Evolutions on Microstructure and Impact Toughness of G115 Steel after Long-Term Aging at 700 °C

Abstract

The microstructure and impact toughness evolution of G115 steel after long-term (ranging from 500 h to 10,000 h) aging at 700 °C were investigated in this study. The results showed that the microstructure of the G115 steel evolved from a finer-grained matrix with minor precipitates to a coarse-grained matrix with more precipitate with aging time, presenting a decrease in the local deformation degree in the matrix. The impact toughness of the steel decreased with aging time, presenting the largest decline at the initial aging times. The decrease in impact toughness was attributed to the coarsening of precipitates (M₂₃C₆ and Laves phase) in the steel matrix. The stable impact toughness during the whole aging process (from 500 h to 10,000 h) should be related to the comprehensive effects, including the precipitation of the Laves phase, the increase in high-angle grain boundaries, and the softening of the metal matrix.

1. Introduction

Reducing CO₂ emissions and improving the efficiency of coal-fired power plants are crucial for promoting the development of the power industry [1,2,3,4]. Ultra-supercritical (USC) power plants as the focus of research and development around the world are an efficient method, involving coal-fired power with an operating temperature ranging from 650 °C to 750 °C [5,6,7,8,9]. However, coal-fired power plants mostly operate at temperatures below 600 °C, which poses a challenge for the traditional 9–12%Cr ferritic steels used in thick-section boiler components for USC [10,11,12,13,14]. Additionally, 9–12%Cr ferritic steels present excellent resistance to high-temperature oxidation, which is mainly attributed to the formation of Cr₂O₃ in the oxide layer [1,3,6,8,14]. It is necessary to simultaneously consider the antioxidant performance and high-temperature mechanical properties during the operation of the USC [4,9,10,11]. ASME-grade T/P92 (9Cr-1.8W-0.5Mo-NbV) is a specific version of 9%Cr heat-resistant steel [13,14]. V and Nb were added to enhance its creep strength, and the Cr content was raised to have better oxidation resistance [15]. Due to the elevated power generation parameters (high temperature and high pressure), there is often a problem of toughness reduction caused by coarse grain size. On the basis of traditional 9Cr steel, by increasing the W content, no more Mo was added, and B, Co, and Cu were added, and excellent creep resistance at high temperature was ultimately achieved [11,15,16,17,18,19,20,21]. G115 martensitic steel (9Cr-3W-3Co steels) is known for its excellent mechanical properties, including high strength, high toughness, and good resistance to corrosion and fatigue. The addition of W and Mo as well as optimization of other alloying elements offer T/P92 steel higher creep rupture strength compared with the older T/P91 steel. Steam generators and their components are often subjected to repeated thermal stress as a result of the temperature gradients that occur during start-ups and shutdowns or during temperature transients. Thus, the risk of fatigue failure for the G115 steel increases under plastic deformation conditions, especially at high temperatures, which can have a significant impact on the structural integrity and reliability of components made from this material [11,15]. The effects of alloying elements and applied temperature on the strength of G115 steel have been reported, and it was found that the formed Laves phases [11,14], M_xC_Y (M-Cr and Fe elements) [11,14,19,20,21] and MX (M-Ni and Ti elements) [22,23,24] precipitates present combinations of various amounts and sizes, which can be sites of local fracture or key obstacles to the dislocation movement. However, the toughness evolution and the mechanism of G115 steel under long-term aging conditions at ultra-high temperatures (650–750 °C) is not clarified, which is one of the critical factors affecting the high-temperature operation process. It is necessary to explore the relationship between microstructural evolution and mechanical properties in G115 steel under various aging conditions.

This study investigated the evolution of microstructure and toughness in aged G115 steel. Figuring out the effects of the aging process on the toughness of the steel is crucial for material selection and component design, especially in applications for USC, where a balance between the service temperature and toughness is required.

2. Materials and Experiments

The experimental G115 steel was obtained after the casting and the hot forging processes, and its chemical composition is made up of C 0.08, Si 0.34, Mn 0.55, Cr 8.98, W 2.72, Co 3.01, V 0.21, Nb 0.05, Cu 0.96, and Fe balance (Central Iron & Steel Research Institute, Beijing, China). Before tests, the G115 steel as the base material (BM) was normalized at 1100 °C for 2 h with air cooling, and then tempered at 780 °C for 2 h with air cooling. After the gradual grinding and polishing processes with abrasive papers and 0.5 μm diamond paste, the microstructures of specimens after aging at 700 °C for 500 h, 1000 h, 3000 h, 5000 h, and 10,000 h were characterized by an optical microscope (OM) and a field emission scanning electron microscope (FESEM). The grain size of specimens after the aging processes was measured by Image-Pro Plus 6.0 software in SEM images. After the electrolytic double-spray process in the solution mixed with 10 vol.% perchloric acid and 90% alcohol, the electron back-scattered diffraction (EBSD) technique was used for characterizing the proportion of the low-angle grain boundaries (LAGBs), high-angle grain boundaries (HAGBs), the phase composition, the proportion of recrystallization microstructure, and the local plastic deformation in specimens after long-term aging processes with different times. The M23C6, MX, LP particles, and the lath martensite were characterized in the thin foils of G115 steel by using the TECNALG220 (Eindhoven, The Netherlands) field emission transmission electron microscope (FETEM) at 200 kV, and the testing specimens were prepared through the same method with the EBSD test.

As shown in Figure 1, Charpy impact tests were performed on the V-shaped notch specimen in accordance with the GB/T 229-2007 standard [25] by using a Charpy impact tester (ZBC2452-CD, SANS, Shenzhen, China) with a 200 kg load, and all tests were maintained at room temperature (20–22 °C). The impact toughness (K_V) was calculated using the following formula [25,26]:

$$K\mathrm{v}=(P\times L)/\left(2W\right)$$

(1)

where P is the load at failure, L is the length of the specimen, and W is the width of the specimen. For each test after the aging process, three specimens were carried out and the average value of the data was obtained. The neck portions of ruptured specimens were observed by FESEM to illustrate the fracture mode.

3. Results

3.1. Microstructure

Figure 2 displays the metallographic microstructure of steels after etching in a mixed solution consisting of FeCl₃ (5 g), HCl (15 mL), and H₂O (80 mL) for 45 s. Obviously, the steel matrix was composed of lath martensite, including the prior austenite grains (PAGs). With increasing aging time, the grain grew and coarsened with the increasing width of lath martensite [16,19,27]. It can be observed that both the grain size and the width of lath martensite increased for specimens after the aging processes from 500 h to 10,000 h. The increasing size of PAG gradually led to unclear grain boundaries and the prolongation of lath martensite.

The grain size evolution of PAG in G115 steel after different aging times is shown in Figure 3, which was measured at three zones containing about 60 grains. It can be found that the grain size of G115 steel is basically the same among the BM, the steel after aging 500 h, the steel after aging 1000 h, and the steel after aging 3000 h, and the average grain size of specimens ranges from 25.5 μm to 26.6 μm. After 5000 h aging time, the average grain size of G115 steel increased obviously to about 30 μm, corresponding to the broadening behavior of lath martensite [19,28,29]. Further, the average grain size of the G115 steel after 10,000 h aging time is about 32 μm, which indicates that the aging process promoted the growth of PAG in G115 steel.

Figure 4 presents the SEM images of G115 steel after different aging times. It can be found that the length of lath martensite was limited by the grain size of PAG. The lath martensite gradually grew with the increase in aging time, which should be related to the reduction in dislocation density during the long-term aging process [7,18]. As for the specimens before 3000 h, the martensitic plates can be observed in PAG, presenting different degrees of broadening. Meanwhile, the same polygonal sub-grain structures can be found in PAG for the specimens after 5000 h and 10,000 h aging time. It has been recognized that the stability of the lath structure mainly depends on the pinning effect of dislocation and fine precipitates, which need further discussion compared with EBSD and TEM results.

Figure 5 shows the magnified back-scattered electron (BSE) images of each specimen, which was used for characterizing the precipitate and second particles in G115 steel after the long-term aging process. According to the mean size of MX (20–50 nm), M₂₃C₆ (100–400 nm), and Laves phase (100–400 nm) particles and the distribution sites [11,15,21], the particles can be roughly identified in steel. It can be found that M₂₃C₆ carbides were the main composition in all steels, which were mainly distributed at the grain boundaries and the martensitic lath boundaries. The presence of M₂₃C₆ carbides showed that these carbides became coarser and more distributed throughout the microstructure as aging time increased [12,13,14,15,16,17]. The microstructure of G115 steel is initially characterized by a fine-grained matrix. However, as the steel is aged at 700 °C, the precipitates within the matrix begin to grow and coarsen. The coarsening of precipitates was attributed to the diffusion of alloying element atoms and the thermodynamic drive, which achieved a more homogeneous microstructure. The growth of precipitates led to more extensive precipitate coverage in the matrix, which can potentially enhance the strength of the material but may compromise its toughness. It has been reported that M₂₃C₆ is Cr₂₃C₆ and the Laves phase is mainly Fe₂W [15,18]. Due to the heavier atom weight of W than Cr, the Laves phase is brighter than M₂₃C₆ in BSE images, which can be used to distinguish the Laves phase from M₂₃C₆ particles [11]. The growth of M₂₃C₆ and the Laves phase at grain boundaries during the aging process should influence the impact toughness, which needs further discussion.

3.2. EBSD Characterizations

To investigate the orientation of microstructures and the recrystallization degree for the BM and the G115 steel after 3000 h, 5000 h, and 10,000 h aging times, the EBSD maps are shown in Figure 6. The random orientation of microstructures was found in the inverse pole figures (IPFs) for all steels. Grain orientation spread (GOS) maps were used to characterize the fraction of recrystallized structure in G115 steel during the aging processes. As for BM, the fraction of grain with GOS less than 1° is 0.064, indicating the lower recrystallization degree for the original G115. With the duration of aging time, the faction of grain with low GOS (<1°) presents a slight increase (from 0.064 to 0.092), which should be related to the local stress release [13,16,19].

Further, the grain boundary misorientation angle distribution of the BM and the G115 steel after 3000 h, 5000 h, and 10,000 h aging times is characterized by the EBSD technique, as shown in Figure 7. In the maps, black lines denote high-angle grain boundaries (HAGBs) for misorientation angles > 15°, and red lines represent low-angle grain boundaries (LAGBs) for values between 2° and 15° [12,18]. It can be found that the proportion of HAGB gradually increases from about 0.35 of BM and the steel after 3000 h aging time to 0.40 of the steel after 5000 h aging time [11]. The proportion of HAGB for the steel after 10,000 h aging time is about 0.39, which is slightly lower than that of the steel after 3000 h. It should be related to the formation of some precipitates and second-phase particles.

To obtain the evolution of local misorientation distributions of G115 steel during the aging process, kernel average misorientation (KAM) maps were used (Figure 8). The yellow area (KAM = 1–3) is given special attention in the map, which can reflect the local deformation. With the aging time, the fraction of yellow area in G115 steel gradually decreased (from 0.47 of BM to 0.24 of steel after 10,000 h aging time), and the local orientation distributions were homogeneous, which implied that the recrystallization of the microstructure occurred during the aging process [11,16], leading to the local stress release.

3.3. TEM Observation

Figure 9 shows the TEM images of G115 steel with different aging times at 700 °C. The slender and fragmentized laths can be found in BM with some particles distributed at the boundaries of martensitic lath. It can be confirmed that these particles are mainly composed of Cr₂₃C₆ and Fe₂W (Laves phase), and the MX precipitates can hardly be found in the matrix [15]. After 3000 h aging time, the width of the martensite lath structures increased, and the size of particles increased obviously compared with BM. The content of W and Fe increased, which indicates the growth of Laves phases during the aging process. As for the G115 after aging for 5000 h at 700 °C, the size of Laves phase particles significantly increased based on the SEM and EDS results. For the G115 after aging for 10,000 h at 700 °C, the particles of G115 steel are Cr₂₃C₆ precipitates, which can be confirmed by EDS results. The martensite lath structures became fuzzy in the G115 steel after the long-term aging process (5000 h and 100,000 h). The evolution of these microstructures should influence the impact toughness of G115, which needs further discussion.

3.4. Impact Toughness Evolution and Analysis

Figure 10 shows the toughness values of the G115 steel after long-term aging at 700 °C, and the statistical analysis of the impact toughness results can be seen in

Table 1. Statistical analysis of the impact toughness results of the G115 steel after long-term aging at 700 °C.

Impact Toughness Results	BM	500 h	1000 h	3000 h	5000 h	10,000 h
Minimum value (J)	48.5	14.8	14.9	19.2	16.5	15.3
Maximum value (J)	53.6	20.1	21.6	21.8	17.1	16.8
Mean value (J)	51.3	17.4	18.2	19.6	16.7	15.7

. The mean values of impact energy were 51.3 J for BM, 17.4 J for 500 h, 18.2 J for 1000 h, 19.6 J for 3000 h, 16.7 J for 5000 h, and 15.7 J for 10,000 h. The impact energy decreased quickly after 500 h, and maintained a basically unchanged state until 10,000 h. The decreased impact toughness of the G115 steel with aging time indicates a potential reduction in the resistance to plastic deformation and crack elongation. The decline in impact toughness was significant after the aging process at 700 °C, suggesting that the microstructural changes had a pronounced effect on the toughness. The decrease in impact toughness was attributed to the coarsening of precipitates, which provided fewer barriers to crack propagation. The development of microvoids within the steel matrix acted as potential sites for crack initiation [17]. The decline in impact toughness at the initial aging time is significant. Generally, the impact toughness can be an evaluation of its ability to resist crack propagation under high-stress and high-temperature conditions. In the case of G115 steel, the coarsening of precipitates and the development of microvoids are believed to be the primary causes of the decrease in impact toughness [11]. The larger precipitates can act as initiation sites for crack formation, and the microvoids can serve as paths for crack propagation, which reduces the overall resistance to impact for steel. The sharp drops in toughness after the 500 h aging process at 700 °C should be attributed to the appearance and development of Laves phase particles, which are large and hard, and have difficulty being coherent with the matrix during the deformation process. The coarsened particles facilitated the crack initiation and propagation, corresponding to the TEM results in Figure 9. The comprehensive effects of the precipitation of the Laves phase (negative effect, Figure 9), increase in HAGB (positive effect, Figure 7), and the softening of the metal matrix (positive effect) led to the impact toughness of the steel basically staying constant from 500 to 10,000 h.

Figure 11 presents the impact fracture for G115 steel after a long-term aging process, including the radiation area and the fiber region. The higher the fraction of fiber region, the better the toughness. The section rate of the fiber region in the fracture is about 70% for BM, which is about 45–55% for the G115 steel after the aging process at 700 °C. There are many dimples in the fiber region for all specimens, while the pore size of the fiber zone gradually decreased with aging time, which indicates the decrease in toughness for G115 steel during the long-term aging process. River patterns can be observed in the radiation area for all specimens, which indicates a typical cleavage fracture characteristic. The cleavage fracture characteristic became obvious with aging time, which indicates the decreasing toughness of G115 steel [18,21]. The counteracting effects of the precipitation of the Laves phase, the decrease in large-angle boundaries (negative effect), and the softening of the metal matrix (positive effect) led to the toughness of the steel decreasing rather slowly from 1000 to 5000 h. Combined with EBSD and TEM results, the Laves phase particles with large size acted as the main factor reducing the toughness, which destroyed the stabilization of the microstructure and weakened its ability to resist the local plastic deformation.

4. Conclusions

How the microstructure and toughness of G115 steel evolved during the aging process at 700 °C was systematically investigated in this work. It can be concluded that the microstructure of the G115 steel evolved significantly with increasing aging time at 700 °C, demonstrating that the microstructure consisted of a coarse-grained matrix with minor precipitates. The precipitates began to grow and coarsen with time, resulting in a finer-grained matrix with more extensive precipitate growth. The impact toughness of the steel presents a sharp decline from 51.3 J to 17.4 J at the initial aging time relating to the formation of particles, and an unobvious numerical fluctuation at about 17 J from 500 h to 100,000 h aging times. The decrease in impact toughness was attributed to the coarsening of Laves phase particles with large sizes in the steel matrix, which decreased the local plastic deformation resistance.