Domestic Development of High-Pressure Pipe Fittings for 600MW Supercritical Power Units

Literature Overview

This brief but significant publication in Power Construction (2004, Vol. 25, No. 12, p. 72) reports the successful domestic development of high-pressure pipe fittings for 600MW supercritical power units. The collaborative effort between Northeast Electric Power Design Institute and Bohai Pipe Fitting Co., Ltd. in Cangzhou, Hebei Province, marked a milestone in China's power generation equipment manufacturing capability. The development of A335 P91 and WB36 high-pressure fittings for supercritical units was achieved after an investment exceeding 10 million RMB, ending China's dependence on imported fittings for this critical application.

The technical parameters specified in the publication are substantial: the main steam system must withstand 25.4 MPa pressure at 576°C, while the high-pressure feedwater system must endure 37 MPa pressure at 285°C. These extreme conditions place demanding requirements on the material properties, manufacturing processes, and quality control systems used in fitting production.

Material Specifications and Technical Requirements

The two material grades specified for this application represent distinct metallurgical challenges:

P91 Steel Fittings

P91 steel (9% Cr, 1% Mo, with V, Nb, and N micro-alloying) is the standard material for main steam piping in supercritical and ultra-supercritical power plants. Its excellent creep resistance and oxidation resistance at temperatures up to 650°C make it indispensable for this application. The forging and heat treatment of P91 fittings require precise control of the microstructure to ensure adequate toughness alongside high-temperature strength. The normalization and tempering cycles must be carefully calibrated to achieve the desired tempered martensitic microstructure.

Key manufacturing considerations for P91 fittings include:

• Forge temperature control within the austenite + carbide region
• Normalization at 1050-1100°C followed by air cooling
• Tempering at 760-780°C for 2-4 hours
• Post-weld heat treatment (PWHT) for welded joints at 760-780°C
• Strict control of carbon equivalent to limit cold cracking susceptibility

WB36 (25Cr-35Ni) Fittings

The 25Cr-35Ni austenitic steel (WB36) is used in applications requiring superior corrosion resistance at elevated temperatures. Its high nickel content provides excellent resistance to sulfidation and oxidation in high-temperature flue gas environments. The manufacturing challenges for WB36 include:

• Prevention of sensitization during welding (avoiding the 600-900°C temperature range)
• Control of grain growth during hot working
• Maintaining solution-treated condition for optimal corrosion resistance
• Careful control of post-weld heat treatment to avoid intergranular corrosion susceptibility

Manufacturing Process Analysis

The development of these high-pressure fittings required mastery of several critical manufacturing processes:

Forging Process

Both P91 and WB36 fittings require precision forging to achieve the required dimensional accuracy and microstructural quality. The forging process must ensure:

• Complete consolidation of the billet without residual porosity
• Appropriate grain size control (typically ASTM grain size 5-8)
• Uniform deformation to avoid localized work hardening
• Proper cooling rates to achieve the target as-forged microstructure

Heat Treatment

The heat treatment regime is critical for achieving the required mechanical properties:

• P91: Normalization + tempering to achieve tempered martensite with dispersed carbides
• WB36: Solution treatment + water quench to achieve single-phase austenite

Welding Considerations

Welded fittings (such as those produced by welding forged blanks) require special attention:

• Preheat temperature control (P91: 150-250°C; WB36: typically no preheat required but interpass temperature control is critical)
• Filler metal selection (P91: matching or slightly lower Cr-Mo grade; WB36: matching austenitic grade)
• Post-weld heat treatment to relieve residual stresses and homogenize the heat-affected zone

Quality Control Framework

The quality assurance requirements for these high-pressure fittings are stringent and multi-layered:

The hydrostatic pressure test is particularly critical for these high-pressure fittings. For the main steam application at 25.4 MPa design pressure, the test pressure would be approximately 38.1 MPa (1.5x design pressure), requiring robust test equipment and careful monitoring.

Engineering Significance and Practice Integration

The successful domestic development of these fittings has profound implications for China's power generation industry:

1. Supply Security: Eliminating dependence on imported fittings for critical power plant components reduces supply chain risk and ensures timely project execution.
2. Cost Reduction: Domestic production eliminates import duties, shipping costs, and foreign exchange exposure, contributing to overall project cost optimization.
3. Technical Capability Development: The process development and quality control experience gained through this project builds organizational competence that can be leveraged for future projects.
4. Standards Harmonization: The development work necessitates alignment with both domestic (GB/T, DL/T) and international (ASTM, ASME) standards, promoting technical exchange and mutual recognition.

From a practical engineering standpoint, the transition from imported to domestically produced fittings requires careful qualification and validation. The following steps are recommended:

• Comparative testing of domestic and imported fittings under identical conditions
• Accelerated creep testing to validate long-term performance predictions
• Field trial installation with extended monitoring periods
• Documentation of any performance differences and their implications for design margins

Key Questions and Reflections

Several questions arise from this development effort:

• What is the long-term creep performance of domestically produced P91 fittings compared to imported equivalents?
• How does the quality consistency of domestic production compare with established international manufacturers?
• What are the implications for warranty and liability if domestically produced fittings fail prematurely?

The investment of over 10 million RMB for this development project must be evaluated against the lifetime cost savings from eliminating imports. For a typical 600MW supercritical unit, the number of high-pressure fittings required is substantial, and the cumulative savings from domestic production over multiple projects would likely justify the initial development investment.

Study Insights and Implications

This publication, while brief, documents a significant milestone in China's power equipment manufacturing capability. The successful development of A335 P91 and WB36 high-pressure fittings for 600MW supercritical units demonstrates that domestic manufacturers can meet the demanding requirements of modern power plant design when given adequate investment, technical support, and quality control infrastructure.

For practicing engineers, the key lesson is that material sourcing decisions should be based on comprehensive qualification data rather than assumptions about geographic origin. Whether imported or domestically produced, fittings must meet the same rigorous specifications and demonstrate equivalent performance under service conditions. The engineering community should actively participate in the qualification process, providing the technical expertise and testing resources necessary to validate domestic production capabilities.

The experience gained from this project should inform future development efforts for ultra-supercritical and advanced ultra-supercritical units, where material requirements are even more demanding. The organizational learning and technical infrastructure established through this project form the foundation for continued advancement in China's power equipment manufacturing sector.