation on the cavity surface.
The casting employed a water glass sand molding system with three open-top risers, three sub-risers, and 26 chill blocks. This setup facilitated smooth mold filling, easy gas venting, reduced metal oxidation, prevented air entrapment and slag inclusion, minimized impact on cores and molds, prevented sand inclusion defects, and maintained uniform temperature differences to avoid shrinkage stresses.
To ensure a smooth inflow of molten steel during mold filling, the inlet velocity was maintained below 0.55 m/s. Pouring parameters were calculated using pouring system design software:
- Pouring Weight: 5.5 tons
- Steel Ladle: #5
- Pouring Temperature: 1,580°C
- Pouring Speed: 0.54 m/s
- Pouring Time: 166 seconds
6. Numerical Simulation
The software AnyCasting was used to simulate the solidification process of the molten steel, optimizing the casting process design. Simulation results indicated no tendency for shrinkage porosity in the casting body, with modulus values complying with sequential solidification requirements. Both the circumferential and axial directions met the criteria for sequential solidification, with the risers and subsidies designed appropriately to meet quality standards.
Simulations of the steel filling process showed a smooth and rapid rise of molten metal, free from turbulence, splashing, and gas entrapment, confirming the合理性 of the pouring system design.
7. Conclusion
Turbine castings produced according to the improved process showed a well-controlled cavity wall thickness, reduced sticking sand, and significantly fewer cracks in areas with large variations in wall thickness such as the back flange. The castings met the specified chemical composition and mechanical properties, as well as customer requirements for non-destructive testing.