Duplex Stainless Steels-An overview

Abstract

Stainless steel is one of the most important materials in the engineering world. The material"s wide applications in chemical, petrochemical, offshore , and power generation plants prove that it is one of the most reliable materials. The Newest fast growing family of stainless steels is duplex alloys. The ferritic-austenitic grades have a ferrite matrix intermix with austenite and in other words island of austenite in a continuous matrix of highly alloyed ferrite commonly called "Duplex" stainless steel. Duplex stainless steel covers ferritic/austenitic Fe-Cr-Ni alloy with between 30% to 70 % Ferrite .Due to high level of Cr, Mo, and N steels shows high pitting & stress corrosion cracking resistance in chloride-containing environments. Hence it is frequently used in oil-refinery heat exchangers & typical applications where there is a risk for SCC and localized corrosion as a result of chloride-containing process streams, cooling waters or deposits. Modern duplex stainless steels have generally good Weldability. Due to a balanced composition, where nitrogen plays an important role, austenite formation in the heat affected zone (HAZ) and weld metal is rapid. Under normal welding conditions a sufficient amount of austenite is formed to maintain good resistance to localized corrosion where as too rapid cooling may result in excessive amounts of ferrite, reducing the toughness. Therefore, welding with low heat input in thick walled materials should be avoided. Welding methods, such as resistance welding, laser welding and electron beam welding, which cause extremely rapid cooling should also be avoided or used with extreme caution. Too slow cooling can in the higher alloyed duplex grades cause formation of inter-metallic phases detrimental to corrosion resistance and toughness. I. INRODUCTION Duplex stainless steels (DSSs), meaning those with a mixed microstructure of about equal proportions of austenite and ferrite have existed for more than 70 years. The early grades were alloys of chromium, nickel and molybdenum. The ternary phase diagram of the metallurgical behavior is shown in Fig.1. Dashed line shows solidification of typical duplex structure while red region denotes the mixture of ferrite & austenite. Fig.2 shows the microstructure of Duplex stainless steels. A section through the ternary at 68% iron illustrates that these alloys solidify as ferrite, some of which then transforms to austenite as the temperature falls to about 1000°C, depending on alloy composition[1]. Fig.1 Fe-Cr-Ni Phase Diagram There is little further change in the equilibrium ferrite austenite balance at lower temperatures. Thermodynamically, because the austenite is forming from the ferrite, it is impossible for the alloy to go past the equilibrium level of austenite. The main problem with Duplex is that it forms very easily brittle intermetalic phases, such as Sigma, Chi, R and Alpha Prime .Prolonged heating in the range 350°C to 550°C can cause 475°C temper embrittlement The first wrought duplex stainless steels were produced in Sweden in 1930 and were used in the sulfite paper industry. These grades were developed to reduce the intergranular corrosion problems in the early high-carbon austenitic stainless steels. Fig.2 Microstructure of Duplex stainless steel RESEARCH ARTICLE OPEN ACCESS