The objective of this paper is to present the detailed procedure of ultrasonic testing of welded joints using the modern phased array ultrasonic testing method and the conventional pulse-echo method. The beginning of the theoretical part of the paper refers to the most common division of non-destructive methods and their description. Also listed are the main characteristics, as well as advantages and disadvantages of each method. Furthermore, the paper describes the physical foundations of ultrasound, which represent the basis of the operation of the ultrasound system. The most commonly used equipment that makes up the ultrasound system is also listed, along with an explanation of its mode of operation. In the experimental part, on the regenerator (heat exchanger), the procedure of ultrasonic testing of the butt-welded V-joint was carried out using the multi-transducer method while, on the sub-assembly of the transformer, the ultrasonic testing of the butt-welded X-joint was carried out using the echo method. In addition, the ultrasound equipment used, as well as the calibration and system adjustment procedures, including the preparation of the materials before testing and the procedure of guiding the probe, are presented in detail. Furthermore, the principle of measuring the thickness of the material wall is presented on the example of the heat exchanger. Finally, based on the obtained results, an analysis was carried out and the testing report was presented.