Enhanced Nitriding of 38CrMoAl Steels with Laser Vibrational Excitation of Ammonia

Abstract

Gas nitriding is a thermochemical surface hardening process that is widely used in industry but suffers from a low decomposition rate of ammonia (NH₃) and extremely long processing cycles. To enhance the gas nitriding efficiency, laser vibrational excitation of NH₃ is introduced into the gas nitriding of 38CrMoAl steels. By matching the laser wavelength with the N–H wagging mode of NH₃, laser energy can be preferentially deposited into NH₃ molecules and facilitates their dissociation. The morphology, composition, and mechanical properties of nitrided 38CrMoAl plates strongly depend on the nitriding time length. Nitrided surfaces experience phase transition from α-Fe solid solution with N through γ′-Fe₄N to ε-Fe_2~3N phase as prolonging the nitriding time length. A 38CrMoAl plate nitrided with laser vibrational excitation of NH₃ for 6 hours possesses a higher nitrogen content (7.1 wt pct) and a harder surface (944 HV_0.1) than that nitrided without laser (6.3 wt pct, and 811 HV_0.1). The study concludes that laser vibrational excitation of NH₃ is an effective way to promote nitrogen diffusion kinetics and shorten the nitriding cycle of gas nitriding, which holds great promise in the development of gas nitriding.