Performance Study and Engineering Application of P91 Steel Tubes from Continuous Casting Billets

Literature Overview

The paper by Jing Shiyu, Liu Qian, Zhang Xinwen, Zeng Hui, Xie Xiaoyuan, Pu Xiaoming, He Xiaoming, and Yang Huachun, published in Steel Pipe (2022, Vol. 51, No. 5, pp. 20–26), presents a comprehensive study on the quality and performance of P91 steel tubes manufactured from continuous casting (CC) billets. The research was conducted by Dongfang Electric Group Dongfang Boiler Co., Ltd., Jiangyin Xingcheng Special Steel Co., Ltd., and Jiangsu Yonggang Group Co., Ltd. The authors performed physical and chemical testing, high-temperature aging, and high-temperature creep tests on P91 tubes from multiple steel mills to evaluate whether CC-billet-based P91 tubes can meet engineering requirements and address existing concerns about their application.

Core Technical Analysis

P91 steel (equivalent to ASTM A335 P91 or Gr.91) is a 9Cr-1Mo-V-Nb-N microalloyed martensitic ferritic steel widely used in high-temperature and high-pressure applications, particularly in supercritical and ultra-supercritical power boilers, where it operates at temperatures up to 620°C and pressures exceeding 25 MPa. The mechanical properties of P91 steel tubes are critically dependent on the quality of the starting material, which is why traditional practice has favored forged billets for manufacturing P91 tubes.

Quality Assessment of Gr.91 Continuous Casting Billets

Quality Indicator	Assessment Result	Standard Requirement
Chemical composition	Meets standard specifications	ASTM A213 T91 / GB 5310
Inclusion content	Low, acceptable levels	Clean steel requirement
Macroscope structure	Uniform, no segregation	No macro defects
Columnar grain morphology	Good, well-developed	Fine, uniform grain
Center cracks	Regular distribution pattern	Within acceptable limits
Segregation	Low degree	Acceptable for P91 application

Performance Test Results

The authors conducted extensive testing on P91 tubes manufactured from CC billets, including:

Test Type	Condition	Result	Comparison with Forged Billet Tubes
Chemical composition	As-received	Meets ASTM A335 P91	Equivalent
Tensile strength	Room temperature	Meets standard	Equivalent
Yield strength	Room temperature	Meets standard	Equivalent
Elongation	Room temperature	Meets standard	Equivalent
Charpy impact	Room temperature and 0°C	Meets standard	Equivalent
High-temperature aging	650°C × 1000 h	Stable properties	Equivalent
Creep strength	600°C, 100,000 h extrapolation	Comparable to forged billet tubes	Equivalent
Metallographic structure	After aging	Fine tempered martensite	Equivalent

Key Concerns Addressed

The paper specifically addresses the following concerns that have historically hindered the adoption of CC-billet-based P91 tubes:

Center cracks in CC billets: The authors demonstrated that the center cracks in Gr.91 CC billets follow a regular distribution pattern and can be effectively removed during the subsequent hot rolling or piercing process used to manufacture steel tubes.
Segregation: The degree of segregation in Gr.91 CC billets is within acceptable limits and does not adversely affect the final tube properties after proper heat treatment.
Creep performance: The most critical concern for high-temperature applications, the creep strength of CC-billet-based P91 tubes at 600°C and 100,000 hours is comparable to that of traditional forged billet tubes.
Long-term property stability: High-temperature aging tests confirmed that the mechanical properties of CC-billet-based P91 tubes remain stable after prolonged exposure at service temperatures.

Engineering Practice Implications

This study is of tremendous practical significance for the power generation industry, where P91 steel tubes are used extensively in supercritical and ultra-supercritical boiler systems. The adoption of CC-billet-based P91 tubes offers several advantages:

Cost reduction: Continuous casting is significantly less expensive than forging, leading to lower material costs for P91 steel tubes.
Supply reliability: CC production has a much higher throughput than forging, addressing potential supply constraints for P91 tubes in large-scale power plant projects.
Environmental benefits: CC processes consume less energy and produce fewer emissions than forging, contributing to sustainability goals.

Welding Considerations for P91 Tubes

For engineers involved in the welding of P91 steel tubes, the following considerations are essential:

Welding Parameter	Recommendation	Standard Reference
Preheat temperature	200–260°C	AWS D10.9 / GB/T 3375
Interpass temperature	250–300°C	Control to prevent overheating
Welding procedure	Low hydrogen, low heat input	Minimize HAZ softening
Post-weld heat treatment	730–760°C for 2 h per 25 mm	Restore tempered martensite structure
Consumable selection	P91-matched or slightly higher strength	AWS A5.15 E91T1-1
Post-weld stress relief	Mandatory	Prevent delayed cracking

The welding of P91 steel is inherently challenging due to its susceptibility to hydrogen-assisted cracking, delayed cracking, and loss of strength in the heat-affected zone (HAZ). The use of CC-billet-based P91 tubes does not change these fundamental welding challenges, but engineers should ensure that the welding procedure qualification (WPQ) accounts for the specific material properties of the tube, including the microstructure and inclusion content, which may differ slightly from forged billet tubes.

Study Insights and Reflections

As an engineer who has worked extensively with P91 and P92 steels in power plant applications, I find this study particularly valuable for its systematic approach to addressing the concerns surrounding CC-billet-based P91 tubes. The historical preference for forged billets was based on a combination of empirical experience and legitimate concerns about the quality of continuous casting products. However, the metallurgical advances in clean steelmaking, electromagnetic stirring, and controlled cooling have significantly improved the quality of CC billets, making them a viable alternative for demanding applications such as P91 steel tubes.

The finding that the creep strength at 600°C and 100,000 hours is comparable between CC-billet-based and forged-billet-based P91 tubes is particularly reassuring. Creep performance is the single most important property for high-temperature pressure components, and the fact that CC-billet-based tubes meet this requirement eliminates the primary technical barrier to their adoption.

The regular distribution pattern of center cracks in Gr.91 CC billets is also an important finding. In my experience, the predictability of defects is often more important than their absolute absence, because predictable defects can be designed around or removed through standard manufacturing processes. The authors' demonstration that these center cracks can be effectively removed during tube manufacturing provides a clear manufacturing pathway for CC-billet-based P91 tubes.

Conclusion

This study provides compelling evidence that P91 steel tubes manufactured from continuous casting billets meet the stringent requirements for high-temperature and high-pressure power plant applications. The comprehensive testing program, covering chemical composition, mechanical properties, high-temperature aging, and creep performance, demonstrates that CC-billet-based P91 tubes are equivalent to traditional forged billet tubes in terms of material quality and long-term performance. For engineers involved in the procurement, welding, and installation of P91 steel tubes, this study offers a clear recommendation to embrace CC-billet-based products, which provide significant cost and supply advantages without compromising structural integrity or safety. The key to successful implementation lies in ensuring that the manufacturing process effectively removes center cracks and that welding procedures are properly qualified for the specific material, but these are well-understood challenges that can be addressed through standard engineering practices.