Conventional welding consumables used to join stainless steels are alloyed with Cr to produce welds with adequate corrosion resistance by promoting a passive oxide surface layer. Vaporization of Cr results in the formation of hexavalent chromium compounds in stainless steel welding fume. Government regulations in the United States and abroad are decreasing allowable exposure levels of hexavalent chromium to welding related personnel. A 2006 OSHA ruling reduced the permissible exposure limit of airborne hexavalent chromium from 52 to 5 micrograms/cubic meter. Achieving the new level may not be practical from an engineering controls standpoint during the fabrication of tightly enclosed stainless steel components. One method of addressing this problem is to implement a chromium-free welding consumable that provides equivalent mechanical performance and corrosion characteristics to current stainless steel welding consumables. This project was aimed at developing such a consumable and evaluating its suitability for replacement of current stainless steel consumables such as E308L-16. A new shielded metal arc welding consumable based on the Ni-Cu-Ru system was developed for austenitic stainless steel welding.
Mechanical properties of welds deposited with the new consumable were found to exceed minimum values of Type 304 stainless steel based on tensile testing. Hot ductility testing revealed a narrow crack susceptible region (33 to 54°C) indicating a low susceptibility to weld metal liquation cracking. A low ductility region was found at intermediate temperatures in the range of 800 to 1100°C. This ductility trough is believed to result in the weld cracking phenomenon known as ductility dip cracking (DDC). Threshold strain levels to initiate DDC were approximately 2 to 3% as determined by strain-to-fracture testing. Varestraint testing revealed that weld deposits have a higher solidification cracking susceptibility than stainless steel consumables used to join Type 304. This was attributed to fully austenitic solidification of the weld metal resulting in increased weld segregation and stabilization of a TiC eutectic reaction at the end of solidification. Solidification cracking susceptibility was shown to increase with dilution by Type 304L base metal.
Fume generation rates (FGR) of the new consumable were measured and bulk fume phases were analyzed with X-ray diffraction. FGR values were found to be similar to current SMAW and flux cored arc welding consumables. No chromium bearing compounds were observed during X-ray diffraction measurements, and the bulk fume consisted primarily of halides and metallic-oxides. Hexavalent Cr content (0.02 wt-%) was reduced by two orders of magnitude compared to E308-16 (2.6 wt-%). The source of this hexavalent chromium was from evaporation of the base metal due to the welding heat source. Fume produced by the new consumable was characterized with an Electrical Low Pressure Impactor to analyze the particle size and mass distributions for comparison to E308-16. This system showed that the fume size and mass distributions were dominated by particle agglomeration. Individual particles collected with this system were subjected to analysis with scanning and transmission electron microscopy. Cr-rich particles were not observed during fume analysis.