“…However, as the cooling rate was more than 0.08 °C/s, the volume fraction of RA in the carburised layer is higher than that of matrix. The bainite transformation Kinetics under non-isothermal conditions has been modeled by the modified Johnson-Mehl-Avrami (JMA) equation, which is expressed as follows [24][25][26],…”
In this study, the phase transformation behaviour of the carburised layer and the matrix of 23CrNi3Mo steel was comparatively investigated by constructing continuous cooling transformation (CCT) diagram, determining the volume fraction of retained austenite (RA) and plotting dilatometric curves. The results indicated that Austenite formation start temperature (Ac1) and Austenite formation finish temperature (Ac3) of the carburised layer decreased compared to the matrix, and the critical cooling rate (0.05 °C/s) of martensite transformation is significantly lower than that (0.8 °C/s) of the matrix. The main products of phase transformation in both the carburised layer and the matrix were martensite and bainite microstructures. Moreover, an increase in carbon content resulted in the formation of lamellar martensite in the carburised layer, whereas the martensite in the matrix was still lath. Furthermore, the volume fraction of RA in the carburised layer was higher than that in the matrix. Moreover, the bainite transformation kinetics of the 23CrNi3Mo steel matrix during the continuous cooling process indicated that the mian mechanism of bainite transformation of the 23CrNi3Mo steel matrix is two-dimensional growth and one-dimensional growth.
“…However, as the cooling rate was more than 0.08 °C/s, the volume fraction of RA in the carburised layer is higher than that of matrix. The bainite transformation Kinetics under non-isothermal conditions has been modeled by the modified Johnson-Mehl-Avrami (JMA) equation, which is expressed as follows [24][25][26],…”
In this study, the phase transformation behaviour of the carburised layer and the matrix of 23CrNi3Mo steel was comparatively investigated by constructing continuous cooling transformation (CCT) diagram, determining the volume fraction of retained austenite (RA) and plotting dilatometric curves. The results indicated that Austenite formation start temperature (Ac1) and Austenite formation finish temperature (Ac3) of the carburised layer decreased compared to the matrix, and the critical cooling rate (0.05 °C/s) of martensite transformation is significantly lower than that (0.8 °C/s) of the matrix. The main products of phase transformation in both the carburised layer and the matrix were martensite and bainite microstructures. Moreover, an increase in carbon content resulted in the formation of lamellar martensite in the carburised layer, whereas the martensite in the matrix was still lath. Furthermore, the volume fraction of RA in the carburised layer was higher than that in the matrix. Moreover, the bainite transformation kinetics of the 23CrNi3Mo steel matrix during the continuous cooling process indicated that the mian mechanism of bainite transformation of the 23CrNi3Mo steel matrix is two-dimensional growth and one-dimensional growth.
“…10,32 Moreover, a large amount of Bainite formed in WM was attributed to the larger CR. 33 Figure 10.Microstructure evolution process in HAZ and WM of multi-pass butt joint of Q690 HSS. (a) HAZ and (b) WM.…”
During multi-pass butt-welding of high-strength steel (HSS) thick plate, it is difficult to estimate the hardness performance of thick weld since multi thermal cycles can drive complex microstructure evolution. In this study, butt-welded joint of 75 mm-thick Q690 HSS was prepared by multi-pass shielded metal arc welding (SMAW) in advance, meanwhile experiments were carried out to characterize microstructure and measure hardness distribution in butt-welded joint. Then the welding temperature history of butt-welded joint was numerically examined. Based on the irreversibility of Martensite and Bainite, the microstructure evolution model was modified to predict the complicated microstructure evolution in multi-pass welded joint. Compared with experimental data, the validity of modified microstructure evolution model was verified. With the application of simulated thermal cycles and material chemical compositions as input parameters, volume fraction distribution of each phase was obtained by modified microstructure evolution model and hardness value was calculated by hardenability algorithm. Both experimental and predicted results exhibited that microstructure in weld metal (WM) is mainly composed of granular Bainite with a small amount of Ferrite and Martensite, and heat-affected zone (HAZ) consists of predominantly lath Martensite. Hardness distribution in butt-welded joint obtained by the prediction method is in accordance with the measurement. Average hardness value in WM and BM is 290 VPN and 250 VPN. A sharp decrease in HAZ hardness can be found from the fusion line to base metal (BM). The maximal value is in the HAZ close to fusion line and the corresponding prediction deviation is 6%.
“…For conventional high-strength low-alloy ultra-heavy plates, a matrix structure with sufficiently high strength, such as lath martensite/bainite is obtained by quenching, which is subsequently tempered to improve toughness [1][2][3][4]. However, it is usually impossible to avoid the low cooling rate in the center of the ultra-heavy plates during cooling, causing inhomogeneous microstructure and mechanical properties across the thickness direction, which highlights the need to enhance the hardenability of the alloy [5][6][7].…”
High hardenability is of great importance to ultra-heavy steel plates and can be achieved by tailoring the composition of steel. In this study, the continuous cooling transformation (CCT) curves of two high-strength low-alloy (HSLA) steels (0.16C-0.92Ni steel and 0.12C-1.86Ni steel) were elucidated to reveal the significance of C–Ni collocation on hardenability from the perspective of morphology and crystallography. At a low cooling rate (0.5 °C/s), the 0.12C-1.86Ni steel showed higher microhardness than 0.16C-0.92Ni steel. The microstructure in 0.16C-0.92Ni steel was mainly granular bainite with block-shaped martensite/austenite islands (M/A islands), while that in 0.12C-1.86Ni steel was typically lath bainite with film-shaped M/A islands, denoting that the 0.12C-1.86Ni steel is of higher hardenability. Moreover, the 0.12C-1.86Ni steel exhibited a higher density of block boundaries, especially V1/V2 boundaries. The higher density of block boundaries resulted from the weakened variant selection due to the larger transformation driving force and more self-accommodation of transformation strain induced by the reduced carbon and increased nickel content.
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