Gas turbines are known as the “crown jewel” of the equipment manufacturing industry and are the core power equipment in the 21st century. According to their structural forms, they can be divided into heavy⁃duty gas turbines, light⁃duty gas turbines, and micro gas turbines. Due to various reasons, there is still a significant gap between China’s gas turbines and the international advanced level, and a true industry has not yet been formed. A gas turbine unit consists of a compressor, a combustion chamber, a turbine, a control system, and auxiliary systems. The compressor, combustion chamber, and gas turbine are the three main components of a gas turbine. Gas turbines are classified into E⁃class, F⁃class, and H⁃class based on the combustion chamber temperature, with the combustion temperature of F⁃class reaching above 1200°C. Compared with traditional large cast steel parts for power stations, the design structure and casting process of F⁃class heavy⁃duty gas turbine outer casing castings are completely new manufacturing technologies. The castings have large sizes, thin walls, and complex structures. Due to the operating conditions, the internal quality and dimensional accuracy of the castings are very demanding, making the casting production highly challenging. This article combines the structural characteristics and quality requirements of the combustor outer casing product, conducts virtual simulation through MAGMA simulation, and studies the casting process of the combustor outer casing based on the production manufacturing conditions and process control requirements. A reasonable casting process scheme is designed, and finally, a qualified casting product is produced.
1. Structure and Quality Requirements of the Outer Casing Casting
1.1 Casting Structure
The rough machining delivery casting product of the combustor outer casing has an outline dimension of 4140 mm × 2075 mm × 1348 mm, with a main wall thickness of 45 mm, a wall thickness tolerance requirement of 0 – + 8 mm, a maximum wall thickness of 85 mm, and a minimum wall thickness of 40 mm. The casting is divided into two half⁃conical structures from the middle flange, with a square inspection window at the bottom of the lower part and 12 evenly distributed circular burner holes in the upper part. It has a complex shape and structure, belonging to a large⁃scale thin⁃walled complex cast steel part, which is prone to deformation during the production process. The structure is shown in Figure 1.
| Part | Features |
|---|---|
| Upper Half | Same main rotary structure as the lower half, but with different positions of the bosses at the upper and lower joint flanges, and differences in the positions of multiple bosses distributed on the rotary outer wall. |
| Lower Half | Same as the upper half, but with different boss positions and a square inspection window at the bottom. |
As shown in Figure 1, the combustor outer casing is divided into two castings: the upper half and the lower half. The main rotary structures of the upper and lower parts are the same, but the positions of the bosses at the joint flange positions are staggered, and the positions of the multiple bosses distributed on the rotary outer walls are different.
1.2 Casting Quality Requirements
The combustor outer casing casting is made of GX23CrMoV12⁃1 heat⁃resistant steel developed on the basis of Cr12 stainless steel and complies with the EN 10213⁃2007 “Technical Conditions for Pressure⁃bearing Cast Steel Parts” standard, which is equivalent to ZG23Cr12MoV steel in GB/T 16253 – 2019 “Pressure⁃bearing Steel Castings”. The material’s chemical composition and mechanical properties are shown in Tables 1 and 2.
| Chemical Composition Requirements (mass fraction, %) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| C | Si | Mn | P | S | Cr | Mo | V | Cu | Al | Ti | Sn |
| 0.26 | 40 – 0.80 | 0.30 – 1.10 | ≤0.030 | ≤0.020 | 11.30 – 12.30 | 0.90 – 1.10 | 0.20 – 0.30 | ≤0.30 | 0.25 – 0.35 | ≤0.25 | Report for reference |
| Mechanical Performance Requirements | ||||
|---|---|---|---|---|
| Test Temperature/°C | R_o.2/MPa | R/MPa | A/% | KV2/J |
| 20 | ≥540 | 740 – 880 | ≥15 | ≥27 |
| 550 | ≥290 | Report for reference | ||
| Note: Among the 3 impact specimens, at most only 1 specimen’s impact absorption energy can be lower than the specified value, and it should not be lower than 70% of the specified value. |
After finishing machining, the surface of the heavy⁃duty gas turbine combustor outer casing casting needs to be installed with a layer of high⁃temperature⁃resistant material. During the operation of the unit, the steam temperature inside the casting reaches 1400 – 1500°C. Since the casting body will eventually need to be drilled with many small holes for installing ceramic insulation sheets, there cannot be shrinkage porosity or inclusions inside the casting. Due to the high temperature and pressure that the casting needs to withstand in its working environment, this product not only has strict dimensional requirements but also has high internal quality requirements: the casting is subject to 100% magnetic particle testing according to EN 1369, with a quality level requirement of Grade 2; the casting is subject to 100% penetrant testing according to EN 1371, with a quality level requirement of Grade 2; the casting is subject to 100% ultrasonic testing according to EN 12680⁃2, with a quality level of Grade 2 + the allowable limit value ESR (equivalent of straight probe) of 6 mm, and RT is performed when the UT inspection results are unable to make a judgment.
2. Design of the Outer Casing Casting Process
2.1 Design of the Casting Feeding Scheme
According to the process design principles of steel castings, the positions of risers and chills should be reasonably determined to form a good temperature gradient in the feeding range of each riser, so that the casting can solidify sequentially from the feeding end area to the riser direction to obtain a dense and sound casting. The combustor outer casing has a large size, complex shape and structure, and the main rotary structure is a large⁃size thin⁃walled design with an equal wall thickness of 45 mm. From the design of the part structure, the part structure itself is not conducive to casting feeding and is prone to deformation. Since almost the entire casting body needs to be drilled, the internal quality requirements of the casting are high, and there cannot be shrinkage porosity or inclusions. Its material grade is GX23CrMoV12⁃1, which belongs to high⁃Cr martensitic heat⁃resistant steel. The flowability of the material is poor, and compared to low⁃alloy materials, its feeding distance is short, the feeding effect is poor, and shrinkage porosity and shrinkage holes are prone to occur during the solidification process. Therefore, in the casting process design, it is necessary to design the distribution and feeding of the risers based on the casting structure and the feeding performance of the material. The distribution of the risers should be set according to their effective feeding distance. According to the structural characteristics of the outer casing product, it is necessary to use segmentation and zoning for feeding. In each riser feeding area, it is also necessary to fully consider the feeding performance of the material, and by increasing the temperature gradient during the solidification process from the end area of each area to the riser position, the casting structure can be made dense to obtain a qualified product.
In the casting process design, based on the structure of the outer casing casting, it is divided into the upper and lower sections from the bottom of the inspection window. The lower section is divided into zones according to the feeding distance to set up blind risers. The upper section is divided into zones according to the positions of the 12 circular burner holes, and open risers are set on the top for feeding. Two riser distribution schemes are designed for the positions of the 12 circular burner holes in the upper section. In Scheme 1, risers are set in two circles in the inner flange of the top surface and the outer flange of the burner hole according to the casting structure, as shown in Figure 2(a). In Scheme 2, every other burner hole is cast solid, and risers are set at the positions of the cast⁃solid burner holes, as shown in Figure 2(b).
| Scheme | Riser Distribution | Advantages | Disadvantages |
|---|---|---|---|
| Scheme 1 | Risers are set in two circles in the inner flange of the top surface and the outer flange of the burner hole. | Relatively more risers, better feeding effect. | Requires more steel water, higher cost. |
| Scheme 2 | Every other burner hole is cast solid, and risers are set at the positions of the cast⁃solid burner holes. | Requires less steel water, lower cost. | Feeding effect may be slightly weaker than Scheme 1. |
Compared to Scheme 2, Scheme 1 has a larger number of risers and requires more steel water. After process calculation and optimization through MAGMA software solidification simulation adjustment, the riser distribution method of Scheme 2 is finally determined. The lower blind risers are adjusted from spherical fiber risers to exothermic insulation risers, and the design of casting the open riser platform and the top surface flange position flat is optimized. After improving the feeding area, feeding gradient, and meat⁃increasing subsidy design of each riser, the scheme is shown in Figure 3.
In the scheme, the lower section is divided into 5 zones, and a blind riser is set in each zone, with external cold partitions between the risers. The upper section is divided into 6 zones, and an open riser is set at every other circular burner hole. A 1:10 slope meat⁃increasing subsidy is set from the bottom of the upper and lower section partition area of the inner cavity inspection window to improve the temperature gradient in the feeding area under each open riser during the solidification process. External cold partitions are used between each feeding area to ensure the internal density of the main body area of the casting.
2.2 Design of the Molding Method
Based on the casting structure and combining the production tooling and auxiliary equipment, molding, and pouring conditions of our factory, the method shown in Figures 4 and 5 is adopted. By adding cutting openings to the joint surface flange, the upper and lower halves of the combustor outer casing are connected and cast into a ring, which can not only improve the molding and pouring production efficiency but also effectively prevent the casting from deformation.
| Molding Method | Details | Advantages |
|---|---|---|
| Using the middle flange as the parting surface | The inner cavity main core is of an integral structure, and the separate core is placed at the joint cutting opening of the upper and lower joint flanges. | Facilitates molding and core setting. |
| Adopting a core for the inner cavity | Ensures the dimensional accuracy of the inner cavity. | Improves the casting quality. |
The parting surface and the core structure are shown in Figures 6 and 7.
2.3 Control of Size and Crack Risks
For the large⁃size thin⁃walled structure of the combustor outer casing, in the casting process scheme design, not only the deformation caused by the inconsistent cooling shrinkage in different areas during the solidification process of the casting needs to be considered, but also the deformation during the heat treatment process of the casting needs to be considered in combination with the loading method of the heat treatment process design. At the same time, for the problem of the high hot cracking tendency of high⁃Cr martensitic heat⁃resistant steel, effective preventive measures need to be taken for the high⁃risk areas of hot cracking in the MAGMA stress simulation results.
Based on the production experience of similar semi⁃annular joint casting structures in our factory and the wall thickness tolerance requirements of the outer casing casting, a reasonable machining allowance and blank surface correction amount are selected, and a shrinkage scale of 1.8% is selected based on the analysis of the MAGMA stress simulation results. According to the heat treatment process, after the molding and shakeout cleaning, the upper and lower halves of the outer casing casting are cut apart from the joint surface of the flange, and the upper and lower halves undergo performance heat treatment separately. To prevent the deformation of the semi⁃annular structure of the casting at the opening during the heat treatment process, the casting process sets upper and lower two layers of tie rods in the opening direction according to the requirements of the heat treatment process. According to the loading method of the casting heat treatment, to prevent the high⁃temperature deformation of the shell during the heat treatment process, a large triangular rib for preventing deformation is set at the inflection point of the inner cavity of the middle flange. For the four rounded corners of the inspection window, the anti⁃deformation tie rods, and the middle parting surface flange position, which show a relatively high tendency of hot cracking in the simulation results, their rounded corners are all increased to R120 mm. All the external cold partitions on the main body wall thickness use round steel external cold to prevent the occurrence of cracks between the external cold partitions.
2.4 Design of the Pouring System
The material GX23CrMoV12⁃1 of the combustor outer casing casting is a high⁃Cr martensitic heat⁃resistant cast steel, with poor fluidity of the molten steel and prone to the generation of oxide inclusions. Therefore, the pouring process needs to be stable and rapid to reduce gas entrapment and slag inclusion during the pouring process. To ensure stable filling and smooth exhaust, a bottom⁃return pouring system is designed. To reduce the turbulence of the molten steel entering the mold cavity and improve the floating ability of slag inclusion, a tangential introduction of the bottom⁃return inner gate is specially designed to ensure the stable and rapid filling of the molten steel into the mold cavity. Combined with the actual production situation, a single ladle with a diameter of 90 mm and double ladle eyes for argon gas protection pouring is adopted. The pouring system design is shown in Figure 8.
2.5 Analysis of the Solidification Simulation Results
Through multiple adjustments and optimizations of the scheme using the MAGMA software simulation, the shrinkage cavity and shrinkage porosity simulation results of the final process scheme are shown in Figures 9 and 10.
| Simulation Result | Details | Conclusion |
|---|---|---|
| Shrinkage Cavity | No shrinkage cavity defects are displayed in the casting body as a whole. | The casting is expected to have good density. |
| Shrinkage Porosity | When the niyama (shrinkage porosity) criterion is set to 0.3, no defects are displayed in the casting body. When the criterion is set to 0.6, only point⁃like defects are displayed in the local positions of the external cold partition, except for the riser and tie rod positions. | Based on the experience of using the MAGMA software, the point⁃like niyama display in the external cold partition area of thin⁃walled castings is considered to have no quality risk. |
3. Production Process Control
(1) Combining the filling simulation results, the bottom-return and tangential pouring system effectively ensures the stable filling of the molten steel during the pouring process, avoiding the occurrence of porosity and slag inclusion defects.
(2) Through the MAGMA stress simulation results, the anti-deformation tie rods and triangular ribs set for the combustor outer casing casting effectively avoid the occurrence of casting deformation and cracks. The shrinkage scale, correction, and machining allowance determined based on the dimensional results of the MAGMA simulation are reasonable, ensuring that the dimensions of the finished casting meet the product delivery requirements.
(3) The upper and lower half joint casting method adopted for the combustor outer casing casting not only improves production efficiency but also prevents the deformation of thin-walled annular parts.
In the sand casting process of the combustor outer casing, the following aspects need to be paid attention to:
- Mold Preparation: The mold should be prepared carefully to ensure its strength and dimensional accuracy. The use of high-quality sand and proper molding techniques can improve the quality of the mold.
- Molten Steel Quality: The quality of the molten steel is crucial for the casting. It should be properly melted and treated to remove impurities and ensure the required chemical composition and mechanical properties.
- Pouring Temperature and Speed: Controlling the pouring temperature and speed is important to ensure the filling of the mold and the solidification of the casting. Too high or too low pouring temperature and speed can lead to defects in the casting.
- Casting Cooling: The cooling process of the casting should be controlled to ensure the proper solidification and microstructure of the casting. Rapid cooling or uneven cooling can cause stress and cracks in the casting.
In conclusion, the successful production of the F-class gas turbine combustor outer casing casting requires a reasonable casting process design, strict process control, and high-quality raw materials. The use of MAGMA software for simulation and optimization can help improve the casting quality and reduce the risk of defects. The experience and conclusions obtained from this study can provide valuable references for the production of similar castings in the future.