Microstructural and mechanical characterization of pearlitic steel after high intensity laser peening and shot peening

Abstract

This study investigated the microstructural/mechanical properties of annealed 4340 pearlitic steel after shot peening (SP) and high intensity laser peening (LP). SP at 0.012–0.016 A resulted in fragmentation of the pearlite colonies ∼1–2 µm from the treated surface and sub-surface cracking as well as significant distortion of pearlite colonies ∼30 µm from the treated surface. LP at a pulse energy of 19 J with a pulse full-width half height of 27 ns and a square spot size of 3x3 mm2 resulted in an intense irradiance on the metal surface of 8 GW/cm2. During this process a thin, micrometer-range layer of surface material is ablated which generates and heats a plasma plume in less than 1 ns to a temperature in the range of 15,000 K. A recast layer ∼5–10 µm in thickness from the treated surface with significant porosity was observed in the LPed specimen. The highest average microhardness (266 ± 9 HV) was observed in the SPed specimen ∼63 µm from the treated surface, however the hardness quickly returned to the baseline hardness (218 HV) ∼250–500 µm from the treated surface. In the LPed specimen, the averaged microhardness values exceeded the baseline hardness up to a depth of ∼1500–2000 µm with an average microhardness of 237 ± 4 HV from ∼60–2500 µm from the treated surface. It is shown that LP not only improves the sub-surface microhardness of the 4340 pearlitic steel up to greater depths than SP, but that it also preserves the pearlitic microstructure near the treated surface.