Effect of Intercritical Annealing on Microstructure and Impact Toughness of Ultra-Low-Carbon Steel after Welding Thermal Simulation

Abstract

In this paper, we compare the differences of impact toughness and microstructure between specimens with and without intercritical annealing after welding thermal simulation. The results show that after welding thermal simulation, the impact energy of the intercritical annealing specimen is 96.6 J, which is much higher than 5.7 J of the specimen without intercritical annealing. This is because after welding thermal simulation, the high-angle (≥ 45°) grain boundary density of the intercritical annealing specimen is 0.079 μm-1, which is nearly twice as high as that of the specimen without intercritical annealing. This grain boundary with high misorientation can deflect or arrest the crack propagation. Furthermore, the maximum size of the M-A constituents in the intercritical annealing specimen is 1 μm, which is half as large as that of the specimen without intercritical annealing, and the intercritical annealing specimen contains more retained austenite. During the deformation process, the M-A constituents with large size lead to stress concentration and promote crack initiation and propagation, while the small M-A constituents, especially those of size less than 1 μm can effectively inhibit the propagation of the cracks. In addition, the more austenite in the M-A constituents of the intercritical annealing specimen also plays an important role in improving the impact toughness.

Keywords: Microstructure, impact toughness, welding thermal simulation, intercritical annealing, grain boundaries