Study on Computational Prediction About Microstructure and Hardness of Q345 Steel Welded Joint Based on SH-CCT Diagram
HU Long1, WANG Yifeng1(), LI Suo1, ZHANG Chaohua1,2, DENG Dean1
1.College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China 2.School of Mechanical and Electrical Engineering, Nanchang University, Nanchang 330031, China
Cite this article:
HU Long, WANG Yifeng, LI Suo, ZHANG Chaohua, DENG Dean. Study on Computational Prediction About Microstructure and Hardness of Q345 Steel Welded Joint Based on SH-CCT Diagram. Acta Metall Sin, 2021, 57(8): 1073-1086.
Q345 steel is a low-alloy high-strength steel that is widely used in production. As the solid-state phase transition of Q345 steel is very sensitive to temperature, the microstructure and hardness of joints fabricated with this steel are difficult to predict. Therefore, studying the welding metallurgy, residual welding stress, and welding deformation of Q345 steel joints is essential for improving the safety and service life of Q345 welding structures. In this study, the microstructure and distribution of hardness of a Q345-steel tungsten-inert-gas welded and re-melted joint were calculated using four models encoded in the general finite element software, ABAQUS, and FORTRAN language. Three models were based on only one of three welding continuous-cooling transformation curves of the simulated heat-affected zone (SH-CCT) of Q345 steel, with peak temperatures of 1300, 1100, and 900oC (hereafter denoted as SH-CCT1300, SH-CCT1100, and SH-CCT900, respectively). The final model was based on the associated diagram consisting of these curves SH-CCT diagram above. Comparing the simulation and experimental results, the capabilities and accuracies of the prediction methods based on the different models were investigated. The microstructural calculations of the single SH-CCT diagram agreed with the experimental results only in the local heat-affected zone (HAZ) and largely deviated in the other areas. In the model based on SH-CCT900, the relative error of the ferrite volume fraction in the inter-critically HAZ (ICHAZ) was 7.7%. In the model based on SH-CCT1100, the relative errors of the ferrite, bainite, and martensite volume fractions in the fine-grained HAZ (FGHAZ) were 11.3%, 17.3%, and 15.5%, respectively. In the model based on SH-CCT1300, the relative error of the martensite volume fraction in the coarse-grained HAZ (CGHAZ) was 29.6%. In contrast, the microstructural calculations of the associated SH-CCT diagram agreed with the experimental results over the whole HAZ. The results showed that models based on the single SH-CCT diagrams met the tested hardness only in several narrow areas of the HAZ, but the hardness computed by the model, based on the associated diagram, was consistent with the tested hardness, with absolute differences ranging from 0 to 14 HV. The calculated microstructure and hardness in the fusion zone (FZ) greatly deviated from the test results in all the four models, indicating that predictions in the FZ require an accurate SH-CCT diagram of the FZ. In practical welding processes, the corresponding single SH-CCT diagram can adequately predict the microstructure and hardness during the phase transition of one HAZ area (such as the CGHAZ).
Fig.1 Schematic of dimensions of welded joint (unit: mm) (a) and arrangement of thermocouple (b)
Fig.2 The hardness test location (a) and distributed distance (b) (unit: mm)
Fig.3 Schematic of finite element method (FEM) (HAZ—heat-affected zone, WM—weld metal)
Fig.4 Schematic of sequential couplings between temperature and metallurgy
Fig.5 The progress of thermal and metallurgical coupling calculation (SH-CCT—continuous cooling transformation curves of simulated heat-affected zone)
Fig.6 The parameters K and n of calculating ferrite and bainite, (a) and (b) got from SH-CCT1300, (c) and (d) got from SH-CCT1100, (e) and (f) got from SH-CCT900 (F—ferrite, P—pearlite, B—bainite)
Case
SH-CCT900
SH-CCT1100
SH-CCT1300
1
Yes
×
×
2
×
Yes
×
3
×
×
Yes
4
Yes
Yes
Yes
Table 1 Simulation cases of microstructure
Fig.7 Interpolation schematic diagram of ferrite starting transition line between 900℃ and 1100℃ for peak temperature
Fig.8 OM images of microstructures of welded joint (M—martensite)
Fig.9 Hardness distribution of welded joint
Fig.10 The temperature field distribution of moving heating source
Fig.11 Comparison between region division of simulation and experiment
Fig.12 The heat cycle curves of simulation and experiment at different distance from weld toe (showing in inset) on upper surface
Fig.13 The fraction of austenite (f) during heating process
Fig.14 Microstructure simulation results of welded joint
Fig.15 The calculated phase fractions at different distances from weld center
Fig.16 Hardness distributions of welded joint (simulation) for Case 1 (a), Case 2 (b), Case 3 (c), and Case 4 (d)
Fig.17 Hardness distributions at 1 mm distance from the upper surface of the welded joint
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