Determination of SHCCT Curve of Low Temperature Steel 09mnnidr and Analysis of Low Temperature Toughness of Welded Joints
By means of thermal-dilatometer, the simulated heat affected zone continous cooling transformation (SHCCT)
curve of 09MnNiDR steel was measured by Gleeble3800 thermal-simulator, and the microstructure of different
cooling rate was observed and analyzed by OM and SEM. According to the results of thermal simulation,
reasonable welding parameters were used for welding, the microstructure of welded joints were observed, and
the low temperature toughness was analyzed at -70℃. The results show that the mainly room temperature
microstructure of 09MnNiDR steel is pro-eutectoid ferrite, ferrite, pearlite, weissner structure, granular bainite,
upper bainite and martensite by different cooling rate. Under slow cooling condition, the mainly room
temperature microstructure is ferrite and pearlite. When the cooling rate reaches above 5℃/s, the mainly room
temperature microstructure is bainite, and irregular polygon M-A group distribute in the ferrite matrix random.
When the cooling rate reaches more than 15℃/s, the mainly room temperature microstructure is martensite. The
microstructure of the weld area and the heat-affected area were mainly ferrite and pearlite. By observing the
microstructure of the impact fracture of the weld area and the heat-affected area at -70℃, the tear dimples and
isoaxial dimples could be observed. The weld area and the heat-affected area were both dominated by ductile
fractures, so the low-temperature toughness was better.