Liquid-Metal-Induced Embrittlement in Turbine Casing Segment Screws of an Aeroengine
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CASE HISTORY—PEER-REVIEWED
Liquid-Metal-Induced Embrittlement in Turbine Casing Segment Screws of an Aeroengine Vaisakhi Nandi • R. R. Bhat • I. N. Yatisha S. V. Suresh
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Submitted: 4 December 2011 / in revised form: 22 February 2012 / Published online: 22 May 2012 ASM International 2012
Abstract One of the 12 screws meant to fasten the diffuser segment to the flange of a turbine casing in an aeroengine was found broken in-service. Cracks were also observed in some of the remaining screws. The screws made of 35NC6 steel material were plated with cadmium. The chemistry, hardness, and microstructure (heattreatment condition) of the screws met the drawing requirements. Heavy oxidation was noticed on the surface as well as on the fracture surface of the screws. Fractographic studies showed brittle intergranular features. Scanning electron microscope–energy dispersive x-ray spectroscopy (SEM–EDS) analysis detected cadmium on the entire fracture surface of the screws. Micro-examination and SEM–EDS analysis of the cracked screws showed the presence of cadmium all along the path of crack propagation including the crack tip. From microscopic and fractographic studies, it was concluded that the screws had failed by liquid-metal-induced embrittlement (LMIE). It appeared that the screws had experienced high service temperature that had resulted in melting of the cadmium plating on the screws. The molten cadmium had penetrated (adsorbed) along the grain boundaries of the screw material causing the screws to eventually fail in the brittle (intergranular) manner by the phenomenon of LMIE.
Background History
Keywords Aeroengine Cadmium plating Intergranular features Brittle failure Liquid-metal-induced embrittlement
The broken head of one of the turbine casing segment screws is shown in Fig. 1. This broken screw is identified as ‘‘screw-1’’. Cracks were noticed in the head to shank region in some of the remaining screws of the engine. Some of these screws are shown in Fig. 2 and are identified as ‘‘screw-2’’ and ‘‘screw-3.’’ The submitted screws were heavily oxidized. The oxidation on the surface of the screws is apparent in Fig. 3. The surface oxidation is explicitly clear when compared to similar screws from another serviceable engine, shown in Fig. 4.
V. Nandi (&) R. R. Bhat I. N. Yatisha S. V. Suresh Central Materials and Processes Laboratory, Foundry and Forge Division, Hindustan Aeronautics Limited (HAL), Bangalore 560017, India e-mail: [email protected]
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During post-flight inspection, physical damage was noticed in some of the third-stage turbine blades of an aeroengine. On strip examination, one of the turbine casing segment screws of the engine was found broken in the head to shank region. The broken head of the screw was found in the engine while the shank portion of the screw had apparently damaged the third-stage turbine blades and escaped from the engine along with the exhaust gas. The broken screw was one of the 12 screws meant to fasten the diffuser segment to the flange of the turbine
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