# Anisotropy Corrected FMC/TFM Based Phased Array Ultrasonic Imaging in an Austenitic Buttering Layer
## 1 Introduction  
To prevent global warming, efforts are being put worldwide to reduce CO\({}_{2}\) emissions from fossil fuel power plants. Reduction in CO\({}_{2}\) emission can be achieved by increasing the efficiency of fossil fuel power plants. In advanced ultra-supercritical (AUSC) thermal plants, desired efficiency will be attained by increasing steam temperature (\(\sim\)700 \({}^{\circ}\)C) and pressure (\(\sim\)300 kg/cm\({}^{2}\)) [1, 2]. In AUSC power plants, the components operating on the steam side have to be made of heat-resistant material. For the Indian AUSC project, Alloy 617 (a nickel base alloy is chosen as the material for high-pressure (HP) turbine rotors to meet the required mechanical properties at elevated temperatures. For low and intermediate temperatures, 10 Cr ferrite steel is chosen. The HP and intermediate turbine rotors are joined by a narrow gap weld joint with alloy 617 buttering on the ferritic steel side [3, 4]. Prior to the welding process, alloy 617 buttering laid on the ferritic steel has to be qualified. The most significant flaws encountered during the buttering qualification are the lack of fusion occurring at the interface and between the layers. Liquid penetrant testing and ultrasonic testing (UT) are used regularly for the examination of the buttering. Though UT is the most prominent technique for volumetric examination, UT of thick austenitic buttering is challenging. According to the metallurgical process, buttering, overlays, and welding processes are basically casting processes. In austenitic cast materials, grains are solidified in a columnar structure leading to a highly anisotropic and heterogeneous microstructure, which leads to difficulty in their UT [5]. Further, the high single-crystal elastic anisotropy of austenitic steel and nickel base alloys leads to large direction-dependent ultrasonicvelocity in these materials in the presence of columnar grain structure. The major physical phenomena making UT of austenitic material challenging are beam skewing, caused by deviation of group velocity direction (energy flow direction) from wave vector direction [6], beam distortion as a result of beam widening in slow direction and vice versa [7] and attenuation due to scattering, which predominates when the grain size is as large as wavelength. Attenuation further depends on grain orientation [8]. These disturbances reduce signal-to-noise ratio (SNR) significantly as well as resolution [9]. Compared to longitudinal wave, shear wave undergoes more beam skewing and is highly attenuated in austenitic weld materials [10]. If any flaw goes undetected during the qualification of the buttering layer interface, it becomes critical due to the cyclic loading of the turbine rotor during its operation. The flaws, which are undetected during buttering qualification, may orient almost radially in the assembled rotor. Detection of such radial planner flaws in thick as well as highly attenuating Ni base alloy using UT is difficult [11]. Hence, it is important to be able to detect the flaws during the qualification stage of buttering, i.e., prior to the welding process. Advanced phased array ultrasonic techniques (PAUT) have been explored for the inspection of anisotropic materials [12; 13; 14; 15; 16; 17]. In contrast to the use of conventional UT for testing anisotropic material, PAUT offers some advantages for controlling physical characteristics of an ultrasonic beam using delay laws as well as offers improved sensitivity and coverage [18]. Among the PA imaging techniques, the application of the full matrix capture/total focusing method (FMC/TFM) is widespread. The fundamental concept of TFM is summing all FMC signals in-phase condition (applying delay and sum rule) to obtain constructive interference in the region of interest (ROI). A number of approaches have been developed to adopt FMC/TFM for enhanced flaw detection in anisotropic materials.