Investigation On The Effect Of Welding Variables On Dilution And Residual Stress Of Hyperbaric Weldfients
Authors: Das N.D. , Parmar R.S. 
Published: October 2008

This paper reports details about a mathematical model developed for predicting dilution as affected by different welding variables in hyperbaric welding. Experimental data was generated for a pressure range of 10 to 40 bars by pulse MIG welding of low carbon steel using the bead-on-plate technique. Four welding variables viz., mean current, nozzle-to-plate distance, speed of the welding and pressure of the chamber were considered for investigation. Mathematiee.1 model for dilution (%) has been developed 8.nd its adequacy tested and the significance of regression coefficients tested. The predicted results and their trends are presented in graphical form. Residua1, stresses measured as per ASTM standard using electrical strain gauge rosettes have been presented in graphical form and analysed. Conclusion arrived at have also been reported.

Increased activities in the off-shore oil exploration and nuclear power generation have led to the increased demand for a reliable fabrication technique which can be effectively used for installation and repairing of off-shore structures. Welding with its intrinsic advantages has offered itself as an obvious choice. Thus, the use of underwater welding techniques for fabrication and repair of off-shore structures and pipelines has increased considerably.

The process of underwater welding is broadly divided into two types viz. wet underwater and dry under-water or hyperbaric welding. Initially, wet underwater welding gained prominence but the demand for better performance from the welded structures has shifted the emphasis to dry underwater welding which can provide desired quality fields. In fact, distinctions are clearly drawn between wet and dry methods as regard their applicability - wet for emergency repairs, dry for initial installations and more elaborate repairs1. Hyperbaric welding that employs a bottomless chamber to enclose the welding site is carried out in a high pressure inert gas environment which can affect the characteristics of weld joints. In hyperbaric welding, there is an additional factor that can complicate hyperbaric weldments due to increasing pressure at water depths. At high pressure, temperature, geometry and stability of the arc can be changed and the metallurgical reactions taking place are influced2.

The study of dilution and residual stress of hyperbaric weldments is important as these parameters determine to a large extent the stress carrying capacity for a weld joint. Dilution is termed as the ratio of the cross? section of the weld metal below the original surface of the base metal, to the total area of the weld bead measured on the cross - section of the weld deposit (fig.1). The study of dilution is also important because it can also help in the determination of the gain or loss of deoxidisers like silicon and manganese.

The welding variables have far reaching effects an the dilution and residual stresses. With a view to achieving the aforesaid aim, factorial technique was used to develop mathematical model for dilution for accurate prediction of the effects of welding variables on dilution.