Investigating Hydrogen Embrittlement in Martensitic Stainless Steel for Improved Steam Turbine Blade Design

There is a high demand for low-cost, high-efficiency power generation systems. In this pursuit, steam turbines play an important role, converting thermal energy into mechanical power. To enhance the efficiency of steam turbine systems, the incorporation of longer last-stage blades has become a well-established strategy. These extended blades not only improve efficiency but also have the potential to significantly reduce costs. However, this goal necessitates the use of high-strength blade materials due to the elevated centrifugal forces these longer blades endure. One such material is precipitation-hardening martensitic stainless steel, exemplified by 15Cr steel, known for its commendable blend of high strength and good ductility. Nevertheless, the practical application of these materials in high-temperature steam environments introduces a significant challenge—hydrogen embrittlement (HE), which poses a substantial threat to the sudden failure of these critical components. Given that materials with higher strength tend to be more susceptible to HE, it becomes paramount to comprehensively investigate the HE behavior of 15Cr steel under real-world service conditions.

To accurately assess the risk of HE in blade materials, it is essential to simulate practical service conditions within a controlled laboratory environment. Two conventional testing methods, slow strain rate tensile (SSRT) testing and constant load tensile (CLT) testing, have been widely employed to evaluate HE susceptibility. These methods differ in their loading modes, potentially leading to significant variations in HE behavior. Understanding these variations is crucial, as the loading conditions on last-stage blades can vary during different service states.

Research into the effect of loading mode on HE susceptibility has garnered substantial attention. In SSRT, previous studies have reported that HE susceptibility increases with decreasing strain rate and increasing hydrogen concentration. Lower strain rates lead to longer durations of hydrogen accumulation, resulting in higher susceptibility to brittle cracking. Conversely, brittle cracks do not occur at higher critical strain rates. These observations highlight the intricate relationship between strain rate and HE susceptibility. Furthermore, research has shown that hydrogen tends to accumulate at prior austenite grain boundaries at lower strain rates. On the other hand, CLT loading conditions, typically used to obtain the threshold fracture strength, introduce another dimension to the understanding of HE susceptibility.

Fracture strength under dynamic stress in SSRT and constant stress in CLT has yielded mixed results in previous studies. Some suggest that fracture strength at small crosshead speeds for SSRT is close to that of CLT, while others propose differences. The reasons for these disparities in fracture strength caused by dynamic stress in SSRT and constant stress in CLT remain unclear. Therefore, elucidating the distinctions in hydrogen-induced fracture processes under various loading modes holds significant value.

In addition to loading conditions, service temperature is another critical factor that must be considered during the service life of last-stage blades, which can operate in a temperature range from 20°C to 200°C. The effect of temperature on hydrogen accumulation and hydrogen-induced fracture is not yet entirely clear. Existing literature suggests that the diffusion rate of hydrogen increases with rising temperature, leading to variations in HE susceptibility. Furthermore, the peak temperatures of HE susceptibility vary among materials and fall within a range of approximately -73°C to 64°C. The influence of temperature on hydrogen diffusion coefficients and trapped hydrogen concentrations further complicates the picture. Hydrogen can either promote or depress crack propagation depending on the temperature, emphasizing the need to understand the temperature effects on HE behavior.

To provide practical guidance for engineering applications and gain a deeper understanding of the influence of loading mode and temperature on HE susceptibility in 15Cr martensitic stainless steel, a new study published in the International Journal of Hydrogen Energy led by Professor Jinxu Li and conducted by Dr. Shenguang Liu, Dr. Weijie Wu, and Dr. Hao Fu from the Corrosion and Protection Center at the Institute for Advanced Materials and Technology at the University of Science and Technology Beijing, provided invaluable insights into HE susceptibility, fracture mechanisms, and hydrogen behavior in 15Cr steel and investigate these factors comprehensively [1]. The research team employed advanced techniques such as scanning electron microscopy combined with electron backscatter diffraction to elucidate the fracture mechanisms under different loading modes. Additionally, they examined hydrogen trappings and distribution using hydrogen microprint technology. The authors conducted SSRT testing and CLT testing at both room temperature and 80°C to simulate real-world service conditions. The outcomes of these investigations have shed light on the complex behavior of hydrogen embrittlement in this critical engineering material.

The hydrogen embrittlement of 15Cr martensitic stainless steel, a material commonly used for steam turbine last-stage blades, has been the focus of systematic study. SSRT testing and CLT testing were employed at both room temperature and 80°C to simulate real-world service conditions. The outcomes of these investigations have shed light on the complex behavior of hydrogen embrittlement in this critical engineering material.

One notable finding from the authors’ research is the disparity in hydrogen-induced fracture strength between SSRT and CLT. Despite the lower hydrogen concentration absorbed during SSRT, it was observed that the hydrogen-induced fracture strength of 15Cr steel for SSRT was lower than the threshold fracture strength for CLT. This discrepancy can be attributed to a remarkable phenomenon: the remarkable enhancement in local hydrogen concentration due to the transportation of hydrogen by mobile dislocations during SSRT. This observation underscores the importance of considering the dynamic interaction between hydrogen and the material’s microstructure under varying loading conditions. During SSRT, where the loading rate is slower, hydrogen is transported by mobile dislocations, leading to the accumulation of hydrogen at specific locations within the material. This local accumulation of hydrogen significantly influences the fracture behavior, rendering the material more susceptible to brittle cracking despite the lower overall hydrogen concentration.

Another purpose of this study is to attempt to establish a correlation between the results of the two experimental methods of SSRT and CLT. Based on the results of this work and the subsequent paper published in Corrosion Science [2], we found that for materials with uniform microstructure, when the strain rate is slow enough, the fracture strength of SSRT is slightly higher than the threshold stress value of CLT under the same hydrogen charging environment (not exceeding 0.1σ_b). Alternatively, the threshold stress of CLT can be conservatively estimated by subtracting 0.1σ_b from the fracture strength of SSRT. Because CLT needs a long experimental time and consumes more samples, while SSRT is relatively fast and requires fewer samples. In this way, in special or emergency situations, SSRT can replace CLT for preliminary evaluation.

Additionally, the authors investigated the effects of temperature on hydrogen embrittlement susceptibility. They found that, although a higher hydrogen concentration was absorbed during SSRT at 80°C compared to room temperature, the hydrogen embrittlement susceptibility of 15Cr steel for SSRT at 80°C was lower than that observed at room temperature. This seemingly counterintuitive result can be explained by the degree of local hydrogen accumulation within the material.

At higher temperatures, the authors found the degree of local hydrogen accumulation decreased. This suggests that the elevated temperature altered the kinetics of hydrogen diffusion and trapping within the material, resulting in a more distributed and less localized distribution of hydrogen. Consequently, even though the overall hydrogen content was higher at 80°C, the material exhibited reduced susceptibility to hydrogen-induced cracking.

The authors findings underscore the important relationship between temperature, loading conditions, and the behavior of hydrogen within the material. They emphasize the importance of considering multiple factors when assessing the risk of hydrogen embrittlement in engineering materials, especially those used in critical applications such as steam turbine last-stage blades. In conclusion, the systematic study of Professor Jinxu Li and colleagues on hydrogen embrittlement in 15Cr martensitic stainless steel for steam turbine last-stage blades has revealed complex and fascinating behavior. The interplay between loading conditions, temperature, and the microstructure of the material plays a crucial role in determining the susceptibility to hydrogen-induced fracture.