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

Literature Overview

The paper by Cheng Xiang, Bao Zheng, Wang Ruo-min, Miao Chun-hui, Chen Guo-hong, and Tang Wen-ming, published in the Journal of Thermal Analysis and Calorimetry in 2024 (Vol. 45, No. 1, pp. 148-156), investigates the microstructure and mechanical properties of TP347HFG high-temperature reheater tubes with different austenite grain sizes after approximately 50,000 hours of service at 600°C. Supported by the Anhui Xinli Electric Power Technology Consulting Co., Ltd. Science and Technology Project (2022-Zi-KJ-02), this study provides valuable insights into the aging behavior and mechanical performance of TP347HFG steel tubes under long-term high-temperature service conditions.

Core Technical Content

TP347HFG is a high-strength, high-temperature resistant austenitic stainless steel tube designed for use in supercritical and ultra-supercritical power plant reheaters. The "HFG" designation indicates a high-temperature grade with improved creep resistance and oxidation resistance. The steel contains approximately 22% chromium and 15% nickel, with additional alloying elements such as molybdenum, niobium, and titanium to enhance high-temperature strength and stability.

Microstructure Evolution

The study compares two TP347HFG steel tubes with different initial austenite grain sizes: 7-grade (coarse grain) and 10-grade (fine grain). After approximately 50,000 hours of service at 600°C, both tubes exhibit varying degrees of microstructural aging, but the aging behavior differs significantly between the two grain sizes.

Coarse grain tube (7-grade):

Larger austenite grain size
Contains more coarse primary MX carbide particles
Nanoscale secondary MX particles precipitate within the austenite grains
M23C6 carbide particles precipitate at the austenite grain boundaries
Overall aging degree is lower than the fine grain tube

Fine grain tube (10-grade):

Smaller austenite grain size
More numerous second-phase particles overall
Predominantly M23C6 carbide particles
Fewer secondary MX particles
No primary MX particles observed
More severe microstructural aging

Parameter	Coarse Grain (7-grade)	Fine Grain (10-grade)
Austenite grain size	Larger	Smaller
Primary MX particles	More coarse particles	None observed
Secondary MX particles	Nanoscale, within grains	Fewer
M23C6 particles	At grain boundaries	Predominant phase, more numerous
Aging degree	Lower	Higher
Room temperature tensile strength	Lower	Higher
High temperature tensile strength	Lower	Higher
Elongation after fracture	Higher	Lower
Yield-to-tensile ratio	Lower	Higher
High temperature yield-to-tensile ratio	Lower	Higher

Mechanical Property Comparison

The fine grain tube exhibits higher tensile strength at both room temperature and high temperature, but this comes at the cost of lower ductility. The elongation after fracture is lower, and the yield-to-tensile ratio is higher, indicating reduced plastic deformation capacity.

The coarse grain tube, despite having lower strength, demonstrates better ductility and a lower yield-to-tensile ratio, which indicates a greater capacity for plastic deformation before fracture. This is particularly important at high temperatures, where the difference in yield-to-tensile ratio between the two grain sizes is more pronounced.

Interpretation of Technical Points

The microstructural evolution observed in this study is consistent with the well-established theory of precipitation hardening and grain boundary precipitation in austenitic stainless steels. The key mechanisms are:

MX carbide precipitation: MX carbides (where M is a substitutional alloying element such as Nb, Ti, or V, and X is a carbonitride former such as C or N) are the primary strengthening phase in TP347HFG steel. The coarse grain tube retains more primary MX particles from the original heat treatment, while the fine grain tube has fewer primary particles but more secondary precipitation.
M23C6 carbide precipitation: M23C6 is a chromium-rich carbide that precipitates preferentially at grain boundaries. Its precipitation can lead to chromium depletion in the adjacent austenite matrix, which may affect corrosion resistance and mechanical properties.
Grain boundary effects: The larger grain size in the coarse grain tube results in a lower grain boundary area per unit volume, which reduces the total amount of M23C6 precipitation. This contributes to the lower aging degree observed in the coarse grain tube.

The relationship between grain size and aging behavior is governed by the balance between nucleation and growth of precipitates. In the fine grain tube, the higher grain boundary area provides more nucleation sites for M23C6 precipitation, leading to more rapid aging. In the coarse grain tube, the lower grain boundary area slows down M23C6 precipitation, but the larger grain size also allows for the retention of more primary MX particles and the formation of secondary MX particles within the grains.

Integration with Engineering Practice

The findings of this study have direct implications for the selection and maintenance of TP347HFG steel tubes in power plant service:

Tube selection: The choice between coarse grain and fine grain TP347HFG tubes should be based on the specific service requirements. Coarse grain tubes offer better long-term ductility and lower aging rate, making them more suitable for applications where long-term reliability is critical. Fine grain tubes offer higher initial strength but may experience more rapid property degradation over time.
Inspection and maintenance: Regular microstructural examination of in-service TP347HFG tubes can provide early warning of aging-related property degradation. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are valuable tools for characterizing the microstructure and identifying aging-related changes.
Welding considerations: The welding of TP347HFG tubes requires careful control of the heat input to avoid excessive grain growth in the heat-affected zone (HAZ). The HAZ microstructure should be examined to ensure that the grain size remains within acceptable limits.
Quality control: The initial grain size of TP347HFG tubes should be specified and verified during manufacturing. The grain size can be measured using standard metallographic techniques (ASTM E112 or equivalent).

From a welding perspective, the HAZ of TP347HFG tubes is particularly sensitive to heat input. Excessive heat input can lead to grain growth, which may result in reduced high-temperature strength and increased susceptibility to intergranular cracking. The welding procedure specification (WPS) should include strict limits on heat input, and the HAZ should be examined after welding to verify that the grain size is within acceptable limits.

The precipitation of M23C6 carbides at grain boundaries in the HAZ can lead to chromium depletion and reduced corrosion resistance. This is particularly important for tubes used in high-temperature, high-pressure service where corrosion resistance is critical. Post-weld heat treatment (PWHT) may be necessary to dissolve grain boundary carbides and restore chromium levels in the adjacent matrix.

Study Insights and Reflections

This study provides valuable insights into the long-term behavior of TP347HFG steel tubes under high-temperature service conditions. The comparison between coarse grain and fine grain tubes highlights the complex interplay between grain size, precipitation behavior, and mechanical properties.

The finding that the coarse grain tube exhibits lower aging degree but also lower strength is particularly interesting. It suggests that there is a trade-off between initial strength and long-term stability, and the optimal grain size depends on the specific service requirements and expected service life.

The observation that the yield-to-tensile ratio is higher in the fine grain tube at high temperatures is concerning from a safety perspective. A high yield-to-tensile ratio indicates that the material has limited capacity for plastic deformation before fracture, which can lead to sudden, catastrophic failure. The coarse grain tube, with its lower yield-to-tensile ratio, provides a greater safety margin in terms of ductility.

For power plant operators, the findings of this study suggest that regular monitoring and inspection of TP347HFG tubes is essential. Microstructural examination can provide early warning of aging-related property degradation, allowing for timely replacement or repair before failure occurs.

In conclusion, this research provides important guidance for the selection, use, and maintenance of TP347HFG steel tubes in high-temperature power plant service, contributing to the safe and reliable operation of these critical components.