The objective of this project was to develop a Cr-free welding consumable for 300-series stainless steels that significantly reduces Cr6+ in the welding fume while maintaining corrosion and mechanical properties of the weld. The Cr-free filler metal composition and accompanying weld properties will replace conventional filler metals containing 18-20% by weight chromium.
Shielded-metal arc welding (SMAW) is widely used for fabrication and repair of stainless steel parts and structures in shipbuilding and other industries associated with national defense. Protecting welding personnel from the health risks associated with welding fume, which is a natural by-product of this process, is a priority. Welding fumes generated using conventional stainless steel welding consumables are particularly harmful, because they may contain hexavalent chromium (Cr6+). Chromium in this oxidation state has been shown to be carcinogenic, and its presence in the environment has been strictly regulated. Since conventional consumables for common stainless steels such as Type 304 contain up to 20% chromium, reducing the level of Cr6+ in welding fumes can be accomplished only by developing chromium-free (Cr-free) consumables. The Occupational Safety & Health Administration recently introduced new guidelines to reduce the Permissible Exposure Limit (PEL) for Cr6+ in welding fumes from 52 to 5μg/m3. This reduced PEL will be extremely difficult to achieve in many situations.
In the SERDP Exploratory Development (SEED) project WP-1346, investigators identified a composition of nickel, copper, and palladium (Ni-5Cu-1Pd) as a potential replacement for 18-20 wt% stainless steel filler materials based on corrosion and mechanical properties. This follow-on project developed a Cr-free consumable compatible with common 300-series base metals such as Types 304 and 316. To ensure that the consumable met or exceeded the properties of existing consumables, numerous corrosion, mechanical property, and weldability tests were conducted.
The final target weld metal composition that meets the design requirements for strength and corrosion resistance is nominally Ni-7.5Cu-1Ru-0.5Ti. This composition is achieved in the coated electrode by over-alloying the core wire with titanium (Ti); the core wire has 4%Ti whereas the deposited metal has only 0.5%Ti as most of the Ti is lost in the arc. This novel Cr-free welding consumable protects the workforce while still meeting the mechanical and corrosion performance requirements of the current consumables used to weld stainless steels on military platforms.
The development of a Cr-free consumable for welding 300-series stainless steels will allow the new lower PEL to be met using both SMAW and gas metal arc welding and reduce the need for expensive fume extraction equipment and other welder protection. The new consumable has sufficient corrosion resistance in most environments in which 300-series stainless steels are used. Furthermore, the mechanical properties and weldability are equivalent to the properties of current consumables used to weld these stainless steels.