SOME ASPECTS OF WELDABILITY AND JOINTABILITY OF DUPLEX STAINLESS STEELS

摘要

Welding and joining of stainless steels, in particular of duplex stainless steels (DSS), has been one of the research lines of the Welding and Joining Group at Metallurgical and Materials Engineering Department in University of Sao Paulo since 1992. This paper is a brief report on the work done in this subject up to 2002. Weldability studies of multipass welding at low temperature HAZ (LTHAZ) as well as at high temperature HAZ (HTHAZ) and FZ (fusion zone) were conducted on samples of UNS S 32304, UNS S31803, UNS S 32550, UNS S32750 and UNS 32760 DSS. Microstructure, mechanical and corrosion properties in simulated (dilatometer and or Gleeble~(~R) equipment) and real welds were characterized using different techniques and tests. Intermetallic phase precipitation (sigma phase and chromium nitride) and secondary austenite were characterized and a model for secondary austenite precipitation in multipass welding of HTHAZ was proposed. Simulated samples were compared to real welds to validate the multipass thermal model and presented a very good agreement between them. Consequently the phase transformations studied by simulated samples represent real weld microstructure. The behavior of multipass FZ using filler metals (AWS E2259-17 and EN 25 9 4L) was also addressed. Intermetallic precipitation diagrams were determined for both filler metals and two DSS to compare reheated weld metal regions for three different welding procedures. The crescent heat input technique presented the best results. Brazeability of UNS S31803 DSS was also considered. Experiments were carried out in a hydrogen continuous furnace using three different nickel based filler metals (AWS BNi-1, BNi-2 and BNi-7). Several brazing conditions were tested using different brazeability tests such as the sessile droplet test, the edge test, and the capillary raise test. Best wetting and spreading results were obtained for BNi-7 for the equipment used in the experiment, but brazing thermal cycling in such furnace impaired base metal corrosion resistance due to intermetallic phase precipitation.