Constitutional Liquation of the Laves Phase in Virgin FB2 Steel The evolutionary behavior of the Laves phase during welding was examined Introduction Elevating steam parameters is the main key to enhancing the efficiency of fossil power plants to reduce fuel consumption and noxious emission (Refs. 1, 2). Therefore, a lot of new creep-resistant martensitic steels have been developed in the frame of the European Cooperation in Science and Technology (COST) program. Among them, FB2 steel is the most promising candidate to be widely used in turbine rotors operated at a temperature range from 600º to 650ºC (Ref. 3). FB2 steel is a forged, boron-added, and 9% Crcontaining martensitic stainless steel (Ref. 4). Welding is now widely applied in manufacturing large components operated at high temperatures to overcome the limit of forging capacity (Refs. 5, 6). Up to now, there is still a limited number of reports on welded FB2 steam turbine rotors, so it is necessary to carry out experiments on FB2 steel welding, gaining experiences to guide the practical production. On the other side, the Laves phase usually occurs in martensitic stainless steels after long-term, high-temperature exposure (Refs. 7–9), but there is still a limited number of reports about the Laves phase in 9–12% Cr martensitic steels in the as-received (or virgin) condition (Refs. 10, 11). Among the numerous martensitic stainless steels, CB2 is a cast steel with a similar chemical composition to FB2 steel, mainly used for steam turbine components. The occurrence of a micron sized (Fe, Cr)2Mo-type Laves phase in virgin CB2 steel was reported by Kasl (Ref. 11) and Jandová (Ref. 12). However, the evolutionary behavior of the Laves phase during welding and the influence of the Laves phase evolutionary WELDING RESEARCH behavior on weld joints of 9–12% Cr martensitic stainless steels have not been clarified in published articles. In the present work, some (Fe, Cr)2Mo-type Laves phase was also found in virgin FB2 steel, which led to constitutional liquation during welding thermal cycles, suggesting a liquation crack tendency in the heataffected zone (HAZ) of FB2 steel. Then the evolutionary mechanism of the Laves phase during heating and cooling was analyzed. This work is important in guiding the practical production of welded turbine rotors made of FB2 steel. Materials and Experiment Procedure The specimens for scanning electron microscopy (SEM) observation of the Laves phase were sampled from virgin FB2 steel (tempered at 700ºC for 4 h), of which the chemical composition is listed in Table 1. The specimens were electrolytic etched at 5V in oxalic acid for 10 s following mechanical polishing. Welding thermal cycle simulation was carried out on dilatometer DIL 805A, and peak temperatures (Tp) were 1150º, 1200º, 1250º, and 1350ºC with a holding time of 0.5 s. Both the heating and cooling rates were 100ºC/s. After thermal simulation, all the specimens were electrolytic etched followed by SEM observation. In specimens experiencing peak temperatures of 1350ºC, some eutectic structures with net-like morphology were found, and focused ion beam (FIB) was employed to sam- BY K. LI, Z. CAI, Y. LI, AND J. PAN ABSTRACT In 9–12% Crcontaining martensitic stainless steel, the Laves phase usually occurs after longterm, hightemperature exposure. However, in the present work, some sparse, relatively large particles of the Laves phase were observed in virgin FB2 steel (a new 9% Cr martensitic stainless steel). It is concluded that the large Laves phase particles formed during casting due to dendritic segregation. Then constitutional liquation resulting from a eutectic reaction between the Laves phase and (Fe) during welding was found, suggesting a liquation crack tendency in FB2 steel. Subsequently, hot ductility tests further confirmed the existence of a liquation crack. The evolutionary behavior of the Laves phase during welding thermal simulation was analyzed, and the Chi phase was found as a eutectic constituent during cooling. In specimens experiencing a peak temperature of 1350˚C, some grain boundaries were surrounded by eutectic microstructures, forming “ghost boundaries,” in which way the grain boundary strength was severely weakened. K. LI, Z. CAI (likejian07@163.com), Y. LI, and J. PAN are with the Department of Mechanical Engineering, Tsinghua University, Beijing, China. CAI is also with the Collaborative Innovation Center of Advanced Nuclear Energy Technology and the State Key Laboratory of Tribology, Tsinghua University, Beijing, China. JULY 2016 / WELDING JOURNAL 257-s KEYWORDS • Laves Phase • FB2 Steel • Constitutional Liquation • Liquation Crack • Chi Phase
Welding Journal | July 2016
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