Effects of Aging at 450°C on the Pitting Corrosion Resistance and Toughness of AISI 317L Steel Welded by GTAW and FSW

Abstract

Austenitic stainless steels are high corrosion resistant alloys widely used in many industrial fields. Among this family of steels, AISI 317L stands out due to its higher localized corrosion resistance when compared to the traditional grades AISI 304L and AISI 316L. In some applications in oil refineries, the AISI 317L is being specified for services at moderately high temperatures. At the same time as it is sought to use new stainless steels, is also desirable to apply and develop emerging welding processes, replacing conventional ones, in order to achieve better behavior in service. In this respect, this work studied the effects of thermal aging on the toughness and resistance to pitting corrosion of AISI 317L steel weld metals produced by the Gas Tungsten Arc Welding (GTAW) and Friction Stir Welding (FSW) joining processes. After prolonged exposures at 450°C, for 200h, 300h and 400h, the microstructural characterization by scanning electron microscopy (SEM), toughness evaluation and anodic polarization tests in 3.5% NaCl solution were performed. The results showed that the increase of the exposure time in both weld metals caused a toughness decrease. The pitting potentials measured in the polarization tests also decreased with the aging at 450ºC.

Effect of the Heat Input and Ageing Treatment on Microstructure and Mechanical Properties of AISI 317L Stainless Steel Dissimilar Welded Joints

“…The formation of the ferrite phase with columnar morphology due to the solidification pattern and three primary austenite morphologies were observed in both conditions, which are grain boundary allotriomorphic, Widmanstätten and intragranular. These morphologies are typically found in duplex and super duplex stainless steels, agreeing with the literature 25,26 , and differ significantly from those observed for the austenitic stainless steel metal welds, including 317L 9,19,27 , because ferrite becomes more stable over a wide temperature range and the solid-state phase transformation from ferrite to austenite stars at a lower temperature.…”

“…Several works [8][9][10] , which studied dissimilar welding among austenitic stainless steels and other types of stainless steels, have reported that higher delta-ferrite formation in the weld metal associated with higher Mo concentration contributes to the precipitation of these deleterious secondary phases, leading to a decrease of the pitting and stress corrosion cracking resistance of the welded joint. Additionally, as the δ-ferrite is more easily electrochemical attacked than austenite, the corrosion process may be accelerated depending on the environment.…”

“…In this sense, an important issue has been the evaluation of proper filler metals for welds of Mo-rich stainless steels, such as the AISI 317L stainless steel, specified for critical applications in onshore and offshore oil and gas processing units, chemical and petrochemical plants, paper and pulp industries and other facilities.For that, an investigation of the filler metal chemical composition relation to the δ-ferrite amount and to the elemental partitioning in the weld metal and HAZ has been carried out to keep the corrosion and mechanical properties of the welded joint similar to the base metal. Recently, duplex stainless steels have been investigated as filler metal to weld Mo-rich austenitic stainless steels to mitigate metallurgical and mechanical problems that arise due to the secondary phases precipitations during welding of austenitic stainless steels [8][9][10][11][12][13] . However, no studies were found in the literature specifically regarding this application for the 317L steel.…”

Influência do metal de adição e do tratamento térmico de envelhecimento na susceptibilidade à corrosão sob tensão de juntas soldadas do aço AISI 317L

“…Para tal, uma investigação da composição química do metal de adição com relação a quantidade de ferrite δ e a partição dos elementos no metal na ZF e ZTA deve ser feita para manter as propriedades de corrosão e mecânica das juntas soldadas mais similar possível à do metal base. Recentemente, os aços inoxidáveis duplex têm sido investigados como metal de adição para soldagem de aços inoxidáveis austeníticos ricos em Mo numa tentativa de mitigar problemas mecânicos, metalurgicos e de corrosão que surgem devido à precipitação de fases deletérias tais como as fases sigma e Chi, durante a soldagem destes aços [6][7][8]. No entanto, não foram encontrados na literature, estudos relacionados especificamente a esta aplicação para os aços 317L.…”