Effect of Compact Height and Compaction Pressure on Densification of Tungsten Dispersed and Yttria Dispersed Duplex Stainless Steel Fabricated by High Energy Planetary Milling Followed by Conventional Sintering

Abstract

In the present investigation, elemental powder of Fe–18Cr–13Ni (all in wt. %) composition was used for the preparation of duplex stainless steel (DSS), tungsten dispersed and yttria dispersed duplex stainless steel (WDSS and YDSS) by adding 1 wt. % tungsten and 1 wt. % nano yttria to the composition respectively followed by 10 h milling in a dual drive planetary mill. The WDSS and YDSS milled powder were characterized by X-Ray diffraction, particle size analysis (PSA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the size, morphology and phase evolution during milling. The gradual transformation from ferrite to austenite is evident from XRD spectra during milling. The crystallite size and lattice strain of yttria dispersed duplex stainless steel after 10 h milling were found to be 7 nm and 1.1% respectively. The crystallite size of tungsten dispersed duplex stainless steel was 5 nm. It has been observed from SEM analysis that particles size has been reduced from 40 to 5 μm in both cases. Annealing of 10 h milled powder was performed at 750°C for 1 h under argon atmosphere to study phase transformation in both tungsten dispersed and yttria dispersed duplex stainless steel. The XRD analysis of annealed stainless steel depicts the phase transformation from α-Fe to γ- Fe with the formation of oxides of Fe and Cr in both cases. The differential scanning calorimetry analysis was conducted by heating the milled powder from room temperature to 1200°C under argon atmosphere to investigate the thermal analysis of both the stainless steel powders.
The milled powders were compacted by uni-axial pressing at pressures of 250, 437 and 625 MPa and sintered at 1200 and 1350ºC for 1 h under argon atmosphere to study the effect of compaction pressure and sintering temperature on densification and properties of the consolidated product. It has been found that with increasing compaction pressure and sintering temperature, density and hardness value increases in both cases. In another set of experiment, powder compacts of various heights of 10 mm diameter were prepared by unaxial pressing to study the effect of compact height on densification of the compacts. It has been observed that density increases, whereas hardness value of sintered product decreases with increasing compact height.