Improvement of cavitation erosion resistance of stainless steels by laser surface treatments

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Improvement of cavitation erosion resistance of stainless steels by laser surface treatments

 

Author: Lo, Ka-hing
Title: Improvement of cavitation erosion resistance of stainless steels by laser surface treatments
Degree: M.Phil.
Year: 2002
Subject: Hong Kong Polytechnic University -- Dissertations
Steel, Stainless -- Thermal properties
Steel, Stainless -- Welding
Metallurgy -- Laser use in
Department: Dept. of Applied Physics
Pages: 1 v. (various pagings) : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1667759
URI: http://theses.lib.polyu.edu.hk/handle/200/3820
Abstract: Martensitic stainless steel AISI 440C and austenitic steel AISI 316 are commonly used in hydraulic machinery and in liquid-handling equipment. In such applications, these materials are liable to cavitation erosion, which is a common mode of degradation of engineering parts in contact with fast-flowing or vibrating liquids. The present project is an attempt to improve the cavitation erosion resistance of stainless steels 440C and 316 by laser surface modification. Laser surface treatments were performed using a high-power Nd:YAG laser system. Based on the composition of the base material, different types of laser treatments were selected for 440C and 316. For 440C, in view of the high carbon content, laser transformation hardening (LTH) and laser surface melting (LSM) were employed. Surface modification of 316 was achieved by laser surface alloying (LSA) using fine tungsten carbide (WC) powder (about 1 um), and by fabrication of a surface metal-matrix composite (MMC) layer using coarse WC powder (about 60 um). The cavitation erosion resistance in 3.5 % NaCl solution was determined using an ultrasonic vibratory facility, while the corrosion behavior was studied by polarization technique. The cavitation erosion resistance of 440C was significantly improved by LTH and by LSM. While the resistance achieved by LTH could be twice that by conventional heat treatment, LSM could only achieve an increase comparable to that by conventional heat treatment. Improvement of cavitation erosion resistance in 440C, both by LTH and LSM, was attributed to a fine microstructure composed of carbide precipitates and appropriate proportions of martensite and retained austenite. Such a microstructure possesses a proper combination of hardness and toughness. The corrosion resistance of 440C treated by LTH and LSM was significantly improved, and was higher than that of the conventionally heat-treated samples, as evidenced by a more noble pitting potential, a wider passive range, and a lower corrosion current density. The improvement in Corrosion resistance of 440C resulted from the reduction in the amount and size of carbides, which was accompanied by an increase of Cr in solid solution, and by the presence of retained austenite. Strong synergistic effect between erosion and corrosion was present, possibly due to the poor repassivation power of the laser-treated samples. The cavitation erosion resistance of samples laser surface-alloyed with fine WC powder could reach about 35 times that of as-received 316, while that of WC-reinforced MMC samples could only reach a value of about 10 times. The increase in cavitation erosion resistance resulted from the dissolution of WC into the substrate, leading to the precipitation of fine carbide and to solution hardening. On the other hand, the coarse WC particles remaining in the melt layer were susceptible to brittle fracture under cavitation attack. Compared with as-received 316, the corrosion resistance of both the alloyed samples and the MMC samples deteriorated as evidenced by the partial disappearance of passivity. The reduction of Cr in solid solution due to the precipitation of Cr-containing carbides and the presence of WC particles were both responsible for the weakening of the passive film. Despite the deterioration in passivity, the corrosion current density remained low and the corrosion potential remained close to that of as-received 316, which guaranteed a low tendency of galvanic corrosion due to local laser treatment. The present study has demonstrated the feasibility of improving the cavitation erosion resistance of stainless steels 440C and 316 by laser surface treatments. The methods are of practical value, as they would prolong the maintenance cycle or service life. An additional value is the total avoidance of the use of strategic or polluting elements like Co or Ni.

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