Investigation On Surface Cracking And Microstrcture Evolution Of Superheavy Forgings During Forging

In recent years,a large number of superheavy forgings have been manufactured and applied in the construction of the latest generation of nuclear power plants,as nozzle shells,the upper head of reactor pressure vessels,and monoblock low-pressure rotors.The emergence of superheavy forgings not only reduced the manufacturing cycle of the nuclear reactor pressure vessels significantly,but also improved the safety and service life of nuclear power plants.The main difficulty in forging superheavy forgings were very low strain rate(the true strain rate was approximately 0.0001s^-1)and slow surface cooling.Typically,strain rates that were selected in investigations of hot deformation and cracking behaviors of heavy forgings ranged from 0.001 to 10s^-1.The lack of basic data on the microstructure evolution and cracking behavior of superheavy forgings limits their manufacture,development,and applications.Based on this,18Mn18Cr0.6N steel with a coarse grain structure was selected as a model material in this study to investigate the surface cracking mechanism and microstructure evolution.In addition,DEFORM-3D software was used to simulate the continuous cooling deformation process of superheavy forgings.The main conclusions are as follows:?1?18Mn18Cr0.6N was tension tested at 0.001s^-11 to fracture from 1200°C to1090°C?fracture temperature??T1200-1090?.It's reduction of area was lower than that of1200°C?T1200?and 1900°C?T1090?.The flow curves of tensile specimens under different deformation conditions had similar trends,and the stress increases with strain and decreased after reaching the peak stress.Moreover,the peak stress of T1200-1090 was much larger than T1200 and T1090.?2?The KAM value and the fraction of LAGBs near the T1200-1090 fracture were higher than those of T1200 and T1090 by EBSD.However,its DRX fraction was much lowe than T1200 and T1090.18Mn18Cr0.6N steel was compressed at 0.001s^-11 to 0.275from 1200°C to 1090°C?C1200-1090-275?.It's DRX fraction was lower than that of1200°C?C1200-275?and 1900°C?C1090-275?,however,it's fraction of LAGBs was lower.Based on these results,the following explanation of the effect of continuous cooling on the microstructural evolution in these materials was proposed.?3?18Mn18Cr0.6N was compressed to different true strains at 1150°C and 1050°C at 0.0001s^-1.The grain refinement effect was remarkable at 1050°C.However,the deformation stress increased about one time from 1150°C to 1050°C.On the basis of the above results,a schematic illustration of the nucleation mechanism of the DRX for18Mn18Cr0.6N steel deformed at 0.0001s^-11 was shown.?4?Crack was easily induced at the grain boundary due to the uncoordinated rotation of coarse grains when 18Mn18Cr0.6N deformed at a rate of 0.0001s^-1.And they propagated along HAGBs,TBs,and the inside of grains,in that sequence.?5?Whether it is constant temperature deformation or continuous cooling deformation,the highest Normalized C&L damage value is found at the half height of the side surface of the cylindrical specimen and gradually decreases toward the core.Besides,the Normalized C&L damage value during continuous cooling deformation is higher than that of constant temperature deformation,and the difference gradually increases with the increase of strain.