Laser surface modification of UNS S31603 stainless steel. Part I: Microstructures and corrosion characteristics

Laser surface alloying using various elements (Co, Ni, Mn, C, Cr, Mo, Si) and alloys/compounds (AlSiFe, Si3N4and NiCrSiB) on austenitic stainless steel UNS S31603 was attemped. Alloying materials in powder form were preplaced on the surface of the substrate by flame spraying or pasting. The surface was then scanned by a high power laser beam to achieve surface alloying. The microstructures of the alloyed layers were studied by scanning electron microscopy, optical microscopy and X-ray diffractometry, and the corrosion characteristics in 3.5% NaCl solution at 23°C were studied by potentiodynamic polarisation. The performance of the laser alloyed surfaces varied depending on the type and amount of alloying materials used, and on the laser processing parameters. The specimens alloyed with Co, Ni, Mn, C or NiCrSiB contained austenite as the main phase, with carbides and carbides/borides as the minor phases in C-alloyed and NiCrSiB-alloyed specimens. For specimens alloyed with Cr or Mo, the major phase was ferrite. In the case of Si or Si3N4, the major phase was an intermetallic Fe3Si. When A1SiFe was used, the major phase could be ferrite or Fe3Al, depending on the dilution ratio. The larggest improvement in corrosion resistance was achieved with Si and Si3N4, leading to a noble shift in the pitting potential of 170 and 211 mV, respectively, and a corresponding noble shift in the protection potential of 130 and 221 mV. For NiCrSiB, the effect on the corrosion resistance depended on the degree of dilution. For all the other alloying materials, the corrosion resistance either remained unchanged or deteriorated mainly due to the presence of some ceramic or intermetallic phases which acted as sites of pit initiation.