Issue 30

D. Nappini et alii, Frattura ed Integrità Strutturale, 30 (2014) 394-402; DOI: 10.3221/IGF-ESIS.30.47 395 based technique for materials characterization are a promising and attractive tool nowadays available in materials engineering and science [5-9]. Instrumented indentation holds several advantages: they are inexpensive, non destructive and it does not require specimen preparation; therefore, it may be used to investigate the material properties on small volumes, such as thin films, and soft tissues on small scales by just one simple impression test. Indentation tests can be used to probe hardness of materials, toughness, elastic modulus, elasto-plastic behaviour and constitutive laws evaluation by different reverse analysis methods of indentation load-displacement curve [10-16]. Aim of the work is to set up the capability of the instrumented spherical indentation testing system to evaluate variations in a gas turbine rotor material properties due to damage resulting from temperature field and stresses acting on components during long time service. To perform this task load-indentation depth curves were acquired in several location of various components made in IN718 and CrMoV from high pressure gas turbine rotor. Conventional hardness measurements were also carried out, in order to verify if a correlation can be obtained between these tests and indentations. Finally, whole results will be analysed to assess the material damage. M ATERIALS AND METHODS Indentation testing apparatus xperimental indentations were carried out using an instrumented indentation testing equipment developed by University of Pisa, University of Trento and Scienza Machinale S.r.l, able to performs a maximum indentation load of 2000 N - with a resolution of the order of 0.2 N - during the experimental tests. With this device it is possible to investigate the behaviour of the hardest metallic materials. Indentations were performed via a spherical tungsten-carbide ball having a diameter of 2.5 mm. The relative displacement between indenter and target was continuously measured by two LVDT transducers ensuring an accuracy of 4 µm and a resolution of 0.2 µm. Tests were carried out in displacement control at a speed of 1µm/s, thus ensuring that the data were collected under quasi static condition and no potential strain-rate effects were involved. The technical specifications of the testing machine are summarized in Tab. 1. A detailed description of the design solutions can be found in [17]. Load cell Loading axis Indenter Maximum load 2000 N Maximum linear excursion 2 mm Tungsten-Carbide Ball Maximum error 1.5 N Accuracy 4 µm Diameter 2.5 mm Resolution 0.2 µm Table 1 : Technical specification of instrumented indentation apparatus (“Diaptometro”). Test procedure overview Indentations were performed on items sampling on 1 st and 2 nd stage turbine discs and on the spacer ring made in nickel- base superalloy (Inconel 718) from an high pressure GE turbine rotor which have seen an extensive service (over 10 5 hours). Moreover some test were carried out on the compressor aft stub shaft made in CrMoV low alloy steel (ASTM A471 type10) belonging to the same machine. The items were machined so as to be free from thermal or mechanical alteration due to working process and to obtain a tolerance parallelism between surfaces minor of 0.05 mm and a tolerance flatness lower than 0.02 mm. Also, in order to achieve the right surface finishing, the items were polished. For each tested item a set of about 40 instrumented indentations and 15 conventional HBW hardness measurements were performed in significant areas, subjected during service to different values of stress and temperature. In order to asses a statistical dispersion of indentation response, before the aforementioned testing campaign a set of measures was carried out on forging ring pieces of ASTM A182F22. To perform this task the items were machined to obtain the same surface conditions and tolerances. Load-indentation depth (P-δ) curves and conventional HBW hardness acquired in the various zones of test samples were analysed as described in the following. E