One of the most studied deleterious phases in stainless steels is the sigma phase, due to its high potential to decrease the toughness and corrosion resistance of these steels. Eight samples of as-received cold rolled UNS S32205 duplex stainless steel were submitted to isothermal heat treatments at 850 o C during 10,15, 30, 60, 120, 180, 240, and 300 minutes in order to study the precipitation kinetics of the sigma and chi deleterious phases. Several complementary microstructural analysis techniques were used to determine the volume fraction of the intermetallic phases, including optical microscopy and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Evaluation was made of the kinetics of isothermal formation of sigma and chi phases in commercial duplex stainless steels (UNS S32205). The results indicated two different mechanisms for sigma phase precipitation: sigma phase formed from the chi phase for shorter isothermal heat treatment times, and sigma phase precipitated at the ferrite-austenite interface for longer isothermal heat treatment times, using a temperature of 850 o C. The phase transformation kinetics determined using the JMA equation indicated that chi phase precipitation caused faster sigma phase formation.
Austenitic-ferritic stainless steels submitted to friction stir welding show good mechanical properties, although there are concerns regarding their application in marine environments, related to their corrosion performance. Friction stir welds of UNS S32101, S32205, S32750, and S32760 steels were successfully produced at 200 rpm and 100 mm/min. The corrosion resistance of the FSW joint was evaluated by application of the ASTM A923 standard method, cyclic polarization measurements, and determination of weight loss during the FeCl 3 immersion test. The sodium hydroxide etch test was used to evaluate the presence of intermetallic phases. Cyclic polarization measurements of the S32205, S32750, and S32760 base metals showed good corrosion resistance, while poorer corrosion resistance was observed for S32101. Cyclic polarization tests at the friction stir welds indicated that for S32205 and S32750, the corrosion resistance was higher, compared to the respective base metals. The S32760 and S32101 welded joints showed lower noble potential and higher corrosion current density, compared to the base metals. NaOH etching indicated affected areas at the ferrite-ferrite grain boundaries for the S32760 FSW joint, which also showed a large loss of mass in ferric chloride solution, associated with the presence of deleterious intermetallic phases.
Duplex stainless steels are largely applied in petrochemical industries due to their high corrosion resistance and mechanical performance. Their applicability also requires a great wear resistance, which can be enhanced by friction stir processing (FSP), a surface hardening technique. In this work, FSP is utilized to process surfaces of UNS S32101 and UNS S32750. The materials were analyzed by EBSD in order to determine grain size and phase fraction. Microhardness tests were used to verify and compare the shifting of hardness values from 296 ± 8 to 314 ± 11 HV 0.2/15 and from 323 ± 8 to 350 ± 8 HV 0.2/15 for UNS S32101 and S32750, respectively. Abrasion tests were executed to study the wear behavior of both processed and unprocessed alloys. Abrasion tests indicated that the hardening by FSP promoted decrement of abrasive wear resistance. Volumetric loss increased after FSP, from 52.1 ± 0.5 to 53 ± 2 mm 3 , for UNS S32101, and from 50 ± 2 to 56.3 ± 0.3 mm 3 for UNS S32750. Tensile tests results were fitted to Hollomon's equation in order to identify mechanical properties and tendencies of strain hardening. SEM images were used to classify the wear micromechanisms acting on the samples. The results suggest that FSP can promote modifications of wear mechanisms and these changes can be correlated to the alloys' microstructure.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.