O&M

Mitigating corrosion in steam turbine engines with engineered compression

Examining the use of deep-engineered compression to combat corrosion in Alloy 450, a stainless steel widely employed in steam turbine blades.
Mitigating corrosion in steam turbine engines with engineered compression
(Steam turbine photo. Source: Mitsubishi Power.)

By Kyle Brandenburg, Research Engineer for Lambda Technologies Group/Lambda Research

Problem: Steam turbines generate most of the world’s electricity, and approximately 42% in the US[1]. Keeping them in operation is vital. Condensation in the low-pressure stage can result in corrosion pitting and corrosion fatigue. These failure mechanisms are two of the most common factors impacting repair and operating expenses. When cracks begin forming at the site of these mechanisms, the component, often a blade, must be replaced. Between the actual component replacement cost and the downtime required, the replacement process can cost millions of dollars. Sometimes replacement blades are new, but they’re often refurbished blades that have been weld-repaired and returned to service. This leads to the recurrence of many failures as condensation and resulting corrosion damage usually form in the same areas[2].  

The primary way to address corrosion damage is by minimizing the chance of it forming. Martensitic stainless steels are often utilized in the production of parts because of the mild corrosion resistance offered by chromium[3]. Coatings are commonly applied to provide further resistance. Shallow compression is provided by shot peening. Operators attempt to control the chemistry of the vapors entering the steam turbines to minimize impurities[4]. All of these efforts offer protection, albeit with some disadvantages. Resistance through material selection is mild. Coatings wear over time and eventually require re-application. Surface damage can easily penetrate the relatively shallow layer of compression provided by shot peening. Ridding the vapors of impurities is challenging and offers no guarantee that corrosion will not still form.

Solution: Engineered compression has been proven to significantly improve the damage tolerance of many materials and components. This study examines the use of deep-engineered compression to combat corrosion pitting and corrosion fatigue in Alloy 450, a martensitic stainless steel widely employed in steam turbine blade manufacturing.

Specimen Design

Fatigue specimens were specially designed to test the benefits of compressive residual stress in 4-point bending. Samples were finished machined using low stress grinding (LSG). To simulate surface damage from any source (handling, FOD, corrosion pitting, or erosion), a semi-elliptical surface notch with a depth of ao = 0.01 in. (0.25 mm) and surface length of 2co = 0.06 in. (1.5 mm) was introduced by electrical discharge machining (EDM). EDM produces a pre-cracked recast layer that is in residual tension at the bottom of the notch, producing a large fatigue debit with a high kf.

Figure 1.

Processing

Low plasticity burnishing (LPB®) was selected to impart the engineered compression due to the depth and stability of compression, as well as the ease of application. Process parameters were developed to impart a depth and magnitude of compression on the order of 0.04 in. (1 mm), sufficient to mitigate the simulated damage. Figure 1 shows a set of eight fatigue specimens in the process of being low plasticity burnished on the four-axis manipulator in a CNC milling machine.

Testing

Active corrosion fatigue tests were conducted in an acidic salt solution containing 3.5 wt% NaCl (pH = 3.5). At the start of cyclic loading, filter papers soaked with the solution were wrapped around the gauge section of the fatigue test specimen and sealed with a polyethylene film to avoid evaporation. There was no exposure to the corrosive solution before the fatigue tests. LPB and LSG baseline samples were tested with and without EDM damage. A few LPB samples were tested with increased damage levels of 2x to analyze the treatment’s effectiveness with deeper damage.

Figure 2.
Figure 3.

X-ray diffraction residual stress measurements were made to characterize the residual stress distribution from LPB. The results of these measurements are shown in Figure 2. Maximum compression is nominally -140 ksi (-965 MPa) at the surface, decreasing to zero over a depth of about 0.035 in. (0.89 mm). The corrosion fatigue performance in acidic NaCl solution is shown in Figure 3. The LSG baseline condition is compared with LPB with and without the EDM notch. With no notch, the baseline fatigue strength at 107 cycles is nominally 100 ksi (689 MPa). The 0.01 in. (0.25 mm) deep EDM notch decreases the baseline fatigue strength to approximately 10% of its original value. The fatigue lives at higher stresses show a corresponding decrease of over an order of magnitude resulting from the notch. Unnotched LPB processed samples have a fatigue strength of about 160 ksi (1100 MPa). The notch had a marginal effect on the LPB fatigue strength, reducing it to 125 ksi (862 MPa), well above the fatigue strength of the undamaged baseline specimens. LPB-treated samples containing the 2x damage depth had fatigue lives comparable to undamaged LSG specimens within the limits of experimental scatter.

Conclusion

LPB imparted highly beneficial compressive residual stresses on the surface, sufficient to withstand pitting and/or surface damage up to a depth of nominally 0.02 in. (0.51 mm). LPB resulted in more than a 50% increase in corrosion fatigue strength without surface damage and a 12x increase in strength with 0.01 in. (0.25 mm) deep damage. The depth and magnitude of surface compression are responsible for improving fatigue strength.

The application of LPB effectively enhances corrosion damage tolerance, as shown by the improved fatigue strength even in the presence of simulated damage. The process has been used successfully in many power applications since the early 2000s. Implementing engineered compression with LPB significantly improves the durability and performance of steam turbine components, ultimately reducing costs associated with maintenance and downtime.

References

[1] US Energy Information Administration, “How Electricity is Generated.” https://www.eia.gov/energyexplained/electricity/how-electricity-is-generated.php October, 2023.

[2] R. Ravindranath, N. Jayaraman & P. Prevey, “Fatigue life Extension of Steam Turbine Alloys Using Low Plasticity Burnishing (LPB).” Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air. Glasgow, UK, June 14-18, 2010.

[3] A. Rivaz, S.H. Mousavi Anijdan, M. Moazami-Goudarzi, “Failure Analysis and Damage Causes of a Steam Turbine Blade of 410 Martensitic Stainless Steel After 165,000 H of Working.” Engineering Failure Analysis, Volume 113, 2020.

[4] Zhou, S, Turnbull, A, “Steam Turbine Operating Conditions, Chemistry of Condensates, and Environment Assisted Cracking – A Critical Review.” NPL Report MATC (A) 95, May, 2002.

 

About the Author: As Research Engineer for both the Surface Integrity and Process Optimization (SIPO) laboratory and the Corrosion Characterization laboratory at Lambda Research, Kyle Brandenburg is part of a team responsible for providing materials testing solutions to clients. Additionally, the SIPO and Corrosion labs conduct in-house research and testing pertaining to the surface enhancement and optimization of materials and components. Laboratory capabilities include high and low cycle fatigue studies, DC electrochemical corrosion testing, stress corrosion cracking, and supporting capabilities like hardness testing, heat treating, SEM and metallographic analysis, and shot peening.

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