Hotwire Tig welding with Inconel 625 (NiCrMo-3)

The Hotwire Tig process uses a (I²R effect) preheated welding wire that is added to the weld pool, this means that the weld pool is undergoing much less cooling and as a result offers higher welding speed with less dilution with the base metal. The process is often applied for internal claddings and can be atomized very well. Standard Mig/Mag wires are often used but are usually not made for this process because they have a lubricant on the surface that was developed for Mig/Mag welding. Ceweld® offers special versions of different alloys that are specialy made for Hotwire Tig welding, these wires are suitable for multi layer welding without intermediate cleaning between the layers because they are cleaned in a special chemical way. Inconel® 625.

1. Hydrogen Shielding Gas
 
The thesis is that the use of 98%Ar 2% H2 shielding for GTA Hot wire ERNiCrMo-3 “625” welding reduces defects and improves productivity. Most 625 defects are oxide inclusions. Hydrogen has a strong affinity for oxygen, so it acts like a flux to reduce oxides from part, wire and shielding gas contamination. There is industry resistance to use hydrogen gas for shielding as 625 overlay is specially intended to mitigate hydrogen induced cracking.
 
However, ERNiCrMo-3 is not a low hydrogen filler material at 8.5ppm hydrogen. Hydrogen in the wire accounts for 90% of the diffusible hydrogen in the weld, when welding with 98Ar 2%H2. Pre- and post-heat reduce diffusible hydrogen by 30%. In the worst case scenarios, with no pre-heat, immediate quench and holding in liquid nitrogen, there is only .9ml/100g in the weld metal. This is less than half of the most stringent published low hydrogen specification. Full weld procedure qualification tests were conducted on both argon & argon/hydrogen weld tests to demonstrate that mechanical properties are not affected. Finally, using a microscopic examination technique, the team has quantified weld quality to show that weld quality improves with hydrogen blends.