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Creep properties of new and aged nickel based alloy for gas turbine blades by means of small punch testing

pubblicazioni - Articolo

Creep properties of new and aged nickel based alloy for gas turbine blades by means of small punch testing

The small punch technique is under development since the 80ies to determine the residual mechanical properties of operated components in power generation industry, when only a very limited quantity of ex- service material is available.

A standardisation effort was recently made by the CEN Workshop 21 which produced a guideline for SP testing both for the determination of tensile and toughness properties of metallic materials and of their creep resistance. This document is based mainly on the published literature related to steel characterisation. Less information can be found about the application of SP to superalloys for gas turbine blading (only some isostress data), even if the small sampling removal required by this technique could be of interest to characterise the residual creep properties of thin aerfoils, whose properties cannot be determined using traditional specimens. This paper describes the activity performed at CESI RICERCA for the evaluation of creep properties on a nickel based alloy by means of small punch creep tests. The test program included characterisation of material in commercial virgin bar, laboratory aged specimens and ex-service rotating blades. In-house testing equipment had been partially modified (preserving conformity to CEN WS21 Agreement suggestion) in order to improve tests repeatability and results accuracy. The activity lets to demonstrate that the small punch procedure can be satisfactory for a so-called “isothermal” creep characterisation of a nickel based alloy and small punch test results can be compared to standard uniaxial creep master-curve when a new correlation among small punch load and uniaxial creep stress is used. The possibility to obtain indications about the degradation of creep resistance due to service is discussed.

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