Laser surface modification of UNS S31603 stainless steel using NiCrSiB alloy for enhancing cavitation erosion resistance

Abstract

Laser surface modification using NiCrSiB alloy (Ni −16.5Cr −15.5Fe −3.5Si −3.8B −0.1C) on austenitic stainless steel UNS S31603 (Fe −17.6Cr −11.2Ni −2.5Mo −1.4Mn −1.4Cu −0.4Si −0.03C) was achieved by using a 2 kW continuous wave Nd-YAG laser. The cavitation erosion and pitting corrosion characteristics of the laser surface modified UNS S31603 in 3.5% NaCl solution at 23 °C were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm and a potentiostat, respectively. The NiCrSiB alloy layer was flame sprayed on the surface of UNS S31603 stainless steel. The surface was then scanned with the laser beam. The melt depth and dilution of the sprayed layer with the substrate material increased with the decrease of scanning speed. At low dilution ratio of sprayed layer thickness to melt depth, less secondary phases such as CrB, CrB 2, Fe 2B and M 7(CB) 3 and low hardness were observed in the laser-modified layer. The cavitation erosion resistance R e (reciprocal of the mean depth of penetration) of the laser-modified specimen with a dilution ratio of 0.65 was improved by 2.7 times as compared with that of the as-received UNS S31603 specimen. In addition, the pitting potential was close to that of the as-received UNS S31603 and the protection potential shifted in the noble direction by a substantial amount (from −3 to 121 mV). The R e of the laser surface modified specimen with dilution ratio of 0.88 was improved by four times as compared with that of the as-received UNS S31603 and very close to that of super duplex stainless steel UNS S32760 (Fe −25.6Cr −7.2Ni −4Mo −0.6Mn −0.7Cu −0.8W −0.3Si −0.2N −0.03C). However, its pitting potential was lowered from 359 to 50 mV and its protection potential shifted in the active direction from −3 to −238 mV. On the other hand, the degree of pitting damage as judged from the pitting morphology was less severe in the laser surface modified specimens. The R e of laser surface modified UNS S31603 was found to be linearly related to hardness. The deformation mechanism by cavitation erosion of the laser-modified specimens was also investigated.