Nanomechanical properties of iron nitrides produced on 42CrMo4 steel by controlled gas nitriding and laser heat treatment

Nitriding, as one of the most widely used surface treatment, was often applied in order to improve the hardness, fatigue strength as well as wear and corrosion resistance of steels. The controlled gas nitriding resulted in a formation of a compound zone ε+(ε+γ′) of limited thickness close to the surface and the diffusion zone below it. It was easy to identify the two separate zones in the microstructure: more porous zone with only ε-Fe3N iron nitrides and zone with ε+γ′ iron nitrides' mixture (Fe3N + Fe4N). In this study, the laser modification without re-melting was carried out after the controlled gas nitriding in order to change microstructure. The nanomechanical properties of the zones with iron nitrides, observed in the compound zone, were examined using the nanoindenter with a Berkovich diamond tip. Young's modulus and hardness were measured and compared to the literature data. The load-displacement curves and the indentation images were shown for the selected measurements. The advantageous influence of laser heat treatment on the Young's modulus and hardness was confirmed for the outer ε zone as well as for the laser quenched diffusion zone. The higher average values of indentation modulus and hardness characterized ε iron nitrides (EIT = 165.72 ± 18.67 GPa and HIT = 9.90 ± 0.47 GPa) and laser quenched diffusion zone (EIT = 199.06 ± 28.80 GPa and HIT = 7.11 ± 0.61 GPa) after laser heat treatment. The diminished porosity of ε zone and the presence of martensite in the diffusion zone were the reason for such a situation. The laser heat treatment almost didn't influence the indentation modulus of zone with ε+γ′ iron nitrides' mixture (EIT = 175.27 ± 4.81 GPa), whereas its hardness was slightly increased (HIT = 11.87 ± 0.43 GPa). The conclusion regarding the percentage of the separate phases (ε and γ′) in this zone was ambiguous.