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High temperature aging method was used to accelerate the aging process of 12Cr1MoV steel which is mainly used as pressure vessel, steam pipes and so on. The evolution of the microstructure and mechanical property was systematically investigated during the process. It is revealed that during the aging process, pearlite was spheroidized gradually and carbide gathered at the grain boundary. The rate of pearlite spheroidization gradually reduced with the high temperature aging time increasing. Based on the hardness measurement and the analysis of the microstructure, the service life of the material was predicted through the combination of the hardness value and the Larson-Miller parameters. Introduction 12Cr1MoV steel is widely used as boiler superheater, steel box and main steam pipes under high pressure, super high pressure and subcritical pressure [1-4]. With high oxidation resistance and thermal strength, the creep limit of 12Cr1MoV steel is very close to the strength value with high plasticity in the case of long lasting tension. 12Cr1MoV steel has high hot strength and oxidation resistance and high durable plastic even at 580C. In addition, the processing and welding property of the steel are well. However, in the long-term use at 580C, the pearlite in the microstructure of steels will be spheroidized, that is, the shape of the Fe3C in the pearlite gradually transforms into granular carbide, mainly including the fracture of the lamellar structure and grain coarsening [5]. In addition to the structural defects of itself, there are other factors affecting the rate of the spheroidization, such as pearlite spacing and alloying elements, which can not be ignored [6,7]. With the increase of pearlite spheroidization and the dilution degree of alloying elements in solid solution, mechanical properties of the steel decrease gradually resulting in material deterioration even failure [8,9]. For the power station boiler steel [10], the life of the steel materials is usually predicted through the combination of the non destructive testing (NDT) and Larson-Miller parameter (LMP) formula [11-13]. Hardness indicates not only the ability of the material to resist deformation, but also the damage of the material in the long term. There is relationship between the hardness tests and creep resistance [14]. H.C. Furtado [15]studied the relationship between hardness and LMP of the 9Cr-Mo ferrite steel and found a method to evaluate the injury degree of the material based on hardness test. The formula HV=961.713-0.020669P was used to describe the relationship between the hardness and LMP[16]. Therefore, the remaining service life of the material can be obtained by the change of hardness data, when the temperature and pressure are known. However, the microstructure imparity at different regions of materials may lead to abnormal value of material hardness. In order to well characterize the damage of material, hardness measurement and microstructure observation should be both considered. Thus, it is very important to investigate the relationship between the microstructure and hardness to predict the remaining service life of 12Cr1MoV steel. In this paper, we systematically investigated the microstructure and hardness of 12Cr1MoV steel during different aging processes. Together with the microstructure characterization, the service life of the steel is predicted based on the analysis of the material hardness. 0195 4th Annual International Conference on Material Science and Engineering (ICMSE 2016) © 2016. The authors – Published by Atlantis Press Experimental Materials and Methods The material is the new tube without use provided by the Tianjin special inspection institute. The composition of the material was detected by ARL4460 spectrometer and HX-HW8B carbon sulfur detection instrument. Components were as follows: 0.119C, 0.232Si, 0.6Mn, 1.02Cr, 0.264Mo, 0.011Cu and a very small amount of P and S, and the residual component is Fe. In order to investigate the influence of temperature and aging time on the spheroidization degree of 12Cr1MoV, the samples were heated at different temperature and aging time. 12Cr1MoV was put into the resistance furnace for the aging heat treatment. The aging time was set as from 2 to 168 h at 500, 550, 600, and 650C, respectively. Then the specimen was moved out under the air cooling. The microstructure and morphology of the materials were characterized by OLYMPUS BX41M optical microscope, S4800 scanning electron microscope, and SU1510 scanning electron microscope. According to the national standard, the spheroidization degree of 12Cr1MoV is divided into 5 levels: non-spheroidization (level 1), mild spheroidization (level 2), moderate spheroidization (level 3), complete (level 4) and serious (level 5) spheroidization. Level 1: pearlite (bainite) region is obvious, the carbide in pearlite is not all of the lamellar, and there is a gray block area. Level 2: the aggregation morphology of the pearlite region has begun to disperse. But its composition is still relatively compact, and the pearlite maintains the original shape of the region. Level 3: the carbide in pearlite region has been significantly dispersed and transferred into small balls, but the original shape of the region still maintains. Level 4: most of the carbide has been dispersed in the ferrite grain boundaries, and there is only trace of pearlite region. Level 5: pearlite regional morphology has completely disappeared, and carbide particles are distributed in the ferrite grain boundaries with dimorphism. Vickers hardness (HV) test was done through the HMV-1000TMB video display automatic rotating tower microhardness tester with the test loading of 50g and loading time of 5s, and the pressure head was regular pyramid type diamond indenter. Results and Discussion Figure 2 shows the metallographic images of the 12Cr1MoV specimens heat-treated at 550, 600, and 650C for 32, 118, 168 h, respectively. In the initial state (Fig.1), the specimen contains black and white phases. The white phase is ferrite as the matrix of material, and the black region is the pearlite phase. Because the content of carbon in the sample is low, the pearlite is evenly distributed in the matrix, which is mainly located at the intersection of the grain boundary and the grain boundary. Fig.1, Metallographic images of the initial samples Compared with the initial microstructure, there is no obvious difference of the distribution of pearlite in the specimens heat-treated for seven days (168h) at 550 C. According to the law of diffusion, the diffusion of carbon needs to reach certain diffusion activation energy. The holding temperature of 550 C is low and close to the actual working temperature. With shorter aging time compared with the actual working time of the steel, there is no apparent diffusion carbon, and thus, 0196 4th Annual International Conference on Material Science and Engineering (ICMSE 2016) © 2016. The authors – Published by Atlantis Press the morphology of the pearlite changes little in Fig.2 (a.b.c). In contrast, after heat treated at 600 C for 168 h, pearlite zone is reduced, and the distribution density of carbides decreases and becomes sparse in Fig.2f. Since the temperature has a significant impact on the diffusion of atoms in the materials, under the same holding time, with the temperature increasing by 50 C, atomic thermal motion of carbon increases. Thus, the diffusion coefficient of carbon increases significantly, and the diffusion rate becomes fast. As a result, pearlite spheroidization is obvious. When the temperature further increases to 650°C, pearlite area and distribution density of carbides obviously decrease after only 32h heat treatment in Fig.2g. With the holding time increasing to 118h, the pearlite area almost disappears. Carbides distributes at the grain boundaries with only a small amount of carbides distributing in the original pearlite area in Fig.2h. With the holding time further increasing, carbide gathers at the grain boundaries, and there is large granular carbides in Fig.2i. Fig.2, Metallographic images of the samples heat-treated at different temperature and time. (a) 550 °C, 32 h; (b) 550 °C, 118 h; (c) 550 °C, 168 h; (d) 600 °C, 32 h; (e) 600 °C, 118 h; (f) 600 °C, 168 h; (g) 650 °C, 32 h; (h) 650 °C, 118 h; (i) 650 °C, 168 h. The microstructures of the samples were further analyzed by scanning electron microscope, as shown in Fig.3. Metallographic images of the samples reveal no obvious change at 550°C. More detailed observation through SEM images shows that the pearlite undergoes significant changes. With the holding time of 38h, the lamellar structure significantly reduces (Fig.3a). There is short rod and little granular carbide. Thus, the pearlite spheroidization begins, namely mild spheroidization (level 2). With the holding time increasing to 118h, the pearlite lamellar structure gradually disappears, showing more short rod and granular carbide, and the amount and area of the pearlite distribution decreases (Fig.3b). When the heat treatment lasts 168h, the carbide distributes in the long strip near the grain boundary, and the carbide at the grain boundary increases. The pearlite spheroidization stage is between the level 2 and 3 (Fig.3c). The microstructure of the specimen heat treated at 600 °C for 38 h exhibits that the pearlite lamellar structure completely disappears with the short rod and granular carbide distributing in the original pearlite region(Fig.3d). The microstructure is similar to that of the sample heat treated at 550°C for 168h, confirming the significant impact of holding temperature on the diffusion of carbon. (a) (b) (c)

Pearlite Spheroidization Mechanism and Lifetime Prediction of 12Cr1MoV Steel used in Power Plant