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

Abstract

Duplex stainless steel contains almost equal proportions of ferrite and austenite phases in its structure. Ferrite phase imparts more strength while austenite phase assures the toughness and better corrosion resistance. Duplex stainless steel has the combining features of two major classes of stainless steel, austenite and ferrite and thus makes it very attractive for numerous applications. These steel possess very good toughness, high corrosion resistance, low thermal expansion, high energy absorption, weldability and high strength compared to single phase austenitic and ferritic stainless steel; hence used in chemical, oil, petrochemical, marine, nuclear power, paper and pulp industries. In this present investigation, elemental powders of duplex stainless steel composition (Fe–18Cr–13Ni) with 1 wt. % nano yttria was milled in a specially built dual drive planetary mill (DDPM) for 10 h to fabricate yttria dispersed duplex stainless steel powders. The milled powder samples were characterized by X-Ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the size, morphology and phase evolution of powder during milling. The gradual transformation from ferrite to austenite is evident from XRD spectra during milling. It has been observed from SEM that particle size has been reduced from 40 to 5 μm after 10 h of milling. TEM shows the smallest average particle size of around 50-100 nm after 10 h of milling. A 10 h milled yttria dispersed duplex stainless steel powders were compacted by using uni-directional pressing at the pressure of 624 MPa. Powder compact of height 2, 6 and 8 mm were prepared and subsequently sintered at 1200C for 1 h under Ar atmosphere to study the effect of compact height on densification. It has been observed that there is not much variation in green density with compact height. In another set of experiment, powder compacts were prepared at the pressure of 250, 437 and 624 MPa and sintered at 1200C for 1 h under Ar atmosphere to study the effect of pressure on densification.