In the pursuit of advancing energy efficiency and environmental sustainability, the development of ultra-high temperature ultra-supercritical steam turbines represents a significant technological leap. These systems operate with main steam and reheat steam temperatures elevated to 620°C, compared to the conventional 580°C in ultra-supercritical units, resulting in a thermal efficiency improvement of 4% to 6%. This enhancement is crucial for reducing emissions and promoting sustainable energy solutions. A key component in these turbines is the reheat main valve casing, which is fabricated from a high-performance steel casting material designated as ZG13Cr9Mo2Co1NiVNbNB, commonly referred to as CB2. This steel casting offers superior high-temperature creep resistance and oxidation performance, making it ideal for demanding applications. The valve casing has a specified weight of 13,070 kg and overall dimensions of 2,700 mm × 2,175 mm × 1,695 mm, featuring a complex structure that presents numerous challenges in manufacturing. In this article, we detail the comprehensive development process of this critical steel casting component, focusing on casting design, simulation, melting, and heat treatment to achieve a high-quality product.
The steel casting development began with a thorough analysis of the technical requirements. The chemical composition of the steel casting must adhere to strict limits to ensure the desired mechanical properties and microstructural integrity. Key elements include carbon, silicon, manganese, chromium, molybdenum, cobalt, vanadium, niobium, boron, and nitrogen, each playing a vital role in enhancing high-temperature performance. For instance, the carbon content is controlled between 0.11% and 0.14% to balance strength and weldability, while alloying elements like chromium and molybdenum contribute to corrosion resistance and creep strength. The allowable deviations for these elements are tightly specified to maintain consistency in the steel casting production. Below is a summary of the chemical composition requirements and permissible deviations for the steel casting material.
| Element | Range | Permissible Deviation |
|---|---|---|
| C | 0.11–0.14 | ±0.01 |
| Si | 0.20–0.30 | ±0.05 |
| Mn | 0.80–1.00 | ±0.05 |
| P | ≤0.020 | 0.005 |
| S | ≤0.010 | 0.005 |
| Cr | 9.00–9.60 | ±0.20 |
| Ni | 0.10–0.20 | ±0.02 |
| Mo | 1.40–1.60 | ±0.02 |
| Co | 0.90–1.10 | ±0.02 |
| V | 0.18–0.23 | ±0.02 |
| Nb | 0.05–0.08 | ±0.005 |
| B | 0.008–0.011 | ±0.001 |
| N | 0.015–0.022 | ±0.003 |
| Altot | ≤0.020 | 0.002 |
In addition to chemical composition, the mechanical properties of the steel casting at room temperature are critical for ensuring structural integrity under operational loads. The yield strength (Rp0.2) must be at least 500 MPa, with tensile strength (Rm) ranging from 630 to 750 MPa. Elongation (A5) should exceed 15%, and reduction of area (Z) must be at least 40%. Charpy V-notch impact energy (KV) requires an average of 30 J across three specimens, with no individual value below 24 J. These properties are achieved through precise control of the steel casting process and subsequent heat treatments. Non-destructive testing, including 100% magnetic particle inspection per ASTM E125 Grade II and ultrasonic testing per ASME B16.34 Grade II, ensures the absence of surface and internal defects. Dimensional tolerances follow GB/T 6414-2017 CT13 level, with machining allowances of 15–20 mm on critical surfaces to facilitate finishing.
The development of this steel casting faced several technical challenges, primarily due to the material characteristics and complex geometry. The steel composition, a low-carbon martensitic steel with multiple alloying elements, leads to high pouring temperatures and poor fluidity, increasing the risk of inclusions and secondary oxides. Nitrogen content introduces susceptibility to gas porosity, while elements like niobium and vanadium can form compounds that precipitate at grain boundaries, promoting hot tearing. The structural complexity of the valve casing, with multiple flanges and transition zones, creates dispersed hot spots that complicate solidification control. Internal cores are subjected to prolonged high-temperature exposure, necessitating high refractoriness, strength, and permeability to prevent defects such as sand sticking and core deformation. Addressing these issues required innovative approaches in the steel casting process to achieve sound and reliable components.
To optimize the steel casting process, we employed computer simulation software Experto-ViewCast to predict and mitigate potential defects like shrinkage porosity and hot tears. The casting orientation was set horizontally, with the largest parting plane selected for mold division. A pattern shrinkage allowance of 2% was applied, and machining allowances were allocated to critical surfaces. The steel casting was divided into three distinct regions—A, B, and C—based on thermal characteristics, with insulating risers and chills strategically placed to control solidification. For example, Region A, resembling a vertical wheel shape, was equipped with an open insulating riser to enhance feeding. The modulus method was used to calculate riser dimensions, ensuring adequate feeding throughout the solidification process. The modulus M is defined as the ratio of volume to cooling surface area, expressed as:
$$ M = \frac{V}{A} $$
where V is the volume and A is the surface area. For Region A, the modulus Ma was calculated as 6.84 cm, requiring a riser modulus Mriser ≥ 1.2 × Ma = 8.21 cm. The riser design accounted for the solidification shrinkage S, approximately 5.8% for this steel casting material, using the formula:
$$ G_{\text{riser}} = G_{\text{casting}} \times \frac{\eta – S}{S} $$
where Griser is the riser metal mass, Gcasting is the casting mass in the region, and η is the riser efficiency, taken as 18%. This yielded a required riser metal mass of approximately 2,910 kg for Region A, met by a rectangular insulating riser of dimensions 600 mm × 900 mm × 900 mm. Similar calculations guided riser placement in other regions, with chills used to accelerate cooling in thick sections and promote directional solidification. Simulation results confirmed that shrinkage defects were confined to the risers, validating the steel casting process design.
The molding process for the steel casting utilized an ester-hardened sodium silicate binder system with quartz sand for the cores and chromite sand for the face layers to enhance refractoriness and surface quality. Chills were pre-treated by shot blasting and baking at 250°C for over 24 hours to remove hydrogen. The mold was compacted uniformly, and the cavity surfaces were coated with a zircon-based alcohol paint to prevent metal penetration. Cores were reinforced with seamless steel tubes for venting and stability. After mold assembly, hot air at 120°C was circulated for more than 24 hours to dry the mold and maintain an internal temperature above 100°C, minimizing moisture-related defects. This meticulous approach in the steel casting molding ensured dimensional accuracy and surface integrity.
Melting of the steel casting was conducted using an electric furnace (EF) followed by ladle furnace (LF) refining and vacuum degassing (VD) to achieve high purity and precise composition control. The charge materials included low-residual scrap steel to minimize impurities like nickel, tin, and copper. Alloy additions, such as metallic chromium, were selected for high purity and low nitrogen content. The LF refining stage focused on desulfurization and deoxidation, achieving sulfur levels below 0.010% and oxygen activity under 20 ppm. Vacuum degassing at pressures ≤133 Pa for over 30 minutes reduced hydrogen to ≤2.0 ppm and nitrogen to ≤60 ppm, critical for avoiding porosity in the steel casting. The chemical composition was tightly controlled within internal standards, as summarized below, to optimize casting performance and mechanical properties.
| Element | Control Target |
|---|---|
| C | 0.11–0.14 |
| Si | 0.20–0.30 |
| Mn | 0.85–0.95 |
| P | ≤0.010 |
| S | ≤0.010 |
| Cr | 9.20–9.45 |
| Ni | 0.10–0.20 |
| Mo | 1.50–1.60 |
| Co | 0.90–1.10 |
| V | 0.20–0.23 |
| Nb | 0.05–0.08 |
| B | 0.008–0.011 |
| N | 0.015–0.022 |
| Altot | ≤0.010 |
Pouring of the steel casting was performed with argon protection to prevent secondary oxidation. The gating system was designed as an open type with bottom pouring to ensure smooth filling and minimize turbulence. Pouring temperature was maintained between 1,570°C and 1,580°C, with a pouring time under 100 seconds to achieve an average rise rate of 19.0 mm/s. The cross-sectional area ratio of the gating system was set as ∑Ssprue : ∑Srunner : ∑Sgate = 1 : 2.3 : 6.4, utilizing two ϕ80 mm pouring cups, two ϕ120 mm sprue, two ϕ120 mm runners, and eight ϕ80 mm gates. After pouring, the steel casting was covered with exothermic material to maintain thermal conditions. The cooling process was carefully managed, with mold dismantling starting after 18 hours and completed by 144 hours, ensuring the casting cooled below 200°C to avoid cracking. Sand removal was conducted after the steel casting reached below 80°C.
Heat treatment of the steel casting involved a preheating stage to refine the grain structure and facilitate hot riser cutting. The annealing process included heating to 500°C at 60°C/h, holding for 20 hours, then cooling to 300°C at 30°C/h. Risers were cut while the casting temperature remained above 200°C to prevent cracking. Subsequently, the steel casting underwent normalizing and tempering treatments to achieve the desired mechanical properties. Normalizing was performed by heating to 1,090°C at 80°C/h, holding for 20 hours, followed by forced air cooling to below 70°C in the thickest sections. Tempering involved heating to 730°C at 80°C/h, holding for 25 hours, and furnace cooling to 300°C before air cooling. Thermocouples were attached to the casting to monitor temperature uniformity, and supports were used to prevent distortion during heat treatment. This comprehensive heat treatment cycle ensured the steel casting met the stringent performance requirements for ultra-high temperature applications.
After processing, the steel casting was subjected to thorough inspection. Chemical analysis confirmed compliance with specifications, as shown in the table below, with low levels of harmful elements and gases. Mechanical testing demonstrated excellent properties, exceeding the minimum requirements. Non-destructive examination revealed no significant defects, with a repair rate of only 0.7%, indicating high quality in the steel casting production. The successful development of this steel casting component underscores the effectiveness of the integrated approach combining simulation, advanced molding, controlled melting, and precise heat treatment.
| Element | Result |
|---|---|
| C | 0.13 |
| Si | 0.28 |
| Mn | 0.89 |
| P | 0.009 |
| S | 0.003 |
| Cr | 9.4 |
| Mo | 1.50 |
| Ni | 0.10 |
| Co | 1.01 |
| V | 0.21 |
| Nb | 0.06 |
| B | 0.010 |
| N | 0.016 |
| Altot | <0.01 |
| Ti | <0.01 |
| Sn | 0.005 |
| Cu | 0.06 |
| H | 0.00003 |
| O | 0.0038 |
| Property | Requirement | Actual Data |
|---|---|---|
| Rp0.2 (MPa) | ≥500 | 615 |
| Rm (MPa) | 630–750 | 742 |
| A5 (%) | ≥15 | 18.7 |
| Z (%) | ≥40 | 59.7 |
| KV (J) | ≥30 (avg), ≥24 (min) | 39/35/38 |
In conclusion, the development of this advanced steel casting for ultra-high temperature ultra-supercritical steam turbine valves demonstrates the importance of a holistic manufacturing strategy. By leveraging simulation tools, optimized molding techniques, precise melting control, and tailored heat treatments, we achieved a high-integrity steel casting that meets all technical specifications. The use of insulating risers, chills, and argon-protected pouring minimized defects, while grain refinement and hot riser cutting prevented cracking. The final steel casting exhibited excellent chemical and mechanical properties, with minimal repair needs, validating the effectiveness of the applied methodologies. This success paves the way for broader adoption of such steel casting components in next-generation power plants, contributing to enhanced efficiency and environmental sustainability.