The Formation of Intragranular Acicular Ferrite in Simulated Heat-affected Zone

Abstract

The preferential conditions of intragranular acicular ferrite (IAF) in heat-affected zone were studied by adding sulfur and calcium into a Ti-killed steel. Sulfur and calcium addition refined the austenite grain due to the increase in the number density of inclusions. An intermediate austenite grain favored the formation of IAF due to the prevention of Widmanstatten ferrite and polygonal ferrite. The inclusions changed from Ti-oxides dominant to Ti-oxysulfides dominant and Ti-Ca-oxysulfides dominant due to small addition of sulfur and calcium. The nucleation potential of inclusions was found to increase in the sequence of Ti-oxides, Ti-oxysulfides, and Ti-Ca-oxysulfides. This phenomenon could be attributed to the increase of the lattice disregistry between inclusions and ferrite matix or the decrease of strain field around inclusions. Both effects prevented the transformation of austenite to ferrite at elevated temperatures, thereby promoting the formation of IAF. The highest volume fraction of IAF was, therefore, obtained at an intermediate sulfur content with small amount of calcium due to the achievement of optimum nucleation potential and grain size.