Recent development of thermally assisted surface hardening techniques: A review

Thermally assisted surface hardening techniques have led to a surge in research efforts and industrial applications, with emphasis on strengthening of metallic materials with high work hardening, high strength and/or poor deformability. This paper reviews the development of common thermally assisted surface hardening techniques, including warm shot peening, warm laser shock peening, and thermally assisted ultrasonic surface hardening. The development and working principle for each of the techniques are discussed. The enhanced mechanical properties and corresponding mechanisms are reviewed in detail. As compared with conventional surface hardening techniques, thermally assisted surface hardening techniques with optimum processing temperatures can further increase the surface and subsurface hardness, thickness of the hardening layer, fatigue life, and wear resistance of mechanical components. The improvements are attributed to the unique microstructures fabricated by the synergistic effects of thermal energy and high-strain-rate plastic deformation. Thermal energy can soften the materials, allowing plastic deformation to produce higher magnitude and deeper region of work hardening. More interestingly, the thermomechanical treatment can also induce dynamic strain aging and dynamic precipitation in some metallic alloys, which leads to precipitation strengthening and enhanced stability of dislocations and compressive residual stress. The coupled thermal-dynamic effect enables a broader design space for alloy hardening. Lastly, future research directions of thermally assisted surface hardening techniques are discussed.