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96419 PERFORMANCE OF HIGH MOLYBDENUM SUPERAUSTENITIC STAINLESS STEEL WELDS IN HARSH CHLORIDE ENVIRONMENTS

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Product Number: 51300-96419-SG
ISBN: 96419 1996 CP
Author: P. Stenvall, M. Li jas, B. Wallen
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Superaustenitic steels are normally welded with nickel-based alloys as filler materials. This practice has been subject to discussions regarding suitable fillers, dilution from parent material, unmixed zone, and their influence on corrosim resistance. To clarify the understanding of weld behaviou. in superaustenitic stainless steels this paper presents the development history of 6Mo and 7Mo steels, and results of Iagoratory tests and field tests on welds of UNS S3 1254 (6Mo) and UNS S32654 (7Mo) in different types of chloride containing environments, The laboratory tests consisted of the well known ferric chloride test (ASTM G 48 Method A). Shielded metal arc welds, gas tungsten arc welds and submerged arc welds in both grades were tested. The critical pitting temperatures were determined and the locations of the attack were noted. Some specimens were sectioned at the position of the attack followed by studies using light optical microscopy, The critical pitting temperatures of the welds in S31254 and S32654 were at normal levels for both grades. i.e. 40-50°C for S31254 anti 60-75°C for S32654. The locations of the attack differed depending on the welding process, In shielded metal arc welds the attack was mostly located in the weld metal. In gas tungsten arc welds the attack was predominantly located next to the fusion line, The field tests showed that the behaviour of welds and parent metal of superaustenitic stainless steels, as well as of nickel-based alloys, is much dependent on the corrosive envirc nment. In oxidizing chloride solutions, similar results to these of the ferric chloride test, are observed. However, crevice corrosion in the parent material is at a greater risk than pitting corrosion in the welds. In very oxidizing solutions of low chIoride concentrations, welds made of nickel-based fillers may corrode faster than the stainless steel base metal due to transpassive uniform corrosion. The opposite situation exists when active uniform corrosion prevails, i.e. welds made of nickel-based fillers corrode less than the stainless steel parent material. It should also be emphasized that corrosion failures in welds have rarely been a problem in practical applications of superaustenitic stainless steels. The general experience is thzt weld corrosion occurrs when alloys are used in conditions close to their corrosion limits. Keywords: UNS S31254, UNS S32654, nickel-based alloys welding, SMAW, GTAW, SAW, pitting corrosion, crevice corrosion, uniform corrosion, weld metal, fusion line, heat affected zone, chlorides.
Superaustenitic steels are normally welded with nickel-based alloys as filler materials. This practice has been subject to discussions regarding suitable fillers, dilution from parent material, unmixed zone, and their influence on corrosim resistance. To clarify the understanding of weld behaviou. in superaustenitic stainless steels this paper presents the development history of 6Mo and 7Mo steels, and results of Iagoratory tests and field tests on welds of UNS S3 1254 (6Mo) and UNS S32654 (7Mo) in different types of chloride containing environments, The laboratory tests consisted of the well known ferric chloride test (ASTM G 48 Method A). Shielded metal arc welds, gas tungsten arc welds and submerged arc welds in both grades were tested. The critical pitting temperatures were determined and the locations of the attack were noted. Some specimens were sectioned at the position of the attack followed by studies using light optical microscopy, The critical pitting temperatures of the welds in S31254 and S32654 were at normal levels for both grades. i.e. 40-50°C for S31254 anti 60-75°C for S32654. The locations of the attack differed depending on the welding process, In shielded metal arc welds the attack was mostly located in the weld metal. In gas tungsten arc welds the attack was predominantly located next to the fusion line, The field tests showed that the behaviour of welds and parent metal of superaustenitic stainless steels, as well as of nickel-based alloys, is much dependent on the corrosive envirc nment. In oxidizing chloride solutions, similar results to these of the ferric chloride test, are observed. However, crevice corrosion in the parent material is at a greater risk than pitting corrosion in the welds. In very oxidizing solutions of low chIoride concentrations, welds made of nickel-based fillers may corrode faster than the stainless steel base metal due to transpassive uniform corrosion. The opposite situation exists when active uniform corrosion prevails, i.e. welds made of nickel-based fillers corrode less than the stainless steel parent material. It should also be emphasized that corrosion failures in welds have rarely been a problem in practical applications of superaustenitic stainless steels. The general experience is thzt weld corrosion occurrs when alloys are used in conditions close to their corrosion limits. Keywords: UNS S31254, UNS S32654, nickel-based alloys welding, SMAW, GTAW, SAW, pitting corrosion, crevice corrosion, uniform corrosion, weld metal, fusion line, heat affected zone, chlorides.
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