The pyrometallurgical reactions are characterized as a function of depth for underwater wet welding. The influence of weld metal compositional variations with depth on weld metal microstructure are described. Metallurgical suggestions as to compositional modifications to underwater wet welding electrodes to achieve the desired microstructure and properties are given.
Underwater welding has been recognized as a difficult and a potentially cost saving engineering task that becomes more important with the maintenance of existing marine structures. Masubuchi et. al. (1,2), Dadian (3), Magarajan and Loper (4), Hart (5) and Inglis and North (6) have reviewed the technologies, characterized the difficulties, and discussed the limitations of underwater welding. Specifications for underwater welding which describe the mechanical requirements that load bearing weld deposits must meet are available (12). Christensen et. al. (7-10) have introduced basic concepts for underwater weld metal chemistry and metallurgy. Their research was primarily on the influence of pressure on hyperbaric shielded metal arc weld metal compositions. Christensen's work (7) did suggest that similar weld metal compositional variations should occur in wet underwater welding. Madator (11) reported on the influences of parameters on the welding process and on the metallurgical reactions.
Christensen et. al (7–10) have shown that with increasing pressure or depth that weld metal manganese and silicon substantially decreased. The weld metal manganese experienced a 0.3 wt. pct. decrease from the surface composition at 30 bars (1000 ft.). Weld metal silicon decreased 0.10 wt. pct. with an increase of 8 bars or 250 ft. depth. Weld metal oxygen and carbon contents increase by factors greater than two when comparing composition of hyperbaric welds made at 1000 ft. to weld metal made on the surface. This manganese and carbon variation can cause significant change in hardenability of the weld metal. The weld metal oxygen went from an acceptable 300 ppm to a very questionable 750 ppm level. High weld metal oxygen has been related to inferior weld metal toughness (15). Mathisen and Gjermundsen (14) reported similar variations for weld metal manganese and silicon for wet welds made down to 70 meters. They also found little variation in carbon for the depth range that they investigated.
There is a major difference between wet and hyperbaric welding. Both experience increase pressure with depth (one bar for each 10 meter increase in depth), but the wet environment also influences the cooling rate during welding which affects the nature of the weld metal phase transformation. Wet underwater welding also introduces hydrogen into the welding arc and into the weld metal. Olson and Ibarra (13) addressed the influence of these compositional variations on the weld metal phase transformation and microstructure. They used metallurgical concepts, such as shifts in the continuous cooling transformation (CCT) diagram, as an instructional procedure to understand how the environmental factors of hydrostatic pressure will change weld metal microstructure and thus the weld metal properties. Increases in weld metal oxygen and silicon should promote weld metal inclusions and better nucleation conditions for weld metal ferrite. This effectively moves the e-shaped CCT curves to shorter times.
