Failure and metallurgical defects analysis of IN-738LC gas turbine blades

https://doi.org/10.1016/j.engfailanal.2021.105213Get rights and content

Highlights

Abstract

Catastrophic and early failure of third stage blades of a gas turbine was investigated. Some areas on the fracture surface of 4 blades were covered with thick black oxide layer. Quantitative microscopic measurements of primary γ′ and MC carbide phases showed that the blades were not exposed to excessive temperature. Scanning electron microscopy and energy dispersive spectroscopy analysis of oxide layer on fracture surface and stress ruptured specimens within 760–980 °C up to 100 h, and heat-treated specimens were used to calculate the blades temperature. The results revealed that oxide bifilms discontinuities and internal cracks accompanied with shrinkage cavities, nucleation and dendritic growth of γ′ precipitates, and non-uniform microstructure led to locally weakened structures at the bottom of airfoils. Accordingly, uncontrolled parameters in casting were the main reasons for commencing the failure.

Introduction

Various mechanisms are involved in failure of gas turbine blades [1], [2]. The mechanisms are vigorously depended on primary microstructure formed during casting and heat treatment of the blades [3], [4]. The elevated temperature mechanical properties of IN-738LC alloy are optimized through modified microstructure, which is affected by shape, size and distribution of γ′ precipitates and metallurgical defects [5], [6]. Metallurgical defects can be related to improper casting processes and/or heat treatment cycles [7], [8], [9]. Once due to improper or insufficient heat treatment, heterogeneous microstructure has obtained, it could cause lower mechanical properties, failure initiation and premature retirement of blades [10], [11]. Casting defects such as porosity on the other hand, have been shown that can results in blades catastrophic failure [12]. The primary microstructure of the blades has undergone significant degradations during high temperature service condition [13], [14], [15]. Correspondingly, degree of microstructural degradations including coarsening of primary γ′ precipitation [16], dissolution and decomposition of MC carbides [17], and the kinetic of oxidation [18] can be correlated to the actual operating condition during failure incident. In this regard, precise quantitative microscopic examination and chemical analysis in conjunction with knowledge of common casting and heat treatment defects are important tools used for conducting failure analysis investigation of gas turbine blades.

Section snippets

Background

Third stage blades of a gas turbine were mostly damaged at tip region after few hours of the engine start. Under normal operating conditions, the output of turbine was 36 MW, the average exhaust temperature was 530 °C and the turbine speed was 5160 rpm. 7 blades in random position were fractured at the bottom of airfoil. On the fracture surface of 4 blades there were some regions covered with thick black oxide layer (Fig. 1). Chemical composition of blades was in accordance with IN738LC

Experimental

Fig. 2a shows the sampling position on the blades which are damaged at the tip. The specimens prepared from the root of the blades, were used to measure the degree of microstructural deterioration at the airfoil during service condition. Sampling were made from blades airfoil at different height from root for quantitative measurements of primary γ′ size and MC carbide area fraction. For blades which fractured at airfoil-root intersection, longitudinal samples were prepared from regions covered

Discussion

Coarsening of primary and secondary γ′ precipitates and decomposition of MC carbides accompanied with morphological and analysis variations have been used to explore the thermal history of blades [21], [22], [23], [24]. It has been shown that primary MC particles forming in solidification remain the skeleton morphology after solution treatment [25]. The morphology of MC carbide does not markedly influenced by chemical composition, however, adding refractory elements such as W, Ta, and Hf

Conclusion

The results of fractography and microstructural studies show that turbine failure occurred because of microstructural defects of blades. The parabolic oxidation rate calculation and time -temperature Arrhenius relationship were used to estimate the temperature as high as 1048 °C. However, the results of metallography quantitative measurements of primary and secondary γ′ particles size and MC particles phase fraction did not conform temperature raise close to solvus temperature. In consistent

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors would like to thank to the Neyrperse Company, Iran for the financial support of project and providing initial data of this research. They also acknowledge the useful discussion with Professor N. Varahram regarding the casting defects.

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