Unveiling the impact of nitriding and PVD coating on the fatigue properties of L-PBF maraging steel

The fatigue properties of 18Ni-300 maraging steel produced by Laser Powder Bed Fusion (L-PBF) were evaluated under axial fatigue loading. The influence of duplex surface treatments (plasma nitriding followed by PVD) was compared with heat treatments (solution annealing and ageing and direct ageing). Microhardness profiles, microstructural analysis (SEM and TEM) of the surface layers, residual stresses measurements and fractographic analysis were carried out to investigate the relationships between surface treatments and fatigue strength. The results demonstrate that surface treatments enhance fatigue strength, with improvements ranging from 30 % to 100 % in dependence on the stress amplitude, compared to heat treated specimens. The effect of surface treatments on fatigue strength is attributed to surface hardening and compressive residual stresses that tend to shift the crack initiation in the interior and to slow down the crack propagation. The hard and brittle coating causes surface crack initiation in some cases, but this effect on fatigue strength is compensated by the prior nitriding. The results demonstrate that duplex surface treatments substantially improve the fatigue performance of L-PBF maraging steel.