Hot deformation characteristics and processing map analysis of a new designed nickel-based alloy for 700 °C A-USC power plant
Introduction
The high-temperature material used for superheater/reheater tubes is the key to operating the 700 °C advanced ultra-super-critical (A-USC) power plant. These materials are required for high stress-rupture strength (>100,000 h rupture life at 100 MPa/760 °C) and high oxidation/corrosion resistance (≤2 mm cross-section loss in 200,000 h at 760 °C) [1], [2]. Up to now, only Nimonic 263, Haynes 282 and Inconel 740 can meet the basic requirement that the stress-rupture strength at 760 °C for 105 h is higher than 100 MPa [3], [4], [5]. However Nimonic 263 and Haynes 282 are in development for superheater/reheater tubes and have not been issued ASME code yet [3], [6], [7], [8]. The microstructure stability of Inconel 740 is not satisfying. Inconel 740H alloy, which is modified from Inconel 740, is a promising alloy for 700 °C A-USC application. However, it is still under development and test loop evaluation also [9], [10], [11], [12], [13], [14]. In other words, there is no fully-matured alloy which can be used as superheater/reheater tubes for 700 °C A-USC power plant up to now. Based on this, a new nickel-based alloy characterized with low contents of Co and Mo for 700 °C A-USC power plant applications was designed and successfully manufactured by metallurgical and hot/cold working processing. The implementation of low contents of Co and Mo in the new alloy can reduce cost, restrain the formation of harmful phases (such as μ, σ, η) and ensure excellent oxidation/corrosion resistance on the basis of high chromium content (wtCr%>20%) [15], [16]. Preliminary researches have been conducted on the new-designed alloy before the large-scale industrial manufacture. There are no harmful phases precipitating after aging for 3000 h at 760 °C, even 800 °C. In addition, the new-designed alloy characterizes with high stress-rupture strength, similar to Inconel 740, according to the stress-rupture strength tests at different temperatures and stresses [17], [18]. These experimental results show that the new-designed alloy characterizes with good microstructure stability and excellent rupture property.
In the new designed nickel-based alloy, the elements of Nb, Ti and Al make a great contribution to the formation of main strengthening phase-γ′ phase, Cr, Co, Mo and W are used as solution strengthening elements, and C is the grain boundary strengthening element. However, the high concentration of alloying elements leads to high deformation resistance, narrow hot working temperature as well as the difficulties of controlling microstructure in the hot working process, similar to many other nickel-based alloys [19], [20]. Usually, the microstructure of the alloy can be controlled by tuning the hot working parameters, such as deformation temperature, strain rate and strain. Thus, hot deformation characteristic of the new alloy was analyzed in this paper to optimize the hot working parameters and control the microstructure under various deformation conditions [21], [22]. The processing map can be a suitable approach to establish a relation between the deformation condition and microstructure evolution to optimize processing parameters without time-consuming and expensive trial-and-error method [23], [24], [25], [26], [27]. The processing map technology, based on the dynamic materials model (DMM) proposed by Prasad and Rao et al. firstly in 1983 [28]. It has been widely accepted that the processing map is an effective method to optimize the hot deformation processes and investigate the deformation mechanism since it can be easily constructed using the flow stress data from the ordinary uniaxial compression tests [29]. This method has been successfully applied in a series of materials such as magnesium alloy [30], [31], aluminum alloy [32], [33], [34], titanium alloy [28], [35], [36], stainless steel [29], [37], [38], [39], [40] and nickel-based superalloy [41], [42], [43], [44], [45], [46], [47].
In recent years, the processing map has been extensively accepted as a powerful tool for optimizing the hot working parameters and controlling the microstructures of nickel-based alloys. Jiang et al. [41] studied the hot deformation behavior and microstructural evolution also established the processing maps for Inconel 617B. The optimum processing parameters of good workability are obtained in the temperature range of 1120–1165 °C with the strain rate range of 0.01–0.1 s−1. Microstructure observations reveal that full DRX occurs in the optimum conditions. Zhang et al. [42] developed the processing maps for a typical nickel-based alloy. The optimum hot working parameters were identified, and the flow instability characteristics were validated by processing maps and micrographs. Shi et al. [43] established the processing maps of GH925 and determined the favorable hot deformation conditions as 1075–1150 °C and 0.01–0.1 s−1. Wen et al. [44] discussed the effects of initial aging time on hot deformation behavior for an aged nickel-based alloy by processing maps. The aged superalloy under 900 °C for 9 h or 12 h is suitable for the hammer forging process, and the optimum deformation window is 1010–1040 °C and 0.1–1.0 s−1. The aged superalloy under 900 °C for 9 h is suitable for the conventional die forging, and the deformation temperature should be controlled in 980–1040 °C with the strain rate lower than 0.1 s−1. By the processing maps, Sun et al. [45] studied the flow behavior and microstructural evolution of IN28 alloy. Two stable flow domains are identified as 950–1080 °C/<0.1 s−1 with maximum dissipation efficiency of 0.50 and 1120–1150 °C/0.1–1.0 s−1 with peak dissipation efficiency of 0.36. Zhang et al. [46] analyzed the hot workability and developed the processing maps of a γ′-hardened nickel-based alloy. The optimum processing parameters for good workability are obtained in the temperature range of 1105–1160 °C and strain rate range of 0.02–0.25 s−1. Guo et al. [47] established the processing maps to investigate the hot deformation behavior of Inconel 690, and the optimum hot working domain is identified as 1050–1200 °C and 0.01–3.0 s−1.
In this study, the hot deformation behavior of the new designed alloy after homogenization was investigated by isothermal compression tests and the constitutive equation as well as processing map was constructed in order to optimize the hot working parameters and control the microstructure under various deformation conditions during the large-scale industrial manufacture. The microstructure evolution was analyzed to validate the established processing map by the method of optical microscopy (OM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Much attention was paid on the multiple DRX process and mechanism corresponding to different hot deformation conditions appearing in various domains in the processing map. On this basis, the optimal hot working domain of the new nickel-based alloy can be finally proposed.
Section snippets
Material
The main chemical composition (wt.%) of the new designed nickel-based alloy for 700 °C A-USC applications in this paper is listed in Table 1. The material was manufactured by vacuum induction melting (VIM) and vacuum arc remelting (VAR) followed by homogenization at 1150 °C for 24 h and 1180 °C for 28 h. The microstructure of the as-received material is shown in Fig. 1, demonstrating uneven and extremely coarse grains with the average grain size more than 200 μm. Cylindrical compression
Flow behavior of the new designed alloy
The typical true stress–strain curves of as-received new alloy at different deformation temperatures and strain rates are shown in Fig. 2. As expected, the flow stress increases with the decreasing temperature and the increasing strain rate, indicating that the new designed alloy is sensitive to the deformation temperature and strain rate.
The shape of flow curve is the competition result of working hardening and softening effect caused by DRV and DRX [41]. The flow curves are generally composed
Conclusions
The hot deformation characteristics as well as the corresponding microstructure evolution of the new designed nickel-based alloy were investigated and the processing map also has been developed over the deformation temperature range of 1000–1200 °C and the strain rate range of 0.01–10 s−1. The following conclusions have been drawn from this investigation:
(1). The flow stress increases with the decreasing temperature and the increasing strain rate. The apparent activation energy of the alloy is
Acknowledgment
The authors would like to thank for the financial support from Shanghai Power Engineering Research Institute (SPERI) and the facilities provided by state key laboratory for advanced metals and materials, University of Science and Technology Beijing.
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