Comparison of the effect of thermal cycles on phase transformation and structure in the heat-affected zone of pipeline steel welding

In this research, X70 (API 5L grade X70)microalloyed steel of oil and gas transmission pipeline was used. In order to draw the temperature-time-transformation diagram, the classical heating and cooling thermal cycle was used. Analysis of the thermal cycles of the heat-affected zone was done based on practical conditions and a dilatometry device was used for simulation. By applying thermal cycles with peak temperatures of 950, 1150 and 1350 °C, the transformation behavior and microscopic structure of different heat-affected areas were studied. Analyzing the results of dilatometry, the kinetics of austenite formation (at different heating rates from 5 to 350 °C/s) and the kinetics of austenite decomposition (in two cases of constant austenite grain size with variable cooling and variable austenite grain size and constant cooling) were investigated. Modeling of austenite transformation was done using modified JMAK equation. It was observed that the “n” parameter does not have much dependence on temperature and is considered constant. The “k” parameter strongly depends on temperature, transformation amount and austenite grain size. Using the modified JMAK, a relationship was created that includes all these variables.