Laser surface modification of stainless steels for cavitation erosion resistance

Abstract

Austenitic stainless steel UNS S31603 (Fe -17.6Cr -11.2Ni -2.5Mo -l.4Mn -0.4Si -0.03C) has higher pitting corrosion resistance but lower cavitation erosion resistance than that of UNS S30400. This is because of its lower tendency for strain induced martensitic transformation and higher stacking fault energy as compared with those of UNS S30400. In order to improve its cavitation erosion resistance, surface modification of S31603 was performed by laser surface melting and laser surface alloying using a 2-kW CW Nd-YAG laser and a 3-kW CW CO2 laser. For laser surface melting, austenitic stainless steel UNS S30400, super duplex stainless steel UNS S32760 and martensitic stainless steel UNS S42000 were also investigated for comparison purpose. For laser surface alloying, alloying materials including various elements (Co, Cr, Ni, Mo, Mn, Si & C), alloys (AlSiFe & NiCrSiB), ceramics (Si3N4, SiC, Cr3C2, TiC, CrB & Cr2O3) and alloys-ceramics (Co-WC, Ni-WC, Ni-Al2O3, Ni-Cr2C3) were used to modify the surface of S31603. The alloyed surface was achieved first by flame spraying or pre-placing of the alloy powder on the S31603 surface and then followed by laser surface remelting. The cavitation erosion characteristics of laser surface modified specimens 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 urn. In addition, their pitting corrosion behaviour was evaluated by electrochemical techniques. The microstmctures, compositions, phase changes and damage mechanisms under cavitation erosion were investigated by optical microscopy, SEM, EDAX and X-ray diffractometry. Mechanical properties such as microhardness profile were also examined. The cavitation erosion resistance Re (reciprocal of the mean depth of penetration rate) of laser surface melted S31603 was found to be improved by 22% and was attributed to the existence of tensile residual stress. Improvement on the Re of S42000 was found to be 8.5 times because of the hardened surface which contained both martensite and retained austenite. Laser surface modification using NiCrSiB, AISi, Si and C for enhancing the Re was also succussfully accomplished. Intermetallic phases, borides and carbides were found to increase both the Re (4-10 times) and the hardness (2-3 times) of the laser treated S31603. The Re of S31603 laser surface modified with CoWC forming a metal-matrix composite (MMC) was highest (45.5 times) among all the materials studied. Laser surface modification with ceramics (e.g. WC and Cr2B) formed ceramic-matrx composite (CMC) but the Re was lower than that of the MMC because of rapid detaching of the ceramics particles from the specimens surface.