| The steam parameters of coal-fired power plant have developed to700°C advanced ultra-supercritical?A-USC?driven by increasing energy efficiency and protecting environment.Only Ni based or Ni-Fe based superalloys with better high-temperature creep strength and excellent oxidation/hot corrosion resistance can meet the requirement of the hottest boiler and turbine components in the 700°C A-USC power plant.The Ni-Fe based superalloy GH984G with proprietary intellectual property rights,low cost and excellent high temperature performance was considered as the candidate materials for the 700°C A-USC power plants in China.The manufacture of large scale forgings for the Ni-Fe based heat resistant alloy is unmanageable due to the following two reasons.Firstly,A significant number of alloying elements?higher than 40%?in the Ni-Fe based alloy lead to high deformation resistance and narrow processing temperature range.Secondly,the crack initiation and inhomogeneous microstructure are easy to appear due to the local stress concentration and the different temperature between the surface and center of the large scale forgings Therefore,it is necessary to simulate the forging process by isothermal compression test to obtain the optimal deformation parameters and avoid macro cracks as well as inhomogeneous microstructure.In general,the initial alloy state of forging is homogenization alloy.However,the influence of initial alloy state during the isothermal compression test is often neglected.Most of the studies took hot-rolled alloys as the experimental materials in the isothermal compression test,which probably resulted in a deviation from the actual forging process.This study investigated the hot deformation behaviors and dynamic recrystallization?DRX?mechanisms of homogenization and hot-rolled alloys in order to establish the correlation between the initial alloy states and hot deformation behaviors.The deformation resistance and apparent activation energy of hot deformation for the homogenization alloy is larger than that of the hot-rolled alloy.The reasonable ranges of processing parameters for the homogenization and hot-rolled alloys are 1100-1200°C/0.01-0.2 s-1 and1100-1200°C/1.0-10 s-1,respectively,based on the processing maps.The flow instability domain of the homogenization alloy is larger than that of the hot-rolled alloy.The different deformation behaviors of the homogenization and hot-rolled alloys result from the discrepant initial grain sizes and DRX mechanisms.In the homogenization alloy with coarsening grains,the poor deformation compatibility results in severe dislocation pile-up and compels the continuous dynamic recrystallization?CDRX?to activate only when the dislocation density exceeds a threshold.The homogenization alloy is vulnerable to the flow localization,wedge cracking and adiabatic shear under this DRX nucleation mode.In the hot-rolled alloy with fine grains,the good deformation compatibility makes the grain boundary migration easier and promotes the occurrence of discontinuous dynamic recrystallization?DDRX?.The hot deformation behavior and dynamic recrystallization of the homogenization and hot-rolled alloys is different.It is reasonable to take homogenization alloys as the experimental materials in the isothermal compression test.The experimental results can optimize the isothermal compression test and provide more reliable forging parameters for manufacturing the large scale forgings of the GH984G alloy. |
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