Effects of multiple normalizing processes on the microstructure and mechanical properties of low carbon steel weld metal with and without Nb

A low carbon microalloyed S355 steel weld metal with and without Nb addition was subjected to normalizing process for different times and the microstructure as well as mechanical properties of the weld metal were evaluated and analyzed. The results showed that there were great differences between the microstructure and mechanical properties of the as-welded and the as-normalized weld metal, and the number of normalizing times played an important role on the microstructure and properties of the Nb bearing weld metal. The strength and plasticity of the as-welded weld metal were improved with the addition of Nb element, while the impact toughness was not affected significantly with the Nb addition. The normalizing treatment at 900 degrees C converted the columnar grain microstructure (CGZ) of weld metal in the as-welded state to an equiaxed grain structure, and the NbC precipitates were observed in the CGZ of the Nb bearing weld metal. The as-normalized weld metal had lower yield and tensile strength, higher elongation and impact energy than the as-welded weld metal. Different normalizing times were performed in order to study the effect of subsequent normalizing on the mechanical properties of the weld metal. With increase of the normalizing times at the normalizing temperature of 900 degrees C for 3 h, the size of NbC precipitates in the Nb bearing weld metal increased significantly, while the strength, elongation and impact energy first increased and then decreased. After three normalizing treatments, the optimal microstructure, strength, plasticity and toughness of the weld metal were obtained. After six normalizing processes, a duplex grain structure appeared, which deteriorated the properties of the Nb bearing weld metal. However, for the Nb free weld metal, the microstructure and mechanical properties did not change significantly with increasing normalizing times. (C) 2011 Elsevier Ltd. All rights reserved.