Safety Evaluation and Life Prediction of Low-Hardness P91 Pipe Fittings

Literature Overview

This paper, authored by Yang Chao, Tang Chun-Po, Gong Hong-Qiang, and Sun Xiong from Jiangsu Fangtian Electric Power Technology Co., Ltd. and affiliated power generation companies, was published in Electric Power in 2017 (Vol. 50, No. 8, pp. 82-86). The research addresses a critical and practical problem in thermal power plant operations: the safety evaluation and remaining life prediction of P91 pipe fittings that have experienced hardness degradation during long-term service. The study was conducted by engineers from operating power plants, which lends strong practical credibility to the findings.

Background and Technical Context

P91 steel (9Cr-0.2V-1Mo-NbN) is widely used in ultra-supercritical (USC) power plant steam pipelines and components due to its excellent creep strength and oxidation resistance at elevated temperatures. P91 components typically operate in the temperature range of 550-650 degrees Celsius, where creep deformation and microstructural aging are the primary degradation mechanisms.

During long-term service, P91 pipe fittings undergo progressive microstructural changes including tempering of martensite, coarsening of precipitates, and carbide agglomeration. These changes manifest macroscopically as hardness reduction. The question of when hardness degradation becomes critical for safety is of paramount importance for power plant asset management.

Creep Rupture Testing Methodology

The authors conducted creep rupture tests on P91 specimens with different hardness levels at 570 degrees Celsius. This temperature is selected because it represents a common operating condition for P91 components in the main steam and reheat steam piping systems.

The key methodological innovation in this paper is the use of logarithmic function fitting to extrapolate creep rupture data, rather than the conventional isochronous line extrapolation method. The authors argue that isochronous line extrapolation can overestimate creep strength, leading to non-conservative safety assessments.

The logarithmic approach provides a more conservative and realistic prediction of long-term creep behavior, which is essential for safety-critical applications where non-conservative estimates could lead to catastrophic failures.

Key Findings and Hardness Thresholds

The research establishes critical quantitative relationships between hardness and creep strength:

1. At 570 degrees Celsius, the 1x10^5 hour creep rupture strength of P91 decreases as hardness decreases.
2. When hardness falls below 165 HBW, the P91 steel creep strength drops below the allowable stress, meaning the component cannot guarantee long-term safe operation.
3. The hardness threshold of 165 HBW serves as a critical safety boundary for P91 components operating at 570 degrees Celsius.

Application-Specific Safety Margins

The authors provide differentiated safety assessments based on operating conditions:

The finding that main steam elbows without thickening design have particularly low strength margins is especially important for maintenance planning. Elbows experience additional stress concentrations due to their geometry, and without compensating wall thickening, they become critical locations for creep failure.

Practical Implications for Power Plant Operations

This research provides actionable guidance for power plant maintenance and safety management:

• Hardness monitoring: Regular hardness testing of P91 components during shutdowns should be institutionalized, with 165 HBW at 570 degrees C operation as the critical threshold.
• Component-specific assessment: Not all P91 components carry equal risk; main steam elbows require the most stringent monitoring.
• Life prediction methodology: The logarithmic fitting approach should be adopted over isochronous extrapolation for more conservative and reliable life predictions.
• Retirement decisions: Components falling below the hardness threshold should be evaluated for retirement or replacement rather than continued operation.

Study Insights and Reflections

This paper stands out for its direct connection between laboratory testing and field operational decisions. The authors are practicing power plant engineers, and the research is driven by real safety concerns rather than academic curiosity. The identification of 165 HBW as a critical hardness threshold for P91 at 570 degrees C provides a clear, actionable criterion for maintenance decisions.

The critique of isochronous line extrapolation is particularly valuable. In practice, many operators rely on this method for life prediction, and the potential for overestimation of creep strength means that some components may be kept in service longer than is safe. The logarithmic fitting approach, while more conservative, provides a safety margin that is essential for nuclear-grade reliability expectations.

The differentiation of safety margins by component type and operating condition reflects mature engineering judgment. Not all P91 components face the same degradation environment, and applying a uniform hardness threshold across all components would be either overly conservative or dangerously non-conservative depending on the component.

Summary

This research provides a rigorous and practical framework for the safety evaluation and life prediction of P91 pipe fittings in thermal power plants. The establishment of 165 HBW as a critical hardness threshold at 570 degrees C, combined with the use of logarithmic function fitting for conservative life prediction, offers power plant engineers reliable tools for asset management. The component-specific differentiation of safety margins demonstrates mature engineering judgment and provides a model for risk-based maintenance strategies in high-temperature power plant applications.