Effect of Cold Work on Cavitation Resistance of an Austenitic Stainless Steel Coating

Abstract

Machining procedures of welding deposits are usual and result on cold work hardened surfaces. The cold work effect on cavitation erosion of an austenitic stainless steel surface is assessed. FeCrMnSiB coatings were processed by PTA on AISI 304 plates. Specimens were grouped as the cold work deformed surface (CWHS) and the undeformed polished surface (UPS) specimens. Top surface and transverse section of coatings were analysed for slip lines and hardness changes by light microscopy and Vickers microhardness measurements. Ultrasonic cavitation tests were conducted in accordance to ASTM G32-10. CWHS specimens exhibited slip lines and hardened surfaces while UPS specimens did not show traces of slip lines and had insignificant changes on microhardness. Cold work prior to cavitation indirectly increased the nominal incubation time and reduced the maximum erosion rate. Cold work increases the duration of the acceleration period postponing the onset of the maximum erosion rate and enhancing cavitation resistance.

Thermal Spraying of FeMnCrSi Alloys: An Overview

“…Also, the peening is faster and does not eliminate coating material to adequate the roughness, as the polishing does. Post-treatment on FeMnCrSi PTA coating was presented by da Cruz, Henke, and d'Oliveira [30], indicating the effectiveness of plastic deformation or cold work on the FeMnCrSi PTA coating cavitation resistance improvement. It increased the coating hardness from 353 HV up to 462 HV and changing the microstructure, Figure 15, presenting strip lines in their grains.…”

“…The cavitation erosion occurs in three stages, in accordance with the ASTM G32-03 standard, the first, called incubation period, which is a time when no significant mass loss is observed; the second is characterized by the mass loss beginning and mass loss increase up to a maximum value, where the third stage starts, characterized as a steady-state mass loss regime [30]. Thermally sprayed FeMnCrSi coatings did not show the first stage, mainly because of the presence of pores and oxides in their microstructure, reducing the cavitation resistance of these coatings [27].…”

“…Cruz, Henke, and d´Oliveira [30] obtained FeMnCrSi coatings on 304 stainless steel by the PTA process with powder feeding, using direct current, where the W+2%ThO2 electrode is in negative polarity, of 160 A and argon as plasma and shielding gas, resulting in a uniform hardness of 350 HV over the very dense coating cross-section, as presented in Figure 8. FeMnCrSi PTA coating had a very low mass-loss rate in ASTM G32-03 accelerated cavitation testing, 0.54 mg•h -1 .…”

“…The authors thank Copel GeT, for financial support; to Lactec, for the execution of R&D projects; UFPR and UTFPR, for the access to the equipment and laboratories for analysis and tests; Brazilian Electricity Regulatory Agency ANEEL, for the regulation of R&D projects; Brazilian National Council for Scientific and Technological Development CNPq, for the fiscal incentives to import equipment to Brazil. after cold work and (b) as deposited [30].…”

Welding and Thermal Spray Processes for Maintenance of Hydraulic Turbine Runners: Case Studies

“…Co content filler metals have superior resistance [30,31], however, in the case of carbon steel runners, the deposition of stainless steel 309L already is a possible and effective solution, due to its higher resistance to cavitation [12]. Will et al [30] evaluated the cavitation resistance of 309LSi and Co-steel alloys coatings welded by PTA with different parameters of the pulsed current, resulting in the lowest erosion rate for the Cavitec alloy, 0.45 mg/h, welded with pulsed current 180/120 A and peak/base time 0.1/0.1 s. By the same process, but with current 160 A, da Cruz, Henke, and d'Oliveira [32] welded the FeMnCrSi alloy [33], obtaining erosion rate of 0.52 mg/h, aswelded, and 0.33 mg/h, after mechanical cold work post-treatment, which converted partially the phase austenite on martensite, increasing the hardness and cavitation resistance. Using the same PTA pulsed arc process, but with pulsed current 180/80 A and peak/base time 0.1/0.1 s, Pukasiewicz et al [34] remelted FeMnCrSi [33] alloy arc-sprayed coatings, reaching the erosion rate of 0.65 mg/h.…”