METALS AND MATERIALS International, Vol. 13, No. 5 (2007), pp. 411~416 Precipitation Behavior of σ Phase in Fusion Zone of Dissimilar Stainless Steel Welds during Multi-Pass GTAW Process Chih-Chun Hsieh 1 , Tao-Chih Chang 2 , Dong-Yih Lin 3 , Ming-Che Chen 1 , and Weite Wu 1,* 1 Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo-Kuang Rd., Taichung 402, Taiwan, R.O.C. 2 Electronics and Optoelectronics Research Laboratory, Industrial Technology Research Institute, 195 Section 4, Chung-Hsing Road, Chutung, Hsinchu 31040, Taiwan, R.O.C. 3 Department of Materials Science and Engineering, I-Shou University, 1, Section 1, Hsueh-Cheng Rd., Ta-Hsu Hsiang, Kaohsiung 840, Taiwan, R.O.C. The purpose of this study is to investigate the precipitation characteristics of σ phase in the fusion zone of stainless steel welds at various welding passes during a tungsten arc welding (GTAW) process. The mor- phology, quantity, and chemical composition of the δ-ferrite and σ phase were analyzed using optical micros- copy (OM), a ferritscope (FS), a X-ray diffractometer (XRD), scanning electron microscopy (SEM), an electron probe micro-analyzer (EPMA), and a wavelength dispersive spectrometer (WDS), respectively. Massive δ-ferrite was observed in the fusion zone of the first pass welds during welding of dissimilar stainless steels. The σ phase precipitated at the inner δ-ferrite particles and decreased δ-ferrite content during the third pass welding. The σ and δ phases can be stabilized by Si element, which promoted the phase transformation of δ→σ + γ in the fusion zone of the third pass welds. It was found that the σ phase was a Fe-Cr-Si intermetallic compound found in the fusion zone of the third pass welds during multi-pass welding. Keywords: σ phase, fusion zone, δ-ferrite, δ→σ + γ , multi-pass welding 1. INTRODUCTION Welding joints between austenitic and ferritic stainless steels with low carbon content are extensively utilized in many high temperature applications, such as energy con- version systems. For instance, in central power stations, sections of boilers subjected to lower temperature are made from ferritic stainless steel for economic reasons [1]. When stainless steels are exposed to high temperatures between 973 K and 1273 K for sustained periods of time, several undesirable intermetallic phases, including σ, χ, and π, can precipitate [2-6]. The σ phase is the most serious of these secondary phases with respect to its impact on the properties of stainless steels, e.g. on mechanical properties, corrosion resistance, weldability, etc [7-13]. It has been demonstrated that the addition of alloying elements, Cr and Mo in particular, can accelerate the precipitation of σ phase at high temperatures [14]. This also enlarges the precipita- tion area in the time-temperature transformation (TTT) dia- gram, as reported by Atamert and King [15]. The σ phase was first observed in the Fe-Cr system [l6] but also was found in Fe-V [17,18], Fe-Mo, [19] and Cr- Mo-Ni [20] metal systems. The crystallographic lattice of the σ phase is a tetragonal structure with 30 atoms in the unit cell [21-22]. The σ phase has been found in more than 50 binary transition metal alloys [23] and is of great interest for both scientific and technological purposes. Scientific interest is related to gathering knowledge of physical prop- erties of the phase, in particular those involved in the mechanism of its formation. Industrial interest originates from the fact that the σ phase often occurs in materials that are technologically important, and its presence drastically deteriorates their properties [24]. Previous studies on the properties of stainless steel con- taining σ phase concentrated on high temperature charac- teristics, and have considered the embrittlement mechanism [25], fracture toughness [26], and corrosion resistance [27] in the heat-affected zone (HAZ). Meanwhile, very few works have addressed the high temperature properties in the fusion zone (FZ). The precipitation mechanism of σ phase in the fusion zone during the welding process is unclear in austenitic and ferritic stainless steels. In the present study, the precipitation behavior of σ phase in the fusion zone of dissimilar stainless steels welds during the multi-pass GTAW *Corresponding author: wwu@dragon.nchu.edu.tw