Performance Implications of High Energy Density Welding of Corrosion Resistant Alloy Heat Exchanger Tubing

The advent of Laser Beam Welding (LBW) has had a significant impact on resultant metallurgical performance of stainless steel welded heat exchanger tubing. While productivity enhancements are well recognized the metallurgical benefits are not. The low total heat input combined with the very high energy density results in ultra-fast quenching of molten metal, super-cooling past normal primary ferrite solidification in common 304/304L and 316/316L grades directly to austenite resulting in weldments with no retained delta ferrite and some inherent corrosion advantages. The same low total heat input and high energy density generates dramatically smaller dendrite spacing making subsequent homogenization by the combined efforts of cold work and solution annealing dramatically more effective. It should be recognized that this paper deals with issues relating to tube production where welds are autogenous or made without filler metal addition and are subsequently cold worked and solution annealed. Resulting benefits of laser welding include a seamless metallographic appearance, and improved field performance. The benefits and limitations of these enhancements are explored relative to other common stainless steel grades, as are alloy characteristics affecting performance.