H.C. Man

Synergistic effect of cavitation erosion and corrosion of various engineering alloys in 3.5% NaCl solution

Abstract The cavitation erosion and corrosion characteristics of various engineering alloys including grey cast iron, steels, copper-based alloys and stainless steels were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm in distilled water and in 3.5% NaCl solution at 23°C. The contributions of pure mechanical erosion, electrochemical corrosion, and the synergism between erosion and corrosion to the overall cavitation erosion–corrosion in 3.5% NaCl solution were determined. It was found that in 3.5% NaCl solution, the effect of corrosion on the overall cavitation erosion–corrosion was most pronounced in mild steel and grey cast iron, and negligible in stainless steels. The stainless steels only suffered pure mechanical erosion in 3.5% NaCl solution in the presence of cavitation owing to the unfavourable local environment for pit growth.

Cavitation erosion and pitting corrosion behaviour of laser surface-melted martensitic stainless steel UNS S42000

Abstract Surface modification of martensitic stainless steel UNS S42000 was achieved by laser surface-melting using a 3.5-kW continuous wave CO 2 laser. The cavitation erosion and corrosion characteristics of laser surface-melted 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 μm and a potentiostat, respectively. In a series of 4-h cavitation tests, the cavitation erosion resistance of laser-melted specimens fabricated under a power of 1.7 kW and a scanning speed of 25 mm/s was 70 times that of the as-received (annealed) S42000 and 1.8 times that of conventionally heat-treated specimens. The excellent cavitation erosion resistance was due to the combined effect of a high volume fraction of retained austenite (89%) and a moderate hardness (450 Hv). By using different processing parameters, it was found that the cavitation erosion resistance of the laser-melted specimens increased with the increase in volume fraction of retained austenite, a result attributable to the high martensitic transformability of the austenite in S42000. On the other hand, cavitation erosion resistance increased with the increase in hardness to a maximum value and then dropped with further increases in hardness. This indicated that martensitic transformability played a more important role than hardness in cavitation erosion. The pitting potentials of all laser-melted specimens shifted in the noble direction, and the pits formed in the laser-melted specimens were shallower as compared with those formed in as-received and hardened S42000 specimens. The improvement in pitting corrosion resistance resulted from the dissolution or refinement of carbide particles and the presence of retained austenite, as evidenced by the fact that the pitting potential increased linearly with the amount of retained austenite. The present study thus shows, that with proper processing parameters, both the cavitation erosion and pitting resistance might be simultaneously improved by laser-surface melting, at least for martensitic stainless steels.

NiTi cladding on stainless steel by TIG surfacing process: Part I. Cavitation erosion behavior

Abstract NiTi was deposited on AISI 316 stainless steel by the tungsten inert gas (TIG) surfacing process, aiming at increasing cavitation erosion resistance. A thick deposit, with a 750 HV microhardness and dilution ratio of 14% was formed, with strong deposit/substrate interfacial bonding. Small pores (size ≤20 μm, total volume fraction less than 1%), and some second phase precipitates were present in the deposit. Even with the presence of such pores, the cavitation erosion rate of the deposit in NaCl solution was lower than that of AISI 316 by a factor of more than nine, and even lower than that of AISI 316 laser-clad with NiCrSiB, a common hard-facing material. The large increase in erosion resistance could be attributed to the partial retention of superelasticity, and also to the high hardness of the deposit. In this preliminary study on the efficacy of the TIG process for NiTi deposition, the main problem identified was the presence of small pores in the deposit, the elimination of which, via more refined processing, would definitely further increase the cavitation erosion of the deposit.

Correlation of cavitation erosion resistance with indentation-derived properties for a NiTi alloy

Abstract An empirical relationship is established between the cavitation erosion resistance R e of a Ni-rich NiTi alloy and the quantity W / δ u , where W is the work of indentation and δ u is the unrecoverable plastic deformation. This relationship also provides evidence that both pseudoelasticity and pseudoplasticity contribute to cavitation erosion resistance.

Cavitation erosion and pitting corrosion of laser surface melted stainless steels

Abstract Surface modification of austenitic stainless steels S31603. S30400 and super duplex stainless steel S32760 was achieved by laser surface melting using 2 kW continuous wave Nd—YAG laser. The cavitation erosion and corrosion characteristics of laser surface melted stainless steels 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 mm and a potentiostat respectively. In a series of 4 h cavitation tests, the cavitation erosion resistance of laser-melted S31603 with 50% overlapping of melted tracks was found to be improved by 22% at 23 °C as compared with the as-received specimen and very close to that of the as-received S30400. No improvement on the cavitation erosion resistance of S30400 and S32760 was found. The pitting potential of laser-melted S31603 increased from 359 to 452 mV at 23 °C. The deformation mechanism by cavitation and the pitting morphology of the laser-melted stainless steels were discussed in terms of microstructure and residual stress.

In-situ synthesis and wear performance of TiC particle reinforced composite coating on alloy Ti6Al4V

A TiC particle dispersively reinforced composite coating on alloy Ti6Al4V has successfully been synthesized by YAG pulse laser induced melting and reaction of a powder mixture, which consisted of pure Ti and Cr2C3 and promptly pre-placed on the alloy surface. Tt follows that with help of optimal process parameters and a suitable composition of the powder mixture, a composite coating can be obtained with the features of good metallurgical bonding with the alloy, and of graded distribution of the fine dispersive particles of TiC in the coating matrix. The TiC particle showed good wettability and compatibility with the matrix of the coating with a sharp and clear interface between them. The microstructure of the composite coating varied with the composition of the powder mixtures and also the laser processing parameters. Furthermore, such a composite material coating offered the alloy Ti6Al4V high hardness and excellent wear resistance

Effect of processing conditions on the corrosion performance of laser surface-melted AISI 440C martensitic stainless steel

Abstract Laser surface melting of AISI 440C martensitic stainless steel was achieved using a 2.5-kW continuous wave Nd:YAG laser. The pitting corrosion behavior of laser surface-melted specimens processed under different processing conditions in 3.5% NaCl solution at 23 °C was studied by potentiodynamic polarization technique. The corrosion resistance of all laser surface-melted specimens was significantly improved, as evidenced by a shift from active corrosion to passivity, a wide passive range and a low passive current density. The pitting potential of the laser surface-melted specimens P08-440C-25 (laser power=0.8 kW, scanning speed=25 mm/s) and P12-440C-25 (laser power=1.2 kW, scanning speed=25 mm/s) was increased to 260 and 200 mV (SCE), respectively, and was much higher than that of the conventionally heat-treated AISI 440C. The pitting corrosion characteristics of the laser surface-melted specimens were strongly dependent on the processing conditions which resulted in different microstructures. The enhanced corrosion resistance was attributed to the dissolution or refinement of carbide particles and the presence of retained austenite. The amount of carbides in the melt layer, which indirectly determine the Cr content in solid solution and hence, the corrosion resistance, was related to the amount of C remaining in solid solution and to decarburization. The pit morphology of the laser surface-melted specimen was also studied.

Cavitation erosion and corrosion behaviour of laser surface alloyed MMC of SiC and Si3N4 on Al alloy AA6061

Abstract Laser surface alloying of SiC/Si3N4on AA6061 aluminium alloy was carried out using a 2-kW CW Nd-YAG laser. Different ratios of SiC and Si3N4 powders were mixed and the layers were preplaced by pasting. Subsequent laser surface melting of the layers gave a surface metal matrix composite (MMC). The microstructure, chemical compositions and phase identification of the modified layers were examined using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD), respectively. The laser surface alloyed MMCs of AA6061-SiC and AA6061-Si3N4 consisted of small amounts of Al4C3/Al4SiC4and AlN, respectively. For specimens alloyed with 100% Si3N4 (AA6061-Si3N4), the cavitation erosion resistance Re was improved by three times as compared to the AA6061 alloy whereas there was no significant improvement in Re of the specimen alloyed with 100% SiC (AA6061-SiC). The surface hardness of the specimens alloyed with SiC/Si3N4 was increased seven times. All the laser alloyed specimens with SiC/Si3N4 showed a decrease in pitting resistance and absence of passivity owing to the aluminium–ceramic interfaces, which favour pit initiation or hindered passivity.

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

Abstract Laser surface alloying using various elements (Co, Ni, Mn, C, Cr, Mo, Si) and alloys/compounds (AlSiFe, Si 3 N 4 and 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 Si 3 N 4 , the major phase was an intermetallic Fe 3 Si. When AlSiFe was used, the major phase could be ferrite or Fe 3 Al, depending on the dilution ratio. The largest improvement in corrosion resistance was achieved with Si and Si 3 N 4 , 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.

Cavitation erosion-corrosion behaviour of laser surface alloyed AISI 1050 mild steel using NiCrSiB

Abstract A Ni-based hardfacing alloy NiCrSiB (Ni–16.5%Cr–15.5%Fe–3.5%Si–3.8%B–1%C) was surface alloyed on AISI 1050 mild steel (Fe–0.2%Cr–0.4%Mn–0.5%C) specimens using a two-step process. NiCrSiB powder was preplaced on the substrate by flame spraying and then remelted by a 2 kW continuous wave Nd:YAG laser to achieve surface alloying. The cavitation erosion–corrosion characteristics of the surface alloyed specimens in 3.5% NaCl solution at 23°C were determined by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm and a potentiostat. At a dilution ratio of 12% (corresponding to a laser scanning speed of 25 mm s −1 ), the overall cavitation erosion–corrosion resistance of the alloyed surface was 8.9 times that of the as-received AISI 1050 specimen. The corrosion resistance was also improved as reflected by a reduction in the current density of least one order of magnitude as compared with the as-received specimen at the same potential. The improvement in cavitation erosion resistance could be attributed to the superior mechanical properties of the NiCr-alloyed matrix formed and the presence of borides and boro-carbides which increased the hardness. The improvement in corrosion resistance was due to the increase in Cr and Ni content in the alloyed layer. The relative contributions of pure mechanical erosion, electrochemical corrosion, and synergism between erosion and corrosion to the overall cavitation erosion–corrosion were also determined.