The Effect of Twice Thermal Cycle on the Microstructure and Mechanical Properties of Coarse Grain Region in X80 Pipeline Steel

Abstract

Based on the multi-pass welding characteristics of X80 pipeline steel, the influence of twice thermal cycle on the microstructure and mechanical properties of coarse grain region in X80 pipeline steel was investigated. The thermal cycles of weld coarse grain region with different peak temperature for the second thermal cycle were simulated with the Gleeble-3500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding optical micrographs and electron micrographs were systematically investigated to study the effect of the peak temperature on microstructure and impact toughness in the coarse grain region. The results of simulated experiment showed that the microstructure in heat affected zone of coarse grain region is granular bainitic and bainitic ferrite. When the peak temperature of the second thermal cycle is 800°C, the types of microstructure and the grain size of original austenite have no change. However, it forms network microstructure with chain structure in grain boundary and the reduction of toughness may be affected by the M–A constituents. With the peak temperature of 1000°C, the micro structure is composed of granular bainitic and a little bainitic ferrite. In this case, the grain size of austenite can be significantly fined, being helpful to increase the impact toughness.