Tensile ductility of a duplex surface treated maraging steel produced by Laser Powder Bed Fusion: Interaction between the inhomogeneous microstructure and transformation induced plasticity

The tensile ductility of a surface-treated 18Ni300 maraging steel produced by Laser Powder Bed Fusion (L-PBF) was investigated in the present work. Both plasma nitriding and duplex treatments (plasma nitriding and Physical Vapour Deposition (PVD) coating) were considered using a direct aged material as a reference. The inhomogeneous microstructure of the surface-treated materials causes an early strain-induced austenite transformation, with a positive effect on the uniform plastic deformation. However, the brittle failure of the hardened surface and the coating promotes an inhomogeneous stress field and inhomogeneous deformation whose effect on tensile ductility prevails on the enhanced TRansformation Induced Plasticity (TRIP), resulting in a negative overall effect on ductility. The study's results suggest a metallurgical approach to optimizing the inhomogeneous microstructure of the surface-treated material, with the aim of preventing the decrease of ductility. • The surface treatment decreases tensile ductility. • The case acts as a metallurgical notch. • The inhomogeneous microstructure promotes inhomogeneous deformation. • Inhomogeneous microstructure causes anticipated austenite strain induced transformation. • TRIP is not effective due to anticipated failure of the case.