Microstructure and Toughness Properties of Subcritically, Intercritically and Supercritically Heat Affected Zones in X80 Microalloyed Pipeline Steel
The influence of the real and simulated thermal cycles with different secondary peak temperatures on the properties of the reheated coarse grained heat affected zone (CGHAZ) in the X80 microalloyed steel has been investigated. The four wires tandem submerged arc welding process with different heat input values was used to generate real double passes thermal cycles. The simulated thermal cycles involved heating to the first peak temperature (TP1) of 1400 °C and then reheating to different secondary peak temperatures (TP2) of 700, 800 and 900 °C with cooling rates of 3.75 and 2 °C/s. The toughness of the simulated reheated CGHAZ with different peak temperature was assessed using Charpy impact testing at 0 °C and -50 °C. It is clear that the reheated CGHAZ thermal cycles with different second peak temperatures have a significant effect on morphology of the martensite/austenite (M/A) constituent. The blocky and connected M/A constituent along the prior-austenite grain boundaries as a brittle phase for crack initiation. The Charpy impact results indicated that intercritically reheated coarse grained heat affected zone had less absorbed energy with higher transition temperature and hardness. In the same prior-austenite grain size, cycles 2 and 4 with lower cooling rate (2 °C/s) have larger size of M/A constituents. The M/A constituent size such as mean diameter and length are important factors influencing Charpy impact properties of the simulated reheated CGHAZ.
S. Moeinifar "Microstructure and Toughness Properties of Subcritically, Intercritically and Supercritically Heat Affected Zones in X80 Microalloyed Pipeline Steel", Advanced Materials Research, Vols. 383-390, pp. 5886-5893, 2012