On the mechanical alloying of novel austenitic dual-precipitation strengthened steels

Austenitic oxide dispersion strengthened steels were developed to serve as a structural material in power plants. Mechanical alloying is used with the aim to homogeneously distribute rare earth elements like yttrium inside a steel matrix to form strengthening nano-scaled particles. Due to the ductile nature of a face centered cubic austenite matrix, mechanical alloying results in a large amount of cold-welding of powder and the milling equipment, which significantly reduces the production yield and the reproducibility of the process. Here, a novel 2-step mechanical alloying process using nitrogen as a process control agent was developed, resulting in a production yield close to 100 %. Vanadium was added to the steel prealloy to sequester the introduced nitrogen and to promote precipitation of nitride precipitates. Optimizations of process parameters were suggested after characterizing chemical compositions, particle sizes, dislocation densities, crystallite sizes, phase compositions and harnesses. A comparison of X-ray diffraction and scanning electron microscopy results revealed a new approach to define an optimized milling duration. After hot-rolling, a successful dual-precipitate formation of nano-scaled oxides and nitrides, with mean diameters of 3 and 37 nm, respectively, was proven using transmission electron microscopy methods.