This article details the research on the casting process of F-class gas turbine combustor outer casing. By combining the structural characteristics and technical requirements of the casing, using MAGMA software for virtual simulation and optimization, a reasonable casting process plan was developed, and preventive measures were formulated for quality risks in the manufacturing process. Through production verification, the selected casting process parameters were proven to be rational, and the final casting products met the technical and quality requirements.
1. Introduction
Gas turbine is known as the “crown jewel” of equipment manufacturing and a core power equipment in the 21st century. It can be classified into heavy-duty gas turbine, light-duty gas turbine, and micro gas turbine based on its structural form. In China, there is still a significant gap compared to the international advanced level in gas turbine, and a true industry has not yet been formed. A gas turbine unit consists of a compressor, a combustion chamber, a turbine, control, and auxiliary systems. Among them, the compressor, combustion chamber, and gas turbine are the three main components. Gas turbines are classified into E-class, F-class, and H-class according to the temperature of the combustion chamber, with the F-class combustion temperature reaching above 1200°C. Compared to traditional large-scale cast steel parts in power stations, the design structure and casting process of F-class heavy-duty gas turbine outer casing are completely new manufacturing technologies. The casting has a large size, thin walls, a complex structure, and very strict requirements for internal quality and dimensional accuracy due to the operating conditions, resulting in high casting production difficulties. This article conducts research on the casting process of the combustor outer casing based on the structural characteristics and quality requirements of the product, through virtual simulation with MAGMA, and designs a reasonable casting process plan according to the production and manufacturing conditions and process control requirements, ultimately producing qualified casting products.
2. Structure and Quality Requirements of the Outer Casing Casting
2.1 Structure of the Casting
The rough-machined delivery casting product of the combustor outer casing has an outline dimension of 4140mm × 2075mm × 1348mm, with a main wall thickness of 45mm, a wall thickness tolerance requirement of 0 to +8mm, a maximum wall thickness of 85mm, and a minimum wall thickness of 40mm. The casting is divided into two semi-conical structures from the middle flange, with a square inspection window following the shape at the lower part, and 12 circular burner holes evenly distributed at the upper part. It belongs 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.
As shown in Figure 1, the combustor outer casing is divided into two castings: the upper half and the lower half of the outer casing. The main rotary structure of the upper and lower halves of the parts is the same, with the difference being that the positions of the bosses at the joint flange of the upper and lower halves are staggered, and the positions of multiple bosses distributed on the rotary outer wall also have structural differences.
2.2 Quality Requirements of the Casting
The combustor outer casing casting is made of GX23CrMoV12-1 heat-resistant steel developed on the basis of Cr12 stainless steel, and it executes the standard of EN10213-2007 “Technical Conditions of Pressure Cast Steel Parts”, which is equivalent to ZG23Cr12MoV steel in GB/T16253-2019 “Pressure Steel Castings”. The chemical composition and mechanical properties of the material are shown in Table 1 and Table 2.
After the finish machining of the heavy-duty gas turbine combustor outer casing casting, a high-temperature resistant material will be installed on the surface, and the steam temperature inside the casting during the unit operation will reach 1400 to 1500°C. Since many small holes need to be machined on the casting body for the installation of ceramic insulation sheets, there cannot be any shrinkage porosity or inclusions inside the casting. Due to the high temperature and pressure that the casting working environment needs to withstand, in addition to the strict size requirements, the internal quality requirements of this product are also very high: 100% magnetic particle inspection is performed on the casting according to EN1369, with a quality level requirement of 2; 100% penetrant inspection is performed on the casting according to EN1371, with a quality level requirement of 2; 100% ultrasonic inspection is performed on the casting according to EN12680-2, with a quality level of 2 + allowable limit value ESR (equivalent of straight probe) of 6mm, and RT is performed when the UT inspection results cannot be judged.
3. Design of the Outer Casing Casting Process
3.1 Design of the Casting Feeding Scheme
According to the process design principles of cast steel parts, the positions of the risers and chills should be reasonably determined to form a good temperature gradient within the feeding range of each riser, so that the sequential solidification from the feeding end area to the riser direction can obtain a dense and sound casting. The combustor outer casing has a large size, a complex shape structure, and a main rotary structure with a large-size thin-wall design of 45mm equal wall thickness. From the perspective of the part structure design, 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, without any shrinkage porosity or inclusions. Its material grade is GX23CrMoV12-1, which belongs to high-Cr martensitic heat-resistant steel, with poor flow performance of the material. Compared with low-alloy materials, its feeding distance is short, the feeding effect is poor, and it is prone to generate shrinkage porosity and shrinkage cavity defects during the solidification process. Therefore, in the casting process design, it is necessary to design the distribution and feeding of the risers by combining the casting structure and the feeding performance of the material. The riser distribution should be set according to its effective feeding distance, and the feeding should be carried out in sections and zones according to the structural characteristics of the outer casing product. In each riser feeding area, the feeding performance of the material also needs to be fully considered, and the temperature gradient of the solidification process from each area end area to the riser position should be increased to make the casting structure dense to obtain a qualified product.
In the casting process design, according to the structure of the outer casing casting, it is divided into the upper and lower sections from the bottom of the inspection window. In the lower section, the riser feeding range is divided according to the feeding distance, and a blind riser is set; in the upper section, the riser is set on the top for feeding according to the positions of the 12 circular burner holes. Two riser distribution schemes were designed for the structure of the positions of the 12 circular burner holes in the upper section. Scheme one is to set the risers in two circles in the area of the inner flange of the top surface and the large flange outside the burner hole according to the casting structure, as shown in Figure 2(a); in scheme two, every other burner hole is cast dead, and the riser is set at the position of the cast dead burner hole, as shown in Figure 2(b).
Scheme one has a relatively larger number of risers than scheme two, requiring more steel water than scheme two. After optimization through process calculation and adjustment with the solidification simulation of MAGMA software, the riser distribution method of scheme two was finally determined. The lower blind riser was adjusted from a spherical fiber riser to a heat-generating insulation riser, and the top riser platform and the casting flat of the top flange position were optimized. After perfecting the riser feeding area, feeding gradient, and meat-increase subsidy design of each riser, the scheme is shown in Figure 3.
The casting process design divides the lower section into 5 zones, each with a blind riser, and the risers are separated by external chills. The upper section is divided into 6 zones, with a bright riser set for every other circular burner hole. A 1:10 slope meat-increase subsidy is set from the position of the upper and lower section partition area at the bottom of the inner cavity inspection window to improve the temperature gradient in the feeding area under each bright riser during the solidification process, and external chills are used between each feeding area to ensure the density inside the main area of the casting.
3.2 Design of the Molding Method
According to the casting structure, combined with the production tooling and molding pouring conditions of our factory, the method shown in Figures 4 and 5 is adopted. By adding a cutting opening to the joint flange of the combustor outer casing upper and lower halves, the upper and lower halves are connected and cast into a ring, which can not only improve the molding and pouring production efficiency but also effectively prevent the deformation of the casting.
The middle flange is selected as the parting surface, the inner cavity main core is an integral structure, and the joint cutting opening part of the upper and lower joint flange is cored separately. The parting surface and main core structure are shown in Figures 6 and 7.
3.3 Size and Crack Risk Control
For the large-size thin-wall structure of the combustor outer casing, in the design of the casting process scheme, not only the deformation caused by inconsistent cooling and contraction of each area during the solidification process of the casting needs to be considered, but also the deformation prevention during the heat treatment process of the casting should be considered in combination with the loading method designed in the heat treatment process. At the same time, in view of the large hot cracking tendency of high-Cr martensitic heat-resistant steel, effective preventive measures need to be taken for the hot cracking high-risk areas in the MAGMA stress simulation results.
Based on the production experience of similar semi-annular joint casting structure products in our factory and the wall thickness tolerance requirements of the outer casing casting, a reasonable machining allowance and blank surface correction amount were selected, and a 1.8% shrinkage scale was selected in combination with the analysis of the MAGMA stress simulation results. According to the heat treatment process, the upper and lower halves of the outer casing casting will be cut apart from the connection of the flange joint surface after molding and shakeout cleaning, and the performance heat treatment will be carried out on the upper and lower halves respectively. In order to prevent the deformation of the semi-annular structure of the casting at the opening position during the heat treatment process, the casting process sets upper and lower two-layer tension ribs in the opening direction according to the requirements of the heat treatment process. According to the loading method of the casting heat treatment, in order to prevent the high-temperature deformation of the shell during the heat treatment process, a large triangular rib for deformation prevention is set at the inflection point position of the inner cavity of the middle flange. The fillet of the four rounded corners of the inspection window, the anti-deformation tension rib, and the middle surface flange position, which show a large tendency of hot cracking in the simulation results, are all enlarged to R120mm. All the external chills on the main wall thickness use round steel external chills to prevent the occurrence of cracks between the external chills.
3.4 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 liquid steel fluidity and easy generation of oxide inclusions. Therefore, the pouring process needs to be stable and rapid to reduce the entrainment of gas and slag during the pouring process. The process design adopts a bottom return pouring system to ensure stable mold filling and smooth exhaust. In order to reduce the turbulence of the liquid steel entering the mold cavity and improve the slag floating ability, a tangentially introduced bottom return inner nozzle access boss is specially designed in the process to ensure that the liquid steel enters the mold cavity smoothly and quickly for filling. Combined with the actual production situation, a single ladle with a diameter of 90mm and two ladle eyes with argon gas protection for pouring is adopted. The design of the pouring system is shown in Figure 8.
3.5 Analysis of the Solidification Simulation Results
Through multiple adjustments and optimizations of the scheme simulated by MAGMA software, the simulation results of the shrinkage cavity and shrinkage porosity of the final process scheme are determined, as shown in Figures 9 and 10.
The simulation results show that there is no display of shrinkage cavity defects in the overall body of the outer casing casting. According to the application experience of the MAGMA software, the niyama (shrinkage porosity) criterion is selected to be 0.3 to 0.6 for the analysis of thin-walled castings. When the niyama criterion is set to 0.3, there is no defect display in the casting body; when the niyama criterion is set to 0.6, except for the positions of the risers and tension ribs, only a few point-like defects are displayed in the local position of the external chill partition. According to the use experience of the MAGMA software, the point-like niyama display in the area of the thin-walled castings’ external chill partition has no quality risk.
4. Production Process Control
In order to ensure the quality of the casting, a reasonable block division was carried out on the model structure, and the model was fabricated by CNC machining to ensure the model size. After the completion, the sizes of the model and core box passed the three-dimensional inspection, all the sizes were qualified, and the deviations were within the tolerance range of the model size. The molding adopts alkaline phenolic resin sand molding, and the surface of the mold cavity is brushed with water-based zircon powder coating. Since the inner cavity structure of the casting is formed by the sand core, the core setting size needs to be strictly controlled during the core setting process of the molding, and the mold cavity size is ensured by means of template inspection and other auxiliary methods during the molding. The upper large conical surface and the upper box surface of the casting are prone to defects such as pores and slag inclusions. Before pouring, an endoscope is used for inspection to remove the floating sand and other debris inside the mold cavity to ensure the cleanliness of the mold cavity.
5. Product Quality Status
The chemical composition and mechanical properties of the combustor outer casing casting all meet the various index requirements in Tables 1 and 2. After the casting is knocked out and shakeout, the surface quality is good, and no visual surface defects such as slag inclusions, pores, cold shuts, and cracks are found. The size deformation situation of the product marked in the processing workshop is consistent with the simulation results, without any size shortage problem. The machined surface after rough machining meets the requirements of this rough machining process allowance, and the product photos after rough machining are shown in Figures 11 and 12.
The casting size is detected by three-dimensional data, and the comparison 余量 between the three-dimensional detection data after rough machining and the part digital model size is shown in Figures 13 and 14, and all the sizes meet the design requirements.
According to the technical requirements, MT, PT, UT, and other inspections were carried out on the overall casting, and from the non-destructive testing results, the surface and internal quality of the casting all meet the design requirements.
6. Conclusions
By using the MAGMA software to assist in designing the determined casting process scheme of the combustor outer casing, it effectively guarantees the feeding and size quality of the casting and avoids the crack risk. Summarizing the casting process and physical quality of the combustor outer casing castings, the following conclusions are drawn:
- For such castings with thin-walled and complex structures, the feeding method of sectioning and partitioning can effectively ensure the internal quality of the castings.
- Combined with the filling simulation results, the bottom return and tangential pouring system is adopted, which effectively ensures the stable mold filling of the liquid steel during the pouring process and avoids the generation of pores and slag inclusions.
- Through the MAGMA stress simulation results, the anti-deformation tension ribs and triangular ribs set for the combustor outer casing castings effectively avoid the generation of casting deformation and cracks; the shrinkage scale, correction, and machining allowance determined according to the size results of the MAGMA simulation are reasonable, ensuring that the size of the finished casting meets the product delivery requirements.
- The way of joint casting of the upper and lower halves of the combustor outer casing castings can not only improve the production efficiency but also prevent the deformation of thin-walled annular parts.
