Microstructure and Toughness of High Current Density Gas Shielded Arc Weld Metals

Abstract

High current density gas shielded arc welding process has been applied successfully to vertical joints of oil storage tank as low heat input and high efficient welding process. The toughness of the welds by this process has to be improved for lower temperature applications. This can be achieved by conducting the fundamental research on the relationship between the microstructure, hardness and toughness by varying major alloying elements and microalloying elements in the weld metals.
Attentions were paid to classifying microstructures quantitatively, and the hardness levels were varied in the wide range.
The main results are as follows:
(1) Amount of acicular ferrite increase with increasing major alloying element for 180-225Hv of hardness levels, which results in improving toughness.
(2) Niobium and Vanadium increase ferrite side-plate microstructure detrimental to toughness, and increase hardness by precipitation of Nb carbide and V carbide for stress relief treatment, which brings about poor toughness. The effects of Nb are influential as compared to V.
(3) Boron increases acicular ferrite, consequently improves remarkably toughness. Stress relief treatment does not bring about precipitation hardening and fixes free nitrogen as BN precipitate, which is expected to result in improving toughness.