Microstructural Analysis of the Laser-Cladded AISI 420 Martensitic Stainless Steel

This study investigates the microstructural evolution and phase transformation of laser-cladded AISI 420 martensitic stainless steel. The microstructural morphologies of the laser-cladded sample were examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM) equipped with energy-dispersive spectroscopy. The three regions of bead zone, dilution zone, and an interface were investigated. The TEM study revealed various morphologies of martensitic phase in each region. The bead zone consisted of plate and lenticular martensite with internal twins, while lath and thin plate martensite and internal twins were presented in the dilution zone. Large lath martensite was observed in the interface zone the boundary between the dilution and heat-affected zones. The selected area electron diffraction identified those forms of martensite as the ά-martensite in three zones. Other microstructural features such as nano-carbide precipitates and high density of dislocations were also observed in each zone. The martensitic transformation with various morphologies provided crucial information about the development of residual stress throughout the three zones. It was concluded that due to the high cooling rate, the martensitic phase transformation first occurred in the bead zone with the formation of plate-like martensite. Combination of the plate and lath-like martensite formed subsequently in the dilution zone created high compressive stress in this zone and high tensile stress in the bead zone.