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

Literature Overview

This 2022 paper published in Steel Pipe by Jing Shiyu and colleagues from Dongfang Electric Group, Jiangyin Xingcheng Special Steel, and Jiangsu Yonggang Group presents a comprehensive evaluation of P91 steel tubes produced from continuous casting (CC) billets rather than the traditional forged billets. The study addresses longstanding industry concerns about the quality and performance of CC-billet P91 tubes for high-temperature, high-pressure applications in power generation. Funded by the Mechanical Industry High-Temperature, High-Pressure Materials and Welding Engineering Laboratory, the research includes chemical composition verification, mechanical property testing, metallographic examination, high-temperature aging tests, and long-term creep rupture testing at 600°C.

Core Technical Findings

The investigation concluded that Chinese Gr.91 steel continuous casting technology has matured to a level where CC-billet P91 tubes can meet all relevant standard requirements and achieve performance comparable to traditional forged-billet tubes. The key results include uniform low-magnification structure, good columnar grain morphology, predictable center crack distribution patterns, and equivalent long-term creep strength at 600°C over 100,000 hours.

Technical Analysis of Continuous Casting Quality Factors

The quality of P91 tubes from CC billets depends critically on several metallurgical factors:

Steel cleanliness: Gr.91 steel contains critical alloying elements (9% Cr, 1% Mo, plus V, Nb, W) that are sensitive to inclusion content. The study confirms that modern Chinese steelmaking technology achieves adequate cleanliness levels, with low oxygen, sulfur, and non-metallic inclusion content. This is essential because inclusions serve as crack initiation sites during hot forming and as stress concentrators during long-term creep service.

Solidification structure: The columnar grain morphology in CC billets is generally favorable for hot rolling, as it promotes texture development that enhances transverse mechanical properties. However, the center region of the billet is susceptible to center cracks caused by shrinkage during solidification. The study found that center cracks follow predictable distribution patterns, which can be managed through appropriate billet quality control and subsequent hot working practices.

Hot working practice: The conversion of CC billet to seamless tube through piercing and hot rolling requires careful control of deformation temperature, reduction ratio, and cooling rate. The microstructural homogenization achieved during hot working must eliminate any residual segregation from the casting process while avoiding excessive grain growth that could compromise creep resistance.

Heat treatment response: P91 steel requires normalization followed by tempering to achieve the desired tempered martensitic microstructure with fine precipitates of M23C6, MX (V,Nb)C, and L12 (Fe3W) phases. The CC-billet tubes demonstrated stable properties after high-temperature aging, indicating adequate precipitate stability and resistance to microstructural degradation during long-term service.

Engineering Application Considerations

The historical reluctance to use CC-billet P91 tubes stems from several concerns that this study addresses:

1. Creep rupture life uncertainty — The 600°C, 100,000-hour extrapolated creep strength equivalence with forged-billet tubes directly addresses the primary concern about long-term reliability. This data provides the basis for design life calculations in boiler and pressure vessel applications.
2. Microstructural homogeneity — The uniform low-magnification structure and controlled grain morphology confirm that CC-billet tubes achieve the microstructural quality required for high-temperature service.
3. Weldability and fabrication — While not directly addressed in this study, the chemical composition compliance ensures that standard P91 welding procedures (preheat, interpass temperature control, post-weld heat treatment) remain applicable.
4. Cost and supply chain — CC-billet production offers significant cost advantages over forging, with shorter production cycles and higher material utilization rates. This economic benefit, combined with proven performance, makes CC-billet P91 tubes attractive for large-scale power plant applications.

Study Insights and Industry Implications

This paper represents a significant contribution to resolving a long-standing debate in the Chinese power generation industry regarding the acceptability of CC-billet P91 tubes. The systematic approach—combining metallurgical quality assessment with long-term performance testing—provides the evidentiary basis needed to overcome institutional resistance to new manufacturing methods. The finding that performance is equivalent to forged-billet tubes at 600°C over 100,000 hours is particularly compelling, as this represents the design life of modern ultra-supercritical boiler tubes. Engineers and procurement specialists should recognize that manufacturing method alone does not determine performance; rather, the final material quality after processing and heat treatment governs long-term reliability. The study's recommendation to eliminate unwarranted concerns and promote CC-billet P91 tube applications reflects a mature engineering judgment based on comprehensive testing data rather than historical precedent alone. This approach—letting data drive material selection decisions—should be the standard practice in high-temperature component procurement.