Experimental investigation on erosion resistance of iron boride coatings for steam turbines at high temperatures

Abstract

In this paper, high temperature erosion tests are performed on four typical anti-erosion coatings for high-parameter steam turbine blades using pneumatic accelerated erosion method to simulate the actual erosion environment in power units. With the employment of scanning electron microscope, energy-dispersive X-ray spectroscopy, glancing-angle X-ray diffraction, and Vickers indentation testing techniques, detailed microscopic analysis of samples is conducted to reveal the underlying causes affecting the anti-erosion performance of coatings. Results show that, under the solid particle erosion at high temperature, boride coatings with more compact structure and higher hardness have stronger anti-erosion ability, whose erosion rate is only 30–50% that of HVOF Cr3C2 coatings under the same erosion condition. Boride coatings composed of duplex phase (FeB and Fe2B) have lower anti-erosion damage capability than that of boride coatings composed of single Fe2B phase. Under high-speed large incidence angle impingement, the surface layer of FeB cracks easily and rapidly propagates to the subsurface layer Fe2B phase under the sustained impingement from particles, resulting in the flaking off and failure of coatings. Single Fe2B phase boride coatings have smaller hardness gradient in the depth direction and higher fracture toughness than boride coatings composed of duplex phase, so that exhibit more stable excellent erosion resistance under different erosion parameters. Therefore, single Fe2B phase boride coatings should be used as the preferred wear resistant coating for turbine blades solid particle erosion problem.

Investigation of the Erosion Damage Mechanism and Erosion Prediction of Boronized Coatings at Elevated Temperatures

“…For the dual-phase boronized coating investigated in this study, the results from the Vickers hardness analysis in reference [ 20 ] show that the fracture toughness of the FeB coating on the top is 1.5 MPa·m 1/2 , whereas the fracture toughness of the Fe 2 B coating on the bottom is 5.42 MPa·m 1/2 . From Equation (1), the critical stress value of the internal crack propagation for the FeB coating is calculated to be in the range 0.85 GPa~2.68 GPa, whereas the critical stress value of the internal crack propagation for the Fe 2 B coating is determined to be in the range 3.06 GPa~9.67 GPa.…”

“…A Buehler MICROMET 5104 automatic microhardness tester was used to measure the hardness of the polished cross-section of the boronized coating, at an applied load of 1 N, as shown in Figure 5 . In addition, the Vickers hardness indentation method was used to measure the fracture toughness of the boronized coating, at an applied load of 3 N. The specific calculation procedure for the determination of the fracture toughness can be found in the literature [ 20 ]. In order to ascertain the reproducibility of the test data, the coating hardness and fracture toughness were reported after averaging 10 measurements under the same conditions.…”

“…The elongation and reduction in area were 16% and 56%, respectively, with a hardness of 320 HB. The boronized coating was obtained by applying the solid boronizing powder to the stainless-steel substrate at a temperature of 1123~1273 K [ 20 ]. The detailed process is as follows: the substrate test piece was kept in the boronizing furnace for 10 h at a temperature of 1273 K, followed by cooling in air to room temperature.…”

“…The coating was noted to swiftly fall off locally and fail. In another study, Cai et al [ 20 ] conducted high-temperature erosion tests on three typical boronized coatings and observed that the single-phase Fe 2 B coatings exhibited a superior erosion resistance to that of the dual-phase boronized coatings. The numerical simulation results [ 21 ] further confirmed the experimental findings.…”

Effect of oblique stator on the aerodynamic performance and erosion characteristics of governing stage blades in a supercritical steam turbine

“…However, hard coatings cannot always guarantee the protection of turbine blades 12,13 due to the limited thickness and unavoidable coating defect. 14,15 In fact, it was already proved by practical engineering that it is impossible to solve the SPE problem of turbine completely only through these passive anti-erosion measures. 11,16 Therefore, initiative measures, such as optimizing the cascade profile, are adopted by different experts.…”