Effects of laser excited ammonia on microstructure and hardness properties of nitriding 38CrMoAl steel (invited)

Objective Nitriding is a common process in the chemical heat treatment of 38CrMoAl steel, which alters the chemical composition and microstructure of the steel surface. It is widely applied to components such as shafts, gears, screws, and hydraulic plungers that operate in harsh environments. By directly coupling laser energy to ammonia molecules through resonant exciting their vibrational modes, the dissociation of active nitrogen atoms from ammonia can be accelerated, reducing heat dissipation during transfer. This enhances the efficient dissociation of ammonia molecules and increases the likelihood of ammonia absorption on the sample surface, leading to a deeper nitrided layer and improved hardness properties. To address the challenges of long treatment times and low efficiency in traditional gas nitriding, lasers were introduced into the gas nitriding process of 38CrMoAl steel. The effect of laser power on the microstructure and hardness of the nitrided samples was analyzed, providing valuable insights for the efficient production of high-quality nitrided layers on 38CrMoAl steel.

Methods The experimental platform for laser-assisted nitriding (Fig.1(a)) consists primarily of a CO₂ infrared laser, a vacuum chamber, vacuum system, a temperature control system, and an ammonia supply system. The experimental material used was tempered and heat-treated 38CrMoAl steels. Using the developed equipment, gas nitriding and laser-assisted nitriding experiments were conducted at laser powers of 50 W, 100 W, 150 W, and 200 W. After the experiments, the cross-sections of the nitrided samples were polished and etched. The surface microstructure was analyzed using an XRD (X-ray diffraction), SEM (Scanning electron microscope), and shape measurement laser microsystem, while the cross-sectional microstructure was examined with an optical microscope. Chemical composition was determined through EDS (Energy-dispersive spectroscopy) and EPMA（Electron probe X-ray micro analyzer） analysis. In addition, the surface hardness and cross-sectional hardness of the nitrided samples were evaluated using a Vickers microhardness tester.

Results and Discussions After gas nitriding and laser-assisted nitriding as varying laser power, nitride particles formed on the surface of 38CrMoAl (Fig.2(a)). As the laser power increased, the nitride particle size grew progressively larger (Fig.2(b)), reaching a maximum size of 865 nm. This increase in particle size also led to higher surface roughness, with a maximum roughness of S_a = 2.92 μm (Fig. 2(c)). EDS analysis was used to study the chemical composition changes on the nitrided surfaces (Fig.3), revealing that higher laser power effectively enhanced the dissociation of ammonia molecules, leading to more active nitrogen atoms being absorbed by the 38CrMoAl. At 200 W, the nitrogen content reached 12.93%, compared to only 6.37% after gas nitriding alone.In addition, after 6 hours of gas nitriding, only a 74.2 μm diffusion layer was observed in the cross-section of the 38CrMoAl, with no visible white compound layer. In contrast, laser-assisted nitriding under the same conditions resulted in both a compound layer and a diffusion layer. The thickness of these layers increased as rising laser power (Fig.4(a)), reaching 232.8 μm for the compound layer and 15.4 μm for the diffusion layer at 200 W. XRD analysis (Fig.1(c)) indicated that the surface of the 38CrMoAl sample underwent a phase transition from α-Fe to γ′-Fe₄N, and then to ε-Fe_2-3N. EPMA analysis of the 38CrMoAl cross-section (Fig.4(c)-(d)) showed that higher laser power promoted the diffusion of nitrogen atoms along grain boundaries, forming nitrides. The increased laser power also resulted in a denser surface compound layer with enhanced hardness properties (Fig.5). At 200 W, surface hardness reached 1102 HV_0.1. Additionally, the higher the laser power, the greater the hardness at equivalent depths below the surface, forming a gradient hardness distribution.

Conclusions In this study, the nitriding layer of 38CrMoAl was produced using laser-excited ammonia-assisted nitriding technology. The microstructure and hardness of the surface and cross-section of samples treated with gas nitriding and laser-assisted nitriding at different laser powers were compared and analyzed. As nitriding progressed, the surface of the 38CrMoAl plate underwent a phase transition from α-Fe to γ′-Fe₄N, and subsequently to ε-Fe_2-3N. As increasing laser power, the average size of nitride particles, surface roughness (S_a), and nitrogen proportion all increased. Additionally, the nitrogen content in the nitriding layer rose with higher laser power, which enhanced the decomposition of ammonia and improved the efficiency of ammonia gas utilization.The average surface hardness of the untreated 38CrMoAl substrate was 256 HV_0.1. After conventional gas nitriding and laser-assisted nitriding at different laser powers, the surface hardness increased to 788 HV_0.1, 843 HV_0.1, 936 HV_0.1, 1057 HV_0.1, and 1102 HV_0.1, respectively. The hardness of the nitrided cross section gradually decreased with depth and eventually approached the hardness of the base material.