PSI - Issue 60

A.H.V. Pavan et al. / Procedia Structural Integrity 60 (2024) 277–285 A.H.V. Pavan/ Structural Integrity Procedia 00 (2024) 000 – 000

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In the present work, LP turbine blade was subjected to laser hardening treatment. This blade has been tested in operating conditions in a test rig setup. Development of residual stresses over the surface of the blade section in X5CrNiCuNb16-4 steel subsequent to laser hardening and testing were evaluated. It was observed that tensile residual stresses existed at certain locations. This was mitigated by carrying out successive shot peening over the LP turbine blade profile post laser-hardening treatment. 2. Experimental Details 2.1. Material X5CrNiCuNb16-4 steel was produced in electric furnace and is further refined by Electro Slag Remelting (ESR) process. Later, these ingots are forged, solution annealed for 1 hour 20 minutes at 1040 ± 5 o C followed by air cooling, intermediate annealed for 2 hours at 820 ± 5 o C and precipitation hardened for 4 hours at 530 ± 5 o C. followed by air cooling. Subsequently, a stress relief treatment was provided for 4 hours at 510 o C ± 5 o C, followed by air cooling. LP turbine blades were made from the heat-treated blocks by Computer Numerical Control (CNC) machining of the blade profile and root sections. Chemical composition of X5CrNiCuNb16-4 steel measured using Optical Emission Spectrometer (OES) in compliance with ASTM E1086:2008 (2008) is provided in Table 1. 2.2. Hardening Treatments Laser hardening was carried out by using two robotic diode laser guns over the blade profile over the region of interest. The hardening was conducted on leading edge up to a distance of ~30 mm and up to a distance of ~ 480 mm from the blade tip on the suction side. Post laser hardening treatment, in one case, shot peening over the complete blade surface profile was conducted to subject the blade profile surface to high compressive stresses. 2.3. Residual Stress Measurements Residual stress measurements were conducted on the location of interest using a StressTech X3000 residual stress analyser operating at 6.7 mA and 30 kV. Cr Kα radiation was used with a 2 mm collimator diameter. A standard sin 2 ψ procedure with 5 tilts from -45° to +45° was utilized for determining residual stress value at each location of interest. The signal intensity of {211} α – Fe peak was measured using two position sensitive detectors mounted on both sides of the incident beam at a pre- set 2θ of 156.4 o . Measurements were made up to a distance of 35 mm in general from the blade leading edge with 5 mm spacing unless otherwise mentioned specifically. Hardness profiling on the cross-section of the laser hardened leading edge was carried out from suction side profile into the material at distances of 5 mm, 10 mm, 15 mm, 20 mm, 25 mm and 30mm. Hardness measurements were performed using 0.5 kg load on Shimadzu HSV-30 Vicker’s hardness tester with 15 seconds dwell time. 2.5. Microstructural Characterization and Fractography In order to understand various microstructural features resulting from laser hardening treatment, a section of the laser hardened blade profile from the leading edge was sectioned and hot mounted in transparent bakelite resin. This mount is then ground by a series of ANSI emery papers in the following sequence: 220, 320, 600 and 1200. Table 1. Chemical composition (in wt.%) of X5CrNiCuNb16-4 steel Element C Si Mn P S Cr Ni Cu Nb Mo Fe X5CrNiCuNb16-4 0.044 0.14 0.39 0.017 0.009 15.47 4.32 3.07 0.29 0.11 Bal. 2.4. Hardness Profiling