Manufacture of Specialty Stainless Steels through Triplex Route at Mukand

Abstract

Specialty stainless steels designed with higher levels of Chromium, Nickel and Molybdenum than the general austenitic grades AISI 304 and AISI 316 have distinctly superior corrosion resistance properties. The commercial production of such high alloyed stainless steels came with advent of improved steel melting, refining and casting technologies. These technological improvements made it possible to produce such steels with low carbon levels and close control of chemical composition necessary to achieve the desired phase balance and avoid formation of undesirable intermetallic phases. Further, ability to achieve low sulphur levels combined with freedom from undesirable level of tramp elements improved the hot workability characteristics. Thermodynamic aspects of steelmaking and refining in Vacuum Oxygen Decarburization ( VOD) process for manufacture of specialty grades 904L (24% Ni, 20% Cr, 4% Mo, 1.2%Cu) , 317LM ( 19% Cr, 14.5% Ni, 4% Mo,) , 2205 ( 22.5% Cr, 5% Ni, 3% Mo) at Mukand through the Triplex process route ( Ultra High Power Furnace Oxygen Top and Bottom Blown Convertor VOD) are outlined. The effects of undesirable intermetallic phases, particularly sigma phase, on mechanical and corrosion properties are discussed. Application areas for such specialty grades are reviewed. Keywords: High alloyed Stainless steels, triplex process, sigma phase, intermetallic phases, tramp elements, hot workability, corrosion properties, vacuum oxygen decarburization.