Abstract
Given the importance of microstructure on the mechanical properties of heavy forgings, the typical microstructure of as-cast 3.5%NiCrMoV steel was confirmed through the dissection of a 380 ton ingot as centimeter-scale grains. Subsequent investigation into the change of this grain structure during forging determined that temperature plays a predominant role in not only breaking up centimeter-scale grains into equiaxed grains, but also controlling the extent to which equiaxed grains are refined. This means that a fine grain structure with an average grain size of less than 60 µm is formed at 900 and 1000°C, whereas coarse grains are usually retained at 1100 and 1200°C. Moreover, at higher temperatures such as 1200°C, centimeter-scale dendrites are completely crushed by a reduction ratio greater than 30%. Grain size homogeneity is improved by dynamic recrystallization (DRX) at a reduction ratio of 10%, but these grains exhibit anisotropy when the reduction ratio is increased to 40%. Microstructural evolution was also found to be influenced by grain growth during reheating, with a significant increase in grain size not observed between 1000 and 1150°C, but grain coarsening occurring at 950°C. Taking into account the complexity and duality of this microstructural evolution, it is concluded that the deformation parameters and reheating conditions of heavy forgings need to be carefully controlled to ensure an appropriate microstructure is formed.
