Coatimg Begrac&sil om off F rst and Second Stage Gas Tan-fern© Bfedes Can Maralc&n, Bora Derin* and Onuralp Yucel Metallurgical and Materials Eng. Dept., Istanbul Technical University, 34469, Turkey (Received August 4,2009) ABSTRACT Degradation study of over-aluminized CoCrAlY coatings (GT-29+) on conventionally cast (CC) GTD- 111 Ni based super alloy substrates was investigated by evaluating first and second stage turbine blades after their 48,000 h of original services. The degradation evidences such as enlargement of the inter-diffusion zone, aluminum depletion from both top aluminide and CoCrAlY coating and related phase formations were observed by using an ΕΡΜΑ. The elemental mapping and quantitative analysis of the phases were also performed using WDS device attached to ΕΡΜΑ. Microhardness changes along the duplex coating thickness for both blades were determined by a Vickers microhardness tester. Keywords: Turbine blade, coating, degradation, GTD- 111.GT-29+ I. INTRODUCTION Heavy-duty gas turbine technology is used for power generation either in simple cycle or in combined cycle with a steam turbine to obtain maximum efficiency. There has been great interest for these turbines in 'Tel: +90 212 285 3096/146, Fax: +90 212 285 3427, e-mail: bderin@itu.edu.tr improving performance in output and heat rate, extension of inspection intervals as shortening their duration, emission reductions, life extension, etc 11-71. Last two decades, a nickel based super alloy, GTD-111, has been used in GE MS9001 turbines as substrate materials of both first and second stage turbine blades providing the industry standard in corrosion resistance and a creep rupture/low-cycle fatigue advantage over the previously used IN-738 alloy /1.21. Besides, CoCrAlY or NiCoCrAlY coatings with top aluminide layer (GT-29+ or GT-33+) are used for GTD-111 bucket's exterior surface and the internal cooling holes. Both coating materials are applied by vacuum plasma spray, PVD or more recently HVOF spray methods. Subsequently, top aluminide layer are deposited onto the coatings by using standard pack aluminizing processes, optionally above the pack (ATP) or CVD processes /3/. There exist some degradation and life- prediction investigation studies on GT29(+)/GTD-lll or GT-33(+) /GTD-111 turbine blades /3-7A For example, Daleo et al. investigated the failure mechanism, life assessment and safe refurbishment of top aluminized CoCrAlY alloy/GTD-lll turbine blades /3,4/. Cheruvu et al. 161 studied on service run GE 600IB and 9000FA turbine blades to identify the in- service degradation mechanism for the dublex coatings. They also conducted a cyclic oxidation test on top aluminized GT-29 or GT-33 coated GTD-111 substrate specimens. They compared their findings with COATLIFE software which was developed for 343 Brought to you by | Purdue University Libraries Authenticated Download Date | 6/16/15 3:09 PM