Correlation between cladding speed, microstructure and hardness of laser-deposited steel coatings: The role of strengthening mechanism

In this study, three wear/corrosion-protective Fe-base alloys with different strengthening mechanisms including AISI 316L (boundary strengthening), Hoganas 3533 (boundary strengthening and carbides) and Carpenter MicroMelt 23 (martensite and carbides) were deposited using laser cladding with powder injection at speeds of 1, 10 a nd 100 m m/s. Microstructure of the coatings and the type and quantity of their constituent phases were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy and Electron Backscatter Diffraction. Hardness of the coatings was measured using Vickers indentation. Microstructural characteristic size, i.e. dendrite arm spacing or cell width of the coatings were measured on SEM images and the cooling rates during solidification were estimated using an analytical model. At higher cooling rates, the characteristic size decreased and the interdendritic phases became refined for all coatings. Nonetheless, hardness of MicroMelt 23 coatings remained unchanged and hardness of AISI 316L and Hoganas 3533 only slightly increased (10-20 percent). The role of cooling rates on the characteristic size, phase constitution, martensitic transformation and the interplay between various strengthening mechanisms in producing the overall hardness of the deposits are discussed.