Effects of Normalizing Processes on Microstructure and Impact Toughness in Ti-bearing Weld Metal of Multilayer MAG Welded HSLA Steel

摘要

The influences of different normalizing heat treatments on microstructure, non-metallic inclusions and impact toughness in MAG Ti-bearing weld metal of HSLA steel has been studied. It has been shown that for the Ti-bearing weld metal the impact toughness after normalizing treatment decreases significantly against prolonging holding time and increasing normalizing temperature. The Mn-depleted zone forms around the Ti-bearing phase (MnTiO_3) precipitated on Mn-Si oxide. Proeutectoid ferrite preferentially nucleates at the Mn-depleted zone and the interface between austenite and proeutectoid ferrite becomes the nucleation sites for pearlite thereafter. Mn-depleted zone formation increases the pearlite nucleation sites, and makes the pearlite fine. The dissolve of Ti-bearing precipitate causes disappear of Mn-depleted zone at strong normalizing processes (longer normalizing time and higher normalizing temperature), and the number of ferrite nucleation sites decreases, then the pearlite become coarser, which causes the deterioration of impact toughness.