Research On Damage Evolution Behavior Of As-cast SA508-3 Low Alloy Steel

Large-size blanks are easily deformed under the action of high temperature and large forming force in the field of metal plastic forming,so that the bearing capacity of the material exceeds its limit value,and cracks occur in the core or surface of the forging,the main fracture form is ductile fracture.At present,most of the existing ductile cracking criteria are applied to predict cold deformation,but there is still a lack of universal criteria for damage and cracking behavior of materials during hot deformation.The damage evolution behavior of the as-cast SA508-3 low-alloy steel of nuclear power is studied in this paper.The basic process of thermal simulation is used to analyze the whole process from damage initiation to cracking from macroscopic and microscopic aspects.The damage evolution model of strain,strain rate and temperature is implanted into the finite element simulation software to predict the damage generation,which provides a theoretical basis for predicting and controlling the forging crack of SA508-3 low alloy steel.The high temperature tensile test of as-cast SA508-3 low alloy steel was carried out by Gleeble-1500 D thermal simulation tester to obtain the stress-strain curves under different deformation conditions.The fracture morphology of the sample and the morphology of the fracture were observed byscanning electron microscopy and optical microscope at the fracture.The results show that the existence of the second phase of the material itself and the precipitation of carbides during the thermal deformation process,leading to the occurrence of damage.On this basis,the influence of temperature and strain rate on the thermal deformation damage behavior is considered for the results of thermal simulation tensile test.Based on the typical difference of axial stress-strain curve of metal thermal deformation,the Arrhenius relationship modified by hyperbolic sine function is used.The critical damage strain model and the fracture strain model are established,and the strain process is described to obtain the damage process model.The concept of damage evolution is introduced based on the stress attenuation damage criterion,and the Zener-Hollomon parameter related to strain rate and deformation temperature isintroduced.In the damage evolution model,the thermal deformation damage evolution equation considering temperature,strain and strain rate is established.The model accurately considers the whole process of damage from initiation to growth.The single-pass isothermal compression test of as-cast SA508-3 low-alloy steel was carried out by Gleeble-1500 D thermal simulation tester to obtain high temperature flow stress curves of materials under different deformation conditions.Based on the Laasraoui two-stage flow stress model,the high temperature flow stress model of as-cast SA508-3 was established by multiple linear regression method.Combined with the material damage evolution behavior equation,the damage flow of material was predicted under the same thermal deformation conditions.A variable behavior model is used to calculate the flow stress curve of the material during hot drawing.In order to prevent the damage cracking behavior of materials during thermal deformation,the damage evolution model of as-cast SA508-3 was implanted into the numerical simulation of high temperature tensile test by using the secondary development interface of finite element simulation software DEFORM-3D.The accuracy of the established equation was verified by the cracking displacement of the specimen under different deformation conditions.Since the main thermoforming method of large-size blanks is the upsetting and drawing process,the damage evolution model of the finite element simulation software DEFORM-3D is used to simulate the upsetting experiment under different deformation conditions,and the casting in the actual forging condition interval is determined.The damage critical deformation and cracking critical shape variables of SA508-3 low alloy steel provide a theoretical basis for the prediction and control of cracking during thermoforming.