Modelling of austenite formation in weld heat affected zone of API-X65 and X70 pipeline steels

In the present work the two microalloyed steel (X65 and X70 ) used in oil and gas transition pipeline, was obtained as a hot rolled plate with accelerated cooling. First, weld heat affected zone thermal cycles, according to four-wire tandem submerged arc welding process were analyzed. The Baehr 805A/D dilatometer was used for weld heat affected zone thermal cycles’ simulation. The thermal cycles simulated process for heated region involved heating the steel specimens to the peak temperatures of 950, 1150 and 1350 °C and transformation behaviour and microstructure is investigated. By analyzing the dilatometry results, continuous heating diagram, austenite grain growth and austenite formation kinetics were investigated. Austenite formation modeling was done using Johnson- Mehl- Avrami- Kolmogorov (JMAK) classic equation. The parameter n was found to be relatively independent on temperature (or heating rate); While the parameter k is strongly dependent to temperature, phase fraction transformed and austenite grain growth