Q. Why we use Nickel Alloys in welding?
Nickel weld metals have high creep-rupture strength but often have lower stress-rupture ductility and lower mechanical and thermal fatigue strengths than wrought material. Placement of the welds should be where the least high temperature deformation occurs, and in low stress areas to minimize the effects of thermal and mechanical fatigue.
All welding slag should be removed as it may melt and become corrosive during high-temperature service. In oxidizing services, the slag becomes fluid and aggressively attacks the metal. In reducing services, the slag may pick up sulphur and cause failures by sulphidation in atmospheres where the sulphur content is low.
When dissimilar metals are welded the composition of the weld metal depends on the choice of filler metal, and the amount of dilution from each of the two base metals. Choosing a filler metal for a particular application first depends upon metallurgical compatibility. If more than one choice is compatible, the selection is then based on required joint strength, service environment, and on cost. Often the filler metal matching the lower alloy is considered adequate.