Zhuojin Pipe Fitting Co., LtdPerformance Testing of P92 Steel Pipes Manufactured from Continuous Casting Billets
Literature Overview
This 2024 paper by Jing Shiyu, Pu Xiaoming, He Xiaoming, Zeng Hui, and Yang Jian from Dongfang Electric Group's Dongfang Boiler Co., Ltd. and the Mechanical Industry High-Temperature and High-Pressure Materials and Welding Engineering Laboratory, published in the journal Steel Pipe (Volume 53, Issue 1, pages 82–86), evaluates the performance of P92 steel pipes manufactured from continuous casting (CC) billets. The study assesses whether CC billet-based P92 pipes can meet the stringent requirements for ultra-supercritical (USC) power boiler applications, where long-term creep strength and microstructural stability are paramount.
Material Background and Testing Scope
P92 is a martensitic 9Cr-2W steel widely used in USC power generation equipment operating at temperatures up to 625 °C and pressures exceeding 25 MPa. The material combines excellent creep strength with good weldability, making it a preferred choice for high-temperature boiler components. Traditionally, P92 pipes have been manufactured from ingot-cast billets, which offer superior cleanliness and homogeneity but at significantly higher cost. The use of CC billets represents a cost-reduction strategy that requires rigorous performance validation.
Three large-diameter, thick-walled P92 steel pipes from different steel mills were tested, covering a comprehensive range of quality assurance requirements:
| Test Category | Specific Tests | Purpose |
|---|---|---|
| Physical and chemical analysis | Composition verification | Confirm alloy specification compliance |
| Mechanical properties | Tensile, hardness, impact | Verify room-temperature strength and toughness |
| High-temperature creep | 625 °C, 100,000 h extrapolated creep strength | Assess long-term service capability |
| Microstructural examination | Grain size, phase distribution, inclusion content | Evaluate metallurgical quality |
| Heat treatment verification | Temper response, phase transformation | Confirm proper thermal processing |
Key Results
The comprehensive testing program yielded the following principal findings:
| Performance Indicator | Requirement | CC Billet P92 Pipe Result | Assessment |
|---|---|---|---|
| Chemical composition | Within ASTM A213/A335 specification limits | Compliant | Acceptable |
| Room-temperature tensile strength | Per applicable standard | Compliant | Acceptable |
| 625 °C, 100,000 h creep strength | ≥ 90 MPa (code recommended value) | 93.1 MPa | Exceeds requirement |
| Microstructural quality | No detrimental phases or excessive segregation | Compliant | Acceptable |
The 625 °C, 100,000-hour extrapolated creep strength of 93.1 MPa exceeded the code-recommended value, demonstrating that CC billet-based P92 pipes possess adequate long-term creep resistance for USC boiler service. All physical and chemical properties met the relevant standard requirements, confirming that the CC billet manufacturing route does not compromise the material's essential performance characteristics.
Engineering Practice Implications
The validation of CC billet-based P92 pipes has significant economic implications for the power generation industry. Continuous casting offers substantial cost advantages over ingot casting, including reduced energy consumption, shorter production cycles, and lower material waste. For large-diameter, thick-walled P92 pipes used in main steam and hot reheat pipelines, the cost savings from CC billet sourcing can be substantial, particularly for large-scale power plant projects.
However, engineers must remain vigilant regarding potential quality concerns associated with CC billets. Continuous casting can introduce centerline segregation, inclusion alignment, and surface defects that may affect the final pipe's mechanical properties, particularly in the thickness direction. The thick-walled pipe manufacturing process, which involves hot rolling or forging of the CC billet, must be carefully controlled to homogenize the microstructure and eliminate casting defects. Quality assurance measures should include strict control of the rolling temperature, reduction ratio, and cooling rate, as well as comprehensive NDT including ultrasonic testing for internal defects and hardness mapping to detect microstructural non-uniformity.
For welding applications, the CC billet-based P92 pipes should exhibit weldability comparable to ingot-cast material, provided that the carbon equivalent and interstitial element content are within acceptable limits. The welding procedure should include appropriate preheating (typically 200–300 °C), interpass temperature control, and post-weld heat treatment to ensure proper tempering of the weld and heat-affected zone.
Study Insights and Outlook
The successful validation of CC billet-based P92 pipes represents an important advancement in the cost optimization of USC power plant materials. The 93.1 MPa creep strength at 625 °C for 100,000 hours, while exceeding the code minimum, should be interpreted with appropriate caution. Creep strength extrapolation to 100,000 hours typically involves significant uncertainty, and the actual long-term performance in service depends on factors such as microstructural stability, operating temperature fluctuations, and stress concentration at welds and fittings. I recommend that power plant operators implement enhanced monitoring programs for CC billet-based P92 components, including periodic in-service inspections and metallurgical evaluation of retired components to validate the extrapolated creep life predictions. The study's findings support the adoption of CC billet-based P92 pipes in new projects, but ongoing research into microstructural evolution during long-term service remains essential to ensure the continued reliability of USC power plant materials.