Zhuojin Pipe Fitting Co., LtdManufacturing Process of Sanicro25 Large-Diameter Steel Pipes for Ultra-Supercritical Power Units
Literature Overview
The paper by Li Yongqing, Qin Ruiting, Yun Xuefeng, Liu Zhengwei, Du Chunjin, and Lei Bingwang, published in Forging and Stamping Technology (2025, Vol. 50, No. 3), describes the manufacturing process of Sanicro25 large-diameter thick-wall seamless steel pipes for advanced ultra-supercritical (USC) power generation units. The research was conducted by Inner Mongolia Northern Heavy Industries Group Co., Ltd., which successfully trial-produced pipes with specifications of Φ500 mm × Φ335 mm × 3000 mm using a 36,000-ton vertical extrusion press. This work addresses a critical supply chain need for high-temperature alloy components in next-generation power plants, where Sanicro25 is specified for superheater and reheater tubes operating at steam temperatures exceeding 650 °C.
Core Technical Content
Material Characteristics of Sanicro25
Sanicro25 is a high-chromium austenitic stainless steel containing approximately 25% chromium, 20% nickel, and significant amounts of niobium and titanium as stabilizing elements. The high chromium content provides excellent resistance to oxidation and corrosion at elevated temperatures, while the nickel ensures austenitic stability and good creep strength. The niobium and titanium additions form fine carbides that contribute to precipitation strengthening and improve creep resistance. Sanicro25 is particularly valued for its superior oxidation resistance compared to conventional austenitic steels such as TP310H, making it suitable for the most demanding service conditions in USC power plants.
The mechanical properties of Sanicro25 are characterized by high yield strength (typically above 300 MPa at 650 °C), good tensile strength, and excellent creep rupture strength. However, the material also exhibits significant anisotropy in mechanical properties, particularly in impact toughness, due to the directional fiber structure developed during hot working. This anisotropy is a critical consideration in the design and qualification of Sanicro25 components.
Manufacturing Process Description
The manufacturing process for the Sanicro25 large-diameter thick-wall pipes involved several key steps. The billet was first heated to the appropriate forging temperature, then extruded through the 36,000-ton vertical extrusion press to produce the pipe blank. The extrusion process developed a strong fiber structure along the longitudinal direction of the pipe, which significantly influences the mechanical properties, particularly the impact toughness in the transverse and radial directions.
After extrusion, the pipes were subjected to solution heat treatment at 1180 °C for either 30 or 60 minutes. The solution treatment is critical for dissolving carbides, homogenizing the microstructure, and achieving the required mechanical properties. The choice of solution treatment parameters directly affects the grain size, carbide distribution, and resulting mechanical properties of the final product.
| Process Parameter | Value | Purpose |
|---|---|---|
| Extrusion press capacity | 36,000 tons | Required for large-diameter thick-wall pipe production |
| Pipe specification | Φ500 mm × Φ335 mm × 3000 mm | Large-diameter thick-wall pipe for USC applications |
| Wall thickness | 82.5 mm | Thick wall for high-pressure USC service |
| Solution treatment temperature | 1180 °C | Carbide dissolution and microstructure homogenization |
| Solution treatment time (short) | 30 min | Rapid treatment for large cross-section |
| Solution treatment time (long) | 60 min | Extended treatment for complete homogenization |
Mechanical Performance Results
The mechanical performance of the Sanicro25 pipes after solution treatment showed excellent results. For the 30-minute treatment condition, the yield strength was 347.5 MPa, tensile strength was 735 MPa, impact energy was 110 J, and grain size was 4.5 grade. For the 60-minute treatment condition, the yield strength was 345.5 MPa, tensile strength was 730 MPa, impact energy was 102 J, and grain size was 4.0 grade. Both conditions produced mechanical properties that exceeded the requirements of the applicable small-diameter pipe standard.
The authors noted that the longitudinal mechanical properties and impact properties of the large-diameter thick-wall pipes reached the level of small-diameter pipes in the as-delivered condition. However, there was a significant difference between the longitudinal and transverse impact properties, which was attributed to the stronger crack propagation resistance along the longitudinal fiber direction developed during extrusion.
Process Analysis and Standards Interpretation
Solution Treatment Optimization
The comparison of 30-minute and 60-minute solution treatment conditions revealed that both treatments produced comparable mechanical properties, with the 30-minute treatment actually showing slightly better performance in yield strength, tensile strength, and impact energy. The 60-minute treatment resulted in a slightly coarser grain size (4.0 grade versus 4.5 grade), which is consistent with the expected grain growth during extended solution treatment at high temperature.
From a production efficiency perspective, the 30-minute treatment condition is preferable because it reduces the heat treatment cycle time by 50%, resulting in significant energy savings and increased throughput. The slightly finer grain size and better impact properties achieved with the shorter treatment time suggest that the 30-minute condition may be the optimal choice for large-diameter Sanicro25 pipe production, provided that complete carbide dissolution is achieved.
The solution treatment temperature of 1180 °C is at the upper end of the typical solution treatment range for Sanicro25 (usually 1120–1200 °C). This high temperature is necessary to dissolve the refractory Nb and Ti carbides that provide precipitation strengthening but must be controlled to avoid excessive grain growth. The grain size of 4.5 grade (30 min) and 4.0 grade (60 min) is within the acceptable range for Sanicro25, which typically requires a grain size of 3.5 grade or finer for optimal creep performance.
Anisotropy in Mechanical Properties
The significant difference between longitudinal and transverse impact properties is a well-known characteristic of extruded steel pipes and is particularly pronounced in large-diameter thick-wall pipes. The extrusion process develops a strong fiber structure along the longitudinal direction, with elongated grains and aligned inclusions that provide high resistance to crack propagation in the longitudinal direction but reduced resistance in the transverse and radial directions.
| Direction | Yield Strength (MPa) | Tensile Strength (MPa) | Impact Energy (J) | Grain Size |
|---|---|---|---|---|
| Longitudinal (30 min) | 347.5 | 735 | 110 | 4.5 |
| Longitudinal (60 min) | 345.5 | 730 | 102 | 4.0 |
| Transverse | Significantly lower | Comparable | Significantly lower | Comparable |
This anisotropy has important implications for the design and qualification of Sanicro25 components. In power plant applications, the tubes are subjected to internal pressure, which creates hoop stresses that are perpendicular to the longitudinal fiber direction. The reduced transverse impact toughness may affect the fracture resistance of the tubes under certain loading conditions. However, for steady-state creep service, the anisotropy is less critical because creep deformation is primarily governed by the matrix and grain boundary properties rather than the fiber structure.
Standards Compliance
Sanicro25 pipes for USC power plant applications are typically qualified against ASTM A213 T25, ASME SA-213 T25, or EN 10217-14 (for the tube variant). These standards specify minimum mechanical properties in the solution-treated condition, including yield strength, tensile strength, elongation, and impact energy. The results reported in this paper exceed the standard requirements, which is important for demonstrating the capability of the manufacturing process.
The qualification of large-diameter thick-wall pipes is more challenging than small-diameter pipes because of the increased difficulty in achieving uniform heat treatment and consistent mechanical properties across the entire cross-section. The thermal gradients during solution treatment of a thick-walled pipe can result in non-uniform microstructures and properties, which must be addressed through careful process control and extensive testing.
Engineering Practice Integration
Production Scalability
The successful trial production of Φ500 mm × Φ335 mm × 3000 mm Sanicro25 pipes using a 36,000-ton vertical extrusion press demonstrates the manufacturing capability for large-diameter thick-wall pipes. However, scaling up to production quantities requires addressing several practical challenges, including consistent billet quality, uniform extrusion parameters, reliable heat treatment of large cross-sections, and comprehensive quality control.
The 36,000-ton extrusion press is one of the largest in the world, and its availability is a significant factor in the ability to produce large-diameter Sanicro25 pipes. The limited number of such presses worldwide means that the supply of large-diameter Sanicro25 pipes is constrained, which has implications for the global supply chain for USC power plant components.
Quality Control Considerations
The quality control of Sanicro25 large-diameter pipes requires comprehensive testing to ensure that the mechanical properties are uniform across the entire cross-section and length. Key quality control activities include:
- Chemical analysis to verify compliance with the specified composition
- Grain size measurement to ensure the required grain size is achieved
- Mechanical testing in both longitudinal and transverse directions
- Impact testing at the required temperature to verify toughness
- Non-destructive testing (UT, MT) to detect internal and surface defects
- Corrosion testing to verify oxidation resistance at service temperature
The anisotropy in impact properties requires that testing be performed in both the longitudinal and transverse directions, with acceptance criteria potentially different for each direction. The transverse impact energy must be verified to meet the minimum requirements, even if the longitudinal impact energy is well above the standard specification.
Key Technical Challenges and Solutions
Thick-Wall Heat Treatment Uniformity
The most significant technical challenge in producing large-diameter thick-wall Sanicro25 pipes is achieving uniform heat treatment across the entire cross-section. The wall thickness of 82.5 mm creates substantial thermal gradients during heating and cooling, which can result in non-uniform microstructures and mechanical properties. The center of the wall may experience different thermal histories than the inner and outer surfaces, leading to variations in grain size, carbide distribution, and mechanical properties.
The solution adopted in this study was to use a high solution treatment temperature (1180 °C) with sufficient holding time to ensure complete carbide dissolution and microstructure homogenization. The 30-minute treatment time was found to be sufficient for achieving the required properties, which is favorable from a production efficiency perspective. However, the heat treatment uniformity must be verified through extensive testing at multiple locations across the wall thickness and pipe circumference.
Surface Quality
The surface quality of the extruded pipes is critical for the subsequent machining and welding operations required for pipe fabrication. The authors report that the surface quality of the trial-produced pipes was good, which is important for ensuring the integrity of the final product. Surface defects such as cracks, seams, or scale can propagate during subsequent processing and lead to component failure.
Surface quality is influenced by several factors, including the quality of the extrusion die, the lubrication conditions during extrusion, and the cooling rate after extrusion. The use of appropriate lubricants and controlled cooling rates is essential for achieving the required surface finish and minimizing surface defects.
Study Insights and Implications
This paper represents a significant achievement in the manufacturing of Sanicro25 large-diameter thick-wall pipes, addressing a critical supply chain gap for USC power plant components. The successful trial production using a 36,000-ton vertical extrusion press demonstrates the technical feasibility of producing these large-diameter pipes, which are essential for the next generation of power plants with higher thermal efficiencies.
The finding that both 30-minute and 60-minute solution treatment conditions produce acceptable mechanical properties is particularly valuable from a production optimization perspective. The 30-minute condition offers the advantage of reduced energy consumption and increased throughput without compromising product quality. This finding can guide the optimization of the heat treatment process for commercial production.
The documentation of the anisotropy in impact properties is an important contribution to the understanding of Sanicro25 pipe behavior. While the longitudinal properties meet or exceed standard requirements, the reduced transverse impact toughness must be considered in the design and qualification of components. This understanding can inform the development of design guidelines and qualification procedures for Sanicro25 components in USC power plant applications.
The successful production of Sanicro25 large-diameter pipes by Inner Mongolia Northern Heavy Industries Group contributes to the global supply chain for USC power plant components and supports the continued development of high-efficiency power generation technologies. As the demand for USC power plants grows globally, the ability to produce high-quality Sanicro25 pipes at scale will be a critical factor in meeting the material requirements of these advanced power generation systems.