Microstructure Evolution and Properties of Gradient Nanostructures Subjected to Laser Shock Processing in 300M Ultrahigh‐Strength Steel

Abstract

Herein, gradient nanostructures (GNs) are created in 300M ultrahigh‐strength (UHS) steel by laser shock processing (LSP). Microstructure evolution and properties of GNs subjected to LSP with different pulse energies are thoroughly characterized on 3D profiler, scanning electron microscope (SEM), transmission electron microscope (TEM), X‐ray diffractometer (XRD), X‐ray stress analyzer, nanoindenter, and tensile tester. Results show successful creations of GNs in 300M steel after LSP treatments. With the increase in pulse energy, the size of the surface layer is refined from 15 nm (3 J) to 10 nm (7 J), and the corresponding grains are amorphized to some extent. Meanwhile, many substructure defects such as dislocation tangles and deformation twins (DTs) are noted in the subsurface. The dislocation density and the number of DTs increase with the pulse energy. Further, the high compressive residual stress is introduced to the 300M steel surface after LSP, and the corresponding hardness is improved substantially. The compressive residual stress, depth of the affected layer, and the hardness rise significantly with the pulse energy. Apart from improvements in strength and plasticity, the fracture morphology is changed from a typical ductile fracture to quasicleavage and ductile mixed fracture.

Laser shock peening rounds influencing microstructural and mechanical properties of 300M steel

“…Such strain and strain rate generated by the laser impacts gradually decayed within material with an increased in distance from the impacting surface. Thus, degree of PD gradually decreased, forming MPD layer in subsurface [21]. Such PD layer with SPD and MPD layers was also obtained for ANSI 304 stainless steel [12].…”

“…At the same time, dislocation movement is hindered by grain boundaries and also dislocation accumulations that form dislocation walls etc., which results in increased and concentrated internal stress. The internal stress is more likely to reach the critical stress for deformation twinning in 300M steel [21], which leads to the formation of deformation twins to accommodate deformation. As the shockwave penetrates into subsurface, strain rate decreases sharply along with a decrease in impact force.…”

“…Such steel has become the most widely used material for manufacturing aircraft landing gear due to it high strength (nominal tensile strength ≥ 1800 MPa), superior toughness and excellent fatigue performance [18][19][20]. In our previous study [21], the effect of LSP pulse energy on the mechanical properties of 300M steel was investigated. It revealed that both increasing laser pulse energy induced enhancement in strength without deteriorating plasticity, which is of significance for the material's application excellence as aircraft landing gear.…”

Effects of Residual Stress Induced by Laser Shock Peening on Fatigue Crack Propagation Behavior in Directed Energy Deposition Stainless Steel

“…To improve the quality of DED parts, industry and researchers commonly utilize posttreatment techniques, including laser shock peening (LSP), laser polishing, traditional machining methods, and heat treatment processes to improve the surface and mechanical properties of the parts. [4][5][6] Among these postprocessing techniques, LSP has widely attracted attention for its advantages of high operational flexibility, no heat-affected zone, and high processing accuracy.…”