Effect of heat input on the microstructure and toughness of heat affect zone of Q690 high strength steel

• 1、Key Lab for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University

Abstract：High strength low-alloy steel Q690 in quenched and tempered conditions was welded by gas shielded arc welding with different welding heat inputs. The influence of heat input on the microstructure, fine structures and impact toughness of the heat affected zone (HAZ) of the joint was investigated via scanning electron microscope, transmission electron microscopy and Charpy V-notch tests. The results indicated that the optimum impact toughness in HAZ was obtained at the heat input about 16 kJ/cm. The formation of lower bainite effectively restricted the size of martensite laths. The microstructure within austenite grains was refined and many large angle lath boundaries were formed, which contributed to enhancing the impact toughness in HAZ. Fibrous zones in fractured surfaces of impact specimens were characterized by dominant elongated dimples. Large cleavage steps were observed in the radical zone. With the heat input increasing, carbide particles paralleled to the habit planes of bainitic ferrite were formed. These carbide particles are detrimental to the toughness of the joint. Slip bands were shown in the fibrous zone. Coarsen carbides along upper bainitic ferrite held back the formation of dimples. The radical zone was composed of small cleavage facets due to more crack initiation sites provided by carbides.

Keywords： high strength steel welding heat affected zone microstructure toughness