The creep crack growth of T92 and a new heat-resistant steel BG12Cr

Liu, Ke (2025). The creep crack growth of T92 and a new heat-resistant steel BG12Cr. University of Birmingham. Ph.D.

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Abstract

The increasing global demand for electricity encourages improvements in the efficiency of fossil fuel-fired power plants to reduce CO2 emissions and conserve resources. A promising strategy to enhance efficiency is by increasing steam temperatures in ultra-supercritical power plants. However, this demands materials with superior creep resistance, thermal stability, corrosion resistance and low cost. T/P92 martensitic heat-resistant steels have been widely used in power plants up to 620℃, but they have limitations in corrosion resistance at higher temperatures such as 650℃.To address this, a new 12Cr martensitic heat-resistant steel, BG12Cr, has been developed, which offers enhanced corrosion resistance and thermal stability for long-term operation at high steam temperatures and pressures. In addition to creep strength, creep crack growth resistance is crucial for the material's longevity in real-world applications under creep conditions. This study aims to examine the creep crack growth resistance of BG12Cr and T92 at various temperatures (600°C, 650°C, and 700°C) and loadings (3 kN, 6 kN, and 10 kN) and research the creep crack growth mechanisms of them. Fracture surface and cross-sectional observations further help the understanding of the creep crack growth mechanism.
To support research on crack growth resistance, this study conducts an in-depth microstructural investigation of both BG12Cr and T92 steels. Optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX) on as-received, heat-treated, and post-test specimens were used in this study. In the as-received BG12Cr, tempered martensite with minor amounts of δ-ferrite was observed. Additionally, M\(_{23}\)C\(_6\) carbide type which is mainly (Cr, Mo, W)-rich carbides and fine MX type of (Nb, V)(C, N) and V(C, N) carbon nitrides were found located on the prior austenite grain boundaries, martensite colony boundaries and lath boundaries. Notably, a significant number of Cu particles (20-30nm in size) were discovered within the martensite lath. And in the as-received T92, the main particles at the lath boundaries and prior austenite grain boundaries are M\(_{23}\)C\(_6\) carbides ((Cr, Mo, W)-rich carbides), (Nb, V)(C, N), V(C, N) and Ti-rich carbides and some (Fe, Cr)\(_2\)(Mo, W) Laves phases were also found at prior austenite grain boundaries.
In the post-test specimens, abnormal acceleration in microstructure degradation was observed adjacent to crack faces. This area showed not only a higher density of precipitates than regions farther from the crack but also a new type of precipitate, Z Phase, not present in the as-received specimens. This acceleration is attributed to stressing and oxidation-induced Cr outward diffusion. Hardness tests conducted on heat-treated specimens showed that BG12Cr had higher hardness values than T92 across all conditions, which can be attributed to its higher content of Cr and Co, a greater number of Cu particles, and finer lath width. Moreover, hardness decreased with increasing temperature and time in both materials, indicating coarsening of precipitates.
Creep crack growth tests were conducted on compact tension (CT) specimens at temperatures of 600°C, 650°C, and 700°C, with stable loadings of 3 kN, 6 kN, and 10 kN over a test period ranging from 158 to 2587 hours. The resistance to creep crack growth was evaluated using two parameters: one from linear elastic fracture mechanics (LEFM), K\(_n\), and another from elastic-plastic time-dependent fracture mechanics, denoted as C*. Although C* correlated well with the crack growth rate, it was not sufficiently sensitive to distinguish the crack growth resistance of BG12Cr and T92. This is due to similar da/dt - C* slopes observed in both steels under equivalent loading conditions. On the other hand, Kn, though not as closely correlated with crack growth rate, provided a more straightforward comparison of material performance under identical conditions and specimen geometry. Overall, BG12Cr displayed better creep crack growth resistance than T92 under investigated test conditions within 2587h, except under loading of 10kN at 600°C.
Based on observations of the fracture surfaces and cross-sections of post-test specimens, the mechanism of creep crack growth is characterized by the formation and growth of microvoids, as well as their subsequent linking and coalescence. These processes are accelerated by elevated temperatures and stress conditions. In T92 steel, microvoids primarily form and grow at the triple points of prior austenite grains. In BG12Cr, microvoids tend to form both at these triple points and along the boundaries of the prior austenite grains. However, compared to T92, the void density is lower, and the growth of coalescences is slower, which contributes to its better creep crack growth resistance.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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