Development of P92 Seamless Steel Pipes for Ultra-Supercritical Power Generation

Literature Overview

This 2014 publication by Guo Yuanrong, Zhan Yong, Hu Maohui, Wu Hong, and Chen Yu from Pangang Group Chengdu Steel and Vanadium Co., Ltd. (published in Steel Pipe, Vol. 43, No. 3, pp. 29-33) documents the successful development of P92 seamless steel pipes for ultra-supercritical (USC) power generation applications. Supported by the National Science and Technology Support Program (Grant 2007BAE51B02), this work represents a significant milestone in China's domestic production capability for high-temperature alloy seamless pipes.

Technical Background

P92 steel (also designated as 9Cr-1Mo-V-Nb) is a modified 9% chromium-molybdenum martensitic heat-resistant steel that represents a major advancement over conventional 9Cr-1Mo (P91) steel for ultra-supercritical boiler applications. The addition of vanadium and niobium microalloying elements provides enhanced creep resistance at elevated temperatures while maintaining good weldability and fabrication properties.

P92 Steel Composition and Properties

Advantages Over P91 Steel

P92 offers approximately 20-30% improvement in creep rupture strength over P91 at equivalent temperatures, enabling:

• Higher steam temperatures (600-620°C vs. 580-600°C for P91)
• Reduced pipe wall thickness for equivalent design pressure
• Improved thermal efficiency of power plants (1-2% absolute improvement)
• Reduced CO2 emissions per unit of electricity generated

Production Process Development

Manufacturing Process Flow

The development of P92 seamless steel pipes at Pangang involved optimization of the complete production chain:

1. Steelmaking: Electric arc furnace melting with strict control of alloy additions and impurity levels (S < 0.015%, P < 0.015%)
2. Refining: LF (Ladle Furnace) + RH (Vacuum Degassing) double refining for steel cleanliness
3. Casting: Continuous casting with controlled cooling rate to produce homogeneous billets
4. Piercing and rolling: Hot piercing followed by multi-pass rolling to achieve target pipe dimensions
5. Heat treatment: Solution treatment + multi-stage tempering for optimal microstructure
6. Inspection: Comprehensive non-destructive testing and mechanical property verification

Heat Treatment Specification

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

The multi-stage tempering process is essential for P92 because it allows controlled precipitation of fine MX-type (V, Nb, Ti) carbides and M23C6-type (Cr, Mo) carbides that provide creep resistance through solid solution strengthening and precipitation hardening.

Performance Evaluation Results

High-Temperature Tensile Properties

The developed P92 seamless pipes demonstrated excellent high-temperature tensile performance:

High-Temperature Creep Performance

The creep performance results confirmed that the developed pipes meet or exceed ASME SA-335/SA-335M requirements:

• At 600°C, the 100,000-hour creep rupture stress exceeded 100 MPa
• The creep rupture life was consistent with the P92 steel design curves
• No abnormal microstructural degradation was observed after long-term exposure testing

Microstructural Characterization

Metallographic examination confirmed:

• Uniform tempered martensitic microstructure throughout the pipe wall thickness
• Fine, uniformly distributed MX-type and M23C6-type carbides
• Absence of abnormal carbide networks or precipitate-free zones
• No microstructural banding or segregation defects

Standards Compliance

The developed P92 seamless pipes were verified against the following standards:

Engineering Practice Significance

The successful development of P92 seamless pipes domestically in China has several important implications:

1. Supply chain security: Reduces dependence on imported high-temperature alloy pipes, which have historically been restricted in export.
2. Cost reduction: Domestic production reduces material costs by 30-50% compared to imported equivalents.
3. Technical capability: Establishes the manufacturing infrastructure for future higher-grade materials (P91, P92, and potential 12Cr steels).
4. Quality traceability: Domestic production enables full traceability from steelmaking to finished pipe, critical for nuclear and power industry applications.

Key Reflections

This development represents a significant achievement in China's metallurgical industry, demonstrating that domestic manufacturers can produce world-class high-temperature alloy seamless pipes meeting international standards. The production process optimization—particularly the multi-stage tempering regime and strict steel cleanliness control—is critical for achieving the required microstructural homogeneity. Engineers working with P92 pipes should pay particular attention to welding procedures, as this material requires strict preheat and post-weld heat treatment protocols to prevent hydrogen-induced cracking and maintain creep resistance in the heat-affected zone. The development of domestic P92 pipe production enables China's ultra-supercritical power generation program to proceed with greater supply security and cost competitiveness, contributing to the nation's energy efficiency and emissions reduction goals.