Analysis of simulated welding thermal cycles S700MC using thermal imaging camera

In this paper an influence of simulated thermal cycle on properties and HAZ structure of 10 mm thick S700MC steel plates. The introduction of thermomechanically processed steels with high yield and relatively low carbon equivalent, will significantly reduce the time of welding works by reducing the preheating temperature, or even complete removal of this processing step, furthermore reduction of cross-sectional areas of structural elements, making welded structures with the same capacity will be more slender and lighter. The simulation of thermal cycles was carried out on a specially built test stand equipped with resistive heating source infrared camera VarioCam Head HR with 50 mm lens and a computer with software IRBIS 3 plus. Simulation was prepared for simple and complex thermal cycle. Simulation studies of thermal cycles consisted of resistive heating of samples prepared for the impact test. Single thermal cycles were simulated at temperatures ranging from 400 to 1300 °C, 100 °C and the cycle complex. For each temperature three repeats were carried out. Specimens were tested on impact, strength test and also hardness and metallographical tests. The study showed that there is a possibility to use a purpose built system to simulate simple and complex thermal cycles of S700 MC steel in specific ranges of the cooling time t_8/5. Analysis of the results of the study showed that the welding thermal cycle strongly influences the structural changes and phase in the HAZ zone of S700 MC steel. Areas of HAZ heated to high temperatures above 1000 [°C], show a sudden drop of toughness to unacceptable levels of impact strength (27 [J/cm²]). This sharp decrease in toughness is associated with uncontrolled separation processes of MX phases and dissolution of carbides, niobium and vanadium carbonitrides in austenite during heating. The study also showed that the chemical composition of steel and especially titanium and aluminium content is sufficient to bind in the HAZ free nitrogen and reduce the aging process. The control of the amount of heat introduced into the joint area during welding will reduce the adverse separation processes in the weld and HAZ which will ensure adequate toughness of the connection. Precise knowledge of the phenomena occurring in the HAZ during the thermal cycle can impact the ability to control properties and structure of the welded joint.