Controlled Rolled Low Alloy Powder Forged Steels

Abstract

Two low alloy powder forged vanadium steels of varying forged blank densities were controlled rolled in the austenitic range. Grain refinement of ferrite was aimed at, by processing the steels below the recrystallization temperature where nucleation takes place extensively on the elongated prior austenite grain boundaries. Effects of initial forged density and deformation on transformation were studied. The cooling rate effect was also examined by using four different cooling media (vermiculite, air, air blast, water) following the deformation. Rolling introduced deformation bands where ferrite nucleation occurred, which aided refinement of the transformed micro-structure. Also, densification achieved with increasing deformation led to almost pore free structures and improved grain refinement. Gas bubble formation observed in pore free zones in these hydrogen annealed powder forged steels was ascribed to a hydrogen gas reaction. Varying initial forged densities had no effect on specimen cooling rate. The lower density materials had higher ferrite transformation temperatures and a high ferrite content relative to fully dense materials. The composition of inclusions, whether oxide or sulphide and containing alloying elements in solution, determined to a large extent the behaviour of the inclusion during rolling. Alloying elements such as vanadium and nickel affect the deformation of inclusions since they aid grain refinement through precipitation and depression of the transformation temperature. Alloy carbides (mainly molybdenum carbides) form at inclusions and at grain boundaries, as carbide films. PM/0330