Improved surface integrity in Inconel 718 using small diameter hammer peening tools

Machine Hammer Peening (MHP) is an emergent treatment that induces high compressive Residual Stresses (RS) which can enhance the in-service performance of components. This paper studies the use of small diameter MHP tools to improve the Surface Integrity (SI) of the nickel-based alloy Inconel 718 used in critical aero-engine components. Complementarily, the relaxation of RS is analyzed by in-situ annealing tests at in-service temperature combined with X-ray diffraction measurement. For this purpose, age hardened Inconel 718 discs were turned as reference condition, and then a pneumatic MHP tool was used under different conditions: two tool diameters (4 mm and 12 mm), feed rates (2.5 m/min and 5.0 m/min) and stepover distances (0.07 mm and 0.35 mm). Subsequently, surface topography characterization, RS measurements, nanoindentation tests, and microstructural observations were conducted. The in-situ annealing tests were done in the X-ray diffractometer at 550 °C for exposure periods from 0 to 20 h. The results demonstrate that 4 mm diameter tools generate smooth surfaces and induce significant compressive RS within a 0.5 mm thick layer. Residual stresses are relaxed, but they remain compressive even after a long thermal exposure. The microstructure of the surface layer (<10–15 μm) was affected by the preceding turning operation, but importantly, MHP did not induce additional damage.