Microstructure and Mechanical Properties of TP347HFG Steel Tubes with Different Grain Sizes in Service Condition

Literature Overview

This paper by Cheng Xiang, Bao Zheng, Wang Ruo-min, Miao Chun-hui, Chen Guo-hong, and Tang Wen-ming from Anhui Xinli Electric Power Technology Consulting, Hefei University of Technology, and State Grid Anhui Electric Power Research Institute, published in the Transactions of Materials and Heat Treatment (Vol. 45, No. 1, 2024, pp. 148–156), investigates the microstructure and mechanical properties of TP347HFG heat-resistant steel tubes with different austenite grain sizes after approximately 50,000 hours of service at 600°C. The research is supported by an Anhui Xinli Electric Power Technology Consulting project (2022-Consulting-KJ-02) and addresses a critical issue in power plant engineering: the long-term performance degradation of heat-resistant steel tubes.

Core Technical Content

Materials and Testing

The study compares two TP347HFG steel tubes with different austenite grain sizes:

Coarse grain tube: Grain size grade 7 (coarse)
Fine grain tube: Grain size grade 10 (fine)

Both tubes have been in service for approximately 50,000 hours at 600°C in a high-temperature reheater application. The investigation employs:

Scanning electron microscopy (SEM): For microstructure observation.
X-ray diffraction (XRD): For phase identification.
Electronic universal testing machine: For mechanical property measurement.

Microstructural Evolution

Both tubes exhibit aging of the microstructure, but the degree of aging differs:

Coarse grain tube (Grade 7):

Larger austenite grain size.
Contains more coarse primary MX phase particles.
Nanoscale secondary MX particles precipitate within austenite grains.
M₂₃C₆ phase particles precipitate at austenite grain boundaries.
Overall aging degree is lower.

Fine grain tube (Grade 10):

Smaller austenite grain size.
More precipitated second-phase particles overall.
Predominantly M₂₃C₆ phase particles.
Fewer secondary MX particles.
No primary MX phase particles.
More severe aging.

Mechanical Properties

The mechanical properties after service show:

Property	Coarse Grain Tube (Grade 7)	Fine Grain Tube (Grade 10)
Room temperature tensile strength	Lower	Higher
High temperature tensile strength (600°C)	Lower	Higher
Elongation after fracture	Higher	Lower
Yield-to-tensile strength ratio	Lower	Higher
Plastic deformation capability	Higher	Lower
High temperature yield-to-tensile ratio difference	Less pronounced	More pronounced

Service Safety Assessment

The coarse grain tube demonstrates higher service safety due to:

Lower aging degree.
Higher ductility.
Lower yield-to-tensile strength ratio, indicating greater ability to undergo plastic deformation before failure.
More pronounced difference in yield-to-tensile ratio between room temperature and high temperature.

Precipitate Phase Analysis

Phase	Coarse Grain Tube	Fine Grain Tube	Role
Primary MX	Present, coarse	Absent	Strengthening
Secondary MX	Nanoscale, within grains	Fewer	Strengthening
M₂₃C₆	At grain boundaries	Predominant, more numerous	Strengthening but reduces ductility

Engineering Practice Implications

For power plant engineering, this study has direct implications for the selection and maintenance of heat-resistant steel tubes in high-temperature reheaters:

Material selection: Coarse grain TP347HFG tubes may be preferred for applications where long-term service at 600°C is required, as they exhibit better ductility and lower aging degree.
Inspection and maintenance: The more severe aging of fine grain tubes suggests that these tubes may require more frequent inspection and earlier replacement.
Failure prediction: The higher yield-to-tensile strength ratio of fine grain tubes indicates a lower margin between yield and fracture, which may lead to sudden failure without significant plastic deformation warning.
Life assessment: The different aging degrees suggest that life assessment models should account for grain size effects, as traditional models may not accurately predict the remaining life of tubes with different grain sizes.

Critical Reflection

The study focuses on two specific grain sizes (grades 7 and 10) after a specific service duration (50,000 hours). The findings may not be directly applicable to other grain sizes or service durations. A more comprehensive study covering a wider range of grain sizes and service durations would provide more generalizable results.

The mechanical properties are measured after removal from service, which means that the tubes have undergone cooling and may have experienced additional microstructural changes during cooling. The actual properties during service at 600°C may differ from the measured properties.

The study does not extensively discuss the effects of other microstructural features, such as inclusion morphology, grain boundary character distribution, and texture, which may also influence the long-term performance of the steel tubes.

The comparison is limited to two grain sizes, and the optimal grain size for long-term service at 600°C is not determined. A systematic study of grain size effects across a wider range would be valuable for material selection and optimization.

Study Insights

The most significant finding of this paper is the demonstration that coarser grain sizes in TP347HFG steel tubes lead to lower aging degree and better ductility after long-term service at 600°C. This finding has direct implications for material selection in power plant applications.

The detailed analysis of precipitate phases provides insight into the mechanisms of microstructural evolution. The presence of primary MX phase particles in the coarse grain tube and their absence in the fine grain tube suggests that the grain size affects the precipitation behavior, possibly due to differences in nucleation sites and diffusion distances.

For engineers involved in the design, operation, and maintenance of power plants, this study provides valuable guidance on the selection and inspection of heat-resistant steel tubes. The findings suggest that grain size is an important factor in the long-term performance of these tubes and should be considered in material specification and life assessment.

This work highlights the importance of microstructural characterization in understanding the long-term behavior of high-temperature materials. The combination of SEM, XRD, and mechanical testing provides a comprehensive picture of the microstructure-property relationship, which is essential for rational material selection and life assessment. Engineers working with heat-resistant steel tubes should consider the findings of this study when making material selection decisions and developing inspection and maintenance strategies.