Secondary Phases Precipitation in the 2510 Duplex Stainless Steel
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Secondary Phases Precipitation in the 2510 Duplex Stainless Steel. Arturo Reyes1, Irene Calliari2, Emilio Ramous2, Michela Zanellato2, Mattia Merlin3 1
Corporacion Mexicana de Investigacion en Materiales COMIMSA, Fracc. Saltello 400, Saltillo, Mexico. 2 Engineering Chemical Process Dept. DPCI, University of Padua, Via Marzolo 9, 35110 Padua, Italy. 3 Engineering Dept.,University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy.
ABSTRACT A lot of duplex and super duplex stainless steels are prone to secondary phases but with different sequence and kinetic which depend on the chemical composition and thermo-mechanical history of the steel. In this paper the results of secondary phase’s determination in a welding grade 2510 duplex steel, heat treated at 850-1050°C for 3-30 min are presented. The precipitation stars at grain boundaries with a consistent ferrite transformation for short times. The noses of the TTP curves are at 1000°C (sigma phase) and at 900°C (chi phase) with a partial transformation of chi to sigma, as evidenced in 2205 and 2507 grades. INTRODUCTION Duplex stainless steels are ferritic-austenitic steels that combine good mechanical and corrosion properties [1, 2]. Good mechanical properties (high strength combined with high toughness) are associated with the presence of a duplex structure with a good balance in the proportion of austenite/ferrite, which is usually about 1.However, owing to their susceptibility to formation of dangerous intermetallic phases, the use of duplex stainless steels have to be restricted, especially in the temperature range over 500 °C [3, 4]. These phase transformations can occur during isothermal ageing, welding or cooling after solubilisation and are mainly concerned with the delta-ferrite decomposition. Moreover the ferrite is enriched with chromium and molybdenum, which are known to promote the intermetallic formation. Given the increased use of duplex stainless steels, we need to gain a better understanding of those metallurgical factors which can affect the weldability. Usually duplex steels can be welded using welding procedures generally used for high alloyed steels [5]. The microstructure of the welded joint mainly depends on chemical composition of the weld metal, on cooling rate after welding and on post weld heat treatment. The cooling rate has a considerable influence on the final austenite-ferrite ratio and on the precipitation of secondary phases. Therefore during cooling a special care must be taken on the actual cooling rate in the temperature range between 900 and 500°C, the more critical temperature range for dangerous phase’s precipitation. Despite its very important technological implications, at now there is not a complete and definitive description of the secondary phase’s formation in DSS and several different precipitation sequences of various intermetallic phases (sigma-phase, chi-phase and carbide or nitrides) have been suggested. Some authors [6, 7] suggest that the precipitation of sigma-phase is associated with the precipitation of chi-phase, m
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