Zhuojin Pipe Fitting Co., LtdManufacturing Technology of Sanicro25 Large Diameter Thick-Walled Steel Tubes for Ultra-Supercritical Power Units
Literature Overview
This paper by Li Yongqing and colleagues from Inner Mongolia Northern Heavy Industry Group documents the manufacturing of Sanicro25 large diameter thick-walled seamless steel tubes for advanced ultra-supercritical (USC) power generation units. Using a 36,000-ton vertical extrusion press, the company successfully produced tubes with dimensions of Φ500 mm × Φ335 mm × 3000 mm. Published in 2025 in the journal Forging and Stamping Technology, this work represents a significant advancement in the domestic production capability for large-diameter heat-resistant alloy tubes in China.
Material and Manufacturing Background
Sanicro25 is a Cr-Mo-Ni austenitic heat-resistant steel developed for ultra-supercritical and supercritical boiler applications. Its composition includes approximately 25% Cr, 10% Ni, 4% Mo, and minor additions of Nb, Ti, and V for precipitation strengthening. The material offers excellent resistance to thermal fatigue, creep, and steam oxidation at temperatures up to 650 °C, making it suitable for high-temperature sections of boiler tubing in advanced power plants.
The challenge of producing large-diameter thick-walled Sanicro25 tubes lies in the combination of the material's high deformation resistance, the difficulty of achieving uniform microstructure in thick sections, and the stringent performance requirements for power plant applications. The use of a 36,000-ton vertical extrusion press enables the necessary deformation force to produce seamless tubes of this size without the need for welding, which would introduce potential failure points in a critical power plant component.
Manufacturing Process and Heat Treatment
The manufacturing process involves the following key stages:
| Process Stage | Key Parameters | Purpose |
|---|---|---|
| Billet preparation | Sanicro25 ingot | Raw material with certified chemistry |
| Homogenization | 1250-1300 °C, several hours | Dissolve segregation and homogenize |
| Hot extrusion | 36,000-ton vertical press | Form seamless tube from billet |
| Solution treatment | 1180 °C, 30 or 60 min | Dissolve carbides and refine grain |
| Quenching | Water or air | Retain solution-treated structure |
| Stress relief | Appropriate temperature | Reduce residual stresses |
The solution treatment temperature of 1180 °C is critical for dissolving secondary carbides and achieving a single-phase austenitic structure. The study compared two solution treatment times: 30 minutes and 60 minutes.
Mechanical Properties and Performance Analysis
The mechanical properties of the large diameter thick-walled tubes after solution treatment are presented below.
| Property | 30 min Solution | 60 min Solution | Small Diameter Reference |
|---|---|---|---|
| Yield Strength (MPa) | 347.5 | 345.5 | Meets standard |
| Tensile Strength (MPa) | 735 | 730 | Meets standard |
| Impact Energy (J) | 110 | 102 | Meets standard |
| Grain Size (ASTM) | 4.5 | 4.0 | Comparable |
Both solution treatment times produced acceptable mechanical properties that exceed the requirements for small diameter tubes. The 30-minute treatment yielded slightly higher yield strength and impact energy, while the 60-minute treatment produced a slightly coarser grain structure (4.0 vs 4.5). The difference between the two treatment times is minimal, suggesting that 30 minutes is sufficient for carbide dissolution at 1180 °C for this tube thickness.
Longitudinal vs Transverse Property Differences
The paper notes that while the longitudinal mechanical properties and impact performance of the large diameter tubes reached the level of small diameter tubes in the delivered condition, there was a significant difference between longitudinal and transverse impact properties. This anisotropy is attributed to the fiber directionality created during extrusion, which provides greater crack resistance in the longitudinal direction. This is an important consideration for pipe fitting applications where transverse welds may be required, as the transverse toughness must be sufficient to resist crack initiation and propagation under thermal cycling conditions.
Quality Control and Engineering Considerations
For the production of Sanicro25 large diameter tubes for power plant applications, the following quality control measures are essential:
- Chemical composition verification of each heat to ensure compliance with Sanicro25 specification ranges, particularly for Cr, Ni, Mo, and Nb content.
- Non-destructive testing of the extruded tubes using ultrasonic testing (UT) to detect internal defects such as seams, inclusions, and cracks.
- Metallographic examination to verify grain size, carbide distribution, and absence of segregation.
- Mechanical property testing in both longitudinal and transverse orientations to characterize the full anisotropic behavior.
- Creep testing and thermal fatigue testing for qualification of the material for long-term power plant service.
The surface quality of the produced tubes was reported as good, which is critical for both aesthetic requirements and corrosion resistance. Surface defects such as scale, scratches, or extrusion marks can serve as initiation sites for corrosion or stress corrosion cracking during service.
Practical Implications for Power Plant Applications
The successful production of Sanicro25 tubes at Φ500 mm × Φ335 mm × 3000 mm dimensions represents a major capability enhancement for domestic power plant equipment manufacturing. These large diameter tubes are typically used for main steam lines, hot reheat steam lines, and other high-temperature piping in ultra-supercritical units operating at temperatures above 600 °C. The ability to produce these tubes domestically reduces dependence on imports and ensures supply chain security for critical power infrastructure.
The finding that 30-minute solution treatment is sufficient is practically significant as it reduces heat treatment cycle time and energy consumption, thereby lowering production costs. However, for the largest tube sizes, thermal uniformity during heat treatment must be carefully managed to avoid property variations across the wall thickness.
Summary
This paper documents a successful manufacturing achievement in producing Sanicro25 large diameter thick-walled seamless steel tubes for ultra-supercritical power units using a 36,000-ton vertical extrusion press. The produced tubes meet or exceed the mechanical property requirements for smaller diameter tubes, with both 30-minute and 60-minute solution treatments at 1180 °C yielding acceptable results. The longitudinal-transverse property anisotropy, while expected, requires careful consideration in design and fabrication of power plant piping systems. This work represents a significant step forward in China's domestic capability for manufacturing critical heat-resistant alloy tubes for advanced power generation.