Study on Behavior of CO2 Gas Shielded Welding Joint of Ultrafine Grain Steel

This research paper mainly studied the microstructure and hardness of the weld bead and welding heat affected zone of ultrafine grain steel after CO2 gas-shielded welding. After normalizing and quenching heat treatment, the changes in microstructure and hardness of welded joints were studied. These experimental results showed that the microstructure of the weld bead of the welded joint was dendritic. The microstructure of weld bead was a mixed structure of proeutectoid ferrite, side plate ferrite, bainite, and pearlite. The microstructure of the overheated zone of HAZ under the as-welded condition was ferrite and pearlite with coarse grains, and the normalized zone was pearlite and ferrite with smaller grains than that of base metal. The microstructure of the normalized + quenched welded joint was a mixture of low carbon martensite, pearlite and ferrite, and the ferrite content was less. The hardness change of these welded joints was that the welded joint hardness after normalizing + quenching was significantly higher than that of welded joints before heat treatment.