Two high-purity tungsten powders, producedviadifferent manufacturing techniques, were characterized to determine size distribution, morphology, thermal properties, and flow characteristics and, thus, the likely suitability for Laser Powder Bed Fusion (LPBF) production. Specimens from duplicate builds were produced with the two powders and characterized for density, defect mechanisms, and thermal penetration into the substrate plate to compare apparent power densities. The first powder was a chemically reduced powder with irregular morphology and the second, a plasma spheroidized powder with highly spherical morphology. The latter was found to have tighter morphological control and size distribution, having a third of particles at the modal particle size in comparison to a fifth of the chemically reduced powder. This led to better flow characteristics, and an increase of 1.5 g cm⁻³(1500 kg m⁻³) in the packing densities seen in the powder bed which corresponds to 57 pct theoretical densityvs50 pct theoretical density in the chemically reduced powder. As a result, the specimens produced from the plasma spheroidized powder had higher densities (97.3vs88.5 pct) and the dominant defect mechanism moved from lack of fusion dominated in the chemically reduced powder to cracking dominated in the plasma spheroidized. The plasma spheroidized powder also showed higher apparent power densities (effective absorptivities) as evidenced by an 80 pct deeper penetration of the laser into the substrate plate.

Large diameter steel line pipes are generally produced by cold deforming hot rolled or thermomechanically controlled rolled (TMCR) plate by the three-stage 'UOE' process. Pipe strength has been found to increase or decrease relative to the plate, depending on the steel grade and plate processing history. The strength increase is due to work hardening whereas any decrease in strength arises from the Bauschinger effect due to the reverse cold deformation of the UOE process. The steel chemistry, through the presence of strengthening microalloy precipitates, and prior processing, through the size and distribution of microalloy precipitates and the presence of retained work hardening, affect the magnitude of the Bauschinger effect. In the present work, the microstructures of two microalloyed (Nb and Nb+V) steel plates have been examined in terms of their grain size distributions, microalloy precipitate type, size and volume fraction to determine the major contributions to the plate strength levels. It has been found that both steels are primarily strengthened by a combination of grain size refinement and precipitate strengthening. Compression–tension testing has been carried out to determine the Bauschinger parameters for the two steels. The Bauschinger stress parameter, defined for the yield point (0˙1% of reverse deformation), was higher for the Nb+V steel (greater microalloying content) than the Nb steel. Transmission electron microscopy (TEM) investigations are being used to determine dislocation–dislocation and precipitate–dislocation interactions and how the spatial inhomogeneity in precipitate distribution affects the Bauschinger parameters.

This paper discusses the role of grain size distribution on the recrystallisation rates and Avrami values for a Fe-30 wt. % Ni steel, which was used as a model alloy retaining an austenitic structure to room temperature. Cold deformation was used to provide uniform macroscopic strain distributions (strains of 0.2 and 0.3), followed by recrystallisation during annealing at 850–950 °C. It was shown that the Avrami parameter was directly related to the grain size distribution, with a lower Avrami exponent being seen for a larger average and wider grain size distribution. A method to predict the Avrami exponent from the grain size distribution was proposed. In situ heating in an SEM with EBSD showed the recrystallisation kinetics to be affected by differences in stored energy and nucleation in the different grain sizes supporting the proposed relationship.