Laser Boronizing of Additively Manufactured 18Ni-300 Maraging Steel Part Surface

The problem of insufficient wear resistance of maraging steels (MSt) has so far been solved mainly by the use of the thermochemical nitriding process, which has a number of limitations and disadvantages. In the present work, for MSt parts manufactured by laser powder bed fusion (LPBF), a more flexible laser alloying process was suggested as an alternative surface hardening process. The purpose of the present work is to give a better understanding on the possible hardening effect obtainable when amorphous boron is used as an alloying additive in relation with microstructural evolution and specific process parameters and to promote further development of this technology. For the alloying, a one kilowatt CO₂ laser was applied at 0.5-4.0 mm laser spot and 250-1500 mm/min laser operating speed, providing 50,955-796 W∙cm^-2 power density and 24.0-4.0 J∙mm^-1 heat input. Before laser processing, surfaces were covered with amorphous boron. The appropriate melt pool geometry was obtained at 0.5 mm laser spot, for which XPS analysis revealed an increase in boron concentration from ~3.1 to ~5.7 wt.% with a laser speed increase from 500 to 1500 mm/min. XRD analysis revealed domination of Fe₃B type borides along with the presence of FeB, Fe₂B, Ni₄B₃ borides, austenitic and martensitic phases. The microstructure of modified layers exhibited evolution from hypoeutectic microstructure, having ~630-780 HK0.5 hardness, to superfine lamellar nanoeutectic (~1000-1030 HK0.2) and further to submicron-sized dendritic boride structure (~1770 HK0.2). Aging of laser-boronized layers resulted in the change of phase composition and microstructure, which is mainly expressed in a plenty precipitation of Mo₂B₅ borides and leads to a reduction in hardness-more significant (by ~200-300 HK0.2) for hypoeutectic and hypereutectic layers and insignificant (by ~50 HK0.2) for near-eutectic. With the application of the laser boronizing technique, the hardness of MSt parts surface was increased up to ~three times before aging and up to ~2.3 times after aging, as compared with the hardness of aged MST part.