Effects of Normalizing Process on the Microstructural Evolution and Mechanical Properties of Low Carbon Steel Weld Metal with Niobium Addition

Abstract

The microstructure and mechanical properties of a Nb bearing weld metal under different normalizing processes had been evaluated and analyzed. The results showed that there was a great difference between the microstructure and mechanical properties of the as-welded and the as-normalized weld metals, and that the normalizing process played an important role in determining the microstructure and mechanical properties of a Nb bearing weld metal. The microstructure of the weld metal was converted from a columnar grain structure in the as-welded state into equiaxed grain, and the degenerated pearlite and NbC precipitates were observed in the weld metal after a 920 degrees C normalizing treatment. Corresponding to the microstructure, the normalized weld metal had lower yield and tensile strengths, higher elongation and higher -20 degrees C impact energy than the as-welded weld metal. With the prolonging of the holding time at the normalizing temperature of 920 degrees C, the grain size in the weld metal remained almost constant, while the size of NbC precipitate increased. The mechanical properties of the weld metal showed no obvious change with the increasing holding time. With an increase of the normalizing temperature, the quantity of the NbC particles decreased and the proportion of Widmanstatten ferrite microstructure in the weld metal increased, which caused the yield and tensile strengths to increase obviously, while the elongation and impact toughness decreased significantly. When normalizing at 1 200 degrees C, the NbC particles in the weld metal disappeared due to dissolution and the twin subplates were formed in the Widmanstatten ferrite.