Development of T23 High-Pressure Boiler Steel Pipe

Literature Overview

The paper by Wang Qijiang, Zou Fengming, and Zhang Ke, published in the journal Steel Pipe in 2005, documents the development and qualification testing of T23 high-pressure boiler steel pipe by Baoshan Iron and Steel Co., Ltd. The research was supported by the Shanghai Science and Technology Commission (Grant 035211025). This study addresses a critical need in the power generation industry: the development of a domestic steel grade capable of meeting the demanding requirements of high-pressure boiler applications specified in ASME SA-213.

Material Background and Specification Requirements

T23 steel is a 9Cr-1Mo-V-Nb (9-1-1) modified alloy steel designed for high-temperature service in power plant boilers. The addition of niobium to the traditional 9Cr-1Mo composition provides enhanced creep resistance and thermal stability, making it suitable for higher-temperature applications compared to its predecessor. ASME SA-213 specifies the requirements for seamless austenitic stainless steel boiler, superheater, and heat exchanger tubes, and T23 is included as a qualified grade for high-pressure boiler applications.

The key specification requirements for T23 boiler tubes include:

Testing and Qualification Results

Baoshan Steel conducted comprehensive testing programs to qualify the T23 pipe against ASME SA-213 requirements. The test program included high-temperature tensile strength testing, creep strength testing, and oxidation resistance testing. The results demonstrated that the T23 pipe produced by Baoshan Steel exhibited stable performance, with all technical indicators and properties meeting or exceeding the ASME SA-213 specification requirements.

The high-temperature tensile strength testing confirmed that the material maintains adequate strength at elevated temperatures, which is critical for the structural integrity of boiler tubes under operating conditions. The creep strength testing verified that the material can withstand prolonged exposure to high temperatures and stresses without excessive deformation. The oxidation resistance testing evaluated the scale growth rate under high-temperature conditions, ensuring that the tube surface remains protected against corrosion.

Metallurgical Considerations

The metallurgical control of T23 steel is critical for achieving the required high-temperature properties. The 9% chromium content provides excellent oxidation resistance through the formation of a protective chromium oxide scale. The molybdenum addition enhances creep strength by solid solution strengthening and by stabilizing carbide precipitates. The vanadium and niobium additions form fine, stable carbide precipitates that provide precipitation hardening and retard grain growth at elevated temperatures.

The heat treatment process is particularly important. The solution treatment temperature and cooling rate directly affect the grain size, the distribution of carbide precipitates, and the residual austenite content. An optimal heat treatment produces a fine-grained ferritic matrix with uniformly distributed fine carbides, which provides the best combination of creep strength and oxidation resistance.

The pipe manufacturing process, whether hot-finished or cold-drawn, also affects the final properties. Cold-drawn tubes typically exhibit better dimensional accuracy and surface finish, which are important for the welding and joining of boiler tubes. The cold work introduced during drawing can improve strength but may reduce ductility, and the final tempering treatment must be carefully controlled to achieve the proper balance of properties.

Engineering Practice Implications

For power plant engineers, the qualification of T23 pipe by Baoshan Steel represents a significant development in domestic material supply capability. Prior to this development, Chinese power plants relied heavily on imported T23 tubes, which created supply chain vulnerabilities and increased costs. The domestic qualification enables greater supply security and cost competitiveness for large-capacity power plant construction.

From a welding perspective, T23 steel requires careful welding procedure qualification. The high alloy content means that the heat-affected zone (HAZ) is susceptible to grain growth and carbide precipitation, which can reduce creep strength. The welding procedure must include a post-weld heat treatment (PWHT) to restore the proper microstructure in the HAZ. The PWHT temperature should be in the range of 760-790°C, and the cooling rate should be controlled to avoid excessive grain growth.

Quality control for T23 boiler tubes should include strict chemical analysis to verify the alloy composition, particularly the niobium and vanadium contents. Non-destructive testing should include eddy current testing for surface and near-surface defects, and ultrasonic testing for internal defects. Hydrostatic testing is mandatory to verify the structural integrity of each tube.

Study Insights and Reflections

This paper documents an important milestone in the domestic development of high-alloy boiler tube materials in China. The comprehensive testing program conducted by Baoshan Steel demonstrates a rigorous approach to material qualification that aligns with international best practices. The results provide confidence that domestically produced T23 pipe can meet the demanding requirements of high-pressure boiler applications.

One observation from this study is the importance of maintaining consistent production quality over time. The stability of properties across multiple heats is critical for ensuring the long-term reliability of boiler tubes. Production parameters such as steelmaking process, rolling schedule, and heat treatment must be tightly controlled to achieve this consistency.

For future development, the extension of T23 steel to even higher temperatures, beyond the current 650°C limit, would require further metallurgical research. The stability of the niobium carbide precipitates at prolonged exposure to high temperatures is a key concern, and the potential for microstructural degradation over extended service lives should be carefully evaluated through long-term aging studies.

This study provides valuable documentation of the domestic development and qualification of T23 high-pressure boiler steel pipe, contributing to the advancement of power generation technology and supply chain security.