Abstract:
This article elaborates on the casting process of a chain steel casting for the ultra-high pressure outer cylinder exhaust end of a 1000MW ultra-supercritical thermal power turbine. Leveraging the structural characteristics, performance requirements, and application demands of the exhaust end casting, the MAGMA software is utilized for mold filling and solidification simulations. Combined with years of experience in manufacturing large steel castings, a reasonable casting and heat treatment process plan is adopted to ensure proper feeding of the casting, dross management during pouring, and high-performance requirements of the casting. After production verification, the casting meets the ultrasonic inspection standards, and no exposed defects are found in the threaded holes after finishing.
1. Basic Parameters, Technical Requirements, and Structural Characteristics Analysis of the Casting
1.1 Basic Parameters and Technical Requirements
The ultra-high pressure outer cylinder exhaust end casting is a crucial component for the 1000MW ultra-supercritical condensing steam turbine of Shanghai Turbine Works, which boasts the largest single-unit power capacity for thermal power generating sets in China. Operating under high temperatures and withstanding high pressure, this product demands rigorous quality standards.
Table 1: Chemical Composition Requirements (% by mass)
| Material | C | Si | Mn | P | S | Cr | Mo | V | Ni | Sn | Al | Ti | Cu |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ZG17Cr1Mo1V | 0.15~0.20 | ≤0.60 | 0.50~0.90 | ≤0.020 | ≤0.015 | 1.20~1.50 | 0.90~1.10 | 0.20~0.30 | ≤0.70 | ≤0.025 | ≤0.025 | ≤0.025 | ≤0.30 |
Table 2: Mechanical Properties Requirements
| Property | R p0.2/MPa | R m/MPa | A (L0=5d0)/% | Z/% | KV2/J | HBW |
|---|---|---|---|---|---|---|
| Standard | ≥440 | 590~780 | ≥15 | ≥40 | ≥27 | 170~230 |
Table 3: High-Temperature Tensile Properties (T/°C)
| T/°C | R p0.2/MPa | R m/MPa | A (L0=5d0)/% | Z/% |
|---|---|---|---|---|
| 500 | ≥300 | – | 560 | ≥260 |
Table 4: High-Temperature Creep Properties (T/°C)
| T/°C | σ0/MPa | tu/h | Au (L0=5d0)/% | Zu/% |
|---|---|---|---|---|
| 550 | 275 | ≥100 | – | – |
The dimensions of the ultra-high pressure outer cylinder exhaust end are 3550 mm × 2760 mm × 1660 mm, with a net weight of 25.6 tons. The maximum wall thickness at the flange end is 345 mm, and at the bearing end, it is 300 mm, while the cylinder wall thickness is 125 mm. The material used is ZG17Cr1Mo1V, which has specific chemical and mechanical property requirements as outlined in Tables 1, 2, 3, and 4.
Non-destructive Testing Requirements:
- 100% UT inspection for the casting.
- UT Grade 1 for the sealing surface and machined flange end, UT Grade 1 for welding areas, and UT Grade 2 for other areas.
- 100% MT inspection, with MT Grade 1 for sealing surfaces and machined surfaces, and MT Grade 2 for others.
- 30 threaded holes to be machined on the flange end for bolting.
1.2 Structural Characteristics Analysis
The detailed structure of the casting. From the structural analysis, there are five main casting difficulties:
- Large Heat Nodes at Both Ends and Small Heat Nodes in the Middle: This makes feeding difficult and can lead to shrinkage porosity and shrinkage cavity defects.
- Significant Wall Thickness Variation: The maximum wall thickness at the cylinder flange end is 345 mm, and at the bearing end, it is 300 mm, with an intermediate cylinder wall thickness of 125 mm. This large difference in wall thickness increases the risk of cracking.
- High-Quality Requirements for Critical Areas: The sealing surface and threaded hole areas on the flange end require UT Grade 1 inspection, making it difficult to deal with exposed defects after processing.
- Challenges in Pouring and Molding: As the molten steel rises to the bearing end, the rapid increase in cross-section reduces the liquid level rise speed, lowering the temperature of the molten steel and making it harder for dross to float, potentially leading to dross defects.
- Semi-enclosed Structure: The barrel-shaped structure makes it difficult to cut off risers and perform other operations. Additionally, the material ZG17Cr1Mo1V has a high V content, increasing the risk of cracking.
2. Casting Process Design
2.1 Determination of Pouring Position
There are two pouring position options.
Option 1: With the flange end placed upwards, a clear riser is set on the upper surface of the flange end, and a blind riser is set on the inner side of the bearing end for feeding. The advantages include simple feeding and convenient boxing operations. However, the critical areas of the flange sealing surface and threaded hole machining areas are at the top, where dross is likely to accumulate. The rapid increase in cross-section at the bearing end reduces the liquid level rise speed, lowering the molten steel temperature and increasing the difficulty of dross floating.
Option 2: With the flange end placed downwards, a clear riser is set on the outer side of the bearing end. Cold iron is used to reduce the modulus of the flange end, and subsidies are provided to ensure feeding of the flange end. The advantages include reduced dross defects in critical areas, fast initial liquid level rise speed during pouring, small temperature drop of the molten steel, and easy inner cavity operations without risers. However, more subsidies are set on the outer surface of the casting, increasing the cutting and grinding workload.
After comprehensive analysis, Option 2 is selected to ensure the quality of critical areas.
2.2 Riser and Cold Iron Design
Based on the structural characteristics and quality requirements of the casting, we ensure sequential solidification of the various feeding zones after pouring by rationally setting risers, subsidies, and cold iron. Below are the specific details of the riser, subsidy, and cold iron design:
Riser Design:
A clear riser is positioned on the outer side of the bearing end. The primary function of this riser is to provide sufficient molten steel to fill the gaps created by shrinkage during the solidification process of the casting.
The selection of the riser should be based on its feeding efficiency, with priority given to insulation risers or exothermic risers, which can more effectively provide the required feeding volume. The feeding efficiency is calculated at 20% to ensure that sufficient molten steel is transported to the feeding areas of the casting.
The modulus of different regions of the casting is calculated using the modulus method, and the size of the riser is determined according to the principle that the modulus of the riser (M_riser) should be at least 1.2 times the modulus of the casting (M_casting). This ensures that the riser has sufficient size and capacity to accommodate the shrinkage needs of the casting.
Subsidy Design:
To effectively feed the flange end, subsidies are set on the outer surface of the casting. These subsidies connect the riser and critical areas of the casting, ensuring that sufficient molten steel flows to these areas during solidification.
The size of the subsidy is determined through the calculation M2 ≥ 1.05~1.1M1, ensuring a reasonable feeding gradient for the cylinder wall and flange. This helps reduce casting defects such as shrinkage cavities and porosity.
Cold Iron Design:
Cold iron is placed on the flange end, primarily to reduce the modulus of this area, thereby accelerating the cooling rate and promoting sequential solidification.
Cold iron can also be used in conjunction with the riser to further optimize the solidification process of the casting by artificially creating a feeding terminal zone.
Calculation of Riser Feeding Volume:
The weight of the feeding liquid provided by the riser (G_riser) is calculated using the formula G_riser = V_riser × 7.85 × (η – S) / S, where V_riser is the volume of the insulation or exothermic riser, S is the solidification shrinkage value (%), and η is the feeding efficiency of the riser.
After verification, the selected insulation riser provides a feeding volume that meets the molten steel requirements for the solidification shrinkage of the casting, thereby ensuring the integrity and quality of the casting.
In summary, through carefully designed configurations of risers, subsidies, and cold iron, this casting process plan can effectively achieve sequential solidification of the casting, reduce the occurrence of casting defects, and improve the overall quality of the casting.