Combined effect of tungsten inert gas welding and roller expansion processes on mechanical and metallurgical characteristics of heat exchanger tube-to-tubesheet joints

Abstract

Tube-to-tubesheet joints are one of the major vulnerable locations prone to cracks in heat exchangers. The manufacturing processes of these joints have an important role in providing structural integrity to the heat exchangers. The main objective of this work is to study the impact of the combined effect of tungsten inert gas weld and expansion percentages of 3%, 5% and 7% on the structural integrity of carbon steel-based tube-to-tubesheet joints. The results show that the pull-out strength of hybrid welded and expanded using 3%, 5% and 7% expansion percentages has exceeded the tube axial strength. The minimum leak path of the welds was satisfactorily above two-thirds of the tube wall thickness. Vickers hardness was restricted at the adjacent regions of weld to below 250 HV. The microstructural studies indicate that the higher the expansion percentage, the smaller the grains at the inner tube surface and the higher the extent the fine grains formed from the inner tube surface. The effect of expansion on the grains at the inner and outer tube surfaces of the transition zone and the unexpanded zone was found negligible. The absence of grain refinement on the outer tube surface using light expansion at a 3% expansion percentage indicated that the contact pressure was inadequate on the tube-to-tubesheet interface. The hardness at the expanded zone and transition zone of the inner tube surface was higher than at the outer tube surface due to the plastic deformation caused by the intensive roller expansion pressure.

Keywords Carbon steel, Heat exchanger, Pull-out load, Roller expansion, Tungsten inert gas welding.