•The overlap welded joint of 18Cr-9Ni austenitic stainless steel with leakage was analysed.•δ-ferrite correct phase composition.•No hot crack or microcracks in the weld.•Macroscopic examination revealed lack of fusion.•Weld shrinkage leads to displacement of the tubes relative to each other. Stainless steel X2CrNi18-9 (304L) radiators were analysed. During detailed examination we proved that tube overlap fillet joint was welded incorrectly and had leaks. Wrong designed jig, incorrect robot movements and non-central tubes position let mounted parts move during welding operation. Shrinkage of the weld caused the tubes to move relative to each other with the formation of a larger gap on one side. Lack of correction of the robot arm movement and fixed stick-out created inadequate fusion into the tube with a smaller cross-section. Identification of non-compliance based on standard NDT methods such as visual inspection, liquid or dye penetrant inspection or X-ray was not possible. Also, destructive methods such as high-pressure water test was not be able to show this type of defect. Only innovative HLT helium leak testing method was able to show non-compliance of the joint. Helium is the ideal element to detect micro cracks in weld as it is an insert gas made up to tiny atoms and has low viscosity, so little atoms can travel fast. This method has a very big potential in the welding industry as a precise and fast NDT test on highest quality requirements.