Pulse frequency density modulation full bridge for induction preheating application of stainless-steel welding
Authors: Chakkuchan P. , Chudjuarjeen S. , Phankong N. 
Published: September 2024

The paper shows the problem of surface cracks from welding (TIG welding) to welding stainless steel pipe (heat-affected zone: HAZ). The study is an experiment of preparing welding with non-heating workpieces and preheating the workpiece using the principle of induction, heating with a fluid inverter circuit that can adjust power by frequency control, pulse frequency density modulation (PFDM) to maintain temperature for industrial.The control circuit is responsible for regulating the functioning of different devices and the speed of operation of the switch device. It divides the control into two closed loops: phase-angle feedback and current feedback. The phase-angle feedback loop ensures frequency tracking during the phase check angle, enabling the inverter to operate at frequencies higher than the resonant frequency throughout its operation. The process of arc welding was employed in the fusion of stainless-steel materials. Preheating is a crucial step in the welding process, as it serves to uphold the integrity of the weld and mitigate the occurrence of undesirable outcomes such as cracking and the subsequent requirement for rework. The Welding Process Specification (WPS) about the task at hand will delineate the lower and upper limits of preheating temperatures and the requisite period for preheating. High-quality products should be devoid of these imperfections and possess comprehensive welding reinforcement. Additional welding certifications encompassed a tensile testing procedure, a microhardness testing procedure, and a comprehensive microstructure analysis.Applying an alternating voltage to an induction coil generates an alternating current (AC) within the coil circuit. The induced currents exhibit a frequency identical to that of the coil current, although they possess an opposite direction to the coil current. These currents facilitate heat generation through the phenomenon known as the Joule effect. The temperature range of 250C-400C can be effectively regulated for preheating stainless steel by using high-frequency electric process heating in induction welding, hence achieving the desired welding preheat. There are two primary classifications for inverters: voltage-source inverters and current-source inverters.The main research limitation is comparing preheat and non-preheat. The size of the heat-affected zones is influenced by the rate of heating and cooling brought on by machining processes. By influencing the microstructural changes in that area, precise control of the variables can impact the integrity of the weld zones. The microstructural characteristics of the metal are different from the rest of the subject because welding heats the metal. The topic is heated up beforehand to ensure smooth welding and structural integrity. Because there is less of a temperature difference between the weld zone and the base material, preheating during welding results in less shrinkage stress. Distortions and crack flaws might result from higher shrinkage stress.NDT, or non-destructive testing, is a weld integrity test to find defects that occur in the weld without damaging that welding line and continuing to use it safely. For the welding line to be strong according to the design of that welding line (conformance to design), the test will use the principles of physical properties, such as light, X or gamma rays, magnetic fields, and high-frequency sound waves.In the research, stainless steel was successfully welded to preheat using induction heating (IH).