Microstructure Properties and Safety Evaluation of Low-Hardness P91 Steel Pipe After Long-Term Service

Literature Overview

This paper, authored by Deng Hui from the Central South Electric Power Test Research Institute of China Datang Corporation, published in 2021 in the journal "Metal Heat Treatment" (Volume 46, Issue 12, pages 209-213), presents a comprehensive study of the microstructure, mechanical properties, and safety evaluation of low-hardness P91 steel pipe after long-term service. The study focuses on P91 steel pipe with a hardness of 157 HBW, which is significantly lower than the normal hardness range for P91 steel, and investigates the effects of long-term service exposure on the microstructure and properties of the pipe. The research includes metallographic examination, energy dispersive spectroscopy (EDS) analysis, room-temperature tensile testing, room-temperature impact testing, high-temperature short-term tensile testing, and high-temperature creep testing. The results reveal that the low-hardness P91 steel pipe has a microstructure consisting of blocky ferrite and large M23C6 carbide precipitates, and that the mechanical properties and creep strength are significantly degraded compared to normal-hardness P91 steel pipe. The safety evaluation indicates that the remaining life of the low-hardness straight pipe section is only 54,075 hours, posing a significant safety risk to the power plant unit.

Technical Background and P91 Steel in Power Plant Applications

P91 steel is a 9% chromium, 1% molybdenum, martensitic heat-resistant steel that is widely used in supercritical and ultra-supercritical power plant boilers for high-pressure steam piping, headers, and other components operating at temperatures up to approximately 620 degrees Celsius. The excellent high-temperature strength and creep resistance of P91 steel are attributed to its martensitic microstructure stabilized by fine carbide precipitates, primarily M2C and MX-type carbides, and the precipitation of M23C6 carbides during tempering. The mechanical properties and creep strength of P91 steel are highly dependent on the microstructure, and any degradation of the microstructure during long-term service can lead to significant reductions in the remaining life of the component.

The hardness of P91 steel is a sensitive indicator of the microstructure and the mechanical properties, and it is commonly used as a screening tool for assessing the condition of P91 components in service. The normal hardness range for P91 steel after proper heat treatment and tempering is typically 280 to 320 HBW, and a hardness below 250 HBW is generally considered to indicate microstructural degradation. The low hardness of 157 HBW reported in this study is well below the normal range and indicates severe microstructural degradation that has occurred during long-term service.

The causes of microstructural degradation in P91 steel during long-term service include the coarsening of precipitates, the transformation of stable MX-type carbides to less stable M23C6 carbides, the formation of blocky ferrite through recovery and recrystallization of the martensitic matrix, and the depletion of carbon and alloying elements from the matrix due to carbide precipitation. These degradation mechanisms are accelerated by high temperatures and long exposure times, and they are particularly severe in components that have been exposed to temperatures above the recommended tempering temperature or that have experienced thermal cycling during start-up and shutdown operations.

P91 Steel Property Comparison

Microstructural Analysis and Degradation Mechanisms

The metallographic examination of the low-hardness P91 steel pipe reveals a microstructure consisting of blocky ferrite and large M23C6 carbide precipitates, which is significantly different from the fine martensitic microstructure with fine carbide precipitates expected for properly heat-treated P91 steel. The blocky ferrite is formed through the recovery and recrystallization of the martensitic matrix during long-term exposure to high temperatures, and it is characterized by a coarse grain structure with low dislocation density and low strength. The large M23C6 carbide precipitates are formed through the coarsening of initially fine carbide precipitates and the transformation of MX-type carbides to M23C6 carbides, and they are characterized by a coarse size and a non-uniform distribution.

The energy dispersive spectroscopy analysis confirms the chemical composition of the precipitates and provides information about the distribution of alloying elements in the microstructure. The analysis reveals that the M23C6 carbides are enriched in chromium and molybdenum, and that the matrix is depleted in these elements due to the precipitation of the carbides. The depletion of alloying elements from the matrix reduces the solid solution strengthening and the precipitation hardening of the matrix, contributing to the reduction in hardness and mechanical properties.

The degradation mechanisms identified in the study are consistent with the established understanding of P91 steel degradation during long-term service, and they provide a clear explanation for the observed reductions in hardness, mechanical properties, and creep strength. The blocky ferrite microstructure is particularly detrimental to the creep strength of P91 steel because it provides easy paths for dislocation motion and grain boundary sliding, which are the primary creep deformation mechanisms at high temperatures. The large M23C6 carbide precipitates are also detrimental to the creep strength because they provide fewer obstacles to dislocation motion and grain boundary sliding compared to fine carbide precipitates.

Degradation Mechanism Summary