The aim of this thesis was to investigate densification and microstructural development of loose packed, 200-grade maraging steel, sintered with ferro-phosphorous additions. This was achieved by characterising the sintered density and hardness of compacts with various phosphorous concentrations, after sintering in nitrogen, argon and vacuum atmospheres at various temperatures. Additionally, samples with 1.00% phosphorous concentration were quenched at various points in the sintering cycle and the microstructure investigated using scanning electron microscope techniques, such that the phase evolution with increasing temperature could be determined.
After a period of constant heating, densification proceeds by liquid phase sintering when an initial liquid forms from a reaction of the Fe3P and residual carbon. Upon further heating, the initial liquid penetrates the maraging steel particles along grain
boundaries, and then significant densification ceases as the liquid dissipates in the matrix. With subsequent heating, a phosphorous rich supersolidus liquid appears at triple points and grain boundaries. The fractional coverage of the grain boundaries by this liquid increases as the temperature increases. Subsequent observations indicate that the particles fragment and the individual grains rearrange, resulting in rapid compact densification.
The onset of rapid densification, associated with fragmentation of the particles and rearrangement of the grains, was studied in terms of fractional coverage of the grain boundaries by liquid. This was performed using existing models of fractional coverage and liquid film thickness. If liquid film thicknesses are within the range of 0.164 to 0.250 µm, then the onset of rapid densification can be explained in terms of the fraction of liquid covering the grain boundaries.
development and densification in this alloy is dominated by the interaction between iron and phosphorous. This is to be expected as iron is the main component in the alloy and the iron-phosphorous binary system has a low temperature eutectic at 1048°C. Additionally, carbon influences the development of the initial and persistent liquid phases, especially their formation temperature.