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Austenite grain size after austenizing treatment at different temperatures with different times (a) 1030 ¦, 20 h (b) 1050 ¦, 6 h (c) 1050 ¦, 20 h (d) 1070 ¦, 3 h (e) 1070 ¦, 15 h (f) 1090 ¦, 2 h (g) 1090 ¦, 12 h (h) 1120 ¦, 2 h (i) 1150 ¦, 1 h (j) 1200 ¦, 20 h
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ABSTRACT The kinetic law of austenite grain growth in the X12CrMoWVNbN10–1–1 ferrite heat–resistant steel, which has been used as the high and medium pressure rotor of ultra–supercritical generating units, has been studied by quantitatively measurement of the austenite grain size after austenitized from 1010 �to 1200 �with holding time from 5 to 12...
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Citations
... Linear regression of ( ) The more phase particles are precipitated, the smaller the radius, the greater the "Pinning effect" resistance generated, and the better the effect of impeding the growth of grains [11] [12]. ...
... Furthermore, the precipitation during hot deformation tended to cause the formation of micro-cracks and large particles, such as M23C6. There are lots of research on this type of steel [1][2][3][4][5]. Cui et al. [2] investigated microstructure evolution of the X12 during short-time(< 400 h) creep within the range of temperatures from 600°C to 750°C and the characteristics of precipitations such as M 23 C 6 (M: Cr, Fe, Mo, W, etc.) type carbides, MX (M: V, Nb, etc. and X: C, N) type carbonitrides and hardening phases. ...
... Cui et al. [2] investigated microstructure evolution of the X12 during short-time(< 400 h) creep within the range of temperatures from 600°C to 750°C and the characteristics of precipitations such as M 23 C 6 (M: Cr, Fe, Mo, W, etc.) type carbides, MX (M: V, Nb, etc. and X: C, N) type carbonitrides and hardening phases. Han et al. [4] focused on discussing the influence of precipitations on the material performance. Additionally, the kinetic law of austenite grain growth in the X12 was studied by quantitatively measurement of the austenite grain size after isothermal austenitized treatment. ...
... As can be seen, the grain size distributions for the S1 to S3 steels at 1000°C are generally uniform, and most of them are below 20 μm. When the heating temperature increases to 1300°C, the multimodal curves in the three steels are observed, indicating that lots of austenite grains grow in an abnormal way at this temperature [21]. By comparison, the number of peaks is found to increase in the sequence of S3, S2 and S1, which suggests that the grain growth is indeed inhibited by Mg addition. ...
To reveal the effect of Mg addition on the austenite grain growth in low-carbon steel, the steels containing different Mg contents were refined with a vacuum induction furnace. First, the steels were subjected to the temperature range of 1000–1300°C for a holding time of 30 min. Moreover, using a confocal scanning laser microscope, the growth of austenite grains was investigated under isothermal holding conditions (1400°C), and the γ–α phase transformation was also identified after the samples were subjected to a cooling rate of 5°C s⁻¹. It reveals that the grain growth is inhibited by Mg addition after increasing the temperature to 1300°C. The kinetic equations of austenite grain growth were further established by regression analysis based on the experimental results. Furthermore, a significant increase in the proportion of intra-granular ferrite takes place in 0.0026%Mg-added steel at the initial stage of γ → α with a cooling rate of 5°C s⁻¹. This is mainly attributed to the plenty of Mg-containing inclusions, which are demonstrated to be effective nuclei for acicular ferrite, being in the Mg-added steel.
... Figures 8 and 9 show the microstructure evolution at different solution temperatures and solution times with corresponding austenitic grain-size distribution histograms, respectively. As shown in Figure 9, the grain size distribution of austenite is close to lognormal distribution, which is consistent with the results presented by Han et al. [16] and Kurtz et al. [17]. When the solid solution time is 0.5 h at 1170 °C, the fine austenite grains distribute inhomogeneously as coarse austenite grains and contact each other, as shown in Figure 8a. Figure 9a indicates the grain size of austenite is mainly in the range of 20-50 μm. ...
... Figures 8 and 9 show the microstructure evolution at different solution temperatures and solution times with corresponding austenitic grain-size distribution histograms, respectively. As shown in Figure 9, the grain size distribution of austenite is close to lognormal distribution, which is consistent with the results presented by Han et al. [16] and Kurtz et al. [17]. When the solid solution time is 0.5 h at 1170 • C, the fine austenite grains distribute inhomogeneously as coarse austenite grains and contact each other, as shown in Figure 8a. Figure 9a indicates the grain size of austenite is mainly in the range of 20-50 µm. ...
In view of the requirements for mechanical properties and service life above 650 °C, a high-Mn austenitic hot work die steel, instead of traditional martensitic hot work die steel such as H13, was developed in the present study. The effect of heat treatment on the microstructure and mechanical properties of the newly developed work die steel was studied. The results show that the microstructure of the high-Mn as-cast electroslag remelting (ESR) ingot is composed of γ-Fe, V(C,N), and Mo2C. V(C,N) is an irregular multilateral strip or slice shape with severe angles. Most eutectic Mo2C carbides are lamellar fish-skeleton-like, except for a few that are rod-shaped. With increasing solid solution time and temperature, the increased hardness caused by solid solution strengthening exceeds the effect of decreased hardness caused by grain size growth, but this trend is reversed later. As a result, the hardness of specimens after various solid solution heat treatments increases first and then decreases. The optimal combination of hardness and austenitic grain size can be obtained by soaking for 2 h at 1170 °C. The maximum Rockwell hardness (HRC) is 47.24 HRC, and the corresponding austenite average grain size is 58.4 μm. When the solid solution time is 3 h at 1230 °C, bimodality presented in the histogram of the austenite grain size as a result of further progress in secondary recrystallization. Compared with the single-stage aging, the maximum impact energy of the specimen after two-stage aging heat treatment was reached at 16.2 J and increased by 29.6%, while the hardness decreased by 1–2 HRC. After two-stage aging heat treatment, the hardness of steel reached the requirements of superior grade H13, and the maximum impact energy was 19.6% higher than that of superior grade H13, as specified in NADCA#207-2003.
Owing to the high strength and hardness of advanced high-strength steels (AHSS), the die materials for AHSS sheet stamping must have superior mechanical properties. A novel carburization technology on high-chromium, low-carbon, low-cost stainless steel XCr13 was used to achieve a hardness of approximately 60 HRC, with a carburized layer of approximately 0.5 mm; these specifications meet the requirements of AHSS sheet stamping. To evaluate the wear characteristics of the treated XCr13, regular pin-on-disk wear tests were performed under dry sliding conditions at room temperature. JIS SKD11, a popular cold work die material, was also tested and compared with XCr13. A numerical simulation model for AHSS sheet flanging with a W-shaped specimen was set up in ABAQUS, and the user subroutine UMESHMOTION was used to evaluate the wear resistance of the two die materials, including the wear surface profile, wear depth, and wear severity index value. For JIS SKD11, the agreement between the simulation and experiment results was good, which verifies the reliability and accuracy of the proposed wear simulation model and method. It was found that the wear resistance of carburized XCr13 was at the same level as that of JIS SKD11 because of the carburized zone. Further, carburized XCr13 can work well as a die material for AHSS stamping.
The precipitation of secondary phases, the austenite grain growth and their effects on the mechanical properties of the HR3C heat-resistant steel in long-term service at 605 °C in an ultra-supercritical power generation unit were investigated. The results show that during the service period of the HR3C steel, the precipitation of M23C6 (M=Cr, Fe, Ni and Mo) follows the Kurdjumov–Sachs orientation relationship. Polygonal M23C6 particles preferentially precipitate and continuously distribute at the austenite grain boundaries. Meanwhile, the nanoscale NbCrN particles precipitate on the dislocations in the austenite grains. The austenite grain growth occurs in three stages with a growth exponent of 5.92 in the third stage, indicating that it is strongly restrained by the precipitates. In the as-served HR3C steel, the precipitation of the secondary phases, rather than the austenite grain growth, results in the strong improvement of the strength and hardness, albeit at the expense of a clear degradation of the toughness and plasticity.
The evolution of the microstructure of P92 steel during heat processing has a significant effect on the final performance of steel products and is thus an area of critical interest. Accordingly, the grain growth behavior of austenite and δ-ferrite changes in P92 steel is comprehensively investigated in this study through thermodynamic calculations and physical simulation. The results show that the AC1, AC3 and γ–δ transformation temperatures of P92 steel are 841 °C, 878 °C and 1095 °C, respectively, which are comparable to the measured values of 850 °C, 879 °C and 1175 °C, respectively. When the temperature exceeds the AC1 temperature, the austenite grain size increases with increasing temperature and longer holding times, and the growth rate of austenite grains increases with higher temperatures but decreases with holding time at a certain temperature. A model that shows the austenite grain growth kinetics is established: Dn − D
n0
= 2.03 × 1016exp(− 498,770/RT)t, in which the time exponent n decreases from 4.52 to 2.11 with an increase in temperature from 1000 °C to 1200 °C due to the reduced pinning effect resultant of the dissolution of precipitates, M23C6, VX and NbX at 887 °C, 1073 °C and 1200 °C, respectively. The formation of δ-ferrite at 1200 °C when held for 600 s and 1300 °C when held for 60 ~ 600 s causes a smaller austenite grain size. The relationship between the evolution of the microstructure of P92 steel and the mechanical properties is also studied. The results show that the prior austenite grain size has an inverse relationship with impact energy and ultimate tensile strength at 620 °C, while the presence of soft δ-ferrite leads to a reduction in the microhardness, impact energy and ultimate tensile strength at 620 °C.
Grain growth behaviors of LZ50 have been systematically investigated for various temperatures and holding times. Quantitative evaluations of the grain growth kinetics over a wide range of temperature (950-1200 °C) and holding time (10-180 min) have been performed. With the holding time kept constant, the average austenite grain size has an exponential relationship with the heating temperature, while with the heating temperature kept constant, the relationship between the austenite average grain size and holding time is a parabolic curve approximately. The holding time dependence of average austenite grain size obeys the Beck’s equation. As the heating temperature increases, the time exponent for grain growth n increases from 0.21 to 0.39. On the basis of previous models and experimental results, taking the initial grain size into account, the mathematical model for austenite grain growth of LZ50 during isothermal heating and non-isothermal heating is proposed. The effects of initial austenite grain size on hot deformation behavior of LZ50 are analyzed through true stress-strain curves under different deformation conditions. Initial grain size has a slight effect on peak stress.
Microstructure and high temperature mechanical properties of X12CrMoWVNbN10-1-1 heat resistant steel were studied by SEM and TEM. The results show that a typical tempered lath martensitic microstructure of the steel is obtained by quenching at 1075°C for 1 h, cooling in oil followed by twice tempering treatments at 570°C for 4 h, cooling in air and then 720°C for 4 h, cooling in air. M23C6-type carbides are distributed along prior austenite grain boundaries and lath boundaries. Nanosized MX-type precipitates are observed in matrix within the laths. There are small amount of undissovled MX-type particles with size up to 0.6 μm during the quenching heat treatment. The tensile and yield strength of the steel decreases while the elongation and reduction in area increase with the increasing temperature when testing in the temperature range of 500-650°C. The stress rupture strength has a parabolic curve relationship with Larson-Miller parameter P=T(25+lg(tf))/1000.
The growth law of austenite grain in 40CrNi2MoE steel at various heating temperatures (850-1200 ℃) and holding time (30-480 min) was investigated on the basis of experimental data and Beck, Hillert and Sellars mathematical models by metallographic technique. The results show that the prior-austenite grain size of 40CrNi2MoE steel increases with increasing heating temperature and holding time, and the austenite grain of test steels begins to coarsen when the heating temperature is over 1050 ℃ or holding time is over 120 min. By comparative analysis of the Beck, Hillert and Sellars models, it can be concluded that Sellars model has high accuracy to predict the austenite grain size of 40CrNi2MoE steel, and the model equation of austenite grain growth can be described: DSellars5.49=7.64×1021texp(-390081/(RT)), (during 850 ℃≤T≤1050 ℃); DSellars8.13=8.04×1041texp(-771322/(RT)), (during 1050 ℃≤T≤1200 ℃).
