Elsevier

Engineering Failure Analysis

Volume 26, December 2012, Pages 211-219
Engineering Failure Analysis

Failure assessment of Nimonic 80A gas turbine blade

https://doi.org/10.1016/j.engfailanal.2012.05.028Get rights and content

Abstract

Nickel base superalloys are widely used in applications requiring strength at high temperature. During the operation of gas turbines, the blades and other elements of hot gas path suffer service induced degradation which may be natural or accelerated due to different causes. The failure analysis of the 3 MW gas turbine first stage blades made of nickel-base alloy Nimonic 80A is presented. The blade experienced base alloy degradation due to operation at high temperature. Initially the blade was sectioned for metallographic and microhardness test. The microstructure and microhardness was compared in four different blade zones i.e. root, 30%, 60%, and 90% of the total height of the hot region of the blade. On the basis of the observed microstructures and the phases present in the alloy, the main cause of failure was found to be creep damage. Finite element method (FEM) analysis is conducted considering peak loading of centrifugal force and surface (aerodynamic) loads. Based on FEM simulation results, the life of turbine blades is predicted using the Larson–Miller method. Finally to improve the blade life two heat treatment cycles were suggested and applied. The effect of heat treatments on grain size, volume friction of γ′ primary phase, and micro hardness were investigated. A detailed analysis of the coating showed that, the loss of coatings resistant to high temperature is due to oxidation, corrosion, erosion and inters diffusion of coating-substrate, which results in diffusion of alloy elements into the coating.

Highlights

► Experimental and numerical data on material used in the power generating plant has been generated. ► Heat treatments have been suggested to revive some mechanical properties and life time of the alloy. ► Analysis has been performed to evaluate the coating deterioration on the blade. ► Data are generated for designing and life assessment of components at high temperature in power generating industry.

Introduction

Nickel base superalloys are widely used in applications requiring strength at high temperature. Most of these alloys are age-hardenable by a fine dispersion of γ′ particles, which have an ordered FCC structure. The mechanical properties of alloy are strongly dependent upon the size and distribution of the γ′ precipitates. Since the γ′ particles can coarsen during the initial heat treatment and during subsequent service, it is important to predict the kinetics of coarsening of this precipitate.

Generally speaking, most blades suffer severe operation conditions characterized by the following factors: operation environment (high temperature, fuel and air contamination, solid particles, etc.), high mechanical stresses (due to centrifugal force, vibratory and flexural stresses, etc.) and high thermal stresses (due to thermal gradients) [1]. There are mainly two types of damage occurring in the blade:- external surfaces damage (corrosion, oxidation, crack formation, erosion, foreign object damage and fretting), and internal damage of microstructure as γ′ [Ni3 (Al, Ti)] phase aging (rafting), grain growth, grain boundary creep voiding, carbides precipitation and brittle phases formation [2]. Nimonic 80A is a casting superalloy frequently used for high temperature applications and the mechanical and creep properties of this alloy are well known [3], [4], [5].

There are several factors such as high gas temperature, high steady state load levels (centrifugal load) and high thermal transients loads (trips, start-ups and slowing downs) which influences the blade material deterioration. In service, the deterioration in individual blades depends on the operation history (number of start-ups, shut-downs and trips), turbine operational modes (temperature, rotational speed, operational conditions (base load mode, cyclic mode)) and material specification (grain size, porosity, alloy composition, heat treatment). Reliable and safe predictions of the lifetime of components operating at high temperature conditions in power plants are essential. There are number of procedures used for life assessment of components at high temperature. Methods of remaining life assessment of components at high temperature can be used for, just in-time blade rejuvenation, safe and cost-effective lifetime extension and to avoid blade catastrophic failure. One of these methods is to correlate mechanical properties to the microstructural changes (deterioration) during service time in a gas turbine blade alloy. This can be used for monitoring and evaluation of extent and degree of material damage and lifetime consumed and to obtain recommendations for blade rejuvenation treatments, operation and reposition [1], [3].

In this paper, first-stage nickel base superalloy Nimonic 80A blades of a gas turbine which had suffered deterioration after 20,000 h service are analyzed. The following failure analysis has been carried out – visual observations, optical microscopy, scanning electron microscopy SEM, Image analyzer, dimensional metrology and microhardness testing. Initially the blade was sectioned for metallographic and microhardness testing. The microstructure and microhardness was compared in four different blade zones i.e. root, 30%, 60%, and 90% of the total height of the hot region of the blade. On the basis of the observed microstructures and the phases present in the alloy the main cause of failure was found to be creep damage. FEM analysis is conducted, based on FEM simulation results; the life of turbine blades is predicted using Larson–Miller method. Finally to improve the blade life, two heat treatment cycles were applied. Also the effect of heat treatment on grain size, volume friction of γ′ primary phase, and micro hardness were investigated.

Section snippets

Service conditions and material composition of the blade

The blade under evaluation was the first stage blade of a 3 MW combustion turbine with a gas inlet temperature of 770 °C. The evaluation was carried out after 20,000 h of blade operation. The combustion turbine operates on natural gas and is located inland. The blade is made of nickel-base Nimonic 80A superalloy by means of conventional investment casting (equiaxed grains) and coated by thermal barrier coating (TBC) by diffusion process. The chemical composition result for the blade is shown in

Experimental work

Specimens were cut and prepared for microstructure evaluation from different zones of the blade, see Fig. 1. The microstructure of the blade hot section (airfoil) was compared to the cold reference zone (blade root) to evaluate the degree of alloy deterioration. The comparative evaluation includes the morphology change of the γ′ particles, carbide precipitation, characterization of grain type, size and volume fraction % γ′ particles.

For microstructural examinations, all samples were ground with

Results and discussion

Samples S1, S2 and S3 are taken from three different sections of the blade airfoil (30%, 60%, and 90% of the total height of the hot region of the blade), and compared with the sample from the blade root which is considered as cold zone (reference zone), because it is not exposed to the hot combustion gases.

Conclusions

High temperature engineering life assessment and component design utilise models based on theoretical principles which always need to be validated under practical and operational circumstances. Due to the complex nature of blade failure, numerous aspects have to be examined. This article has focused specifically on the possible causes of creep failure. Based upon experimental and numerical results the following conclusions are drawn:

  • 1.

    Metallographic investigation for the top zone showed

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