L Journal of Alloys and Compounds 313 (2000) 168–173 www.elsevier.com / locate / jallcom Magnetic property changes during embrittlement of a duplex stainless steel a, b c * S.S.M. Tavares , M.R. da Silva , J.M. Neto a ˆ Departamento de Engenharia Mecanica, Universidade Federal Fluminense, Rua Passo da Patria 156, 24210-240 Niteroi, Brazil b ´ ´ Escola Federal de Engenharia de Itajuba ( EFEI), Itajuba, Brazil c ´ Instituto de Fısica, Universidade Federal do Rio de Janeiro ( UFRJ), Rio de Janeiro, Brazil Received 10 April 2000; accepted 1 June 2000 Abstract Changes in magnetic properties were used to investigate the ferrite decomposition that occurs in wrought duplex stainless steel (DSS) UNS S31803 at high (8008C) and low temperatures (4758C). At 8008C the saturation and residual induction, the coercive force and the differential permeability decrease with time, due to the increase in the austenite content. Firstly, secondary austenite (g ) forms in the a / a 2 grain boundaries causing embrittlement, but not hardening. Ferrite then decomposes into s and austenite phases ( a→s1g ), producing 2 hardening and severe embrittlement. During embrittlement at 4758C, only a small increase in the coercivity was observed, even on aging up to 500 h. On the other hand, the Curie temperature increased with aging time, as a consequence of spinodal decomposition ( a→a1a9 ). The results obtained show that the mechanical property changes of DSS UNS S31803 due to exposures at 475 and 8008C may be monitored by magnetic measurements.  2000 Elsevier Science B.V. All rights reserved. Keywords: Structural materials; Transition metal alloys; Phase transitions; Magnetic measurements; X-ray diffraction 1. Introduction lent corrosion resistance and mechanical properties (strength and toughness). Nevertheless, two embrittlement Duplex stainless steels (DSS) have been used in many phenomena may occur when the steel is heated: sigma industrial processes for special purposes in the chemical, phase ( s) precipitation in the 700 to 9008C range and petrochemical, fertilizer, and cellulose industries [1–3]. precipitation of the a9 phase, which may occur in the range Because of their austenitic-ferritic microstructure they 300 to 6008C. offer an excellent combination of corrosion resistance, Many authors [8–11] have studied the detrimental formability and mechanical properties [4–6]. One of the effects of s precipitation on the toughness and corrosion most used DSS of the last generation is UNS S31803, also resistance of DSS. The s phase occurs preferentially at the known as ‘2205’. It has the average composition 22% Cr, ferritic sites, where the concentrations of chromium and 5% Ni, 2.5% Mo, 0.03% C and 0.14% N (wt.%). Its molybdenum are higher. During s phase formation the microstructure can be described as islands of austenite in a surrounding regions become depleted in chromium and ferritic matrix. The steel solidifies as ferrite, and the molybdenum and transform into a secondary austenite austenite phase appears during thermomechanical treat- phase. This happens simultaneously in the eutectoid re- ment and cooling below about 13008C. The amount of action ( a→s1g ). The formation of an allotriomorphic 2 austenite phase increases with decreasing cooling rate. To austenite phase ( g ) at the a / a grain boundaries is also 2 achieve the best properties, wrought DSS may contain common [12], which is favorable when the austenite nearly 50% of each phase, which is obtained by appro- fraction present in the steel is lower than the equilibrium priate adjustment of the chemical composition and cooling content. rate [7]. In general, duplex stainless steels are not used to work In the solution treated condition the material has excel- in the range of temperatures where g or s occur (600– 2 9008C). However, as the kinetics of the reactions a→g 2 and a→g 1s are both fast, embrittlement in this tempera- 2 ´ *Corresponding author. Universite Joseph Fourier, Laboratoire de ture range may occur during hot forming or welding Cristallographie, B.P. 166, 38042 Grenoble Cedex, France. E-mail address: tavares@polycnrs-gre.fr (S.S.M. Tavares). fabrication processes. 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388(00)01062-8