Microstructure transformation of X70 pipeline steel welding heat-affected zone

The continuous cooling transformation curve of heat-affected zone (HAZ) of X70 pipeline steel was measured by Gleeble-3500 thermal mechanical simulator, optical microscope (OM) and hardness analysis. The microstructure transformation rule at different cooling rates and solution behaviors of microalloy carbonitride during heating process of simulated specimens were investigated. When the cooling rate changes from 10 to 20 °C·s−1, microstructures at HAZ are identified as granular bainite, lathy bainite, and quasi-polygonal ferrite. This microstructure is featured with fine ferrite grains, martensite/austenite islands dispersed, high-density dislocations, and fine carbonitride particles, resulting in improving the strength and toughness of HAZ. With the cooling rate increasing to above 40 °C·s−1, the microstructure is predominantly coarse lathy bainite with clear primary austenite grain boundary. While the cooling rate decreases to below 1 °C·s−1, a fairly small amount of pearlite can be observed at the boundaries. The strength and toughness of HAZ are deteriorated because of coarse grains among these microstructures. Most of microalloy carbonitrides in HAZ could be dissolved in the matrix during heating process. A few of TiN particles existing as residues in the matrix can prevent austenite grain from growing, and then improve the strength and toughness of HAZ.