Performance Testing of P92 Steel Pipe Manufactured from Continuous Casting Billets

Literature Overview

This paper by Jing Shiyu, Pu Xiaoming, He Xiaoming, Zeng Hui, and Yang Jian from Dongfang Electric Group Dongfang Boiler Co., Ltd. and the Mechanical Industry High-Temperature and High-Pressure Materials and Welding Engineering Laboratory, published in Steel Pipe (2024, Vol. 53, No. 1, pp. 82-86), presents a comprehensive performance evaluation of P92 steel pipe manufactured from continuous casting (CC) billets. P92 is a 9Cr-1Mo-V-Nb (also known as 9-1-2) high-strength low-alloy martensitic steel widely used in supercritical and ultra-supercritical power boiler applications, where it must withstand extreme temperatures and pressures over extended service lives. The study examined three large-diameter, thick-walled P92 steel pipes supplied by different steel mills, all manufactured from CC billets, and conducted physicochemical testing and high-temperature creep strength testing at 625°C.

Material Background and Industry Context

P92 steel is a critical material for modern power generation, particularly in supercritical (SC) and ultra-supercritical (USC) coal-fired power plants, where steam temperatures exceed 580°C and pressures exceed 25 MPa. The material's excellent high-temperature strength, creep resistance, and oxidation resistance make it suitable for boiler headers, main steam pipes, and other high-temperature components. Traditionally, P92 steel pipe has been manufactured from ingot-cast billets, which involves a time-consuming and expensive solidification and refining process. The use of continuous casting billets offers a potential cost reduction by eliminating the ingot casting step, but raises concerns about the quality of the resulting steel pipe, particularly regarding macrosegregation, inclusion distribution, and long-term high-temperature performance.

Physicochemical Property Results

The physicochemical testing encompassed chemical composition analysis, mechanical property testing (tensile, hardness, impact), and microstructural examination. The results confirmed that all three CC-billet P92 pipes met the relevant standard requirements for chemical composition, mechanical properties, and microstructure. The microstructural examination likely revealed a fine martensitic matrix with dispersed carbides, which is the expected microstructure for properly heat-treated P92 steel. The absence of excessive non-metallic inclusions, banding, or segregation in the CC-billet pipe is critical for ensuring uniform high-temperature performance.

Long-Temperature Creep Strength Results

The most significant finding of this study is the 625°C, 100,000-hour extrapolated long-term creep strength of 93.1 MPa. This value exceeds the recommended minimum value specified in relevant standards and design codes. This result is particularly important because:

1. Long-term reliability: The 100,000-hour extrapolated strength represents the expected material performance over a typical power plant design life of approximately 10-12 years of continuous operation. Exceeding the code minimum provides a safety margin against premature creep rupture.
2. Design margin: The creep strength value of 93.1 MPa provides engineers with a quantifiable basis for stress rating calculations in accordance with codes such as ASME BPVC Section VIII, NB/T 47003, or EN 13275. The margin above the code minimum allows for additional safety factors in the design.
3. Cost reduction potential: The demonstration that CC-billet P92 pipe meets or exceeds performance requirements supports the use of this lower-cost manufacturing route, which can significantly reduce the overall cost of the steel supply chain.

Quality Control and Engineering Practice

For the engineering application of CC-billet P92 pipe, the following quality control measures should be implemented:

The welding of P92 pipe presents specific challenges due to the material's susceptibility to hydrogen-induced cracking (HIC) and delayed cracking in the heat-affected zone (HAZ). Proper welding procedures must include preheating to 200-250°C, interpass temperature control, and post-weld heat treatment (PWHT) at 740-760°C for a sufficient holding time to achieve full tempering of the martensitic structure. The PWHT is critical for relieving welding residual stresses and restoring the toughness of the HAZ.

Study Insights and Reflections

This study provides strong evidence that CC-billet P92 steel pipe is a viable and reliable material for high-temperature power generation applications. The finding that the 100,000-hour creep strength exceeds the code minimum is particularly encouraging, as it suggests that the continuous casting process does not compromise the long-term high-temperature performance of P92 steel. The economic implications are significant, as the use of CC billets can reduce the steelmaking cost by eliminating the ingot casting and descaling steps, leading to lower material costs for power plant owners and engineers.

However, several considerations should be kept in mind. First, the study examined only three specimens from different mills, and the results may not be fully representative of all CC-billet P92 production. The variability between mills—due to differences in CC parameters, rolling practices, and heat treatment schedules—should be investigated through a larger sample size. Second, the creep test data is extrapolated to 100,000 hours, which inherently involves uncertainty. Engineers should apply appropriate safety factors and consider the scatter in creep data when using these values for design. Third, the long-term oxidation and corrosion resistance of CC-billet P92 pipe in high-temperature steam environments has not been addressed in this study, yet this is a critical performance criterion for power boiler applications. Finally, the welding and fabrication performance of CC-billet P92 pipe—particularly the weldability and post-weld heat treatment response—should be evaluated in conjunction with the material performance data to ensure that the full fabrication chain is qualified for engineering use.

Overall, this research represents an important step toward the widespread adoption of CC-billet P92 pipe in power generation, offering a pathway to cost reduction without compromising structural integrity. Engineers should carefully evaluate the material qualification data for each specific supply and apply appropriate quality assurance measures to ensure reliable long-term performance in service.