When stainless steels are fabricated into structures, shielded metal arc welding is often used to join the stainless steel components. To ensure that these welds exhibit sufficient corrosion resistance, consumables containing filler metals rich in chromium (Cr) must be used. Evaporation and oxidation of chromium from molten weld pools results in the generation of carcinogenic hexavalent chromium (Cr6+) in the welding fume. It has been well documented that significant quantities of Cr6+ can form when using conventional stainless steel filler metals, resulting in a health hazard for the welder. Welding is a major capability used in all of the Services' depots, shipyards, arsenals, and air logistics centers. As much as 200 pounds per year of Cr fumes are generated at a single site from welding repairs.
The objective of this SERDP Exploratory Development (SEED) project was to demonstrate the viability of a novel approach for welding stainless steel using Cr-free welding consumables.
Considerable effort has been applied to the development of Cr-free stainless steel over the years. The most successful development has been iron-manganese-aluminum (Fe-Mn-Al) steels, which show improved corrosion resistance in chloride solutions over carbon steel. However, Fe-Mn-Al steel is unsuitable for use as a weld metal for standard Cr-containing stainless steels because of galvanic corrosion considerations. Galvanic interactions between the large area of stainless steel base metal and the small weld metal area dominate the corrosion properties of welds. The corrosion potential of the whole welded structure is set by the corrosion potential of the stainless steel base metal. This project focused on the development of a stainless steel welding process using a weld metal that is similar in composition to Monel, an alloy composed of nickel and copper, which is galvanically compatible with 304 and 316 stainless steel and has good corrosion resistance in seawater. Monel also met the two main design criteria that stated (1) the breakdown and repassivation potentials of the weld metal should be higher than the corrosion potential of the substrate to prevent localized attack of the weld metal and (2) the corrosion potential of the weld metal should be slightly higher than that of the substrate so that the weld metal is cathodically protected.
High-quality welds in 304L stainless steel were produced using Monel weld wire. The welds passed bend tests and exhibited mechanical properties comparable to welds made with standard 308L stainless steel consumables. The 304L/Monel welds survived exposure tests in mildly aggressive environments such as immersion in aerated 0.1 M NaCl for 2 weeks with no evidence of attack. Electrochemical data was collected to support these observations. The optimized composition for a new Cr-free consumable for welding type 304 stainless steel is Ni-(5-10)Cu-1Pd. The corrosion potential of this alloy in 0.1M NaCl was slightly higher than that of type 304L stainless steel. The localized corrosion breakdown of this alloy was slightly lower than that of a conventional weld made with 308L filler metal, but its repassivation potential was much higher. A follow-on SERDP project (WP-1415) is ongoing to further investigate the development of a Cr-free welding consumable for use on stainless steels.
Release of carcinogenic Cr fumes will be greatly reduced if a suitable Cr-free welding electrode and process can be developed, thereby reducing the human health hazard associated with shielded metal arc welding. (SEED Project Completed – 2004)