Microstructural characterizations and wear and corrosion behaviors of laser-nitrided NAK80 mold steel

The effects of laser nitriding on the microstructural evolution and wear and corrosion behaviors of the NAK80 mold steel (AISI P21) were investigated. A high-power diode laser with a laser energy density of 1125 J/mm² was employed for laser nitriding of NAK80. To examine the microstructure of laser-nitrided NAK80, diverse characterization methods such as scanning electron microscopy with electron backscatter diffractometry, high-resolution transmission electron microscopy with electron energy loss spectrometry, and atom probe tomography were utilized. These analyses revealed that the laser-nitrided region consisted of rod-like aluminum nitride (AlN) precipitates with various nitrides and carbides in the martensite matrix, resulting in the surface hardening of NAK80 from 409 to 536 HV. As a result, the laser-nitrided NAK80 exhibited an enhancement in the wear resistance in comparison to that of the base metal in the fretting test. The wear loss of laser-nitrided NAK80 was reduced by 33.4% compared to that of the base metal after 40,000 fretting cycles. However, a deterioration in the corrosion resistance was observed for the laser-nitrided NAK80 compared to the base metal. It may be associated with the elemental segregation in the vicinity of the AlN precipitates, which was observed in the atom-probe tomography.