A two-step laser-sustained plasma nitriding process for deep-case hardening of commercially pure titanium

A two-step, laser-sustained plasma (LSP) process was developed to form deep, hard, nitrided cases on commercially pure titanium. A laser-sustained plasma is plasma generated in a gaseous atmosphere that can be sustained indefinitely by the laser beam away from any potentially interacting surface. The first step utilized a nitrogen LSP associated with a defocused CO2 laser beam in pure nitrogen gas flow to melt and nitride the titanium surface. The second step used an argon LSP to remelt and refine the nitrided layer. Twenty experimental cases with varying nitriding and remelting speeds were studied. Optical and scanning electron microscopy, energy dispersive spectroscopy, optical profilometry, and x-ray diffraction were used to characterize the remelted nitrided layer. An analytical moving heat source solution was used to model the heat transfer in the melt pool during the nitriding and remelting processes. The mass flux of nitrogen into the melt pool and the efficiency of nitrogen intake were estimated. The remelting treatment was found to eliminate the surface cracks formed during nitriding, decrease the surface roughness of the nitrided trail, homogenize the hardened surface layer, and refine the microstructure. The microstructures and hardness of the nitrided layers could be tailored by changing the combination of LSP nitriding and remelting scan speeds. Case depths of up to 0.8 mm and average case hardness values in the range of 475-729 HV0.3 were achieved in this study.